Symetrix 425 Service manual

SymetrixSymetrix

425

Dual Compressor / Limiter

Owner’s Manual

Signal Processing at it’s Best!

Revision 2.0, 10/29/93
Part number: 530201
Subject to change at our whim, and without notice.
Copyright 1992–1993 by Symetrix Inc. All rights Reserved.
Batteries not included. Void where taxed or prohibited. Ground ain’t ground!

Table of Contents

1. Introduction...........................................................................................1-1

1.1 About this manual...........................................................................................1-1

1.2 Operator Safety Summary..............................................................................1-2

1.2.1 Equipment Markings....................................................................................1-2

1.2.2. Terms .........................................................................................................1-2

1.3. Other Safety Information ...............................................................................1-3

2. Dynamics Processing Tutorial and Functional Basics.....................2-1

2.1 Dynamics Processing Basics..........................................................................2-1

2.1.1 Compressors and Limiters...........................................................................2-1

2.1.2 Expanders and Gates..................................................................................2-2

2.1.3 Ratio............................................................................................................2-2

2.1.4 Gain vs Output ............................................................................................2-2

2.1.5 Attack Time..................................................................................................2-2

2.1.6 Release Time ..............................................................................................2-3

2.1.7 Threshold Setting ........................................................................................2-3

2.1.8 Interpreting the Displays..............................................................................2-3

2.2 Using the Sidechain........................................................................................2-3

3. Technical Tutorial ................................................................................3-1

3.1. Matching Levels vs Matching Impedances .....................................................3-1

3.2. Signal Levels ..................................................................................................3-2

3.3. I/O Impedances ..............................................................................................3-2

3.4. Polarity Convention ........................................................................................3-2

3.5. Input and Output Connections........................................................................3-3

4. Front Panel Overview..........................................................................4-1

5. Rear Panel Overview...........................................................................5-1

6. Fast First Time Setup ...........................................................................6-1

6.1 Connections ...................................................................................................6-1

6.2 Settings ..........................................................................................................6-1

6.3 Initial Setup.....................................................................................................6-2

6.4 Refining Your Settings....................................................................................6-2

6.5 Stereo Coupling..............................................................................................6-2

7. Using the 425.........................................................................................7-1

7.1 Block Diagram................................................................................................7-1

7.2 Installation......................................................................................................7-1

7.3 The 425 as a Compressor..............................................................................7-1

7.4 The 425 as a Ducker......................................................................................7-2

7.5 The 425 as a Limiter.......................................................................................7-2

7.6 The 425 as an Expander................................................................................7-3

7.7 Using the Expander and Compressor Simultaneously ...................................7-3

i

8. Applications.........................................................................................8-1

8.1 Vocal Level Smoothing...................................................................................8-1

8.2 Removing Noise from Vocal Tracks................................................................8-1

8.3 Constant Level Paging ...................................................................................8-1

8.4 Paging with Ducking.......................................................................................8-2

8.5 Stage Monitors ...............................................................................................8-2

8.6 Keyed Bass....................................................................................................8-3

8.7 Sibilance Control ............................................................................................8-3

9. Troubleshooting Chart ........................................................................9-1

10. 425 Dual Compressor Limiter Limited Warranty..............................10-1

11. Repair Information...............................................................................11-1

11.1. Return Authorization....................................................................................11-1

11.2. In-Warranty Repairs.....................................................................................11-1

11.3. Out-of-Warranty Repairs..............................................................................11-1

12. Specifications......................................................................................12-1

13. PCB Layouts and Schematics............................................................13-1

13.1 Troubleshooting Hints...................................................................................13-1

13.2 Additional Reading .......................................................................................13-1

Appendix A. Architects and Engineers Specification............................A-1

Appendix B. Disassembly Instructions ..................................................B-1

ii

EXPANDER

RELEASE

THRESHOLD (dBu)

-20

-5 -30

FAST

+5

BYPASS

SLOW

0VU CLIP-20 -3

-8 -6 -10 -12 -25 -15 -2 -6 -12 -18

-4

COMPRESSOR

RELEASE RELEASE

THRESHOLD (dBu)

-10

+10 -25

FAST

-40

BYPASS

SLOW

RATIO (X:1)

1.3 2.5

THRESHOLD (dBu)

-2.5

-10 1

10

LIMIT

+5 2

+12.5 +5 +15

0

-10 -6 -9 -12 -2

GAIN (dB)

+10

DUAL STEREO

LINK

THRESHOLD (dBu)

-20

STEREO

IN

DUAL MONO

BYPASS

+20 BYPASS

-5 -30

+5

BYPASS

EXPANDEROUT

RELEASE

FAST

SLOW

THRESHOLD (dBu)

-10

-25 +10

FAST

-40

BYPASS

-4 -6 -10 -12 -8 -25 -15 -6 -12 -2 -18

COMPRESSOR

RATIO (X:1)

1

-6 -9 -12 -2

THRESHOLD (dBu)

+5 2

-2.51.3 2.5

-10

10SLOW

LIMIT

+12.5 +5 +15

0

BYPASS

GAIN (dB)

+10

0VU CLIP-10 -20 -3

OUT

IN
BYPASS

+20

1. Introduction

The Symetrix 425 is a dual channel compressor/limiter/expander. The two channels may be
operated in dual-mono mode, or in stereo mode. In stereo mode, both channels receive
identical control signals, which minimizes image shifts caused by unequal gains in the two
channels.

At Symetrix, we feel that the 425 represents the distillation of our 15 years of signal
processing experience into a product that is at once versatile and easy to use without having
important controls removed in the interest of simplicity. The 425 is unique because its
processor sections are always in-line. There is no selector switch to select between expander,
compressor, or limiter. Now you can expand the low-level parts of a signal to improve its
signal-to-noise ratio, then apply compression to add punch and density and still have the
security of a peak limiter for overload protection. We call this approach to total level control
IDP or Integrated Dynamics Processing.

IDP also means powerful, streamlined controls that make the 425 easy to learn and quick to
set up, even with three types of processing in one rack space. Each section has the controls
that you need to respond to any audio situation fast. Individual LED meters show you what's
going on inside each section.

The 425 is Listed by Underwriters Laboratories Inc. (UL). Samples of this product have been
evaluated by UL and meet the applicable UL Standards for Safety.

In the final analysis, Integrated Dynamics Processing means clean, quiet sound that meets
professional demands in any situation. High-quality components and a minimalist signal path
make the 425 exceptionally transparent.

1.1 About this manual

We recommend that you read this manual from cover to cover. Somewhere between the
confines of the two covers you should find the answers to most (98%) of your questions, both
technical as well as musical.

If you're in a hurry (like most of us), or if you really don't believe that someone could write a
decent owners manual that you can read and understand, then do us both a favor and read
section 6, "Fast First Time Setup." This section will help you get connected, tell you what the
knobs do, and send you on your way.

This manual contains the following sections:

Chapter 1. Introduction: introduces the 425 and this manual.
Chapter 2. Using the 425: describes how to use the 425. Covers installation, signal levels,

input and output impedances, and physical features.
Chapter 3. Technical Tutorial: a basic and not-so-basic discussion of signal levels, input and

output impedances and connection polarity.
Chapter 4. Front Panel Overview: a brief look at the controls and switches located on the front

panel of the 425.

Chapter 5. Rear Panel Overview: a brief look at the rear panel of the 425.
Chapter 6. Fast First Time Setup: is the setion to read if you can't wait.

Rev 2.0, 10/29/93

1-1

Chapter 7. Using the 425: describes the use of the 425 in detail.

p

Chapter 8. Applications: gives some hints for using the 425 in a variety of situations and
provides a short introduction to dynamics processing..

Chapter 9. Troubleshooting: tells what to do when the 425 doesn't work.
Chapter 10. Limited Warranty: describes the 425's limited warranty.
Chapter 11. Service Information: tells how to get your 425 repaired, how to obtain parts, and

how to contact the factory.

Chapter 12. Specifications: lists the technical specifications of the 425's performance.
Chapter 13. Schematics: contains the printed circuit board layout(s) and schematic diagrams.
Appendix A. Appendix A: contains the Architects and Engineer's specifications.
Appendix B. Appendix B: contains disassembly instructions.

1.2 Operator Safety Summary

The information in this summary is intended for persons who operate the equipment as well
as repair personnel. Specific warnings and cautions are found throughout this manual
wherever they may apply; they do not appear in this summary.

The notational conventions used in this manual and on the equipment itself are described in
the following paragraphs.

1.2.1 Equipment Markings

CAUTION

RISK OF ELECTRIC SHOCK

DO NOT OPEN

ATTENTION:

No user serviceable parts inside. Refer servicing to qualified service personnel.

Il ne se trouve a l’interieur aucune piece pourvant entre reparée l’usager.

S’adresser a un reparateur compétent.

The lightning flash with arowhead symbol
within an equilateral triangle is intended to
alert the user of the presence of uninsulated
"dangerous voltage" within the product's
enclosure that may be of sufficient
magnitude to constitute a risk of electric
shock to

ersons.

RISQUE DE CHOC ELECTRIQUE
NE PAS OUVRIR

The exclamation point within an equilateral
triangle is intended to alert the user of the
presence of important operating and
maintenance (servicing) instructions in the
literature accompanying the appliance (i.e.
this manual).

Caution

To prevent electric shock, do not use the polarized plug supplied with this appliance
with any extension cord, receptacle, or other outlet unless the blades can be fully
inserted to prevent blade exposure.

1-2

Rev 2.0, 10/29/93

1.2.2. Terms

Several notational conventions are used in this manual. Some paragraphs may use Note,
Caution, or Warning as a heading. These headings have the following meaning:

Convention Description

Caution

NOTE

Warning

In addition, certain typefaces and capitalization are used to identify certain words. These
situations are:

Identifies information that, if not heeded, may cause
damage to the 425 or other equipment in your system.

Identifies information that needs extra emphasis. A
Note generally supplies extra information to help you
use the 425 better.

Identifies information that, if ignored, may be
hazardous to your health or that of others.

Convention Meaning

CAPITALS

Boldface

Controls, switches or other markings on the chassis.

Strong emphasis.

1.3. Other Safety Information

Power Source

This product is intended to operate from a power source
that does not apply more than 250V rms between the
power supply conductors or between either power
supply conductor and ground. A protective ground
connection, by way of the grounding conductor in the
power cord, is essential for safe operation

Grounding

Danger from Loss of
Ground

Proper Power Cord Use only the power cord and connector specified for the

Rev 2.0, 10/29/93

The chassis of this product is grounded through the
grounding conductor of the power cord. To avoid
electric shock, plug the power cord into a properly
wired receptacle before making any connections to the
product. A protective ground connection, by way of the
grounding conductor in the power cord, is essential for
safe operation.

If the protective ground connection is lost, all accessible
conductive parts, including knobs and controls that
may appear to be insulated, can render an electric
shock.

product and your operating locale.
Use only a cord that is in good condition.

1-3

Proper Fuse

The fuse is mounted internally and is not considered
user serviceable. The fuseholder accepts American sized
fuses (1/4 in dia.) or European sized fuses (5mm dia).
For 117 VAC operation, the correct value is 1/4A,
250VAC, fast blowing (bussman type AGC)
For 230 VAC operation, the correct value is 1/8A,
250VAC, slow blowing (Bussman type MDL or GDC.

Operating Location

Stay Out of the Box

User-serviceable parts

Do not operate this equipment under any of the
following conditions: explosive atmospheres, in wet
locations, in inclement weather, improper or unknown
AC mains voltage, or if improperly fused.

To avoid personal injury (or worse), do not remove the
product covers or panels. Do not operate the product
without the covers and panels properly installed.

There are no user serviceable parts inside the 425. In
case of failure, refer all servicing to the factory.

1-4

Rev 2.0, 10/29/93

2. Dynamics Processing Tutorial and Functional Basics

The 425 brings together three widely-used signal processors: a downward expander, a
compressor, and a peak-limiter. It's important to note that although the 425 is three
processors in one box, it contains one-third less circuitry than there would be if each
processor was in its own box. The reason is simple: there is only one VCA (voltage-controlled
amplifier) in each channel of the 425. Smart analog computer signal processing circuitry (the
sidechain by any other name) combines the three control signals from the downward
expander, compressor, and limiter to control the one VCA. There is no compromise involved in
doing this and the signal passes through two fewer VCAs.

This part of the 425 tells how to use your 425. If you are new to signal processors, read on; the
remainder of this section is a primer on the three basic parts of the 425.

2.1 Dynamics Processing Basics

Audio signals possess several basic properties: amplitude or volume (measured in volts or dB),
frequency or pitch (measured in Hertz), duration (measured in hours:minutes:seconds) and
waveform (described graphically, like sine, square, triangle, pulse). Complex signals like
musical sounds are made up of simpler waveforms such as sine waves, mixed in the proper
proportions.

Signal processors allow you to manipulate various parameters of an audio signal. Equalizers
change the amount of amplification given to different frequencies (a perfect amplifier amplifies
all frequencies by the same amount). Dynamics processors change the dynamic range of audio
signals.

The dynamic range of an audio signal is the difference between its loudest and softest
moments. For audio equipment, this is the difference between the noise floor (residual noise
output, with no input signal) and peak clipping (the point at which the output clips or
distorts). A hypothetical black-box having a noise floor of -90 dBu and a maximum peak
output level of +24 dBu would have a dynamic range of 114 dB (+24 minus -90). Audio storage
devices like tape machines have worse limitations, a typical professional analog two-track tape
machine may have a dynamic range of 65 to 70 dB.

If you've used an analog tape recorder before, then you are already familiar with the problem of
setting recording level. Record too hot and you get distortion; record too cold and get noise in
return. Many musical instruments have dynamic ranges that exceed that of most tape
recorders. So, how do we squeeze a 80 or 90 dB signal into a 60 or 70 dB window?

The answer lies in a common audio signal processor: the compressor.

2.1.1 Compressors and Limiters

A compressor or limiter monitors the level or amplitude of a signal and reduces the amplitude
according to a rule whenever the signal level exceeds a predetermined level. The
predetermined level is known as the threshold level and is usually set by a front panel control.
The rule by which the compressor lowers the level is the compressor's compression ratio and
this parameter is also usually set via a front panel control.

Compression ratio refers to the ratio of a change at the input versus the change at the output
of the device. Thus, if we apply a signal that changes 10 dB to the input of a hypothetical
compressor, and measure a 2 dB change in the output signal, that compressor would have a
compression ratio of 10:2, or 5:1 (reduce the fraction). Different compression ratios have
different uses. Use lower ratios (6:1 or less) for level control, intermediate ratios (8:1 to 12:1)
for leveling (making the signal level more or less constant), and higher ratios for limiting
(putting an absolute ceiling on the signal level).

Limiters are nothing more than compressors, but being possessed of much higher
compression ratios (20:1 or higher). Limiters are typically used to stop occasional peaks which

Rev 2.0, 10/29/93

2-1

would have otherwise caused overload or distortion. Typically you set a limiter (via its
threshold control) so that it "stays out of the way" until a peak comes along.

2.1.2 Expanders and Gates

While a compressor or limiter reduces the dynamic range of a signal by reducing its level once
it has exceeded a threshold level, an expander does the opposite (well, almost). The easiest
way to visualize an expander is to think of it making loud signals louder. This is fine, except
that in the real world, you run into the limitations of processors and amplifiers after the
expander going into terminal overload.

The solution is to make soft signals softer, or downward expansion. This is what the 425's
downward expander does. When the signal level falls below the level set by the threshold
control, the expander reduces the gain by the amount dictated by its expansion ratio. Thus,
for a below-threshold signal, a 10 dB output change results from a 5 dB change in the input
signal, if the expansion ratio is 1:2.

A gate is similar to an expander except that its ratio is much higher; thus the action is more
like a switch once the signal falls below threshold. Some expander applications for the 425
may be quite similar to a gate, like tending a lone announce microphone, but the 425's
expander isn't well suited to typical gate applications like removing leakage from drum
microphones.

2.1.3 Ratio

The compression ratio of the 425 tells how much the output changes for a change in the
input. A linear amplifier (like a simple preamp) has a ratio of 1:1 because a change of 1 dB at
its input results in a 1 dB change at its output. A compressor alters the input/output
relationship by its compression ratio. Thus a 20:1 ratio means that a 20 dB change at the
input results in a 1 dB change at the output. In other words, a very audible change at the
input (20 dB) turns into a barely discernible change at the output (1 dB).

Compressors are not the only devices to have an input/output ratio. Any device that is
capable of changing the input/output relationship can be said to have a ratio. Thus
expanders, gates, compressors and limiters all fit this category.

An expander maginifies output changes for a given input change. Thus, once the input signal
falls below threshold, the expander changes the output by the amount of the ratio. The 425's
downward expander has an expansion ratio of 1:2, which means that an input signal that gets
5 dB quieter turns into a 10 dB quieter output change.

A gate can be looked at as an expander with an infinite expansion ratio. Thus the slightest
change in the input signal, above threshold, turns the gate full-on.

2.1.4 Gain vs Output

The gain control allows compensating for signal level lost to compression. As an example, try
setting the 425 for a 4:1 ratio. Now adjust the
as read on the compressor's gain reduction display. The output level should be significantly
lower than what it was. You supply the additional gain (make-up gain) by adjusting the
control until the input and output signal levels match.

THRESHOLD control for 10 dB of gain reduction

GAIN

2.1.5 Attack Time

The attack time represents the amount of time that a compressor (or limiter, or expander, or
gate) needs to react to an input change. You might ask, why not just make it lightning fast?
Because you may sometimes want to let occasional peaks through, which helps maintain the
dynamic characteristics of the input signal. Usually, what we're after is control of the input
signal, not total homogenization.

2-2

Rev 2.0, 10/29/93

2.1.6 Release Time

Most dynamics processing equipment has a knob marked release on the panel. This refers to
release time, and affects the length of time required for the gain to recover to the no-signal
state.

For compressors, the no-signal point is unity gain and this applies to any signal whose level is
below threshold. For expanders the no-signal point is the gain reduction set by the threshold
control under no-signal conditions and this applies to any signal whose level is above
threshold. In this case, the release time control governs how long it takes for the expander to
reduce the gain when the signal disappears.

The release time control allows tailoring the compressor's recovery time to the program
material. Generally, peak limiting is associated with short release times and compression or
leveling associated with longer release times.

The 425's compressor release circuitry has a unique dual-release time feature designed to
make life easier. The dual-release feature makes the release time partially program dependent,
giving you the best of both worlds: fast release for short-duration peaks and a longer release
time for longer-duration peaks. The release time control adjusts the speed of the longer of the
two.

2.1.7 Threshold Setting

The threshold control sets the audio signal level where the compressor/expander/limiter
begins working. In the case of the compressor or limiter, the processor begins working once
the signal has exceeded the threshold level. For the expander, it begins working once the
signal has fallen below the threshold level.

For any of the three processors, the threshold control setting also determines the degree or
amount of gain reduction. Thus, for the compressor, rotating the control counter-clockwise
(towards -40) results in increasing amounts of compression. For the expander, counter­clockwise rotation raises the level that the signal must exceed to pass through the expander
untouched. This has the effect of "shutting off" the signal once you reach and then pass the
threshold level.

For most compressor applications, moderate amounts of gain reduction are all that is
required, 3-9 dB at the most. If you are using the compressor to minimize level changes of a
wide range of program material (automatic level control), then higher amounts of gain
reduction are needed; the amount of gain reduction corresponds to the range of change that
you can respond to (slower release times are also indicated, too).

2.1.8 Interpreting the Displays

The 425 has many displays; one per processing section. The three displays associated with the
expander, compressor and limiter indicate a parameter called gain reduction. Simply stated,
the gain reduction indication shows how far the gain or amplification was reduced from unity.
Another way of looking at this is: if the gain reduction display says 10 dB of gain reduction,
switching the unit to bypass will result in a 10 dB increase in the output level.

The output display indicates output level, in VU (0 VU = +4 dBm = 1.23V RMS measured at
the balanced outputs). For most applications, just make certain that you never see the CLIP
LED illuminate.

2.2 Using the Sidechain

The sidechain is a patch point in the control circuit of a dynamic range processor, which
provides access to the part of the circuitry that tells the VCA what to do. The 425's sidechain
is routed through a TRS jack located on the rear panel that provides both a send and return
via the same jack. The sidechain connection affects all three processors in the 425.

Rev 2.0, 10/29/93

2-3

Look at the block diagram in Section 7. Notice the sidechain connections that come from the
balanced input stage. They allow access to the control circuit's input signal. The control signal
is derived from, but kept totally separate from, the audio signal path. This means the control
signal can be processed outside the 425 without actually processing the signal that's going
through the VCA (the audio signal itself). This presents some very interesting possibilities for
changing or improving the operation of the dynamic range processor.

The best use of the sidechain is to make the action of the 425's compressor/limiter/expander
frequency dependent, that is, to make it respond more (or less) to certain frequencies. Because
the audio signal and the control signal remain completely separate (even while the control
circuit tells the VCA whether to turn the gain up or down), you can equalize the sidechain
without changing the EQ in the main audio path.

Removing unwanted frequencies from the control signal before it actually reaches the VCA
prevents those frequencies from being used to create gain changes. Perhaps most importantly,
this happens without actually equalizing the signal being processed through the 425.

To make the 425's processors more sensitive to high frequencies, use an equalizer (graphic or
parametric) to boost the high frequencies in the sidechain signal. This increases the sensitivity
of the control circuits to those particular frequencies so the compressor/limiter/expander
responds more to those frequencies than any others. If the offending frequencies produce a
control signal of greater amplitude than the desired frequencies they will control how the
compressor/limiter/expander behaves with the rest of the signal as well. However, if the
offending signals are of significantly greater amplitude than the rest of the signal, careful
adjustment of the corresponding threshold control (combined with the boost provided by the
EQ in the sidechain) will make the compressor/limiter/expander respond only to the boosted
frequencies.

Keep in mind that the threshold level becomes a function of the amount of overall gain
through the equalizer, including the boost. This technique can be used with any frequency
that can be controlled by the equalizer.

Cutting a frequency creates the inverse effect, making the 425 less sensitive to the frequencies
that were removed from the control signal.

Since the expander only discriminates between different levels (not different sounds), it can be
fooled by signals whose levels are nearly the same, even if the frequency content of those
signals is fundamentally different. When the 425's expander is used to shut out unwanted
sounds, any signal exceeding the threshold setting triggers the expander. When this happens,
it's often possible to eliminate the false triggering by equalizing the control signal.

For example, if low frequency signals transmitted through a desk or podium are triggering the
425's expander unnecessarily:

❒ Use an equalizer in the sidechain to remove the low frequencies from the control

signal.

and/or

❒ Use the equalizer to boost the voice-range frequencies in the control signal.

When the offending frequencies are removed or minimized, the relative level of the desired
frequencies increases and the expander can now tell the difference between the wanted and
unwanted signals. Use this technique in any situation where levels are nearly the same, but
the fundamental frequencies involved are different.

2-4

Rev 2.0, 10/29/93

NOTE The ability of the expander to discriminate between wanted and unwanted

signals is partially determined by mic technique. Be particularly careful of high
frequency sounds entering the side or rear pattern of a cardioid mic. Most
cardioid mics exhibit a sharply rising off-axis response characteristic at higher
frequencies. Check the off-axis curve (the lower one) in the manufacturer's
literature. If there's only a 3dB to 6dB difference between the on-axis (frontal)
response and the off-axis (side or rear) response in the 5kHz to 10kHz reason,
high frequency sounds will be picked up by the side or back of your mic.

Use the mic's directional pattern to keep other sources as far off-axis as
possible - do everything you can do to extract all the source-to-source
discrimination possible through good mic technique. The sounds picked up by
individual mics must be primarily the sound of the desired signal, or the
expander won't be able to tell the difference.

Hint: You can save time, and make life easier by listening to the output of the

equalizer (instead of the 425's output) that you're using in the sidechain. Doing
this allows you to hear the signal that will control the 425, and perhaps to find
the range that you wish to emphasize or de-emphasize more easily.

Rev 2.0, 10/29/93

2-5

Notes

2-6

Rev 2.0, 10/29/93

3. Technical Tutorial

This section discusses a multitude of things, all related to getting signals in and out of the

425.

3.1. Matching Levels vs Matching Impedances

In any audio equipment application, the question of "matching" inevitably comes up. Without
digging a hole any deeper than absolutely necessary, we offer the following discussion to
(hopefully) clarify your understanding of the subject.

Over the years, we have all had impedance matching pounded into our heads. This is
important only for ancient audio systems, power amplifiers, and RF. Technically speaking, the
reason is power transfer, which reaches a maximum when source and load are matched.
Modern audio systems are voltage transmission systems and source and load matching is not
only unnecessary, but undesirable as well.

❒ Ancient audio systems operate at 600 ohms (or some other impedance value), and must

be matched, both at their inputs and at their outputs. Generally speaking, if you are
dealing with equipment that uses vacuum tubes, or was designed prior to 1970, you
should be concerned about matching. These units were designed when audio systems
were based on maximum power transfer, hence the need for input/output matching.

❒ Power amplifiers are fussy because an abnormally low load impedance generally means a

visit to the amp hospital. Thus, it's important to know what the total impedance of the pile
of speakers connected to the amplifier really is.

❒ RF systems are matched because we really are concerned with maximum power transfer

and with matching the impedance of the transmission line (keeps nasty things from
happening). Video signals (composite, baseband, or otherwise) should be treated like RF.

Some folks seem to believe that balanced/unbalanced lines and impedances are related; or
even worse that they are associated with a particular type of connector. Not so. Unbalanced
signals are not necessarily high-impedance and balanced signals/lines are not necessarily
low-impedance. Similarly, although 1/4 inch jacks are typically used for things like guitars
(which are high-impedance and unbalanced), this does not predispose them to only this
usage. After all, 1/4 inch jacks are sometimes used for loudspeakers, which are anything but
high-impedance. Therefore, the presence of 3-pin XLR connectors should not be construed to
mean that the input or output is low-impedance (or high-impedance). The same applies to 1/4
inch jacks.

So, what is really important? Signal level, and (to a much lesser degree), the impedance
relation between an output (signal source) and the input that it connects to (signal receiver).

Signal level is very important. Mismatch causes either loss of headroom or loss of signal-to­noise ratio. Thus, microphone inputs should only see signals originating from a microphone, a
direct (DI) box, or an output designated microphone-level output. Electrically, this is in the
range of approximately -70 to -20 dBm. Line inputs should only see signals in the -10 to +24
dBm/dBu range. Guitars, high-impedance microphones, and many electronic keyboards do
not qualify as line-level sources.

The impedance relation between outputs and inputs needs to be considered, but only in the
following way:

Always make sure that a device's input impedance is higher than the output
source impedance of the device that drives it.

Some manufacturers state a relatively high-impedance figure as the output impedance of their
equipment. What they really mean is that this is the minimum load impedance that they
would like their gear to see. In most cases, seeing a output impedance figure of 10,000 (10K)
ohms or higher from modern equipment that requires power (batteries or AC) is an instance of

Rev 2.0, 10/29/93

3-1

this type of rating. If so, then the input impedance of the succeeding input must be equal to or
greater than the output impedance of the driving device.

Symetrix equipment inputs are designed to bridge (be greater than 10 times the actual source
impedance) the output of whatever device drives the input. Symetrix equipment outputs are
designed to drive 600 ohm or higher loads (600 ohm loads are an archaic practice that won't
go away). You don't need to terminate the output with a 600 ohm resistor if you aren't driving
a 600 ohm load. If you don't understand the concept of termination, you probably don't need
to anyway.

The two facts that you need to derive from this discussion are:

r

Match signal levels for best headroom and signal-to-noise ratio.

r

For audio, impedance matching is only needed for antique equipment and power
amplifier outputs. In all other cases, ensure that your inputs bridge (are in the range of
2 to 200 times the output source impedance) your outputs.

3.2. Signal Levels

The 425 is designed around studio/professional line levels: +4 dBu or 1.23 volts RMS. The
unit is quiet enough to operate at lower signal levels such as those found in semi-pro or
musical-instrument (MI) equipment (-10 dBu or 300 millivolts).

3.3. I/O Impedances

The 425 is designed to interface into almost any recording studio or sound reinforcement
application. This includes:

r

600 ohm systems where input and output impedances are matched.

r

Unbalanced semi-professional equipment applications.

r

Modern bridging systems where inputs bridge and outputs are low source impedances
(voltage transmission systems).

The 425's input impedance is greater than 30-kilohms balanced or unbalanced. The inputs
may be driven from any source (balanced or unbalanced) capable of delivering at least -10 dBu
into the aforementioned impedances.

The 425's output impedance is 300 ohms balanced, 150 ohms unbalanced. The output line
driver delivers +23 dBm into 600 ohm balanced loads or +18 dBm into 600 ohm unbalanced
loads.

3.4. Polarity Convention

The 425 uses the international standard polarity convention of pin 2 hot. Therefore:

If your system uses balanced inputs and outputs,

XLR Tip-Ring-

Signal

Sleeve

1 Sleeve Ground
2 Tip High
3 Ring Low

and uses the 425 this way, then the polarity
convention is unimportant. If your system is both
balanced and unbalanced, then you must pay
attention to this, especially when going in and
coming out through different connector types
(like input on an XLR, output on a phone jack).

3-2

Rev 2.0, 10/29/93

3.5. Input and Output Connections

Figure 3-1 illustrates how to connect the 425 to balanced and unbalanced sources and loads.
To operate the 425 from unbalanced sources, run a 2-conductor shielded cable (that's two

conductors plus the shield) from the source to the 425. At the source, connect the low/minus
side to the shield, these connect to the source's ground; connect the high/plus side to the
source's signal connection. At the 425, the high/plus wire connects to pin 2, the low/minus
wire connects to pin 3, and the shield (always) connects to pin 1. This is the preferred method
as it makes best use of the 425's balanced input (even though the source is unbalanced). The
other alternative shown in Figure 3-1 converts the 425's balanced input into an unbalanced
input at the input connector. This works, but is more susceptible to hum and buzz than the
preferred method. There is no level difference between either method.

You can drive unbalanced loads with the 425's outputs by using the XLR connector with pin 3
left open. In an emergency (the show must go on), you can ground pin 3, but if you have the
choice...leave it open. If you must ground pin 3, it is must be grounded at the 425, rather than
at the other end of the cable. The price, regardless of whether or not pin 3 is grounded is 6 dB
less output level. This can be easily made up via the output gain controls. If your system is
wired with pin 3 hot, pin 2 must float if you are driving an unbalanced load.

The 1/4-inch input jack is paralleled with the XLR-input and the screw terminals. In a large
installation, it is permissible to use one of the connectors as the input connection and to use
either or both of the remaining connections for paralleling other inputs with the 425.

The 1/4 inch output jack is a TRS (tip-ring-sleeve) jack wired for unbalanced operations. That
is, the tip is the signal connection, and the ring and sleeve connections both go to circuit
ground. This style of connection assures operation (passage of signal) regardless of the type of
plug inserted into the jack. The unbalanced output is always 6 dB lower in level than the
balanced output.

Rev 2.0, 10/29/93

3-3

FROM BALANCED OUT

FEMALE XLR

PIN 1 = GROUND

PIN 2 = HIGH

PIN 3 = LOW

FROM BALANCED OUT

MALE TRS PLUG

TIP = HIGH

RING = LOW

SLEEVE = GROUND

FROM UNBALANCED OUT

MALE TS PLUG

SLEEVE = GROUND +

TIP = HIGH

LOW

2

3

1

TO BALANCED IN

1

3

2

TIP

RING

SLEEVE

TO UNBALANCED IN FROM

TRANSFORMER COUPLED OR

FLOATING BALANCED OUTPUT

MALE XLR
PIN 1 = GROUND
PIN 2 = HIGH
PIN 3 = LOW

MALE TS PLUG
TIP = HIGH
SLEEVE = GROUND +

LOW

FROM BALANCED OUT

TERMINAL STRIP

(+) = HIGH

(-) = LOW

= GROUND

FROM UNBALANCED OUT

TERMINAL STRIP

(+) = HIGH

(-) = NOT USED

= GROUND

FROM NON-TRANSFORMER (ELECTRONIC)

PIN 1 = GROUND + LOW

PIN 3 = NOT USED

BALANCED OUTPUT

(TYPICAL OF SYMETRIX PRODUCTS)

FEMALE XLR

PIN 2 = HIGH

TO BALANCED IN

TIP

RING

SLEEVE

MALE TRS PLUG
TIP = HIGH
RING = LOW
SLEEVE = GROUND

TO BALANCED IN

TERMINAL STRIP
(+) = HIGH
(-) = LOW

= GROUND

TO UNBALANCED IN

2

3

1

MALE TS PLUG
TIP = HIGH
SLEEVE = GROUND +

REV-B

LOW

3-4

Figure 3-1. Input and output connector wiring. These diagrams represent

the majority of connectors used in modern audio equipment. Locate the

source connector in the left column and match it up with the destination

connector in the right column. Wire your cable according to the diagrams.

Rev 2.0, 10/29/93

4. Front Panel Overview

EXPANDER

THRESHOLD (dBu)

-20

-5 -30

+5

BYPASS

RELEASE

FAST

SLOW

THRESHOLD (dBu)

-10

+10 -25

-40

BYPASS

-8 -6 -10 -12 -25 -15 -2 -6 -12 -18

FAST

-4

RELEASE

COMPRESSOR

1.3 2.5

SLOW

RATIO (X:1)

10

THRESHOLD (dBu)

+5 2

-2.5

-10 1

0VU CLIP-20 -3

-10 -6 -9 -12 -2

LIMIT

GAIN (dB)

+10

+12.5 +5 +15

0

+20 BYPASS

OUT

IN
BYPASS

DUAL STEREO

LINK

STEREO
DUAL MONO

The 425 has the following controls, switches, and indicators on its front panel:

Expander

T

HRESHOLD

RELEASE

LED Display The expander display indicates how much the expander has

Sets the signal level below which the expander begins to
operate. When the input signal falls below the level indicated
on the knob, the expander begins reducing the gain. The LED
display above the knob indicates how much the expander has
reduced the gain.

Determines the speed at which the expander reduces the gain
for an instantaneous change in the input signal (below
threshold). Use the

RELEASE control to prevent the expander

from punching holes in the input signal.

reduced the level of the input signal.

Compressor

T

HRESHOLD

Sets the level above which the compressor begins reducing the
output level.

RELEASE

Determines the speed at which the compressor restores the
gain for an instantaneous change in the input signal. Use the

RELEASE control to smooth the action of the compressor on

staccato material.

RATIO

Determines the amount of change in the output for a given
change in the input. If the ratio control is set to 10:1, this
means that the output will change 1 dB (not much) for a 10 dB
(3.16 times) change in the input. Use higher ratios to control
peaks and lower ratios to smooth out average levels.

LED Display Indicates how much the output level has been reduced by the

compressor.

Limiter

T

HRESHOLD

LED Display Indicates how much the output level has been reduced by the

Sets the level above which the limiter begins reducing the
output level.

limiter. -3 to -6 dB indications are a good place to start.

Rev 2.0, 10/29/93

4-1

Other Controls Switches and Displays

G

AIN(DB)

LED Display Indicates the output level of the 425.

Increases the output level of the 425. Use this control to
restore signal level lost by compression or limiting.

Note: the unity gain position of the control (0 dB) is referenced
to the balanced output connector. If you are using the
unbalanced output, unity gain occurs when the the
control is set to +6 dB.

GAIN

IN/BYPASS

DUAL / STEREO

This switch bypasses the 425. This is NOT a hard-wire bypass;
the 425 will not pass signal unless the unit is powered and
turned on.

This switch selects dual-mono or stereo-coupled mode. In
dual-mono mode, each channel of the 425 operates
independently.

In stereo-coupled mode, the channel 1 controls determine the
settings for both channel 1 and channel 2. Channel 1's
sidechain receives a mono mix of the channel 1 and channel 2
signals. The LED displays for channel 2 blank (except for the
output display) to remind you that the channels are stereo­coupled.

The remaining controls and switches on the front panel
duplicate those previously described for channel 1.

4-2

Rev 2.0, 10/29/93

5. Rear Panel Overview

O

D

12.5W MAXIMUM

MANUFACTURED IN
SEATTLE WASHINGTON,
UNITED STATES OF AMERICA

POWER

TIP=RETURN
RING=SEND
SLEEVE=GROUND

SIDECHAIN

IN/OUT

OUTPUT

UNBALANCED

BALANCED

SIDECHAIN

IN/OUT

UNBALANCED/

INPUT

BALANCED BALANCED

The following connectors and features are found on the rear panel of the 425.
Serial Number Do yourself a favor and write this number down somewhere

safe, and while you're at it, please send us the completed
warranty card?

power cord IEC power receptacle. Connect the power cord to an

appropriate source of AC power. Observe the marked power
supply voltage on rear panel.

UNBALANCED OUTPUT

1/4 inch tip-ring-sleeve (TRS) phone jack (wired tip = hot, ring
& sleeve = ground). Use this jack when you need an
unbalanced output. The nominal signal level here is +4 dBu.

Note: the
lower in level than the
easily make up the difference in level via the front panel

UNBALANCED OUTPUT connector is normally 6 dB

BALANCED OUTPUT connector. You can

GAIN

control.l

BALANCED OUTPUT

XLR-3 male connector. Use this connector when you need a
balanced output. The nominal signal level here is +4 dBu.

XLR PIN 2 = HI

XLR PIN 3 = L

XLR PIN 1 = GN

SIDECHAIN IN/OUT

1/4 inch tip-ring-sleeve connector. This is both an input and
output on the same connector (tip = input/return, ring =
output/send). Use this connector to connect equalizers and/or
filters into the 425's sidechain for frequency-selective
processing. The wiring diagram for the sidechain in/out
connector is reproduced on the back panel of the 425.

UNBALANCED/BALANCED
INPUT

1/4 inch TRS phone jack. This connector is wired in parallel
with the XLR input connector. Connect either balanced or
unbalanced sources here. Use a tip-sleeve plug for unbalanced
sources and a tip-ring-sleeve (stereo) plug for balanced
sources.

BALANCED INPUT

XLR-3 female connector. This connector is wired in parallel
with the TRS input connector. Connect balanced sources here.

The remaining connectors on the rear panel are the channel 1 input/output/sidechain
connectors. These connectors duplicate those used for channel 2.

Rev 2.0, 10/29/93

5-1

Notes

5-2

Rev 2.0, 10/29/93

6. Fast First Time Setup

Follow these instructions to get your 425 up-and-running as quickly as possible. The intent of
this section is fast setup. If you need something clarified, then you'll find the answer
elsewhere in this manual.

6.1 Connections

Connect the line-level signal source to either the female XLR connector or the 1/4 inch TRS
(tip-ring-sleeve, stereo, 3 conductor) input jack. If the source is unbalanced, then use a 1/4
inch TS (tip-sleeve, mono, or guitar) plug fully inserted into the TRS input jack.

Connect the line-level signal return to either the male XLR connector or to the 1/4 inch TRS
output jack. If you use the 1/4 inch jack, then use a TRS plug for balanced circuits or use a
TS plug for unbalanced circuits. Additional information on the signal connections may be
found in the Installation section of this manual.

Repeat for the second channel. Ignore the sidechain connections for now.
Connect the AC input to an AC power source of the proper voltage and frequency, as marked

on the rear of the unit.

Caution: Failure to connect the 425 to the proper AC mains voltage may
cause fire and/or internal damage. There are no user serviceable parts inside
the chassis. Refer all service to qualified service personnel or to the factory.

Warning: Lethal voltages are present inside the chassis. There are no
user serviceable parts inside the chassis. Refer all service to qualified
service personnel or to the factory.

AMP

= HIGH = 1/4" TIP
LOW = 1/4" RING
GND = 1/4" SLEEVE

UNBALANCED UNBALANCED/

BALANCED

SIDECHAIN

IN/OUT

LINE LEVEL

BALANCED

INPUTOUTPUT

BALANCED

LINE LEVEL

Rev 2.0, 10/29/93

6-1

6.2 Settings

Make your initial switch and control settings as follows:

Usage Switch Setting

Dual mono

IN/BYPASS

depressed

10

out
depressed

depressed

-12 -9 -6 -3

+5

-2.5 +12.5

-10

BYPASS

LIMIT

CLIP0VU-10 -20 STEREO

GAIN (dB)

+10

+5 +15

0

+20

OUT

IN
BYPASS

DUAL

Stereo

-18 -12 -6 -2 -10 -12 -15 -25

THRESHOLD (dBu) RELEASE

-20

-5 -30 +10 -25

+5

BYPASS

FAST

THRESHOLD (dBu) THRESHOLD (dBu)

-10

-40

SLOW

BYPASS

STEREO/DUAL
IN/BYPASS

STEREO/DUAL

-8 -6 -4 -2

FAST

SLOW

COMPRESSOREXPANDER

RATIO (X:1)RELEASE

2

1.3 2.5

1

Control Setting Control Setting Control Setting

Expander: Compressor: Peak Limiter:

THRESHOLD
RELEASE

-20
12 o'clock

THRESHOLD

RELEASE
RATIO

-10
12 o'clock
10:1

THRESHOLD

GAIN

+5

0

6.3 Initial Setup

LINK

STEREO
DUAL MONO

The 425's controls and switches are now set according to the preceding section. All
connections listed in Section 6.1 are now made. The 425 should now pass signal. The LED
display should be illuminated.

Set the input level by increasing the setting of the input level control until the amber LEDs in
the

HEADROOM display illuminate. Ideally, the highest signal level should illuminate the -1 dB

LED, and the

CLIPPING LED should almost never illuminate (the CLIPPING LED operates 1 or 2

dB below clipping.).

6.4 Refining Your Settings

For most signals, the above settings should produce activity in each section of the 425. The
expander and compressor sections tend to be complementary, that is, you probably won't be
showing expansion when the signal level is high enough to cause compression.

Whenever the operation of two of the sections of the 425 overlaps (especially the compressor
and limiter sections), whichever section has the highest gain-reduction reading on its display
is the section controlling the signal at that instant.

Refine the expander
signal falls below the threshold setting. You can make the expander "ignore" momentary
lapses in signal by increasing the setting of the
release once the signal falls below-threshold.

THRESHOLD setting so that the expander reduces the gain when the input

RELEASE control. This also affects the rate-of-

6-2

Rev 2.0, 10/29/93

Refine the compressor THRESHOLD setting so that the compressor display shows the amount of

gain reduction desired when the signal is within the operating range of the compressor. For
most applications, 3 to 6 dB are all that's necessary. Choose a
your application: gentle level control demands low ratio settings (below 6:1), peak leveling
requires higher settings (8:1 or higher). Choose a
sounds like fast release times, speech and music like slower times. These settings are just
guidelines, you can use whatever setting actually works.

If you're using a large amount of compression, for leveling or automatic gain control, you may
need to adjust the output
lost to the compressor. You can set the makeup gain easily by comparing signal levels via the

GAIN control to add makeup gain to compensate for the signal level

RELEASE time accordingly: percussive

RATIO setting appropriate for

IN/BYPASS switch and adding gain via the GAIN control

Refine the peak limiter
reduction during the highest peaks, and then only momentarily. Large amounts of gain
reduction showing on the peak limiter's display indicate that the limiter
may be too low. If this is what you want, fine, but bear in mind that peak limiters are designed
to stop occasional peaks dead, and may not sound very pretty used semi-continuously. May
we suggest using the compressor with the ratio control set at maximum.

THRESHOLD setting so that the peak limiter display only shows gain

THRESHOLD setting

6.5 Stereo Coupling

If you are using the 425 for a stereo signal, say for a stereo mixdown, depress the LINK switch

so that the
VCAs together. Channel 1 's sidechain receives an equal mix of the two input signals. Only
Channel 1's controls work (they control both channels now). The end result is the gain
reduction applied to the two channels is identical, which prevents stereo image shifts caused
by unequal gain reduction in the two stereo channels.

STEREO LED is lit. This disables the channel 2 control channel, and slaves the two

Rev 2.0, 10/29/93

6-3

Notes

6-4

Rev 2.0, 10/29/93

7. Using the 425

The 425's design emphasizes ease-of-use. This doesn't mean that we made it easy to use by
removing everything except the power switch; instead we concentrated on keeping the controls
that really make a particular feature work well and eliminating those that didn't. This section
contains installation information and descriptions of each of the front and rear panel controls,
switches, and connectors.

7.1 Block Diagram

Figure 7-1 is the block diagram of the 425.

SSERP

BALANCED

INPUT

UNBALANCED/

BALANCED

INPUT

PEAK

CIRCUIT

THRESHOLD

LIMITER
CIRCUIT

IN/OUT

SWITCH

VCA

BALANCED
OUTPUT

UNBALANCED
OUTPUT

SIDECHAIN

IN/OUT

TO/FROM CH2

SIDECHAIN

TO/FROM CH2

VCA CONTROL VOLTAGE

STEREO LED

DUAL LED

TO ALL CH2 LEDS

(EXCEPT OUTPUT LEDS)

LOG
AMP

LINK SWITCH

(SHOWN IN STEREO)

CIRCUIT

+V

+V

THRESHOLD

RMS

RELEASE

EXPANDER

CIRCUIT

COMPRESSOR

CIRCUIT

LIMITER LEDS

OUTPUT LEDS

GAIN

RATIORELEASETHRESHOLD

EXPANDER LEDS

COMPRESSOR LEDS

OUTPUT

LEVEL

Figure 7-1. Block Diagram (one channel shown).

Please take a moment and take note of the following:

❒ There is only one VCA per channel.
❒ Bypass mode is not a hard-wire bypass for each channel.
❒ The TRS, and XLR input connectors are all paralleled.
❒ The TRS output jack is wired for unbalanced operation (tip hot, ring and sleeve grounded).

7.2 Installation

The 425 may be installed free-standing or rack mounted. No special ventilation requirements
are necessary.

Installation Requirements

Mechanical One rack space (1.75 inches) required, 10 inches depth (including

connector allowance). Rear chassis support recommended for road
applications.

Electrical 105-125 VAC, 50-60Hz, 12.5 Watts maximum.

Connectors XLR-3 female for inputs, XLR-3 male for outputs, Pin 2 of the XLR

connectors is "Hot." TRS female connectors are also provided.
Note: the unbalanced output uses a TRS output jack with the ring

and sleeve connections connected to circuit ground.

Rev 2.0, 10/29/93

7-1

7.3 The 425 as a Compressor

O

O

You can use the 425 as a compressor in two different ways:

1. the compressor operates occasionally, dispatching occasional peaks.

2. the compressor operates continuously, making the dynamic range smaller at the output.

For the first scenario, pick a threshold setting that results in occasional gain reduction, as
displayed on the compressor display. Use a ratio setting suitable for your application; low
(1.3:1 to 2.5:1 for gentle compression, higher for more drastic squashing). Low ratios are
harder to hear, consequently you can operate with 6 dB or so of compression, without too
many audible effects. Higher ratios require subtlety, especially in threshold selection. Pick
your threshold setting so that no more than 6 dB of gain reduction occurs and you'll get the
signal control that you need, without being too obvious. Pick a release setting that lets the
gain recover fairly quickly.

Scenario two requires low ratio settings (unless you don't care about being obvious), and a
lower (-10 or lower) threshold setting. Choose a threshold setting that results in more or less
continuous gain reduction. Pick the ratio setting so that the dynamic range of the output
signal corresponds to the maximum and minimum signal levels that you want. Pick a release
setting that allows gain recovery during longer pauses.

7.4 The 425 as a Ducker

Well, almost. First, what is a ducker? (Hint: It's not something twisted utilizing waterfowl.) It is
a way of making the level of one audio signal follow that of another. A prime example would be
an announcer (the ducker) talking over a music bed (the duckee). What most people do is to
simply ride the music facer when the announcer is talking. You can do the same thing
automatically by using a compressor having a sidechain connection.

Now for some people, a "true" ducker must reduce the level of the duckee to some preset level
whenever the ducker signal is present. The 425 will not do this. It will,however, reduce the
level of the duckee whenever the ducker is present, but the amount of gain reduction follows

the envelope of the
ducker signal. Get it?
See Figure 7-1 for a

DUCKEE

LUME
V

TIME

LUME
V

TIME

DUCKER

OUTPUT

Figure 7-1. Signal relationships in a ducker.

more graphic
description.

Anyway, if this
"limitation" isn't a
problem, then refer to
Section 8.4 for
hookup. The ratio
control determines the
talkover ratio (how
much the duckee gets
reduced for a change
in the ducker) and the
threshold control
determines the total
amount of ducking
action. The release
time control govern's
how rapidly the

7-2

Rev 2.0, 10/29/93

duckee recovers after being ducked. Don't be meek with the knobs, most ducker applications
require 20+ dB of gain reduction to be effective.

7.5 The 425 as a Limiter

The peak limiter section of the 425 is a fast-acting (200µS), steep slope (20:1) limiter. Peak
limiters are generally used to stop signal peaks dead in their tracks. The peak limiter in the
425 has only one control: threshold. Set the control for occasional -3 dB gain reduction
indications (you can let it hit 6 dB once in a great while). If you allow more gain reduction

than 3-6 dB, that's ok, but
the results may be quite

O

10

u

+2.5

t

0

p
u

-10

t

-20

Downward
Exp.

-10

Linear

1:1

Compression

4:1

Limiting

10:1

Input

Figure 7-2. Simultaneous downward-expansion, compresion and limiting.

0dB or higher, and limiter threshold high enough to catch the highest peaks (or at recording
media overload). The compressor catches most peaks, using a moderate ratio (which is more
ear-pleasing), while the limiter gives you the freedom from overload that only a peak limiter
can give.

audible (peak limiters are
designed to stop peaks, not
to sound pretty).

You can use the 425 as a
dual-threshold peak
limiter, with a gentle slope
for lower-level peaks and a
steep slope (20:1) for high-

40302520151050-5-20 -15

level peaks by setting the
compressor ratio at 4:1,
compressor threshold at

7.6 The 425 as an Expander

As described before, the expander section of the 425 is a downward expander. When the input
signal falls below the threshold control setting, the 425 reacts by reducing the gain of the
VCA. This increases the dynamic range of the output signal (since the soft just got softer).

You can use the expander to reduce body noises picked up by an announce microphone, or to
reduce amp noise from a noisy guitar amp, or how about reducing the amount of room sound
in a guitar track that was recorded a little too loosely in a room that was a little too noisy? For
sound reinforcement, how about using the expander to tend an announce mic that needs to
be left on, but you really don't want it picking up every whisper in its vicinity.

Setup is easy, with no signal, adjust the

THRESHOLD control counter-clockwise until you begin

to see gain reduction on the expander's LED display, continuing until you hear the degree of
expansion that you want. When the signal is present, you may need to refine the setting so
that the expander doesn't reduce the gain during momentary pauses. You can slow things
down a bit via the

RELEASE control, which helps prevent expansion during momentary pauses.

7.7 Using the Expander and Compressor Simultaneously

There are times when you might want to expand and compress simultaneously. Now this isn't
a game of tug-of-war between the expander and compressor. How about a track that has too
much dynamic range and needs to be squashed to fit into the space available in your mix?
After squashing, you notice a lot of room noise, amp noise, etc. whenever the track is not
active. 425 to the rescue. Simply adjust the expander threshold to cause gain reduction
during the quiet parts of the track and you'll make those ugly noises disappear.

Rev 2.0, 10/29/93

7-3

Another application for simultaneous expanding and compressing is voice processing. Close
mic'ed voices tend to be a little un-natural. Things that we don't hear at conversational
distance are heard quite well by the announce microphone. A bit of expansion will help to
reduce lip-smacking noises and some compression will help even levels out and generally
tighten up the sound.

7-4

Rev 2.0, 10/29/93

8. Applications

Here are a few applications that the 425 lends itself to. The following applications make one
assumption: settings for the threshold control(s) are a function of each individual system's
actual operating level. Wherever specific threshold settings are mentioned, they are referenced
to either a particular gain reduction level as indicated by the meter, or to a O dBu input level.

8.1 Vocal Level Smoothing

This application assumes that the voice source does not vary tremendously in level. Examples
of this include narration, commercial voice-over production, seminar speaking, etc.

-25 -15 -12 -10

THRESHOLD (dBu)

-10

-25 +10

-40

BYPASS

-4 -6 -8

-2

COMPRESSOR

RELEASE RATIO (X:1)

FAST

SLOW

1

q

Lower the threshold setting until the gain reduction display
begins to show compression, then continue lowering the

2

2.51.3

10

setting until the display shows 6-9 dB of gain reduction on
peaks.

q

To eliminate noises like paper rattling or breathing, try
using the downward expander.

8.2 Removing Noise from Vocal Tracks

Sometimes, due to the more abrupt nature of gating, it may be more appropriate to clean up a
noisy vocal track with a downward expander. The 425's expander is smooth, and can clean up
vocals without cutting into phrasing.

-2 -6 -12 -18

EXPANDER

FAST

RELEASETHRESHOLD (dBu)

SLOW

-20

-30 -5

+5

BYPASS

q

Set the expander threshold for 16 dB (or more) of gain reduction
when the signal source is silent, and 0 dB of gain reduction when the
signal is present.

q

Use the release control to make the expander follow the phrasing.

8.3 Constant Level Paging

All too often, paging announcements are either sub-audible, or distorted. The problem is the
result of changing input levels from different users (timid Tom vs Sam Screamer) and
unpredictable environmental circumstances. To optimize system levels for intelligibility
without overload, use the compressor section to even out levels, and the peak limiter to put a
clamp on Sam Screamer.

q

-25 -15 -12 -10

THRESHOLD (dBu)

-10

-25 +10

-40

BYPASS

Rev 2.0, 10/29/93

-4 -6 -8

RELEASE RATIO (X:1)

FAST

-2

COMPRESSOR

SLOW

2

2.51.3

1

10

Set the compressor threshold for 6 dB of gain reduction
with normal paging levels.

q

Set the peak limiter threshold for 6 dB of gain reduction
when Sam Screamer is on the system.

q

All normal signals will be slightly compressed, and really
loud signals will activate the peak limiter. With these
settings a shy person will be audible, and the guy who
thinks he has to shout won't be too loud, or cause
distortion.

8-1

8.4 Paging with Ducking

Many situations involving a "talking host" and any sort of musical background will require the
background music to drop in level whenever the host signal is present. By splitting the host
signal, part of it can be used to tell the 425 when to take action and lower the background
source by an amount determined by the threshold control.

-18 -12 -6 -2

THRESHOLD (dBu) RELEASE

-20

-5 -30 +10 -25

+5

BYPASS

FAST

THRESHOLD (dBu) THRESHOLD (dBu)

-10

-40

SLOW

BYPASS

-8 -6 -4 -2

FAST

SLOW

COMPRESSOREXPANDER

1.3 2.5

-12 -9 -6 -3

RATIO (X:1)RELEASE

2

1

10

+5

-2.5 +12.5

-10

BYPASS

LIMIT

CLIP0VU-10 -20 STEREO

GAIN (dB)

+10

+5 +15

0

+20

OUT

-10 -12 -15 -25

AUDIO

INPUT

AUDIO

OUTPUT

LINE
INPUT

PMA

PAGE

MICROPHONE

SIDECHAIN

INPUT

PRE-AMP

MIC

INPUT

MIC

SPLITTER

q

Set the compressor threshold so that all the gain reduction LEDs for the compressor are illuminated
when the host speaks. Although the display won't show it, you can set the threshold control even
lower for more ducking action.

q

Use the release control to vary the speed at which the music returns when the host stops talking.

8.5 Stage Monitors

IN
BYPASS

DUAL

LINK

STEREO
DUAL MONO

Public address and sound reinforcement situations that require comp/limiting are often
plagued by feedback problems. The overall increase in level that results from compression can
cause feedback in the absence of signal, when the compressor releases and brings levels back
up to normal.

-10 -12 -15 -25

BYPASS

-8 -6 -4 -2

FAST

SLOW BYPASS

COMPRESSOREXPANDER

RATIO (X:1)RELEASE

1.3 2.5

1

2

10

-18 -12 -6 -2

SLOW

THRESHOLD (dBu)

-10

-40

THRESHOLD (dBu) RELEASE

-20

-5 -30 +10 -25

+5

q

Set the compressor threshold for no more than 9 dB of gain reduction on normal signals.

q

Set the expander threshold for 16 dB (or more) of gain reduction when the signal source is silent,

FAST

and 0 dB of gain reduction when the signal is present.

q

Use the release control to make the expander follow momentary pauses.

q

Since the successful implementation of this technique requires careful settting of the two threshold
controls, be prepared to fine tune the settings to match the levels of your system and the vocalist's
style.

8-2

Rev 2.0, 10/29/93

8.6 Keyed Bass

As a special effect, try using the signal from a kick drum to key a gated bass. This technique
will tighten the "signal start" relationship between the two instruments. Since the bass will
not be heard until the drum is played, the result is a perfectly tight kick/bass combination.

SIDECHAIN

EQ

EQ INPUT

EQ OUTPUT

q

The expander threshold control varies the amount of action; higher settings are crisp, lower settings

IN/OUT

SEND(RING)

RETURN(TIP)

THRESHOLD (dBu)

merely increase the volume when the kick drum keys the expander.

q

Use the release control to make the expander follow the music.

-10 -12 -15 -25

-8 -6 -4 -2

-10

+10 -25

-40

BYPASS

FAST

COMPRESSOR

1.3 2.5

SLOW

RATIO (X:1)RELEASE

2

1

10

8.7 Sibilance Control

Patching an equalizer into the sidechain can cause the 425 to respond more or less to selected
frequencies, giving it the ability to make sibilance problems less apparent. Fine tuning
between the compressor threshold, ratio, and the EQ boost applied in the sidechain will have
to be made to arrive at premimum results.

q

To find basic settings, start with a fairly high ratio (5:1 or so), and a compressor threshold
setting between -20 and -5. Cut the low frequencies on the equalizer and give a 15 dB
broadband boost to the EQ at around 5 or 6kHz. Now, carefully "tweak" the threshold
setting as you count "four, five, six." What you're looking for is no compression on "four,
five," and somewhere around 9 dB of gain reduction on the word "six."

q

You can refine the setting by listening to the equalizer output and adjusting the EQ to
emphasize the sibilance in the source. Remember that you're equalizing the signal to
emphasize the sibilance, not to sound groovy. Let the 425 do that.

q

Do you have a recording where the cymbals drive you nuts?...try the same technique on
the overall mix.

q

Set the peak limiter threshold for 6 dB of gain reduction when Sam Screamer is on the
system.

q

All normal signals will be slightly compressed, and really loud signals will activate the
peak limiter. With these settings a shy person will be audible, and the guy who thinks he
has to shout won't be too loud, or cause distortion.

Rev 2.0, 10/29/93

8-3

Notes

8-4

Rev 2.0, 10/29/93

9. Troubleshooting Chart

j

Symptom Probable Cause

No output Check cables and connections.

Are inputs driven by outputs, and outputs driving
inputs?
Verify cables, source and load by patching input and
output connections together, at the unit.
Check for AC power presence.
Check input by plugging headphones halfway into the
sidechain
Check output by plugging headphones into output
connector .
Do either of the output LED displays show anything?

Hum or buzz in output Check input and output connector wiring (refer to

Figure 3.1).
Ground loop. check related system equipment
grounding. Are all system components on the same AC
ground?

Distortion Check input signal. Is it too hot, or is it already

distorted?
Does the output LED
reduce setting of gain control and make up lost gain
after the 425.
Check output loading. Should be above 600 ohms.
Are the power amplifier(s) clipping?
Is something else clipping?

ack and listening for input signal.

display indicate clipping? If so,

Noise (hiss) Check input signal levels. The 425 is intended to

operate at or near "line" level (-10 dBu or greater).
Check gain settings on downstream equipment.
The system gain structure should be such that the
601 operates at or near unity gain.
Is the input signal already noisy?

No LED display Is the unit plugged in, and turned on?

Is the AC outlet OK?

No nothing Is the unit in

Unit not plugged in, but works
anyway

Call us.

BYPASS mode?

9-1

Notes

9-2

10. 425 Dual Compressor Limiter Limited Warranty

This Symetrix product is designed and manufactured for use in professional and studio audio
systems. Symetrix, Inc. (Symetrix) warrants that this product, manufactured by Symetrix,
when properly installed, used, and maintained in accordance with the instructions contained
in the product's operator's manual, will perform according to the specifications set forth in the
operator's manual.

Symetrix expressly warrants that the product will be free from defects in material and
workmanship for one (1) year. Symetrix' obligations under this warranty will be limited to
repairing or replacing, at Symetrix' option, the part or parts of the product which prove
defective in material or workmanship within one (1) year from date of purchase, provided that
the Buyer gives Symetrix prompt notice of any defect or failure and satisfactory proof thereof.
Products may be returned by Buyer only after a Return Authorization number (RA) has been
obtained from Symetrix and Buyer will prepay all freight charges to return any products to the
Symetrix factory. Symetrix reserves the right to inspect any products which may be the
subject of any warranty claim before repair or replacement is carried out. Symetrix may, at its
option, require proof of the original date of purchase (dated copy of original retail dealer's
invoice). Final determination of warranty coverage lies solely with Symetrix. Products repaired
under warranty will be returned freight prepaid via United Parcel Service by Symetrix, to any
location within the Continental United States. Outside the Continental United States,
products will be returned freight collect.

The foregoing warranties are in lieu of all other warranties, whether oral, written,
express, implied or statutory. Symetrix, expressly disclaims any IMPLIED warranties,
including fitness for a particular purpose or merchantability. Symetrix's warranty
obligation and buyer's remedies hereunder are SOLELY and exclusively as stated herein.

This Symetrix product is designed and manufactured for use in professional and studio audio
systems and is not intended for other usage. With respect to products purchased by
consumers for personal, family, or household use, Symetrix expressly disclaims all implied

warranties, including but not limited to warranties of merchantability and fitness for a
particular purpose.

This limited warranty, with all terms, conditions and disclaimers set forth herein, shall extend
to the original purchaser and anyone who purchases the product within the specified
warranty period.

Warranty Registration must be completed and mailed to Symetrix within thirty (30) days of the
date of purchase.

Symetrix does not authorize any third party, including any dealer or sales representative, to
assume any liability or make any additional warranties or representation regarding this
product information on behalf of Symetrix.

This limited warranty gives the buyer certain rights. You may have additional rights provided
by applicable law.

Limitation of Liability

The total liability of Symetrix on any claim, whether in contract, tort (including negligence) or
otherwise arising out of, connected with, or resulting from the manufacture, sale, delivery,
resale, repair, replacement or use of any product will not exceed the price allocable to the
product or any part thereof which gives rise to the claim. In no event will Symetrix be liable for
any incidental or consequential damages including but not limited to damage for loss of
revenue, cost of capital, claims of customers for service interruptions or failure to supply, and
costs and expenses incurred in connection with labor, overhead, transportation, installation
or removal of products or substitute facilities or supply houses.

10-1

Notes

10-2

11. Repair Information

Should you decide to return your 425 to Symetrix for service, please follow the following
instructions.

11.1 Return Authorization

Symetrix will service any of its products for a period of five years from the date of manufacture.
However, no goods will be accepted without a Return Authorization number.

BEFORE SENDING ANYTHING TO SYMETRIX, CALL US FOR AN RA NUMBER. JUST ASK,
WE'LL GLADLY GIVE YOU ONE! CALL [phone] WEEKDAYS, 8AM TO 4:30 PM PACIFIC
TIME.

11.2 In-Warranty Repairs

To get your unit repaired under the terms of the warranty:

1. Call us for an RA number.

2. Pack the unit in its original packaging materials.

3. Include your name, address, etc. and a brief statement of the problem. Your daytime
telephone number is very useful if we can't duplicate your problem.

4. Put the RA number on the outside of the box.

5. Ship the unit to Symetrix, freight prepaid.

Just do those five things, and repairs made in-warranty will cost you only the one-way freight
fee. We'll pay the return freight.

If you choose to send us your product in some sort of flimsy, non-Symetrix packaging, we'll
have to charge you for proper shipping materials. If you don't have the factory packaging
materials, then do yourself a favor by using an oversize carton, wrap the unit in a plastic bag,
and surround it with bubble-wrap. Pack the box full of Styrofoam peanuts. Use additional
bubble-wrap if you must ship more than one unit per carton. Be sure there is enough
clearance in the carton to protect the rack ears (you wouldn't believe how many units we see
here with bent ears). We won't return the unit in anything but original Symetrix packaging. Of
course, if the problem turns out to be operator inflicted, you'll have to pay for both parts and
labor. In any event, if there are charges for the repair costs, you will pay for return freight. All
charges will be COD unless you have made other arrangements (prepaid, Visa or Mastercard).

11.3 Out-of-Warranty Repairs

If the warranty period has passed, you'll be billed for all necessary parts, labor, packaging
materials, and any applicable freight charges.

Remember, you must call for an RA number before you send the unit to Symetrix.

Rev 2.0, 10/29/93

11-1

Notes

11-2

Rev 2.0, 10/29/93

12. Specifications

Input/Output

Inputs XLR-female, >30-kilohms line-level balanced bridging, >30-kilohms

unbalanced bridging.

TRS-female paralleled with XLR connector.

Outputs 300-ohm source impedance, balanced. XLR-male

TRS-female wired tip hot, ring & sleeve grounded (unbalanced), 150­ohm source impedance.

Maximum input level

Maximum output level (onset
of clipping/1% THD)

+20 dBu, balanced
+25 dBu balanced
+23 dBm balanced (600 ohms)
+20 dBu unbalanced
+18 dBm unbalanced (600 ohms)

Sidechain

100-ohm source impedance, 6800-ohm input impedance.
TRS jack, tip is return.

Compressor

Type RMS responding, soft-knee

Attack time 2ms

Release time 180ms to 2.5s long term

Auto-release time 20ms to 1s (based on 20ms tone burst)

Threshold -40 dBu to +20dBu (bypass)

Ratio 1:1 to 10:1

Limiter

Attack time 200µs

Release time 100ms

Threshold -10dBu to +20dBu

Ratio 20:1

Downward Expander

Attack time 4ms

Release time 250ms to 5s

Threshold 0dBu to -40dBu (bypass)

Ratio 1:1.5

Rev 2.0, 10/29/93

12-1

Performance Data

Frequency Response

Distortion (THD) .04 %, +4 dBu in, +4 dBu out, 10 dB gain reduction, 1 kHz, 30

+0, -3 dB 10_Hz to 60 kHz

kHz low-pass filter
.01 %, +4 dBu in, +4 dBu out, 10 dB gain reduction, 20 kHz,

30 kHz low-pass filter
.02 %, +18 dBu output, 0 gain reduction, 20-20 kHz, 30 kHz

low-pass filter

Crosstalk

-95 db @ 1k, -95 dB @ 10k, + 4dBu in, remaining channel
terminated in 600 ohms, 20k rolloff in analyzer.

Dynamic Range

Input CMRR

Output Noise

115 dB (difference of maximum output and noise floor)
greater than 40 dB @ 1kHz

-90 dBu, measured at balanced output, input terminated in
600 ohms, 20 kHz rolloff in analyzer.

Physical

Connectors

Polarity pin 2 of XLR is hot, tip of TRS jacks is hot.

Size (hwd), in & cm 1.75 x 19 x 7.25 in

Weight, lbs & kg 8 lbs (3.63kg) net

input: XLR-3F, 1/4" TRS
output: XLR-3M, 1/4" TRS
sidechain: 1/4" TRS (one)

4.44 x 48.3 x 18.4 cm

11 lbs (4.6kg) shipping

Electrical

Approvals

Power requirements

Listed by Underwriters Laboratories, Inc. Control number
2T38.

117V AC nominal, 105-125VAC 50-60 Hz, 12.5 Watts
maximum
230V AC nominal, 205-253V AC 50 Hz, 12.5 Watts maximum

In the interest of continuous product improvement, Symetrix Inc. reserves the right to alter,
change, or modify these specifications without prior notice.

12-2

Rev 2.0, 10/29/93

13. PCB Layouts and Schematics

Note: The printed circuit board layouts and schematics in this section are
intended for use only by qualified personnel.

Caution: These servicing instructions are for use only by qualified personnel. To
avoid electric shock do not perform any servicing other than that contained in the
operating instructions portion of this manual unless you are qualified to do so.
Refer all servicing to qualified service personnel.

These schematics and layouts are provided for reference use only, for use by qualified service
personnel and for use in answering certain technical questions that are beyond the scope and
intent of this manual. The schematics and layouts in this manual were accurate at the time
that this manual was written. Your actual product may contain changes not shown on these
drawings. The inclusion of this material in this manual in no way obligates Symetrix to
provide updated information or to inform users of any changes, past or pending.

A complete service manual is available from the factory for a nominal charge. Please contact
the Symetrix Service Department at the address listed in Section 11 of this manual.

13.1 Troubleshooting Hints

If you are attempting repair of your 425, the following tips and hints may be useful, especially
if you are not familiar with operational amplifiers.

The first thing that you should check are the power supplies. Their nominal voltage should be
within 5% of the noted value on the schematic. The IC regulators used are current limited and
short-circuit protected; their output voltage drops under excessive load (like something
downstream that draws excessive current).

In audio amplifiers that utilize operational amplifiers as their active gain element, the two
feedback resistors establish the working gain of the circuit. In Symetrix equipment, and other
equipment using bipolar (separate plus and minus) power supplies, the nominal DC output
voltage of each stage should be at or very near zero volts.

If an opamp's output is at or near one of the power supply rails, this usually means that the
opamp has failed. The exception to this rule is when the circuit configuration uses the opamp
as a DC amplifier or as a comparator.

When used as a DC amplifier, the output should follow the input signal (modified by the
circuit's gain equation).

When used as a comparator, the opamp's output swings between the two supply rails and
there should be no intermediate output state.

Units using substantial amounts of digital circuitry are probably best serviced at the factory.

13.2 Additional Reading

You can find additional information on the design and applications of operational amplifiers in
the following text:

"Audio IC Op-Amp Applications," Walter G. Jung, Copyright 1987, Howard W. Sams &
Company, Indianapolis, IN.

13-1

13-2

CHANNEL 1

BALANCED

C45

C12

R22
50KB

EXP THRES

R48
D9
R50

INPUT

R6
R5

R1
R2
R3
R4

C46

R47

R21
R19

R18

D1
R20
R31

D4

R38
R37
R36
R39
D5

D6

D7
R170

R40

UNBALANCED

BALANCED

INPUT

J1

INPUT

C4
C3

C1

C2

C13

50KB 500KA 50KB 500KA 50KB 50KB 50KB

INPUT

C7

C5

BP1

U1

DISTORTION

TRIM CH1

BP2

R16

U11

TP

BP3

U3

BP4

U4

R41

Q5

500KA 50KB

EXP REL COMP THRES COMP REL COMP RATIO

SIDECHAIN

IN/OUT

J2

R77

R15
R156
R155

R104

R23

R67

R8

C6

R13
R12
R10
R9
R11

R7
R157
R69

R68
R70

C28

R35

D3

R34
R33

R32

R171
R172

R166

R167

C14

J3

SIDECHAIN

C8

C29

C27

1

U2

U28

C52

C57

R46

50KB

D10
R113

R115
R42

R43
R44
R49

R45

BALANCED

OUTPUT

OUTPUT

C32

C11

C9

R17
R14
R26

R24

C50

BP9

U21

BP10

R106

UNBALANCED

OUTPUT

J13

J4

OUTPUT

U5

D2
R25

CH1 LOG
CONVERTER

BP5

U6

BP6

C16

C15

R52
R51
R54
R53

D13

R56

TRIM

R29
C10

R30
R27
R28

C58

R169
R168

D17
R66
R65
R64

R62

C25

LIMITER

R158

THRESHOLD TRIM

R164

R55

R173

50KB 50KB

IGNORE THIS (ERROR ON SILKSCREEN)

BP13

U12

BP14

U7

R75

LIM THRESH

BALANCED

INPUT

J11

INPUT

DISTORTION

C31

R73

R72

C30

D20

C17

R57

D14
D15
R60
R59
R58

C26

C36

C35

R82
R83

C33

R78
R79
R80
R81

TRIM CH2

D21

R71

R74

D22

R61

50KB

GAIN

GAINTHRESHOLDRATIORELEASETHRESHOLDRELEASETHRESHOLD

C34

C48

J12

R76

UNBALANCED

BALANCED

INPUT

INPUT

C39

C37

BP15

U14

BP16

R91

TP

U27

D11
R110

IN/BYPASS

BYPASS

CHANNEL 2

OPTIONAL

SW1

C19

C20

THRESHOLD

CH2 LIMITER

R201
50KB

50KB

230

115

115

U8

R117

-15VA

D38
R152
R151
R150
R149

C55

R162
R163

R105

POWER
SWITCH

J5

OPTIONAL

A.C. SWITCH

CAUTION HAZARDOUS VOLTAGE

SW2

CAUTION

N

J7

1/4AMP FOR 117V

1/8AMP FOR 230V

GND

FUSE

120018

D47

BP19

D53
R100

U20

R99
R98

C44

BP20

R102

R101

R153
50KB

LIM THRESH

THRESHOLD

50KB

50KB

GAIN

GAIN

L

50KB

J6

CAUTION

T1

PIN 1 PIN 12

SW6

IN/BYPASS

IN/

BYPASS

SIDECHAIN

IN/OUT

J8

J10

BALANCED

OUTPUT

UNBALANCED

OUTPUT

J14

J9

P/N 022001 REV ’E’
COPYRIGHT 1991
SYMETRIX, INC.

022001 REV E
COMPONENT SIDE

A.C. RANGE SELECT

C18

R63

C54
R147

D36

R146

R145
C53

D35

R148
D37

C56

R154

500KA

RELEASE

R107

U9

78M15

C23
C21

HS1

U10

79M15

C24

C22

C62

C63

R198

U25

R197
R200
R199
D46
R202

500KA

D39
R175

CH2 LOG
CONVERTER
TRIM

R180
R177
R178

R216
R217
R207

D48
R206
R205
R204

R219
R220
R214
R215

C67

COMP THRES COMP REL COMP RATIO

THRESHOLD

BP17

R192

50KB

R116

+15VA

U15

BP18

R111
D12
R112

C49

C51

R114
R188
R189

R190
R195
R109

50KB

HS2

RATIO

SW4

IN/

R90

R161
R160
R103

R92

R94

R93

C40

R89

R88

R86

R85

R87

R84
R159

R96

R95

C42

R97

STEREO/

DUAL MONO

STEREO/

DUAL MONO

SIDECHAIN

C38

C41

U29

C43

1

U13

C47

SW5

C64

R187
50KB

EXP THRES

THRESHOLD RELEASE

R182
R181
R176
R174

R108
R186

R184
R183

D40

R185
R203
R165

D49
R210
R211
R209
R208

D50

D51

D52
R218
R212

50KB

OUTPUT

C61

C60

C65

BP12

Q8

U22

BP7

U23

BP8

BP11

U24

U26

R213

500KA

EXP REL

C68

C66

500KA

OUTPUT

R179

C59

CHANNEL 2CHANNEL 1

REV#

REVISIONS: DATE DATEREVISIONS:

A

11/20/91BOARD REV’D TO REV#A

REV#

11/22/91B BOARD REV’D TO REV#B
12/16/91BOARD REV’D TO REV#CC
3/10/99BOARD REV’D TO REV#D (ECO#170)D

E 12/1/92BOARD REV’D TO REV#E

7/6/93NYLON STANDOFFS NOW METALE1

ENG.

8/27/93ERROR ON SILKSCREEN CALLED OUTE2

MANF.
PURCH.
DATE

PRINTED
DRAWN

James Husted

BY

TITLE

425MBOARD ASSY

DWG
NO.
DATE

10-1-91

DWG
REV.

SHEET 1 OF 1

E2022001-ASY

REV# REV#

DATEREVISIONS:

11/22/91PCB REV’D TO REV ‘B’A
2/13/92ADDED C4 & C5 IN TOUCH-UP (ECO #169)B
3/10/92BOARD REV’D TO REV ‘C’ (ECO #170)C

REVISIONS: DATE

12/2/92D BOARD REV’D TO REV ‘D’
7/6/93NYLON STANDOFFS NOW METALD1

COPYRIGHT 1991
SYMETRIX, INC.

Q1

R7

C6

R8

C3

D1
D2
R5

U1
R4
R3
R2
R1
R6

D27D26D25D24

RED

EXPANDER

MOTHERBOARD
SCREW ACCESS

POLARITY

OF ALL

LEDS

022002 REV ’D’
COMPONENT SIDE

R19

Q2

R20

D3
D4

R14
R15
R16

U3

R17

R18

D29D28

RED

D17
D18
R33
R69

R13
R12
R11

R10

R9

C1

D35D34D33D32D31D30

C12

U11

C11

U2

COMPRESSOR

R26
R27

D5
D6

R25

R24

R158

R23

ACCESS TO CH1

R22
R21

LIMITER THRESHOLD TRIM

R36

D38

D37D36

RED

C2

J5

Q3

U4

D39

LIMITER

R34
R35

D7
D8

R32

R31
R30
R29
R28

D43D42

D41D40

GREEN RED

Q4

OUTPUT

MOTHERBOARD

SCREW ACCESS

U5

C4

D65

D64

DUAL MONO

STEREO

Q5

D21

C9

R43
R44

D9
D10
R41
R40
R39
R38

R37
R42

D47

D46D45D44

REDREDRED

EXPANDER

D22

Q6

R55

C7

R56
D11

U6

U8

D12
R50
R51
R52
R53

R54

ALL RED

D19
D20
R73
R74

R49
R48
R47
R46
R45

D55D54D53D52D51D50D49D48

C13

U12

C14

U7

POLARITY

OF ALL

LEDS

COMPRESSOR

D23
R62
R63
D13
D14
R61
R60

R162

R59

ACCESS TO CH2

R58

LIMITER THRESHOLD

R57
R72

D56

RED

RED REDREDRED RED RED REDGREEN

DUAL/

STEREO

EXPANDER COMPRESSOR LIMIT OUTPUTOUTPUTLIMITCOMPRESSOREXPANDER

Q7

U9

MOTHERBOARD
SCREW ACCESS

LIMITER

P/N 022002 REV ’D’

Q8

R70
R71
D15
D16
R68
R67
R66
R65
R64

D63D62D61D60D59D58D57
REDGREEN

OUTPUT

GREEN RED

C8

C10

U10

C5

CHANNEL 2CHANNEL 1

ANODE CATHODE

ENG.
MANF.

LED

ORIENTATION

PURCH.
DATE

PRINTED
DRAWN

James Husted

BY

7/6/93

TITLE

425DBOARD ASSY

DWG
NO.
DATE

10-2-91

DWG
REV.

SHEET 1 OF 1

D1022002-ASY

13-3

CHANNEL 1 INPUT

BALANCED

XLR-FEMALE

J1

BALANCED/

UNBALANCED

J2

SIDECHAIN

SEND/RETURN

J3

S/C_IN_CH1

2

1

3

4

R104
100K

2K00

470pF

R6

2K00

R15

6K98

REVISION RECORD

REVISIONS DONE OR ECO NUMBER:

UNBALANCED

OUTPUT

J13

1

2

3

EXPLEDS_CH1

4

COMPLEDS_CH1

5

6

7

8

9

10

12

13
14

15

16

(SHOWN IN STEREO LINK)

11

LIMLEDS_CH1

EXPLEDS_CH2

COMPLEDS_CH2

LIMLEDS_CH2

O/PLEDS_CH2

SW5D

11

LINK

3

1

(SHOWN IN

2

STEREO LINK)

+15VA

10

-15VA

12

LINK

SW5A

R165
2K2

DATE:

7/21/92

2/16/93

37

EC1

2150A

IN

S

V

Y

E

M

E

R13
5K1

R10

51

R11

61.9

C26

.001uF

R59

49K9

6

-

5

+

+15VA

VCC

OUT

E

G

C

N

2

D

6254

U7B

RC4560

C6
82pF

R8

13K0

U1B

6

­7

5

+

RC4560

R70

13K0

R157

1K00

CHANNEL 1

IN/OUT

(SHOWN IN)

SW4A

7

3

1

2

S/C_JACK_CH2

C32

4.7NP

O/PLEDS_CH1

R67

+

150 OHM

100/35

C28
82pF

R68

13K0

U11B

6

­7

5

+

RC4560

(SHOWN IN STEREO LINK)

CH2INOUT.2

(SHOWN IN STEREO LINK)

R77

6K98

C27

R155
100K

7

9

6

4

R69

150 OHM

LINK

SW5C

LINK

SW5B

BALANCED

OUTPUT

XLR-MALE

2

3

4

+
100/35

8

CV_IN_CH2

5

C29

J4

1

LTR

D RESISTOR VALUE CHANGES AS PER ECO 169 2/13/92

E RESISTORS AND CAPS AS PER ECO 170 3/9/92
E1

RESISTORS AND CAPS AS PER ECO 425-002
R158, R162, R163, R164 CHANGED

E2

PER ECO 425-003

CHANNEL 1 OUTPUT

R156

100K

J12A

J12B

J12C

J12D

J12E

J12F

J12G

J12H

J12I

J12J

J12K

J12L

J12M

J12N

J12O

J12P

C1

2

-

3

+

R4
15K0

R23

100 OHM

100pF

R1

15K0

U1A

RC4560

C2
100pF

1

R5

C3

R2

13K0

C4

470pF

R3

13K0

C8

4.7NP

100K

C7

4.7NP

CH1_CTRL

R76

50KB

R16

4.7NP

+15VA

1

3

-15VA

C5

CHANNEL 1
DISTORTION TRIM

1

2

3

R9

61.9

U2

R7

18

13K0

R12
510K

2

R60

100K0

R61
300K

-15VA

R58

357K0

CHANNEL 1

MASTER

OUTPUT LEVEL

(FRONT PANEL)

-15VA

DRAWN:

DRAWN:

RELEASED:

RELEASED:

DATED:

DATED:

DATED:

DATED:

COMPANY:

COMPANY:

TITLE:

TITLE:

SYMETRIX INC.

4211 24th Ave W

SEATTLE, WA 98199

425 DUAL COMP/LIMITER/EXPANDER

DRAWING NO:

DRAWING NO:

022001-SCH

SHEET:

REV:

REV:

E2

OF

7

1

S/C_IN_CH1

CHANNEL 1

LOG CONVERTER

TRIM

R47
13K0

+

C45

4.7/50

R29

100K

+15VA

1

3

-15VA

CW

2

R26

10K0

-15VA

3

2

R30
200K0

R28

82K

131214

U3A
RC4560

+

-

1N4148

R18

200K0

R19
6K98

D1

-15VA

U5B

MC3346P

6

5

R27
100 OHM

10K0

2

-

3

+

U6A
RC4560

MC3346P

1

R21
1M5

10911

7

8

6

C10
22pF

­7

+

U6B

RC4560

R24

15

1

15

U28D

R20

560K

MC3346P

-15VA

2

2

MC3346P

D2

1N4148

R25

2K2

U5D

13

12

3

U5A

MC3346P

U28A

MC3346P

3

PEAK_DET_CH1

+15VA

1

2

3

-15VA

U5C

14

4

4

EXPANDER

R22

THRESHOLD

50KB

(FRONT PANEL)
CW
(BYPASS)

R31

+15VA

75K

D9

1N4148

R48

2

3

D4

1N4148

C13

22pF

R38

1M

U4A

2

-

3

+

RC4560

R40
10K0

R41

500KA

1

2

RELEASE

(FRONT PANEL)

1M

U11A

-

+

RC4560

1

CW
3

R17
2K7

+

+

3.3/25t

C9
10/35

-15VA

+

R32

4K7

R34

2M0

C11

R115

3K3

C52
33/35

R37

2K7

6

5

U3B
RC4560

-

+

R33

10M

U21A

RC4560

1K00
R43

3

+

2

-

R42

30K1

R44

158K0

1

-15VA

2

+15VA

-15VA

C12
.22uF

R113

2K2

C50

.22uF

COMPRESSOR

3

CW

THRESHOLD

R46

(FRONT PANEL)

50KB

1

R45

2K00

R36

20K

-15VA

CH1 EXPANDER SIDE CHAIN

D3

7

1N4148

R35

10K0

DRAWN:

RELEASED:

R14
1M

CH1 COMPRESSOR

C46

22pF

1

R106

R39

1K00

DATED:

DATED:

500KA

1

RELEASE

(FRONT PANEL)

+15VA

R170
100 OHM

D5

1N4148

R49

6K2

D6

1N4148

SIDECHAIN

R50

200 OHM

CW
3

2

+

C15

4.7/50

+

Q5
2N4401

D7
1N4148

COMPANY:

TITLE:

425 DUAL COMP/LIMITER/EXPANDER

DRAWING NO:

D10

1N4148

R51
2K2

R52

C16

10M

10/35

-15VA
5

+

6

-

R172

150K0

R166

2K2

+

C14

3.3/25t

-15VA

SYMETRIX INC.

022001-SCH

5

+

6

-

RC4560

R54

10K0

R53

10K0

D13
1N4148

U4B
RC4560

R171

10K0

R168

7

10K0

-15VA

+

C57

4.7/50

4211 24th Ave W

SEATTLE, WA 98199

U21B

R167
10M

7

CHANNEL 1

IN/OUT

(SHOWN IN)

8

COMPLEDS_CH1

3

CW

R55

50KB

1

C58

22pF

2

-

3

+

SW4C

7

9

R56

30K

2

RATIO

(FRONT PANEL)

CH1_CTRL

R169

10K0

U12A

1

RC4560

SW4B

6

4

CHANNEL 1

IN/OUT

(SHOWN IN)

SHEET:

D14

1N4148

5

D15

1N4148

EXPLEDS_CH1

REV:

E2

2

OF

7

DETACHABLE

A.C. CORD

IEC A.C. JACK

J5

2

J7

J6

3

1

MAINS FUSE

(INTERNAL PC MOUNTED)
1/4 AMP FUSE FOR 117V

1/8 AMP FUSE FOR 230V

OPTIONAL

MAINS POWER

SW2A

3

2

1

SW2B

4

5

6

J7

T1,1

SW1

100/115 VAC

230 VAC

A.C.RANGE SELECT

SWITCH (INTERNAL)

POWER SUPPLY

120026

T1

1

8

2

7

3

6

45

C18

.01uF

R63

18 OHM

U9

78M15

1

VIN3VOUT

1

U8

4

VM48

2

3

+

+

C19
1000/35

C20
1000/35

GND

2

GND

2

VIN3VOUT

U10

79M15

+

C21

4.7/50

+

C22

1

4.7/50

C23
.01uF

C24
.01uF

+15VA

R116

0 OHM

R117

0 OHM

-15VA

8

4

U26C
RC4560

8

4

U23C

RC4560

8

4

BP1
.1uF

U1C

RC4560

BP2
.1uF

8

4

BP3
.1uF

U3C

RC4560

BP4
.1uF

8

4

U6C

RC4560

BP5
.1uF

BP6
.1uF

8

4

BP7
.1uF

U4C

RC4560

BP8
.1uF

8

4

U21C

RC4560

BP9
.1uF

BP10
.1uF

8

4

BP11
.1uF

U24C

RC4560

BP12
.1uF

DRAWN:

RELEASED:

8

4

U11C

RC4560

BP13
.1uF

BP14
.1uF

8

4

U12C

RC4560

DATED:

DATED:

BP15
.1uF

8

4

BP16
.1uF

RC4560

U14C

COMPANY:

BP17
.1uF

8

4

BP18
.1uF

U15C

RC4560

BP19
.1uF

BP20
.1uF

8

4

U20C

RC4560

8

4

SYMETRIX INC.

4211 24th Ave W

TITLE:

425 DUAL COMP/LIMITER/EXPANDER

DRAWING NO:

SEATTLE, WA 98199

022001-SCH

U7C

RC4560

8

U25C

RC4560

4

SHEET:

+15VA

8

U27C

RC4560

4

-15VA

REV:

E2

37

OF

CH1 LIMITER SIDE CHAIN

LIMITER

THRESHOLD

(FRONT PANEL)

50KB

R75

+15VA

3

1

-15VA

R173
12K

BYPASS

CW

2

R64

249K0

+15VA

2

CW

LIMITER
THRESHOLD
TRIM

1

R158
100K

3

R164
220K

PEAK_DET_CH1

U7A

RC4560

3

+

2

-

R65

24K3

D17

1N4148

R66
1K00

LIMLEDS_CH1

R62

C25

1M

.047

1

-15VA

D20

1N4148

R71
2M0

C30
.039

D21

1N4148

5

6

R73

10K0

U12B
RC4560

+

-

R72

15K8

C31

10pF

COMPANY:

SW4D

7

10

11

12

SYSTEM IN/OUT

(SHOWN IN)

R74

200 OHM

D22

1N4148

R57

CH1_CTRL

51K

C17
.01

SYMETRIX INC.

4211 24th Ave W

DRAWN:

RELEASED:

DATED:

DATED:

TITLE:

SEATTLE, WA 98199

425 DUAL COMP/LIMITER/EXPANDER

DRAWING NO:

022001-SCH

SHEET:

REV:

E2

4

OF

7

CHANNEL 2 INPUTS

BALANCED

XLR-FEMALE

J11

1

4

BALANCED/

UNBALANCED

J8

SIDECHAIN

SEND/RETURN

J10

R82

2

3

2K00

C35

470pF

R83

2K00

R90

6K98

C36

470pF

R79

13K0

R80

13K0

C38

4.7NP

2

3

R81
15K0

R92

100 OHM

C33

100pF

R78

15K0

-

+

U14A

RC4560

C34

100pF

S/C_IN_CH2

R85

61.9

C37

R84

18

+15VA

3

2

1

CHANNEL 2

DISTORTION TRIM

-15VA

CV_IN_CH2

13K0

R88
510K

4.7NP

1

R91

100K

C39

4.7NP

37

U13

EC1

2150A

IN

V

S

E

Y

E

M

R89
5K1

-15VA

R86

51

R159
1K00

E
C
2

+15VA

VCC

OUT

G
N
D

R87

61.9

6254

C40
82pF

R93

13K0

U14B

6

­7

5

+

RC4560

R97

13K0

CHANNEL 2 (SLAVE)

OUTPUT LEVEL

(FRONT PANEL)

R94

150 OHM

R95

13K0

6

-

5

+

C42
82pF

U27B

RC4560

CH2_CTRL

O/PLEDS_CH2

C41

+
100/35

7

150 OHM

1

R102

50KB

3

R96

CHANNEL 2 OUTPUTS

J14

UNBALANCED
OUTPUT

C44

U20B

RC4560

7

XLR-MALE

2

3

R161

100K

J9

1

4

CHANNEL 2

IN/OUT

(SHOWN IN)

SW6D

10

11

12

BALANCED
OUTPUT

CH2INOUT.2

R160

100K

C43

+
100/35

R100

100K0

R98

R101
300K

357K0

2

.001uF

R99

49K9

6

-

5

+

R103
100K

S/C_JACK_CH2

DRAWN:

RELEASED:

DATED:

DATED:

-15VA

COMPANY:

SYMETRIX INC.

4211 24th Ave W

TITLE:

425 DUAL COMP/LIMITER/EXPANDER

DRAWING NO:

SEATTLE, WA 98199

022001-SCH

SHEET:

REV:

E2

57

OF

CH2 LIMITER SIDE CHAIN

LIMITER

THRESHOLD

(FRONT PANEL)

R153

50KB

+15VA

3

1

-15VA

CW

R105
12K

2

R150

249K0

2

CW

+15VA

R163

220K

LIMITER
THRESHOLD
TRIM

1

R162
100K

3

PEAK_DET_CH2

3

2

R152
1K00

U20A

RC4560

+

-

R151

24K3

D38

1N4148

LIMLEDS_CH2

U15B
RC4560

5

+

6

D36

R147
10K0

-

R146

15K8

C54

10pF

R145

C55

R149

.047

1M

1

-15VA

D35

1N4148

2M0

C53
.039

1N4148

7

COMPANY:

SW6B

5

SYSTEM

IN/OUT

(SHOWN IN)

R148

6

200 OHM

4

D37

1N4148

R154

CH2_CTRL

51K

C56
.01

SYMETRIX INC.

4211 24th Ave W

DRAWN:

RELEASED:

DATED:

DATED:

TITLE:

SEATTLE, WA 98199

425 DUAL COMP/LIMITER/EXPANDER

DRAWING NO:

022001-SCH

SHEET:

REV:

E2

6

7

OF

CHANNEL 2

LOG CONVERTER

TRIM

R108
13K0

+

C47

4.7/50

S/C_IN_CH2

R179
100K

R176

10K0

13

-15VA

3

2

R184
6K98

+15VA

1

3

-15VA

U24A

RC4560

+

-

D40

1N4148
R183

200K0

2

3

12

-15VA

2

CW

R174

10K0

-

+

U23A
RC4560

MC3346P

1

8

R180
200K0

U22B

MC3346P

6

-

5

+

RC4560

R177
100 OHM

3

15

U22A

2

MC3346P

U29A

2

MC3346P

15

3

+15VA

EXPANDER
THRESHOLD

1

R187
50KB

2

3

CW

(BYPASS)

R203

75K

-15VA

14

U29D

R186
1M5

R178

82K

1

R185

560K

10

7

6

C59
22pF

7

U23B

4

4

PEAK_DET_CH2

(FRONT PANEL)

R204

4K7

+15VA

9

D39

1N4148

R175

2K2

R206

2M0

11

U22C

MC3346P

R209

2K7

-15VA

U22D

MC3346P

6

-

5

+

RC4560

R205

-15VA

131214

R114

3K3

+

C51
33/35

U24B

10M

R181

7

1M

-15VA

C64
.22uF

D48

1N4148

D11

R182
2K7

+

C61

3

2

R208

20K

C60
10/35

+

-

R189
1K00

U25A

RC4560

R188

30K1

R190

158K0

R207

10K0

3.3/25t

R112

2K2

1

+15VA

3

CW

R192

2

50KB

1

R109
2K00

-15VA

C49
.22uF

-15VA

COMPRESSOR
THRESHOLD

(FRONT PANEL)

D49

1N4148

C65

22pF

2

-

3

+

R212
10K0

CH2 EXPANDER SIDE CHAIN

DRAWN:

RELEASED:

+

2

3

R210

1M

U26A

RC4560

R213

500KA

1

R110

1M

-

+

1

(FRONT PANEL)
CW

3

2

1N4148

C48

22pF

1

U27A

RC4560

D51

1N4148

RELEASE

DATED:

DATED:

200 OHM

R195

6K2

+15VA

R211

1K00

D50

1N4148

CH2 COMPRESSOR

SIDECHAIN

R111

RELEASE

(FRONT PANEL)

1

500KA

+

-15VA

D12

1N4148

R107

CW
3

2

C62

4.7/50

+

C63
10/35

R218
100 OHM

Q8
2N4401

1N4148

D52

COMPANY:

TITLE:

425 DUAL COMP/LIMITER/EXPANDER

DRAWING NO:

5

+

6

-

RC4560

7

R220

150K0

+

R199

10K0

1N4148
D46

C67

4.7/50

R200

10K0

R216

10K0

-15VA

+

-15VA

5

6

R197
2K2

R198
10M

U26B

RC4560

+

-

R214

2K2

C66

3.3/25t

R219

10K0

SYMETRIX INC.

4211 24th Ave W

SEATTLE, WA 98199

022001-SCH

U25B

7

R215
10M

R201
50KB

CHANNEL 2

IN/OUT

(SHOWN IN)

SW6A

3

2

1

COMPLEDS_CH2

R202

30K

3

CW

2

RATIO

1

(FRONT PANEL)

C68

22pF

R217

10K0

U15A

2

­1

3

+

RC4560

7

9

CHANNEL 2
IN/OUT

(SHOWN IN)

SHEET:

CH2_CTRL

SW6C

8

D53

1N4148

EXPLEDS_CH2

D47

1N4148

REV:

E2

OF

77

D2
1N4148

R7
120 OHM

D1
1N4148

Q1
2N4403

R8
10K

R1
2K67

9

+

8

11

10

5

4

7

6

EXPLEDS_CH1

-

+

-

+

-

+

-

-15VA

+15VA

R2
3K01

R3
4K22

R4
4K22

R5
35K7

R6
1M

CH1 EXPANDER LEDS

U1D

LM339

U1C

LM339

U1B

LM339

U1A

LM339

14

13

2

1

-15VA

D27

(RED)

D26

(RED)

D25

D24

(RED)

+15VA

R20

10K

-2dB

-6dB

-12dB

(RED)

-18dB

+15VA

D4

R9
10K5

R10
6K49

R11
5K76

R12
5K62

R13
5K62

R14
5K62

R15
8K45

R16
28K0

R17
124K

1N4148

D3
1N4148

R18
1M

R19
120 OHM

Q2
2N4403

U2D

9

+

14

8

-

LM339

U2C

11

+

10

-

LM339

U2B

5

+

4

-

LM339

U2A

7

+

6

-

LM339

U3D

9

+

8

-

LM339

U3C

11

+

10

-

LM339

U3B

5

+

4

-

LM339

U3A

7

+

6

-

LM339

D35

13

D34

2

D33

1

D32

14

D31

13

D30

2

D29

1

D28

-15VA

CH1 COMPRESSOR LEDS

-2dB

(RED)

-4dB

(RED)

-6dB

(RED)

-8dB

(RED)

-10dB

(RED)

-12dB

(RED)

-15dB

(RED)

-25dB

(RED)

-15VA

+15VA

LIMLEDS_CH1

COMPLEDS_CH1

D6
1N4148

R26
120 OHM

D5
1N4148

Q3
2N4403

R27
10K

R21
8K45

9

8

R22
5K90

11

10

R23
5K49

5

4

R24
5K76

7

6

R25
100K0

R36
1M

CH1 LIMITER LEDS

DRAWN:

RELEASED:

+

-

+

-

+

-

+

-

U4D

LM339

U4C

LM339

U4B

LM339

U4A

LM339

14

13

2

1

-15VA

D39

D38

D37

D36

-3dB

(RED)

-6dB

(RED)

-9dB

(RED)

-12dB

(RED)

DATED:

DATED:

C4

.01uF

DETECT_CH1

-15VA

+15VA

D8
1N4148

D7
1N4148

R35
10K

R28
549 OHM

R29
1K21

R30
3K83

R31
12K1

R32
30K1

COMPANY:

TITLE:

DRAWING NO:

REVISION RECORD

REVISIONS DONE OR ECO NUMBER:

VALUE CHANGES IN OUTPUT LED STRING 12/19/91

CHANGES AS PER ECO 425-001 7/9/92D

ECO 425-8 SWAPPING LED REF DES.

D40-D43 & D60-D63

EXPLEDS_CH1

D40

D41

D42

D43

-20

(GREEN)

-10

(GREEN)

0VU

(GREEN)

CLIP

(RED)

COMPLEDS_CH1

LIMLEDS_CH1

O/PLEDS_CH1

+15VA

R34
120 OHM

Q4
2N4403

9

8

11

10

5

4

7

6

LTR DATE:

B

C C4 & C5 ADDED AS PER ECO 169 2/13/92

E

U5D

+

14

-

LM339

U5C

+

13

-

LM339

U5B

+

2

-

LM339

U5A

+

1

-

LM339

-15VA

CH1 OUTPUT LEDS

SYMETRIX INC.

4211 24th Ave W

SEATTLE, WA 98199

425 LED DAUGHTERBOARD

022002-SCH

SHEET:

FROM MOTHERBOARD

STEREO

D65

(RED)

DUAL

D64

(RED)

1

1

2

3

4

5

6

7

8

REV:

OF

8/24/93

B.G.

J5A

J5B

J5C

J5D

J5E

J5F

J5G

J5H

E1

3

D10
1N4148

R43
120 OHM

D9
1N4148

Q5
2N4403

R44
10K

R37

D21
1N4148

R38
3K01

R39
4K22

R40
4K22

R41
35K7

+15VA

2K67

R42
1M

9

8

11

10

5

4

7

6

U6D

+

-

LM339

U6C

+

-

LM339

U6B

+

-

LM339

U6A

+

-

LM339

EXPLEDS_CH2

CH2 EXPANDER LEDS

D22

1N4148

14

13

2

1

-15VA

LEDBUSS

+15VA

LEDBUSS

-2dB

D47

(RED)

-6dB

D46

(RED)

-12dB

D45

(RED)

D44

-18dB

(RED)

COMPLEDS_CH2

R56

20K0

+15VA

R45
10K5

R46
6K49

R47
5K76

R48
5K62

R49
5K62

R50
5K62

R51
8K45

R52
28K0

R53
124K

D12
1N4148

D11
1N4148

R55
120 OHM

LEDBUSS

10

10

R54
1M

D23

Q6
2N4403

1N4148

U7D

9

+

14

8

-

LM339

D55

U7C

11

+

13

2

1

14

13

2

1

-15VA

(RED)

D54

(RED)

D53

(RED)

D52

(RED)

D51

(RED)

D50

(RED)

D49

(RED)

D48

(RED)

-

LM339

U7B

5

+

4

-

LM339

U7A

7

+

6

-

LM339

U8D

9

+

8

-

LM339

U8C

11

+

-

LM339

U8B

5

+

4

-

LM339

U8A

7

+

6

-

LM339

R63

10K

-2dB

-4dB

-6dB

-8dB

-10dB

-12dB

-15dB

-25dB

CH2 COMPRESSOR LEDS

D14
1N4148

D13
1N4148

+15VA

LIMLEDS_CH2

R57
8K45

R58
5K90

R59
5K49

R60
5K76

R61
100K0

R62
120 OHM

Q7
2N4403

U9D

9

+

14

8

-

R72
1M

LM339

U9C

11

+

10

-

LM339

U9B

5

+

4

-

LM339

U9A

7

+

6

-

LM339

13

2

1

-15VA

D59

D58

D57

D56

-3dB

(RED)

-6dB

(RED)

-9dB

(RED)

-12dB

(RED)

CH2 LIMITER LEDS

DRAWN:

RELEASED:

R71
10K

-15VA

C5

.01uF

+15VA

DETECT_CH2

DATED:

DATED:

D16
1N4148

D15
1N4148

R64
549 OHM

R65
1K21

R66
3K83

R67
12K1

R68
30K1

R70
120 OHM

Q8
2N4403

U10D

9

+

14

8

-

LM339

U10C

11

+

10

-

LM339

U10B

5

+

4

-

LM339

U10A

7

+

6

-

LM339

-20

D60

13

(GREEN)

-10

D61

2

(GREEN)

D62

0VU

(GREEN)

1

CLIP

D63

(RED)

-15VA

CH2 OUTPUT LEDS

COMPANY:

SYMETRIX INC.

4211 24th Ave W

TITLE:

425 LED DAUGHTERBOARD

DRAWING NO:

SEATTLE, WA 98199

022002-SCH

EXPLEDS_CH2

COMPLEDS_CH2

LIMLEDS_CH2

O/PLEDS_CH2

+15VA

-15VA

9

J5I

10

J5J

11

J5K

12

J5L

13

J5M

14

J5N

15

J5O

16

J5P

LEDBUSS

SHEET:

REV:

E1

OF

23

-15VA

U5E
LM339

3

12

U10E

LM339

+15VA

O/PLEDS_CH1

O/PLEDS_CH2

DRAWN:

RELEASED:

R69
20K0

R74
20K0

AGND

AGND

R33
20K0

R73
20K0

DATED:

DATED:

U11A

2

­1

3

+

RC4560

U11B

6

­7

5

+

RC4560

U12A

2

­1

3

+

RC4560

U12B

6

­7

5

+

RC4560

COMPANY:

D18

1N4148

D20

1N4148

D17
1N4148

D19
1N4148

+

AGND

+

AGND

C2

3.3/25t

C1

3.3/25t

DETECT_CH1

DETECT_CH2

AGND

C12

.1uF

8

RC4560

4

C11

.1uF

C13

.1uF

8

RC4560

4

C14
.1uF

AGND

+15VA

U11C

-15VA

+15VA

AGND

U12C

-15VA

SYMETRIX INC.

4211 24th Ave W

TITLE:

425 LED DAUGHTERBOARD

DRAWING NO:

022002-SCH

SEATTLE, WA 98199

SHEET:

REV:

E1

3

3

OF

C3

AGND

.1uF

.1uF

C6

C7
.1uF

C9
.1uF

3

U2E

LM339

12

3

U7E

LM339

12

3

U3E
LM339

12

3

U8E
LM339

12

3

12

AGND

3

12

U4E
LM339

U9E

LM339

C8
.1uF

C10
.1uF

3

12

AGND

3

U1E

LM339

12

3

U6E
LM339

12

13-4

Appendix A. Architects and Engineers Specification

The Compressor/Limiter/Expander shall be a dual channel model that controls the dynamic
range of wide range, wideband audio signals, providing compression, peak limiting, and
downward expansion simultaneously. The unit shall occupy one rack space (1U).

The threshold of the compressor section shall be adjustable over a range of -40dBu to
+20dBu via a front panel control. When the control is fully clockwise the section shall revert
to bypass mode. The input-to-output ratio (compression ratio) shall be adjustable from 1:1 to
10:1. Control of the compressor release time shall be program dependent within a range set
by the front panel release time control. The compressor section shall have a dedicated eight
segment LED ladder that displays the degree of gain reduction.

The Compressor/Limiter/Expander shall contain an integral peak limiter having a 20:1
compression ratio and adjustable threshold level. A dedicated four segment LED ladder shall
be provided to indicate the amount of peak limiter activity.

A front panel switch, with LED indicator, shall select between dual mono and stereo
master/slave operation. Each channel shall have a bypass switch which defeats all front
panel controls for that channel.

The Compressor/Limiter/Expander shall also contain a downward expander having a 1:1.5
expansion ratio with threshold, and release time controls. A dedicated four LED display shall
be provided to indicate the amount of downward expansion.

The inputs shall be active balanced bridging designs terminated with 3-pin XLR (AES/IEC
standard wiring), and 1/4" TRS female. The input circuitry shall incorporate RFI filters. The
outputs shall be active balanced designs having equal source impedances and terminated with
3-pin XLR (AES/IEC standard wiring), and 1/4" TRS female.

The balanced inputs shall accommodate +20 dBu signals without distortion, and the balanced
outputs shall be capable of delivering +23 dBm into a 600 ohm load.

Overall frequency response shall be 10 Hz - 60 kHz (+0dB, -3dB). THD shall be .02%
measured under the following conditions: +4 dBu input, +18 dBu output, BYPASS switch out,
20 Hz to 30kHz, 30kHz low-pass filter, 0dB gain reduction. Residual noise output shall be no
greater than -90 dBu, measured with a 20 kHz noise bandwidth, input terminated in 600
ohms.

Access to each channel's sidechain shall be provided via a single 1/4" TRS female connector
(per channel). The ring connection shall be the sidechain output and the tip connection shall
be the sidechain return.

The unit shall be capable of operating by means of its own built-in power supply connected to
117V nominal AC (105-130V) 50/60 Hz (230V nominal, 207-253V AC, 50 Hz where
applicable).

The unit shall be a Symetrix Incorporated model 425 Dual Compressor/Limiter/Expander.

Rev 2.0, 10/29/93

A-1

Notes

A-2

Rev 2.0, 10/29/93

Appendix B. Disassembly Instructions

Caution: These servicing instructions are for use by qualified personnel only. To
avoid electric shock do not perform any servicing other than that contained in the
operating instructions portion of this manual unless you are qualified to do so.
Refer all servicing to qualified service personnel.

Warning: Lethal voltages are present inside the chassis. Perform all service
work with the unit disconnected from all AC power.

Tools Required

1. #2 Phillips screwdriver.

2. 5/64" allen wrench.

Top Cover Removal

1. Ensure that the 425 is disconnected from the AC power source.

2. Remove four 6-32 x 1/2 inch screws from the rear-panel.

3. Remove two 6-32 x 1/2 inch screws from each side of the chassis.

4. Remove one 6-32 x 1/2 inch screw from the top-middle of the front panel. Use caution to
prevent the screwdriver from slipping and marring the front panel.

5. Lift the top cover free of the chassis.

Circuit Board Removal

1. Ensure that the 425 is disconnected from the AC power source.

2. Remove the top cover using the procedure described previously.

3. Remove the four screws from the four corners of the front panel and one screw in the
bottom-center of the front panel.

4. Locate the LED board near the front panel.

5. Remove the LED board by removing the two screws located at the ends of the board,
then applying upward pressure at the connector located at the middle-rear of the board
to disengage the board.

6. Remove four 6-32 x 1/4 inch SEMS machine screws from the printed circuit board.
Three are located near the front panel, the remaining screw is located near J13.

7. Remove the two 6-32 x 1/2 inch machine screws that fasten the IEC power connector to
the chassis.

8. Remove four 6-32 x 1/2 inch screws from the ends of the front panel (two on left, two on
right).

9. Disconnect the green safety ground wire at the chassis screw terminal by removing the
nut. Ensure that this wire is reconnected when reassembling the 425.

10. Lift the circuit board, with the front panel attached clear of the chassis.

Rev 2.0, 10/29/93

B-1

Notes

B-2

Rev 2.0, 10/29/93