Symetrix 304 User manual

User’s Guide
304 Headphone Amplifier

User’s Guide
304 Headphone
Amplifier
1/9
Before You Begin
What Ships in the Box
RA 304 hardware unit.
ROne PS-3 (117 VAC), PS-3E (230
VAC), or PS-3J (100 VAC) power
supply.
RThis User’s Guide.
Getting Help
If you have questions beyond the scope
of this User’s 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 support.symetrixaudio.com
Important Safety Instructions.
1. Read these instructions.
2. Keep these instructions.
3. Heed all warnings.
4. Follow all instructions.
5. 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 appara-
tus.
6. Clean only with dry cloth.
7. Do not block any ventilation open-
ings. Install only in accordance with
the manufacturer’s instructions.
8. Do not install near any heat sources
such as radiators, heat registers,
stoves, or other apparatus (including
amplifiers) that produce heat.
9. 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 electri-
cian for replacement of the obsolete
outlet.
10.Protect the power cord from being
walked on or pinched particularly at
plugs, convenience receptacles, and
the point where they exit from the
apparatus.
11. Only use attachments/accessories
specified by the manufacturer.
12.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 com-
bination to avoid injury from tip-over.
13.Unplug this apparatus during light-
ning storms or when unused for long
periods of time.
14.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.
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
The lightning flash with arrowhead
symbol within an equilateral triangle is
intended to alert the user of the pres-
ence 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 User’s Guide).
CAUTION 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.
Power Source. The 304 Head-
phone Amplifier hardware uses a
separate external power supply for
domestic and export markets. Ensure
that your AC mains voltage matches
that of your power supply. 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.
User Serviceable Parts. There
are no user serviceable parts inside the
304 Headphone Amplifier. 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
www.symetrixaudio.com.

User’s Guide
304 Headphone
Amplifier
2/9
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" plugs
are inserted. The outputs are 1/4" stereo connectors.
Fast First Time Setup
The 304 is a four-output headphone amplifier. A stereo input drives all four headphone amplifiers. Each amplifier has an inde-
pendent 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.
1.
2.
3.
Introduction, Fast First Time Setup

User’s Guide
304 Headphone
Amplifier
3/9
Front Panel
Item Description
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”.
Rear Panel
Item Description
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 “Y” 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 “Y” 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.
Front & Rear Panel Overview
INPUT HEADPHONE OUT HEADPHONE LEVEL
304
HEADPHONE
AMPLIFIER
LEVEL LEVEL 1 LEVEL 2 LEVEL 3 LEVEL 4
OUTPUT 1 OUTPUT 2 OUTPUT 3 OUTPUT 4
PWR
MODE
+12
[phone]
d
MONO
STERO
304 HEADPHONE
AMPLIFIER
CONNECT TO
SYMETRIX
PS-3 SERIES
POWER
SUPPLY ONLY
THIS PRODUCT CONTAINS NO USER SERVICEABLE PARTS.
PAS DES ELEMENTS SERVIABLE PAR UTILISATEUR.
POWER
INPUT
MANUFACTURED BY SYMETRIX INC. MOUNTLAKE TERRACE, WA USA
FABRIQUE PAR SYMETRIX INC.
WWW.SYMETRIXAUDIO.COM
THRU B THRU A INPUT B INPUT A
PASS THRU LINE INPUTS

User’s Guide
304 Headphone
Amplifier
4/9
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, be-
cause the amplifiers are capable of very high output voltage swing. However, to get a loudness equivalent to that of low imped-
ance 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 imped-
ance should be used on the same channel. This allows each listener to properly adjust loudness, without regard to the imped-
ance and sensitivity 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 exceed-
ed.

User’s Guide
304 Headphone
Amplifier
5/9
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? 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 “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
304 Headphone
Amplifier
6/9
Block Diagram
HEADPHONE
OUTPUT 1
HEADPHONE
OUTPUT 2
HEADPHONE
OUTPUT 3
OUTPUT 4
HEADPHONE
HIGH
AMP
HIGH
PHONE
LEVEL 1
HEAD-
HIGH
AMP
LEVEL 2
PHONE
HEAD-
AMP
AMP
HIGH
AMP
PHONE
HEAD-
LEVEL
INPUT
MODE
HIGH
EMI
FILTER
CIRCUITRY
LINE
INPUT A
THRU A
PASS
AMP
MONO/STEREO
HIGH
AMP
AMP
HIGH
AMP
HIGH
AMP
LEVEL 4
PHONE
HEAD-
FILTER
CIRCUITRY
LINE
INPUT B
PASS
THRU B
AMP
EMI LEVEL 3
AMP
VOLTAGE
VOLTAGE
VOLTAGE
VOLTAGE
VOLTAGE
VOLTAGE
VOLTAGE
VOLTAGE

User’s Guide
304 Headphone
Amplifier
7/9
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
304 Headphone
Amplifier
8/9
Specifications and Declaration of Conformity
In the interest of continuous product improvement, Symetrix, Inc.
reserves the right to alter, change, or modify these specifications
without prior notice.
©2006, Symetrix, Inc. All rights reserved.
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 117V nomi-
nal AC (95 to 130V), 50/60 Hz or 230V nominal AC, 207 to 253V AC, 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 +18dBu Balanced
Maximum Output Level 21dBm, 600 Ohm load,
>6VRMS, 200 Ohm load
Input Impedance 20k Ohms Balanced, 10k Ohms Unbalanced
Output Impedance 100 Ohms
Performance Data
Frequency Response 20Hz to 20kHz, +0, -1 dB
THD+Noise .01% (1kHz, 0dBm, 600 Ohms)
.02% (1kHz, +20dBm, 600 Ohms)
Signal-to-Noise Ratio 95dB
Maximum Gain 25dB
Connections
Input 1/4" in. TRS
Output 1/4" in. 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 Requirements 117V nominal, 95 to 130V AC, 50 to 60 Hz
230V nominal, 207 to 253V AC, 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.

User’s Guide
304 Headphone
Amplifier
9/9
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 workman-
ship 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 determina-
tion 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 warran-
ties, whether oral, written, express, implied or statutory.
Symetrix, Inc. expressly disclaims any IMPLIED warran-
ties, including fitness for a particular purpose or mer-
chantability. Symetrix's warranty obligation and buyer's
remedies hereunder are SOLELY and exclusively as stated
herein.
This Symetrix Lucid product is designed and manufactured
for use in professional and studio audio systems and is not
intended for other usage. With respect to products pur-
chased 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 disclaim-
ers set forth herein, shall extend to the original purchaser and
anyone who purchases the product within the specified war-
ranty 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 Lucid 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, con-
nected 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 the 304 Headphone Amplifier
If you have determined that your 304 Headphone Amplifier
requires repair services and you live outside of the United
States please contact your local SymNet 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 manufac-
ture.
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:
support.symetrixaudio.com.
In-warranty repairs
To get your 304 Headphone Amplifier 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 rec-
ommend 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 neces-
sary parts, labor, packaging materials, and freight charges.
Please remember, you must call for an RA number before
sending the unit to Symetrix.
Warranty and Service

6408 216th St. SW, Mountlake Terrace, WA, USA • Tel [phone] • Fax [phone] • Web www.symetrixaudio.com
© 2006, Symetrix, Inc. All rights reserved. Printed in the United States of America. Symetrix Part Number 53304-1A02. The information in this User’s
Guide 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 User’s Guide 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. Product names mentioned herein may be trademarks and/or registered trademarks of their respective companies.