Symetrix 304 User manual

304 Headphone Amplifier
Users Guide
304

i
Table of Contents
Chapter 1 Introduction 1
Chapter 2 Operator Safety Summary 2
Chapter 3 Fast Setup 3
Chapter 4 Front & Rear Panel Overview 4
Chapter 5 Using the 304 5
Chapter 6 Applications 6
Chapter 7 Block Diagram 7
Chapter 8 Troubleshooting 8
Chapter 9 Specifications 9
Chapter 10 Warranty and Service 10
Chapter 11 Declaration of Conformity 13
Rev A, 31 October, 1998
Symetrix part number 53304-0A00
Subject to change without notice.
©1998, 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 photo-
copying, scanning, recording or by any information
storage and retrieval system, without permission, in
writing, from Symetrix, Inc.
14926 35th Ave.West
Lynnwood, WA 98037 USA
Tel [phone]
Fax [phone]
Email: [email protected]
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IntroductionChapter 1
Front panel
Rear panel
LEVEL 4
10
PWR
LEVEL 3
10
0100100
LEVEL 1 LEVEL 2
0
HEADPHONE LEVEL
OUTPUT 3
MODE
MONO
STEREO
OUTPUT 1 OUTPUT 2
LEVEL
+12
+25
∞
HEADPHONE
AMPLIFIER
304
INPUT HEADPHONE OUT
OUTPUT 4
LINE INPUTS
INPUT BTHRU A INPUT A
POWER
INPUT OR PS-3E POWER
SUPPLY ONLY.
THIS PRODUCT CONTAINS NO USER SERVICABLE PARTS.
RÉFÉREZ TOUTE RÉPARATION À UN TECHNICIEN QUALIFIÉ.
FABRIQUÉ AUX E.-U. PAR SYMETRIX INC., LYNNWOOD, WASHINGTON.
MANUFACTURED BY SYMETRIX INC. LYNNWOOD, WA USA
CONNECT TO
SYMETRIX PS-3
THRU B
PASS THRU
The Symetrix 304 Headphone Amplifier is a 1-
in 4-out amplifier designed to drive multiple
headphones of any impedance. Symetrix
proprietary high voltage drive technology
gives the 304 the ability to drive high imped-
ance headphones with the equivalent output
voltage of a much larger power amplifier, yet
provides more than ample current for low
impedance phones. The four stereo outputs
automatically adjust output power to match the
load impedance.
Controls provided are input level, output level
for each individual amplifier, and a stereo/
mono switch.
The STEREO/MONO switch allows the amplifier
to be used several ways. As a normal stereo
amplifier (with four outputs), the left and right
inputs are used for stereo signals, which feed
the left and right outputs. Switching to mono
with a stereo input provides a quick check of
the mono compatibility of a stereo mix. For
that matter, you may want to monitor in mono.
As a mono amplifier, one signal fed to either
input will feed both outputs. Also in mono
mode, when two different mono signals are fed
to the left and right inputs, they will be mixed
to feed both the left and right outputs.
The inputs are electronically balanced, but will
operate normally in an unbalanced configura-
tion when 2-conductor 1/4" plugs are inserted.
The outputs are 1/4" stereo connectors.
Do you hate reading manuals? Chapter 3 of
this manual was written just for you.
Pleeeeeze! Skip the rest of this and just read
Chapter 3. It will get you up and running with
minimum reading and minimum pain. Try to
read the rest of the manual later. Its worth it.
Should you have any comments or questions,
please do not hesitate to contact us. Your calls
and e-mail are always welcome.
Phone: [phone]
Fax: [phone]
Email: [email protected]
Website: www.symetrixaudio.com

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2
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.
The notational conventions used in this manual
and on the equipment itself are described in the
following paragraphs.
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 mainte-
nance (servicing) instructions in the literature
accompanying the 304 (i.e. this manual).
Caution To prevent electric shock, do not
use the polarized plug supplied
with the 304 with any extension
cord, receptacle, or other outlet
unless the blades can be fully
inserted.
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 304.
Equipment Markings
Operator Safety Summary Chapter 2
Caution Identifies information that, if not
heeded, may cause damage to the
304 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 304s chassis.
Boldface Strong emphasis.
Power source - This product is intended to
operate from a power source that does not
apply more than 255Vrms 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.
Danger from loss of ground - If the protec-
tive ground connection is lost, all accessible
conductive parts, including knobs and controls
that may appear to be insulated, can render an
electric shock.
In-line power supply - This product receives
its operating power from the Symetrix PS-3 or
PS-3E power supply. This is the only power
supply approved for use with the product. Do
not connect the product to any other in-line, or
plug-in, transformer. The use of other power
sources may cause damage to the equipment or
present a shock hazard to the operator.
Operating location - Do not operate this
equipment under any of the following condi-
tions: explosive atmospheres, in wet locations,
in inclement weather, improper or unknown
AC mains voltage, or if improperly fused.
Stay out of the box - To avoid personal injury
or injury to others, do not remove the product
covers or panels. Do not operate the product
without the covers and panels properly
installed.
AVIS:
NE PAS OUVRIR
Il ne se trouve a l’interieur aucune piece pourvant entre reparée l’usager.
SEE OWNERS MANUAL. VOIR CAHIER D’INSTRUCTIONS.
S’adresser a un reparateur compétent.
RISQUE DE CHOC ELECTRIQUE
N
o user serviceable parts inside. Refer servicing to qualified service personnel
.
CAUTION
WARNING:
TO REDUCE THE RISK OF FIRE OR
ELECTRIC SHOCK DO NOT EXPOSE
THIS EQUIPMENT TO RAIN OR MOISTURE
DO NOT OPEN
RISK OF ELECTRIC SHOCK

304
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Fast First-Time Setup
This section is dedicated to users who hate to read manuals.
The 304 is a four-output headphone amplifier. A stereo input drives all four headphone amplifiers.
Each amplifier has an independent level control. There is an overall master level control, and an
overall STEREO/MONO switch.
Wiring
To install the 304:
Plug the PS-3 power supplys DIN connector into the 304. Plug the PS-3 into a
suitable source of AC power. Refer to the labelling on the PS-3 to determine what is
suitable.
Connect your source to the 304s tip-ring-sleeve input jacks. The inputs are balanced.
They will work with either balanced or unbalanced sources.
Connect your headphones to the four output jacks. The 304 can handle headphone
impedances ranging from 8-ohms to over 2000 ohms. 60-ohm headphones are
optimal (they get the loudest). It is not necessary for each set of headphones to be the
same (although it might help if they were).
Go!
Set the individual level controls to minimum (max ccw rotation). Set the master level control to the
halfway position (12 noon). Set the stereo/mono switch as appropriate.
With the source signal active, adjust the individual level controls for comfortable listening levels.
Caution The 304 is capable of driving many headphones past the threshold of pain. This can
also result in hearing damage. For your own hearing health, monitor at the lowest
SPL that you can use and still get the job done.









Fast Setup
Chapter 3

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Front & Rear Panel Overview Chapter 4
LEVEL 4
10
PWR
LEVEL 3
10
0100100
LEVEL 1 LEVEL 2
0
HEADPHONE LEVEL
OUTPUT 3
MODE
MONO
STEREO
OUTPUT 1 OUTPUT 2
LEVEL
+12
+25
∞
HEADPHONE
AMPLIFIER
304
INPUT HEADPHONE OUT
OUTPUT 4
INPUT LEVEL Sets the input signal strength. Fully counterclockwise, no signal. Fully
clockwise, input signals are boosted by 25dB.
MONO/STEREO Combines stereo, or two mono, input signals into one mono output signal
(fed equally to both left and right headphone outputs).
HEADPHONE OUTPUTS 1/4" stereo jacks for headphones. The minimum impedance is 8 ohms
(see Table 1 for combined headphone impedances).
HEADPHONE LEVELS Volume controls set the signal strength at the corresponding output jack
(1-4).
PWR Red LED indicates power on.
LINE INPUTS
INPUT BTHRU A INPUT A
POWER
INPUT OR PS-3E POWER
SUPPLY ONLY.
THIS PRODUCT CONTAINS NO USER SERVICABLE PARTS.
RÉFÉREZ TOUTE RÉPARATION À UN TECHNICIEN QUALIFIÉ.
FABRIQUÉ AUX E.-U. PAR SYMETRIX INC., LYNNWOOD, WASHINGTON.
MANUFACTURED BY SYMETRIX INC. LYNNWOOD, WA USA
CONNECT TO
SYMETRIX PS-3
THRU B
PASS THRU
7-PIN DIN CONNECTOR Connect the 7-pin DIN plug end of a Symetrix PS-3 (117V) or PS-3E
(230V) here. Connect the AC power connector end of the PS-3 or
PS-3E to an AC power source that is of the correct voltage and
frequency as marked on the PS-3 or PS-3E.
THRU A TRS jack connected in parallel with INPUT A jack. Use this jack to
connect to other 304s or other audio equipment. Its a built-in wye
cord.
THRU B TRS jack connected in parallel with INPUT B jack. Use this jack to
connect to other 304s or other audio equipment. Its a built-in wye
cord.
INPUT A (Left channel) 3 conductor 1/4" jack for line level balanced or unbalanced input
signals. Tip = high (+), Ring = low (-), Sleeve = ground.
INPUT B (Right channel) 3 conductor 1/4" jack for line level balanced or unbalanced input
signals. Tip = high (+), Ring = low (-), Sleeve = ground.

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5
Chapter 5 Using the 304
Impedance, Sensitivity, Loudness
Loudness is directly related to the impedance and sensitivity of the headphones. Impedance
determines how much power the headphones will draw, while sensitivity indicates how much of
the electrical signal delivered to the headphones is converted into sound.
Because low impedance headphones draw more power from the amplifier they will sound louder,
at the same control settings, than high impedance headphones. Also, headphones of higher
sensitivity will sound louder than those of lower sensitivity. Check the manufacturers spec sheet
for impedance and sensitivity information.
Proprietary high voltage driver technology allows the 304 to drive even high impedance head-
phones to maximum levels, because the amplifiers are capable of very high output voltage swing.
However, to get a loudness equivalent to that of low impedance headphones, the controls will have
to be set to higher settings when using high impedance headphones. This is normal.
If low and high impedance headphones are combined on the same channel, the low impedance
headphones will be much louder. As a result, when a combination of high impedance and low
impedance headphones is used, only those of like impedance should be used on the same channel.
This allows each listener to properly adjust loudness, without regard to the impedance and sensi-
tivity of the other headphones. (Its ok to mix impedances on the same amplifier, its just not
optimum. You wont hurt anything.)
Maximum Amplifier Load
The minimum operating impedance, or maximum load, for each output of the 304 is 8 ohms. Use as
many pairs of headphones as desired, as long as the total combined impedance (per output) does not fall
below 8 ohms. Table 1 shows how many headphones of various impedances may be used with each
output of the 304.
Table 1 - Combined Impedance Values (approx. 8:)
Impedance of Headphones Maximum Number/Channel
8 (see note 1) 1
16 2
32 4
100 10
200 20
400 40
600 60
Note 1: The quantities given above reflect usage with typical wide-range musical signals. If you
are driving low-impedance headphones (16-ohms or lower actual impedance), and you intend to
drive them to the brink of amplifier clipping, then you should revise the number of headphones
possible (per 304) to 2 8-ohm headphones or 4 16-ohm headphones (for the 8-ohm case, youll
have two amplifier channels that arent doing anything). It is OK to use any remaining amplifier
channels with higher impedance phones (50 ohms or higher).
For maximum output with all channels driven, try to restrict the lowest combined load impedance
to 50 ohms per channel. Lower impedance loads simply reduce the maximum output possible.
Note 2: Many headphones are specd as being suitable for eight-ohm outputs. They are, but this
does not mean that their actual impedance is 8-ohms. In most cases, the actual impedance is
substantially higher. This is good (rather than bad). For whatever its worth, most decent
headphones have actual impedances ranging from 50 ohms and up.
Note 3. When multiple headphones are required with each output, use headphone type Y cords,
or a breakout box. A breakout box may have as many paralleled 1/4" stereo jacks as necessary,
as long as the numbers in Table 1 are not exceeded.

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Applications Chapter 6
The 304 can be used in the following applications. Please note that this list is far from comprehensive
and your imagination may create new uses that we havent listed here.
PROJECT STUDIO Let the 304 expand your mixers headphone capacity. You can drive it
via a tip-ring-sleeve to dual tip-sleeve cable from your mixers
headphone jack, or from one or two of your mixers aux sends. If you
only use one aux send, then press the stereo/mono switch to route the
signal to both channels.
BROADCAST Use the 304 to drive the guest headphones in your studio.
Does your console have an anemic headphone amplifier? The 304 will
make it roar.
LIVE SOUND Does your console have an anemic headphone amplifier? The 304 will
make it roar.
HOUSE OF WORSHIP Create assistive listening stations at several pews. Drive the 304 with a
mixture of the sound reinforcement signal and a room microphone.
No, this isnt as sophisticated as one of the RF or infrared systems, but
it doesnt cost like they do either!
BALANCED Heres how to use the 304 as a one-in, four out balanced line amplifier.
LINE DRIVER Make a wye cable so that you can drive both inputs in parallel. Wire the
plug for the right-channel input so that it is reverse polarity from the
left-channel input.
Connect your source to the wye-cables input.
Connect each of your balanced loads to the tip-ring-sleeve output
connectors. The tip connection is hot.

304
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Block DiagramChapter 7
OUTPUT
CHANNEL 1
CHANNEL 2
CHANNEL 3
CHANNEL 4
CHANNEL
LEVEL
MONO
STEREO
INPUT A
INPUT B
MASTER
VOLUME
Rev-B

304
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Troubleshooting Chapter 8
Troubleshooting Chart
SYMPTOM PROBABLE CAUSE
No output Check cables and connections. Are inputs driven by outputs, and
outputs driving inputs? Verify cables, source and load. Check for AC
power presence.
Hum or buzz in output Check input and output connector wiring. 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? Check the
output loading. Is something else clipping?
Noise (hiss) Check input signal levels. The 304 is intended to operate at or near
line level (+4dBu is nominal). Make sure that the signal you are
feeding to the 304 is a line level signal, not mic level. Make sure that all
systems components are on the same AC ground. Check gain settings on
upstream equipment. The system gain structure should be such that
the 304 receives +4dBu nominal signal. Is the input signal already
noisy?
No LED display Is the unit plugged in? Is the AC outlet OK?