Symetrix 302 User manual

302 Dual Microphone
Preamplifier
User’s Guide
302

i
Table of Contents
Chapter 1 Introduction 1
Chapter 2 Operator Safety Summary 2
Chapter 3 Fast Setup 3
Chapter 4 Front Panel Controls 4
Chapter 5 Rear Panel Connections 5
Chapter 6 Using the 302 7
Chapter 7 Applications 9
Chapter 8 Troubleshooting 11
Chapter 9 Specifications 12
Chapter 10 Warranty and Service 13
Chapter 11 Declaration of Conformity 15
Rev A.01, 11 September, 1998
Symetrix part number 53302-0A01
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]
302

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IntroductionChapter 1
Front panel
Rear panel
Congratulations on your purchase of the
Symetrix302DualMicrophonePreamplifier.
The 302 is an ultra-clean two channel dual
mono preamp for use in critical digital and
analog recording situations. The 302 delivers
exceptional sonic performance even at
extremelyhighinputlevels,makingitthe
ideal mate to the newest transformerless
condenser microphone designs.
When compared with older mic preamps, the
302 offers substantial sonic advances, such
as:
1) Its solid stereo imaging (less than 10
degree phase shift at 20kHz);
2) Excellent transient handling (its positive
and negative slew rates are symmetrical);
3) Very low noise (approaching the theoreti-
callimit);and
4) Almost undetectable distortion (.007%).
Variable gain inputs with 15dB pads allow
the 302 to handle any input level up to
+14dBV. Phantom power is available at both
inputs. In addition, both channels include a
polarity switch to correct for improperly
wired cables or unresolvable mic placement
problems.
Balanced low-Z line drivers are on 3-conduc-
tor 1/4" TRS connectors and Euroblock
connectors. These drivers are capable of
+24dBmmaximumoutput.The output stage
can also drive unbalanced lines of up to a
maximumof +18dBm.
We recommend that you read this manual
cover-to-cover. You will find the answers to
most of your questions inside. However, if
you are in a hurry, go directly to Chapter 3
(Fast Setup). It will get you started quickly.
Please feel free to contact us if you have
questions, comments or suggestions.
Phone: [phone]
Fax: [phone]
Email: [email protected]
Website: www.symetrixaudio.com
PWR
OFF
ON
POLARITY
0°
180°
15dB PAD
OUT
IN
40
GAIN (dB)
60
MICROPHONE 2 PHANTOM POWER
CLIP
0°
180°
POLARITY
IN
40
GAIN (dB)
20 60
OUT
15dB PAD
302
DUAL MICROPHONE
PREAMPLIFIER
CLIP
MICROPHONE 1
20
MICROPHONE INPUTS
INPUT 1INPUT 2
OUTPUT 1
OUTPUT 1
OUTPUT 2
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.
CONNECT TO
SYMETRIX PS-3
MANUFACTURED BY SYMETRIX INC. LYNNWOOD, WA USA
OUTPUT 2 OUTPUT 1+2
BALANCED LINE OUTPUTS

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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 per-
sons.
Theexclamationpointwithinanequilateral
triangle is intended to alert the user of the
presence of important operating and mainte-
nance (servicing) instructions in the literature
accompanyingthe302 (i.e. thismanual).
Caution To prevent electric shock, do not
use the polarized plug supplied
with the 302 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 useNote,
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 302.
Equipment Markings
Operator Safety Summary Chapter 2
Caution Identifies information that, if not
heeded, may cause damage to the
302 or other equipment in your
system.
Warning Identifiesinformationthat,if
ignored,maybehazardousto
yourhealthorthatofothers.
CAPITALS Controls, switches or other
markings on the 302’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.
Dangerfromlossof ground-Iftheprotective
ground connection is lost, all accessible
conductive parts, including knobs and
controls that may appear to be insulated, can
render an electric shock.
In-linepowersupply-Thisproductreceivesits
operatingpowerfromtheSymetrixPS-3,PS-3E
or PY-3 DC power supply. These are the only
power supplies 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.
Operatinglocation-Donotoperatethis
equipment under any of the following condi-
tions: explosive atmospheres, in wet locations,
ininclementweather,improper orunknownAC
mainsvoltage,orif improperly fused.
Stayoutofthebox-Toavoidpersonal 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

302
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Fast First-Time Setup
Follow these instructions to get your 302 up-and-running as quickly as possible. The intent of this
section is fast setup only. Please refer to later chapters for explanation of the 302’s controls and
functions.
Set the Microphone 1 and Microphone 2 GAIN controls completely counter-clockwise.
Set the Microphone 1 and Microphone 2 PAD switches to the “OUT” position.
Set the Microphone 1 and Microphone 2 POLARITY switches to the “OUT” position.
Set the PHANTOM POWER switch to the “OUT” position, unless you will be using a
condenser microphone which requires phantom power.
Connect your first microphone to the Microphone 1 XLR Input. If you are using two mics,
connect your second microphone to the Microphone 2 XLR Input.
Caution: Use only balanced microphones and cables!
Connect Output 1 and Output 2 of the Symetrix 302 to the inputs of the destination device.
Plug the 302 into an AC outlet using the external power supply (the Symetrix PS-3, PS-3E or
PY-3).
Caution:Failure to connect the 302 to the proper AC mains voltage may
cause fire and/or internal damage.
Turn the Microphone 1 and Microphone 2 GAIN controls slowly clockwise until you
achieve the desired output level from the 302.
Fast setup is now complete. Please refer to later chapters of this user’s guide or the
Symetrix Customer Service Department if you have additional questions.



Fast Setup
Chapter 3







302
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Front Panel
CLIP LED-The CLIP LEDlightswhenthe signalreaches3dBbelow clipping. ThisLEDshould
never be on solidly. It is usually all right if the CLIP LED flashes occasionally, on peaks in the
program material. If you see the CLIP LED light, monitor your final signal destination to be sure
that you are not creating distortion by running audio levels that are too hot.
GAINcontrol-Setsthegainofthepreamplifierfromaminimumof20dBtoamaximumof60dB.
15dB PAD - Reduces the input signal by 15dB when the switch is set to the “IN” position.
POLARITY push-button - Inverts the phase of the signal being amplified when the switch is set to
the “IN” position.
PHANTOM POWER - Applies a positive DC voltage to pins 2 and 3 of the XLR input jacks when
the switch is set to the “IN” position. This provides the ability to power condenser microphones
which require phantom power.
Caution: When Phantom Power is engaged, always turn your monitoring speakers and
headphones completely off before connecting or disconnecting any microphones to
the 302. This is to avoid a loud “pop” which occurs due to the sudden change in
voltage on XLR pins 2 & 3 when they are connected to, or disconnected from,
Phantom Power. Such a loud “pop” can result in speaker and/or hearing damage.
Caution: Never connect an unbalanced microphone or mic cable to the input of the 302 when
the Phantom Power is engaged. This will short out the Phantom Power.
See page 8 for more Dos and Don’ts of Phantom Powering.
Front Panel Controls Chapter 4
PWR
OFF
ON
POLARITY
0°
180°
15dB PAD
OUT
IN
40
GAIN (dB)
60
MICROPHONE 2 PHANTOM POWER
CLIP
0°
180°
POLARITY
IN
40
GAIN (dB)
20 60
OUT
15dB PAD
302
DUAL MICROPHONE
PREAMPLIFIER
CLIP
MICROPHONE 1
20

302
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Rear Panel ConnectionsChapter 5
MICROPHONE INPUTS
INPUT 1INPUT 2
OUTPUT 1
OUTPUT 1
OUTPUT 2
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.
CONNECT TO
SYMETRIX PS-3
MANUFACTURED BY SYMETRIX INC. LYNNWOOD, WA USA
OUTPUT 2 OUTPUT 1+2
BALANCED LINE OUTPUTS
Rear Panel
POWER INPUT- 7-pin DINreceptacle.Connectthis to the7-pinDINplug of aSymetrixPS-3,
PS-3EorPY-3only.ConnecttheAC power connectorofthePS-3 or PS-3Eto
an AC power source that is of the correct voltage and frequency, as marked
onthePS-3orPS-3E.
Note The PS-3 is a 117V AC power supply, used to power a single unit. The PS-3E is a
230V AC power supply, used to power a single unit. The PY-3 is a power supply with
a “Y” cable that is used to power two units from one supply. The PY-3isonly
availablefor117VACoperation. ThePS-3,PS-3EandPY-3areSymetrixaccessories,
availablefromyourSymetrixdealer.
BALANCED LINE OUTPUTS
TRSjacks-1/4-inchtip-ring-sleeve(TRS)phonejack,wired:
Tip=high(+),Ring=low(-)andSleeve=ground
Use these jacks when you need 1/4-inch balanced or unbalanced outputs (Output 1 or
Output 2). The nominal signal level here is +4 dBu. If you need an unbalanced output
use a tip-ring-sleeve phone plug for best results and connect only to the tip and sleeve.
Note Connecting an unbalanced cable here will result in an output level that is 6dB
lower than the output level achieved using a balanced cable.
Euroblock connectors - Low-profile balanced (+, - and ground) connectors, typically used in
contractor sound installations. Use these jacks when you need Euroblock balanced outputs
(Output 1, Output 2 or Output 1+2).
Note The signal at Output 1+2 is a combination of Output 1 and Output 2 signal, with
the level balance between Output 1 and Output 2 determined by the front-panel
GAIN and PAD settings.
MICROPHONE INPUTS
XLRinputs- Balanced XLR input jacks for Microphone 1 and Microphone 2.
(Pin1=ground, Pin 2 =+,Pin3 = -)
These are mic-level inputs, designed to accept the balanced output from one
microphone.
Caution: When Phantom Power is engaged, do not use unbalanced microphones or
mic cables.
Caution: When Phantom Power is engaged, do not use switched mics which short
Pin 2 and /or Pin 3 to ground.
Caution: When Phantom Power is engaged, always turn down monitors/headphones
before connecting or disconnecting microphones.

302
6

302
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Using the 302Chapter 6
The Preamplifiers
The 302’s ultra-low noise, low distortion mic preamps provide direct-coupled balanced low-Z
inputs for optimum transient response and phase coherency. The positive and negative slew
rates are symmetrical, ensuring sonic integrity.
Gain Controls, Clip LEDs, Pads
The 302’s gain controls set the gain of the preamp circuitry. They operate like the trim controls
found on most mixers. On a mixer the trim controls set the initial amount of amplification for each
mic input. The faders govern how much of the amplified signal is allowed to pass into the rest of
the system. Using a separate mic preamp such as the 302 assumes that somewhere downstream
there will be a volume control that functions like the fader on a mixing console.
The 302 offers 40dB overall gain range, from 20 dB to 60dB, when used without the pad.
Including the 15dB pad, the total range is 75dB, with a minimum overall gain of 5dB.
Any mic output level is easy to match with the 302’s stepless gain control, regardless if it is
the -60dBV to -80dBV levels common with ribbon mics or the near line level often delivered by
transformerless condenser mics. Input signals as high as +14dBV can by handled by the 302.
(The front panel clip LEDs fire at 3dB below clipping.)
Both source level and preamp gain are directly related to the noise floor, as well as to head-
room at the output. The key to using the 302 at its optimum performance level is to:
1. Usethemaximumallowablesourcelevel.
2. Use only as much gain as necessary.
The combination of the highest possible input levels with the lowest possible gain always
resultsin minimumnoiseandmaximumheadroom.
The PAD push-buttons on the 302 reduce the input level at each channel by 15dB. Use these pads
when a very high output mic is used or when the incoming signal is so hot that the clip indicator is
lighted with the gain control at minimum. As previously mentioned, the 302 uses gain controls, not
volume controls or faders. It is not necessary to use the pads to reduce input levels in order to get
the gain controls to operate in some theoretical “best” position. The only “best” operating position
for a gain control is the minimum gain that delivers the required output level.
In addition to their more common use in reducing input signal strength, the pads can also give
the user a wider gain control-adjustment range for more accurate channel-to-channel level
matching. For very close level matching, the resolution of the gain controls at the very lowest
end of their range may be too coarse if the pads are not engaged. There is a trade-off here,
however: When using the pads the apparent noise floor is higher.
PolarityPush-buttons
Both channels include a polarity push-button. In the normal “out” position, signal source
polarity is maintained to the outputs. In the “in” position, the wiring from the XLR connector
pins 2 and 3 is reversed. This allows for correction of miswired cables, and for polarity match-
ing of various manufacturers’ microphones.
Powering for Condenser Microphones
All condenser microphones require some kind of electrical power. This power may be supplied
by internal batteries, an external power supply that is connected to the microphone by a
special multiwire cable, or through a standard microphone cable by phantom or “T” system
powering. Phantom powering and T system powering are incompatible systems.
Phantom power derives its name from its invisibility to audio signals, even though the micro-
phone cable carries both phantom power (as direct current) and audio signals (as alternating
current). Specifically, the term phantom power means a positive DC voltage sent to the microphone
on both audio leads, through current-limiting resistors which also serve to isolate the audio leads

302
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from one another. The front panel phantom power button applies phantom power via pins 2 and 3
of the mic input XLR connectors.
The phantom power technique uses the two signal conductors in a standard balanced mic cable to
deliver the power required by the microphone, eliminating the need for internal batteries or an
external power supply. Because the voltage is applied equally to both sides of a floating balanced
circuit, no current flows through the microphone’s transformer, or through the microphone element
itself.
It is often said that the sound of some dynamic microphones is affected by phantom power and
that ribbon mics cannot be plugged into an input that is phantom powered. For the most part, these
are myths that grew out of difficulties created by other problems in the mic circuit:
1. When XLR connectors are mated there is no guarantee that both pins 2 and 3 will make
contact at exactly the same time. It is possible that a damaging current could flow through
the mic for a brief moment under these conditions. However, this is a connector problem,
not a problem with the mic itself or phantom power in general.
2. In the past, it was a common practice to ground the center tap of the mic’s output
transformer. However, this practice should be avoided in phantom powered systems. The
solution: Locate the center tap and cut the connection between it and pin 1 of the XLR
connector.
3. If the mic’s output transformer has developed leakage, the microphone may become noisy
when phantom power is turned on. Crackling, sputtering or even humming noises may
occur. The leakage, not the power, is the problem. The solutions are:
A. Turn off the phantom power.
B. Put a 1:1 low-impedance transformer between the mic and the input.
C. Getthemicrepaired.
The dominant remote powering system in use today is the phantom power system, which is
compatible with both condenser and non-condenser microphones (dynamics, ribbons, etc.). If your
microphone’s specifications sheet claims that it requires phantom power, the Symetrix 302 can
powerit.
The technical requirements for operation and/or compatibility are:
1. The microphone must have a balanced, low-impedance output.
2. The balanced output must be floating with respect to ground. If there is a center tap, it
must not be grounded.
In a nutshell, here are the do’s and don’ts of phantom powering:
DO: Verify that your microphone can be phantom powered (if it is a condenser mic).
Ensure that your microphone’s output is low impedance, balanced and floating. This is
especiallyimportant forribbonmics.
Turn the phantom power off when connecting vintage ribbon microphones.
Mute your monitor speakers or headphones when turning the phantom power on or off.
If you don’t, there will be a loud, nasty pop.
Mute your monitor speakers or headphones whenever you plug in or unplug a phantom
powered microphone. If you don’t, there will be a loud, nasty pop.
DON’T: Plug in an A-B powered microphone without a suitable adapter.
Worry about your dynamic or ribbon microphones, as long as they are wired so that the
output is balanced and floating.
Use the microphone input for line-level sources, especially those that are transformerless.
Use the microphone input with unbalanced sources. Use a direct box to feed an
unbalanced source into the microphone input.
Worry about your tube condenser mics. They are compatible (although they cannot be
phantom powered).