Symetrix 420 User Manual

420 Stereo Power Amplifier

User’ s Guide

420

Table of Contents

Chapter 1 Introduction 1
Chapter 2 Operator Safety Summary 2
Chapter 3 Fast Setup 3
Chapter 4 Front & Rear Panel Overview 5
Chapter 5 420 Basics 7
Chapter 6 Using the 420 10
Chapter 7 Applications 13
Chapter 8 Technical Tutorial 16
Chapter 9 Troubleshooting 19
Chapter 10 Specifications 20
Chapter 11 Warranty & Service 21
Appendix A Disassembly Instructions 23

& Bridged-Mono Mode Switch

Rev A.00, 25 February, 1999
Symetrix part number 53420-0A00
Subject to change without notice.
©1999, Symetrix, Inc. All right reserved.
Symetrix is a registered trademark of Symetrix, Inc.
Mention of third-party products is for informational

purposes only and constitutes neither an endorsement nor
a recommendation. Symetrix assumes no responsibility
with regard to the performance or use of these products.

Under copyright laws, no part of this manual may be
reproduced or transmitted in any form or by any means,
electronic or mechanical, including photocopying,

420

scanning, recording or by any information storage and
retrieval system, without permission, in writing, from
Symetrix, Inc.

Mountlake Terrace, WA 98043 USA

6408 216th St. SW

Tel [phone]

Fax [phone]

Email [e-mail]

IntroductionChapter 1

The Symetrix 420 is a two-channel power amplifier intended for use in professional and commercial
audio systems. The 420 may be operated as a two channel amplifier with 20-watts per channel, or as
a single channel amplifier capable of 40-watts output (mono-bridged mode). The amplifier is
intended for use with near-field monitors, small speakers used for “radio reference” audio-for­video, and with headphones. In stereo mode, the minimum load impedance is 4 ohms, in mono­bridged mode the minimum load impedance is 8 ohms.

For convenience, the 420 is equipped with both 1/4" TRS (tip-ring-sleeve) and XLR connectors,
which accept either balanced or unbalanced signals. The 420 produces full output with a 0.5V input
signal (balanced or unbalanced). The 420 can be used as a stereo amplifier, with ganged level
controls, as a 2-channel amplifier, with independent level controls, or as a single-channel mono
amplifier.

The front-panel MODE switch mixes the two inputs together. Broadcast and recording applications
can use this feature to check the mono compatibility of their signals. Commercial sound applica­tions can use this feature to mix paging signals or paging and music signals.

In recording studios and other similar applications, the 420 makes an ideal headphone amplifier.
You can drive one pair of phones from the front-panel jack, or many pairs from the rear-panel
terminals. A front panel switch allows you to turn off the rear-panel speaker terminals, which is
useful for near-field monitor applications or for headphone driver applications.

We recommend that you read this manual cover-to-cover. You will find the answers to most of
your questions inside. Should you have any comments or questions, please do not hesitate to
contact us at the numbers/addresses below. Your calls are always welcome.

Phone:[phone]
Fax:[phone]
Email: [e-mail]
Website:www.symetrixaudio.com

420

STEREO POWER
AMPLIFIER

Front panel

CLIP

GAIN

5

2738

GAIN

GAIN

64

DUAL TRACKING
INDEPENDENT

91

100

654

37

2

CLIP

1

0

10

STEREO

HEADPHONESCONTROLS

MONO
STEREO

8

9

MONITORMODECHANNEL 2CHANNEL 1

SPEAKER

MUTED
ACTIVE

POWER

100 WATTS MAXIMUM

Rear panel

FUSE

MANUFACTURED IN
SEATTLE WASHINGTON,
UNITED STATES OF AMERICA

TYPICAL INPUTS

TIP

=HIGH(+)=
=LOW(-)=

=GROUND=

SLEEVE

RING

PIN 2
PIN 3
PIN 1

12

TIP

3

RING

FUSE

SLEEVE

OUTPUTS

CHANNEL

CHANNEL

1

2

20 WATTS PER CHANNEL 8 OHMS (CLASS 2 WIRING)

UNBALANCED/

BALANCED

INPUT 2

BALANCED

UNBALANCED/

BALANCED

INPUT 1

BALANCED

420

1

Operator Safety Summary Chapter 2

N

.

Equipment Markings

CAUTION

RISK OF ELECTRIC SHOCK

DO NOT OPEN

TO REDUCE THE RISK OF FIRE OR

WARNING:

SEE OWNERS MANUAL. VOIR CAHIER D’INSTRUCTIONS.

o user serviceable parts inside. Refer servicing to qualified service personnel

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

The lightning flash with arrowhead 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
persons. 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
product (i.e. this manual).

Caution To prevent electric shock, do not use

th e polarized plug supplied with the unit
with any extension cord, receptacle, or
other outlet unless the blades can be
fully inserted.

ELECTRIC SHOCK DO NOT EXPOSE
THIS EQUIPMENT TO RAIN OR MOISTURE

RISQUE DE CHOC ELECTRIQUE

AVIS:

NE PAS OUVRIR

S’adresser a un reparateur compétent.

Terms

Several notational conventions are used in this
manual. Some paragraphs may use Note, Caution,
or Warning as a heading. Certain typefaces and
capitalization are used to identify certain words.

These are:

Note Identifies information that needs

extra emphasis. A Note generally
supplies extra information to help
you to better use the 420.

Caution Identifies information that, if not

heeded, may cause damage to the
420 or other equipment in your
system.

Warning Identifies information that, if

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

CAPITALS Controls, switches or other markings

on the 420’s chassis.

Boldface Strong emphasis.

Important Safety Instructions

Please read and keep these instructions. Heed
and follow all warnings and instructions.
Install in accordance with the manufacturer’s
instructions.

Power Source This product is intended to
operate from a power source that does not apply

420

more than 250V rms between the power supply
conductors or between either power supply
conductor and ground. A protective ground

2

connection, by way of the grounding conductor
in the power cord, is essential for safe operation.

Grounding 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. Do not defeat the
safety purpose of the grounding plug. The
grounding plug has two blades and a third
grounding prong. The third prong is provided for
your safety. When the provided plug does not fit
your outlet, consult an electrician for replacement
of the obsolete outlet.

Danger from Loss of Ground 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.

Proper Power Cord Use only the power cord
and connector specified for the product and your
operating locale. Use only a cord that is in good
condition. Protect the power cord from being
walked on or pinched, particularly at plugs,
convenience receptacles, and the point where
they exit from the apparatus.

Proper Fuse The user accessible fuse is a part of
the IEC AC inlet connector. The fuseholder
accepts 5 x 20mm diameter fuses. For 117VAC
operation, the correct value is 1.0A, 250VAC,
slow blowing. For 230VAC operation, the correct
value is 0.5A, 250VAC, slow blowing.

Operating Location 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. Do not
install near any heat source such as radiators,
heat registers, stoves, or other apparatus
(including amplifiers) that produce heat. Unplug
this apparatus during lightning storms or when
unused for long periods of time.

Stay Out of the Box 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. Only use
accessories specified by the manufacturer. Clean
only with a damp cloth.

User-serviceable parts There are no user
serviceable parts inside the 420. In case of failure,
refer all servicing to the factory. Servicing is
required when the 420 has been damaged in any
way, such as when a power supply cord or plug
is damaged, liquid has been spilled or objects
have fallen into the apparatus, the apparatus has
been exposed to rain or moisture, does not
operate normally, or has been dropped.

Chapter 3

Fast Setup

Follow these instructions to get your 420 up-and-running as quickly as possible. The intent of this
section is fast setup. Refer to later chapters for explanation of the 420’s controls and functions.

Connections

Connect your input source to the either the XLR connectors or to the TRS phone jacks. Connect
the 420’s output to the loudspeakers (or other load) using the barrier strip connector located at the
rear of the unit. If the 420 is driving a headphone distribution system, you’ll get better results (read
louder) if you use the screw terminals on the rear of the unit. The front-panel headphone jack is
really only intended to drive one pair of ‘phones.

Fast Setup

CHANNEL CHANNEL

20 WATTS PER CHANNEL 8 OHMS (CLASS 2 WIRING)

Figure 3-1. 420 generic hookup diagram.

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 420 to the proper AC mains voltage may cause fire and/or

Warning Lethal voltages are present inside the chassis. There are no user serviceable parts

12

internal damage. There are no user serviceable parts inside the chassis. Refer all
service to qualified service personnel or to the factory.

inside the chassis. Refer all service to qualified service personnel or to the factory.

INPUT 2OUTPUTS

UNBALANCED/

BALANCED

INPUT 1

UNBALANCED/

BALANCED

BALANCEDBALANCED

13212 3

Settings

Set the controls and switches on the front and rear panel as follows:

Front Panel Control Setting Rear Panel Connectors Connect

CHANNEL 1 GAIN 9:00 o’clock Outputs Connect loudspeakers,

headphone distribution
system, or other load.

CHANNEL 2 GAIN 9:00 o’clock Input 2 Connect balanced or

unbalanced source here.

MODE Out/Stereo Input 2 Connect balanced or

unbalanced source here.
SPEAKER MUTE Out/Active
POWER In/On

420

3

Initial Setup

The 420’s controls and switches are now set according to the preceding section. All connections
listed in the settings table on the previous page are now made. The 420 should now pass signal.
The amplifier’s CLIP indicators should not be illuminated.

Generally speaking, the gain controls on a power amplifier should be operated at as high a setting
as possible. This ensures sufficient amplifier sensitivity that the preceding unit can drive the
amplifier into clipping. If the gain controls are turned down sufficiently, it is possible to either clip
the balanced input buffer amplifiers or to cause the preceding unit to clip.

With the signal level of the source at a reasonable level, increase the gain of the 420 until either the
CLIP indicators illuminate or the acoustical output of the loudspeakers is sufficient.

420

4

Front & Rear Panel OverviewChapter 4

420

STEREO POWER
AMPLIFIER

CLIP

GAIN

5

2738

MONITORMODECHANNEL 2CHANNEL 1

GAIN

GAIN

64

DUAL TRACKING
INDEPENDENT

91

100

654

37

2

CLIP

1

10

0

STEREO

SPEAKER

HEADPHONESCONTROLS

MONO
STEREO

8

9

MUTED
ACTIVE

POWER

Channel 1

CLIP LED Indicates the onset of clipping at the output of the 420.
GAIN Controls the gain, or input sensitivity, of the 420. A volume control by

any other name. This control is active regardless of the 420’s operating
mode. Refer to the DUAL TRACKING / INDEPENDENT switch for more
information.

GAIN CONTROLS switch In DUAL TRACKING mode, the gain control for channel 1 sets the gain for

both channels. The inputs are still separate, there’s just one control for
both channels. Use this mode for stereo applications.
In INDEPENDENT mode, the gain controls for both channels are separate.

Channel 2

CLIP LED Indicates the onset of clipping at the output of the 420.
GAIN Controls the gain, or input sensitivity, of channel 2 of the 420. A volume

control by any other name. This control is active only in INDEPENDENT
mode.

Mode

MONO / STEREO In STEREO mode, both channels of the 420 are separate. In MONO mode,

both inputs of the 420 are mixed (ahead of the GAIN controls).

Monitor

STEREO HEADPHONES ¼ in. TRS jack. Connect stereo headphones here. This jack will drive

headphones of any impedance, from 4 ohms and up.

SPEAKER MUTE Mutes (disconnects) the rear-panel OUTPUT terminals.

Power

SWITCH Turns the 420 on.
LED Indicates the presence of AC power.

420

5

100 WATTS MAXIMUM

MANUFACTURED IN
SEATTLE WASHINGTON,
UNITED STATES OF AMERICA

FUSE

FUSE

TIP
RING
SLEEVE

TYPICAL INPUTS

=HIGH(+)=
=LOW(-)=

=GROUND=

SLEEVE

RING

TIP

OUTPUTS

CHANNEL

PIN 2
PIN 3
PIN 1

12

3

CHANNEL

1

2

20 WATTS PER CHANNEL 8 OHMS (CLASS 2 WIRING)

UNBALANCED/

BALANCED

INPUT 2

BALANCED

UNBALANCED/

BALANCED

INPUT 1

BALANCED

SERIAL NUMBER [Please send in the completed warranty card.]
AC POWER INPUT IEC-power connector. Connect only to appropriate AC power source.

Refer to rear-panel marking for correct AC source value.

FUSE 1Ampere Slow-blow fuse. Replace only with same type of fuse 117V ac:

1A, 250V AC, slow blowing (Bussman type MDL-1) 230V AC: 0.5A,
250V ac, slow blowing (Bussman type GDC-500ma).

OUTPUTS #6 screw terminals. This is the output of the 420. These terminals may be

turned on/off via the front-panel SPEAKER MUTE switch. For stereo or 2­channel applications, connect one load to the channel 1 output and
connect the other load to the channel 2 output. The minimum load
impedance is 4-ohms. For mono-bridge applications, connect the
positive load connection to the channel 1 + output terminal, connect the
negative load connection to the channel 2 + output terminal. The

minimum load impedance is 8-ohms.
INPUT 2 XLR-female (pin-2 hot) paralleled with TRS phone jack.
INPUT 1 XLR-female (pin-2 hot) paralleled with TRS phone jack. Use this input

for mono-bridge applications.
MONO BRIDGE switch Located internally.

420

6