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