Symetrix 304 User manual

www.SymetrixAudio.com
User’s Guide
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6408 216th St. SW | Mountlake Terrace, WA 98043 | USA | Tel: [phone] | Fax: [phone]
User’s Guide: 304 Headphone Amplifier

www.SymetrixAudio.com
®
1
!
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 (servic-
ing) instructions in the literature accompanying
the product (i.e. this User’s Guide).
1
!
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.
1
!
Power Source: The 304 Headphone Ampli-
fier 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 connec-
tion, 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.
1
!
User Serviceable Parts: There are no user
serviceable parts inside your 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.
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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 appa-
ratus (including amplifiers) that produce heat.
(This apparatus shall be connected to a mains
socket outlet with a protective earthing con-
nection. 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 speci-
fied 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 person-
nel. 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.
Safety
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Before You Begin
What Ships in the Box
øA 304 hardware unit.
øOne PS-3 (115 VAC), PS-3E (230 VAC)
power supply.
øThis User’s Guide.
Getting Help
If you have questions beyond the scope of this
User’s Guide, contact our Customer Support Group
inthe 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.

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6408 216th St. SW | Mountlake Terrace, WA 98043 | USA | Tel: [phone] | Fax: [phone]
Introduction: First Time Setup
Fast First Time Setup
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 posi-
tion (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.
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User’s Guide: 304 Headphone Amplifier
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 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 differ-
ent 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-conduc-
tor 1/4 inch plugs are inserted. The outputs are 1/4 inch
stereo connectors.

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Front and 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 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).
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.
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6408 216th St. SW | Mountlake Terrace, WA 98043 | USA | Tel: [phone] | Fax: [phone]
Using the 304
Impedance, Sensitivity, Loudness
Loudness is directly related to the impedance and sen-
sitivity 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 equiva-
lent 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 loud-
ness, without regard to the impedance 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.
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 imped-
ance 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.
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
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User’s Guide: 304 Headphone Amplifier

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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.
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6408 216th St. SW | Mountlake Terrace, WA 98043 | USA | Tel: [phone] | Fax: [phone]
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
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User’s Guide: 304 Headphone Amplifier

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www.SymetrixAudio.com
Troubleshooting
Troubleshooting Chart
Symptom Probable Cause
No output: Check cables and connections. Are inputs driven by outputs, and outputs driv-
ing 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:
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6408 216th St. SW | Mountlake Terrace, WA 98043 | USA | Tel: [phone] | Fax: [phone]
Specifications • Statement 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 Requirements.......115 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 ap-
paratus 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 Com-
munity 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.
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User’s Guide: 304 Headphone Amplifier