Symetrix 304 User manual

304 Headphone Amplifier
User’s Guide

User’s Guide
2
6408 216th Street SW |Mountlake Terrace, WA 98043 USA
T+1.425.778.7728 F+1.425.778.7727 |www.SymetrixAudio.com
Safety

304 Headphone Amplifier
3
Safety
GThe 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 Quick Start Guide).
GCAUTION: To prevent electric shock,
do not use the polarized plug supplied
with the unit with any extension cord,
receptacle, or other outlet unless the
prongs can be fully inserted.
GPower Source: This Symetrix hardware
uses a switching power supply that
automatically adjusts to the applied
voltage. Ensure that your AC mains
voltage is somewhere between 100-240
VAC, 50-60 Hz. Use only the power
cord and connector specified for the
product and your operating locale. A
protective ground connection, by way of
the grounding conductor in the power
cord, is essential for safe operation. The
appliance inlet and coupler shall remain
readily operable once the apparatus has
been installed.
GUser Serviceable Parts: There are no
user serviceable parts inside this Symetrix
product. In case of failure, customers
inside the U.S. should refer all servicing to
the Symetrix factory. Customers outside
the U.S. should refer all servicing to an
authorized Symetrix distributor. Distributor
contact information is available online at:
http://www.SymetrixAudio.com.
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
No 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
Important Safety
Instructions
!Read these instructions.
@Keep these instructions.
#Heed all warnings.
$Follow all instructions.
%Do not use this apparatus near water.
This apparatus shall not be exposed to
dripping or splashing and no objects
filled with liquids, such as vases, shall be
placed on the apparatus.
^Clean only with dry cloth.
&Do not block any ventilation openings.
Install only in accordance with the
manufacturer’s instructions.
*Do not install near any heat sources such
as radiators, heat registers, stoves, or
other apparatus (including amplifiers) that
produce heat.
(This apparatus shall be connected to
a mains socket outlet with a protective
earthing connection. Do not defeat
the safety purpose of the polarized or
grounding-type plug. A polarized plug has
two blades with one wider than the other.
A grounding type plug has two blades
and a third grounding prong. The wide
blade or the third prong are provided for
your safety. If the provided plug does not
fit into your outlet, consult an electrician
for replacement of the obsolete outlet.
BL Protect the power cord from being
walked on or pinched particularly at
plugs, convenience receptacles, and the
point where they exit from the apparatus.
BM Only use attachments/accessories
specified by the manufacturer.
BN Use only with the cart, stand,
tripod, bracket, or table
specified by the manufacturer,
or sold with the apparatus.
When a cart is used, use
caution when moving the cart/
apparatus combination to avoid injury
from tip-over.
BO Unplug this apparatus during lightning
storms or when unused for long periods
of time.
BP Refer all servicing to qualified service
personnel. Servicing is required when
the apparatus has been damaged in any
way, such as power-supply cord or plug
cord 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.
What Ships in the Box
• A304hardwareunit.
• OnePS-3(115VAC),PS-3E(230VAC)
power supply.
• ThisUser’sGuide.
If you have questions beyond the scope
of this Quick Start Guide or the help
module, contact our Customer Support
Group in the following ways:
Tel: [phone]
8:00 am to 4:30 pm
Monday through Friday,
Pacific Time
Web: http://www.SymetrixAudio.com
This device complies with part 15 of
the FCC Rules. Operation is subject
to the following two conditions: (1)
This device may not cause harmful
interference, and (2) this device must
accept any interference received,
including interference that may cause
undesired operation.
This Class B Digital apparatus meets
all requirements of the Canadian
Interference-Causing Equipment
Regulations
Cet appariel numerique de la classe
B respecte toutes les Exigences du
Reglement sur le materiel brouilleur
du Canada.

User’s Guide
4
6408 216th Street SW |Mountlake Terrace, WA 98043 USA
T+1.425.778.7728 F+1.425.778.7727 |www.SymetrixAudio.com
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.
Warning: 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.
First Time Setup
Fast First Time Setup
Introduction
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 impedance
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 configuration when 2-conductor
1/4 inch plugs are inserted. The outputs are 1/4 inch stereo
connectors.

304 Headphone Amplifier
5
Front/Rear Panel Overview
Front Panel
Item Description
Input Level Sets the input signal strength. Fully counterclockwise, no signal. Fully clockwise, input signals are boosted by 20 dB.
Mono/Stereo Combines stereo, or two mono, input signals into one mono output signal (fed equally to both left and right head-
phone outputs).
Headphone Outputs 1/4" stereo jacks for headphones. The minimum impedance is 8 Ohms (see Table 1 for combined headphone imped-
ances).
Headphone Levels Volume controls set the signal strength at the corresponding output jack (1-4).
Power Red LED indicates power “on”.
Rear Panel
Item Description
7-pin DIN Connector Connect the 7-pin DIN plug end of a Symetrix PS-3 (115 VAC) or PS-3E (230 VAC) 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.
Thru B TRS jack connected in parallel with Input B jack. Use this jack to connect to other 304’s or other audio equipment.
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.

User’s Guide
6
6408 216th Street SW |Mountlake Terrace, WA 98043 USA
T+1.425.778.7728 F+1.425.778.7727 |www.SymetrixAudio.com
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 headphones 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 sensitivity of the other head-
phones. (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

304 Headphone Amplifier
7
Applications
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?
Live Sound: Does your console have an anemic headphone amplifier?
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 “Y” 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 “Y” cable’s input. Connect each of your balanced
loads to the tip-ring-sleeve output connectors. The tip connection is hot.

User’s Guide
8
6408 216th Street SW |Mountlake Terrace, WA 98043 USA
T+1.425.778.7728 F+1.425.778.7727 |www.SymetrixAudio.com
Block Diagram
HEADPHONE
OUTPUT 1
HEADPHONE
OUTPUT 2
HEADPHONE
OUTPUT 3
HEADPHONE
OUTPUT 4
LINE
INPUT A
PASS
THRU A
LINE
INPUT B
PASS
THRU B
EMI
FILTER
CIRCUITRY
MODE
MONO / STEREO
HIGH
VOLTAGE
AMP
HIGH
VOLTAGE
AMP
HIGH
VOLTAGE
AMP
HIGH
VOLTAGE
AMP
EMI
FILTER
CIRCUITRY
HIGH
VOLTAGE
AMP
HIGH
VOLTAGE
AMP
HIGH
VOLTAGE
AMP
HIGH
VOLTAGE
AMP
HIGH
VOLTAGE
AMP
HIGH
VOLTAGE
AMP
HIGH
VOLTAGE
AMP
HIGH
VOLTAGE
AMP
HEAD-
PHONE
LEVEL 2
HEAD-
PHONE
LEVEL 1
INPUT
LEVEL
HEAD-
PHONE
LEVEL 3
HEAD-
PHONE
LEVEL 4
HEADPHONE
OUTPUT 1
HEADPHONE
OUTPUT 2
HEADPHONE
OUTPUT 3
HEADPHONE
OUTPUT 4
LINE
INPUT A
PASS
THRU A
LINE
INPUT B
PASS
THRU B
EMI
FILTER
CIRCUITRY
MODE
MONO / STEREO
HIGH
VOLTAGE
AMP
HIGH
VOLTAGE
AMP
HIGH
VOLTAGE
AMP
HIGH
VOLTAGE
AMP
EMI
FILTER
CIRCUITRY
HIGH
VOLTAGE
AMP
HIGH
VOLTAGE
AMP
HIGH
VOLTAGE
AMP
HIGH
VOLTAGE
AMP
HIGH
VOLTAGE
AMP
HIGH
VOLTAGE
AMP
HIGH
VOLTAGE
AMP
HIGH
VOLTAGE
AMP
HEAD-
PHONE
LEVEL 2
HEAD-
PHONE
LEVEL 1
INPUT
LEVEL
HEAD-
PHONE
LEVEL 3
HEAD-
PHONE
LEVEL 4

304 Headphone Amplifier
9
Troubleshooting
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?
NOTES:

User’s Guide
10
6408 216th Street SW |Mountlake Terrace, WA 98043 USA
T+1.425.778.7728 F+1.425.778.7727 |www.SymetrixAudio.com
Specs•DeclarationofConformity
Architects and Engineers Specifications
The headphone amplifier shall have a single stereo input and four stereo outputs. Each output shall have its own amplifier and volume control. An
overall volume control (master) shall also be provided. There shall be an overall stereo/mono switch.
All inputs are balanced bridging inputs. Outputs shall be suitable for driving headphones in the range of 8 Ohms to 2000 Ohms. The output source
impedance shall be 100 Ohms.
The headphone amplifier shall be capable of delivering 35 volts peak-to-peak into a 2000 Ohm load (or greater).
The unit shall be capable of operating by means of a Symetrix PS-3 or PS-3E external power supply connected to 115 VAC nominal (95 to 130
VAC), 50/60 Hz or 230 VAC nominal, 207 to 253 VAC, 50 Hz where applicable.
The unit shall be a Symetrix, Inc. model 304 Headphone Amplifier.
Specifications
INPUT/OUTPUT
Input Type............................Bridging, Balanced or Unbalanced
Maximum Input Level...................................+18 dBu Balanced
Maximum Output Level....................... 21 dBm, 600 Ohm load,
>6 VRMS, 200 Ohm load
Input Impedance 20k Ohms Balanced, 10k Ohms Unbalanced
Output Impedance .................................................. 100 Ohms
Performance Data
Frequency Response.................... 20 Hz to 20 kHz, +0, -1 dB
THD+Noise............................. .01% (1kHz, 0 dBm, 600 Ohms)
.02% (1kHz, +20 dBm, 600 Ohms)
Signal-to-Noise Ratio...................................................... 95 dB
Maximum Gain ............................................................... 25 dB
CONNECTIONS
Input.......................................................................... 1/4" TRS
Output....................................................................... 1/4" TRS
Power In....................................................................7-pin DIN
PHYSICAL
Size (hwd)............................................................... ½ rack unit
1.75 x 8.5 x 6.5 in., 4.445 x 21.59 x 15.875 cm.
Shipping Weight ............................................................ 4.5 lbs
ELECTRICAL
Power Requirements115 VAC nominal, 95 to 130 VAC, 50 to 60 Hz
230 VAC nominal, 207 to 253 VAC, 50 Hz
Declaration of Conformity
We, Symetrix Incorporated, 6408 216th St. SW, Mountlake Terrace,
Washington, USA, declare under our sole responsibility that the
product:
304 Headphone Amplifier
to which this declaration relates, is in conformity with the following
standards:
EN 60065
Safety requirements for mains operated electronic and related
apparatus for household and similar general use.
EN 50081-1
Electromagnetic compatibility - Generic emission standard
Part 1: Residential, commercial, and light industry.
EN 50082-1
Electromagnetic compatibility - Generic immunity standard
Part 1: Residential, commercial, and light industry.
The technical construction file is maintained at:
Symetrix, Inc.
6408 216th St. SW
Mountlake Terrace, WA, 98043 USA
The authorized representative located within the European Community
is:
World Marketing Associates
P.O. Box 100
St. Austell, Cornwall, PL26 6YU, U.K.
Date of issue: October 31, 1998
Place of issue: Lynnwood, Washington, USA
Authorized signature:
Dane Butcher, President, Symetrix Incorporated.

304 Headphone Amplifier
11
Warranty and Service
The Symetrix Limited Warranty
Symetrix, Inc. expressly warrants that the product will be free from
defects in material and workmanship for eighteen (18) months from
the date the product is shipped from the factory. Symetrix’s obligations
under this warranty will be limited to repairing or replacing, at Symetrix’s
option, the part or parts of the product which prove defective in material
or workmanship within eighteen (18) months from the date the product
is shipped from the factory, 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. Buyer will prepay all
freight charges to return the product 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, Inc.
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.
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.
Note: Some Symetrix products contain embedded software and may
also be accompanied by control software intended to be run on a
personal computer. Said software is specifically excluded from
this warranty.
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 allocatable 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, substitute facilities or supply houses.
Servicing Your Symetrix Product
If you have determined that your Symetrix product requires repair
services and you live outside of the United States please contact your
local Symetrix dealer or distributor for instructions on how to obtain
service. If you reside in the U.S. then proceed as follows:
Return Authorization
At the Symetrix factory, Symetrix will perform in-warranty or out-of-
warranty service on any product it has manufactured for a period of
three (3) years from date of discontinued manufacture.
Before sending anything to Symetrix, please contact our Customer
Service Department for a Return Authorization (RA) number. The
telephone number is [phone]. Additionally, support is
available via the web site: http://support.SymetrixAudio.com.
In-warranty Repairs
To get your Symetrix product repaired under the terms of the warranty:
1. Call us for an RA number (have the serial number, shipping and
contact information and description of the problem ready).
2. Pack the unit in its original packaging materials.
3. Include your name, address, daytime telephone number, and a
brief statement of the problem.
4. Write the RA number on the outside of the box.
5. Ship the unit to Symetrix, freight prepaid. We do not accept
freight collect shipments.
Just do these five things, and repairs made in-warranty will cost you only
one way freight charges. We’ll pay the return freight.
If you don’t have the factory packaging materials, we recommend using
an oversize box. Wrap the unit in a plastic bag, surround it with bubble-
wrap, and place it in the box surrounded by Styrofoam peanuts. Be sure
there is enough clearance in the box to protect the rack ears. We won’t
return the unit in anything but Symetrix packaging for which we will have
to charge you. If the problem is due to operator misuse or error, you will
have to pay for both parts and labor. In any event, if there are charges
for the repair, you will pay for the return freight. All charges will be COD
unless you have made other arrangements (prepaid, Visa or Mastercard).
Out-of-warranty Repairs
If the warranty period has passed, you’ll be billed for all necessary parts,
labor, packaging materials, and freight charges. Please remember, you
must call for an RA number before sending the unit to Symetrix.

Item No. 53-0001
304 Headphone Amplifier User’s Guide
© 2009 Symetrix, Inc. All rights reserved. Printed in the United States of America. The information in this document is subject to change without notice.
Symetrix, Inc. shall not be liable for technical or editorial errors or omissions contained herein; nor is it liable for incidental or consequential damages
resulting from the furnishing, performance, or use of this material. 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 brochure may be reproduced or transmitted in any form or by any means, electronic or mechanical, without permission
in writing from Symetrix, Inc. If, however, your only means of access is electronic, permission to print one copy is hereby granted. The following are
either Trademarks or Registered Trademarks of Symetrix, Inc.: Symetrix, SymNet, SymNet Designer, SymLink and CobraLink. Windows is a Registered
Trademark of Microsoft, Inc.. Other product names mentioned herein may be trademarks and/or registered trademarks of other companies and are
property of their respective owners.
6408 216th Street SW |Mountlake Terrace, WA 98043 USA
T+1.425.778.7728 F+1.425.778.7727 |www.SymetrixAudio.com