Symetrix 302 User manual

302 Dual Microphone
Preamplifier
Users 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 Usin the 302 7
Chapter 7 Applications 9
Chapter 8 Troubleshootin 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 chan e without notice.
©1998, Symetrix, Inc. All ri ht reserved.
Symetrix is a re istered 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 re ard to the performance or use
of these products.
Under copyri ht laws, no part of this manual may be
reproduced or transmitted in any form or by any
means, electronic or mechanical, includin photo-
copyin , scannin , recordin or by any information
stora e and retrieval system, without permission, in
writin , from Symetrix, Inc.
6408 216th St. SW
Mountlake Terrace, WA 98043 USA
Tel [phone]
Fax [phone]
Email: [email protected]
302

302
1
IntroductionChapter 1
Front panel
Rear panel
Congratulations on your purchase of the
Symetrix 302 Dual Microphone Preamplifier.
The 302 is an ultra-clean t o channel dual
mono preamp for use in critical digital and
analog recording situations. The 302 delivers
exceptional sonic performance even at
extremely high input levels, making it the
ideal mate to the ne est transformerless
condenser microphone designs.
When compared ith 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 sle rates are symmetrical);
3) Very lo noise (approaching the theoreti-
cal limit); and
4) Almost undetectable distortion (.007%).
Variable gain inputs ith 15dB pads allo
the 302 to handle any input level up to
+14dBV. Phantom po er is available at both
inputs. In addition, both channels include a
polarity s itch to correct for improperly
ired cables or unresolvable mic placement
problems.
Balanced lo -Z line drivers are on 3-conduc-
tor 1/4" TRS connectors and Euroblock
connectors. These drivers are capable of
+24dBm maximum output. The output stage
can also drive unbalanced lines of up to a
maximum of +18dBm.
We recommend that you read this manual
cover-to-cover. You ill find the ans ers to
most of your questions inside. Ho ever, if
you are in a hurry, go directly to Chapter 3
(Fast Setup). It ill 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: .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 ÀUN TECHNICIEN QUALIFIÉ.
FABRIQUÉAUX 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

302
2
The information in this summary is intended
for persons ho operate the equipment as ell
as repair personnel. Specific arnings and
cautions are found throughout this manual
herever they may apply.
The notational conventions used in this
manual and on the equipment itself are
described in the follo ing paragraphs.
The lightning flash ith arro head symbol
ithin an equilateral triangle is intended to
alert the user of the presence of uninsulated
dangerous voltage ithin the product’s
enclosure that may be of sufficient magnitude
to constitute a risk of electric shock to per-
sons.
The exclamation point ithin 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 302 (i.e. this manual).
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 use Note,
Caution, or Warning as a heading. Certain
typefaces and capitalization are used to
identify certain ords. 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 infor ation that, if not
heeded, ay cause da age to the
302 or other equip ent in your
syste .
Warning Identifies information that, if
ignored, may be hazardous to
your health or that of others.
CAPITALS Controls, s itches or other
markings on the 302's chassis.
Boldface Strong emphasis.
Power source - This product is intended to
operate from a po er source that does not
apply more than 255Vrms bet een the po er
supply conductors or bet een either po er
supply conductor and ground. A protective
ground connection, by ay of the grounding
conductor in the po er cord, is essential for
safe operation.
Danger from loss of ground - If the protective
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 po er from the Symetrix PS-3 or PS-3E
po er supply. These are the only
po er supplies approved for use ith the
product. Do not connect the product to any
other in-line, or plug-in, transformer. The use
of other po er 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 follo ing condi-
tions: explosive atmospheres, in et locations,
in inclement eather, improper or unkno n AC
mains voltage, or if improperly fused.
tay 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
ithout 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
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

302
3
Fast First-Time Setup
Follo 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.
1. Set the Microphone 1 and Microphone 2 GAIN controls completely counter-clock ise.
2. Set the Microphone 1 and Microphone 2 PAD s itches to the OUT position.
3. Set the Microphone 1 and Microphone 2 POLARITY s itches to the OUT position.
4. Set the PHANTOM POWER s itch to the OUT position, unless you ill be using a
condenser microphone hich requires phantom po er.
5. Connect your first microphone to the Microphone 1 XLR Input. If you are using t o mics,
connect your second microphone to the Microphone 2 XLR Input.
Caution:Use only balanced icrophones and cables!
6. Connect Output 1 and Output 2 of the Symetrix 302 to the inputs of the destination device.
7. Plug the 302 into an AC outlet using the external po er supply (the Symetrix PS-3 or PS-3E).
Caution:Failure to connect the 302 to the proper AC ains voltage ay
cause fire and/or internal da age.
8. Turn the Microphone 1 and Microphone 2 GAIN controls slo ly clock ise until you
achieve the desired output level from the 302.
9. Fast setup is no complete. Please refer to later chapters of this users guide or the
Symetrix Customer Service Department if you have additional questions.
Fast Setup
Chapter 3

302
4
Front Panel
CLIP LED - The CLIP LED lights hen the signal reaches 3dB belo clipping. This LED should
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.
GAIN control - Sets the gain of the preamplifier from a minimum of 20dB to a maximum of 60dB.
15dB PAD - Reduces the input signal by 15dB hen the s itch is set to the IN position.
POLARITY push-button - Inverts the phase of the signal being amplified hen the s itch is set to
the IN position.
PHANTOM POWER - Applies a positive DC voltage to pins 2 and 3 of the XLR input jacks hen
the s itch is set to the IN position. This provides the ability to po er condenser microphones
hich require phantom po er.
Caution: When Phanto Power is engaged, always turn your onitoring speakers and
headphones co pletely off before connecting or disconnecting any icrophones 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 fro ,
Phanto Power. Such a loud pop can result in speaker and/or hearing da age.
Caution: Never connect an unbalanced icrophone or ic cable to the input of the 302 when
the Phanto Power is engaged. This will short out the Phanto Power.
See page 8 for more Do's and Don'ts of Phantom Po ering.
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
5
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 ÀUN TECHNICIEN QUALIFIÉ.
FABRIQUÉAUX 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 DIN receptacle. Connect this to the 7-pin DIN plug of a Symetrix PS-3 or
PS-3E only. Connect the AC po er connector of the PS-3 or PS-3E to
an AC po er source that is of the correct voltage and frequency, as marked
on the PS-3 or PS-3E.
Note The PS-3 is a 117V AC po er supply, used to po er a single unit. The PS-3E is a
230V AC po er supply, used to po er a single unit. The PS-3 and PS-3E are Symetrix
accessories, available from your Symetrix dealer.
BALANCED LINE OUTPUT
TRS jacks -1/4-inch tip-ring-sleeve (TRS) phone jack, ired:
Tip= high(+), Ring =lo (-) and Sleeve=ground
Use these jacks hen 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 ill result in an output level that is 6dB
lo er than the output level achieved using a balanced cable.
Euroblock connectors - Lo -profile balanced (+, - and ground) connectors, typically used in
contractor sound installations. Use these jacks hen 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, ith
the level balance bet een Output 1 and Output 2 determined by the front-panel
GAIN and PAD settings.
MICROPHONE INPUT
XLR inputs - Balanced XLR input jacks for Microphone 1 and Microphone 2.
(Pin 1 = ground, Pin 2 = +, Pin 3 = -)
These are ic-level inputs, designed to accept the balanced output from one
microphone.
Caution: When Phanto Power is engaged, do not use unbalanced icrophones or
ic cables.
Caution: When Phanto Power is engaged, do not use switched ics which short
Pin 2 and /or Pin 3 to ground.
Caution: When Phanto Power is engaged, always turn down onitors/headphones
before connecting or disconnecting icrophones.

302
6

302
7
Using the 302Chapter 6
The Preamplifiers
The 302's ultra-lo noise, lo distortion mic preamps provide direct-coupled balanced lo -Z
inputs for optimum transient response and phase coherency. The positive and negative sle
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 ho much of the amplified signal is allo ed to pass into the rest of
the system. Using a separate mic preamp such as the 302 assumes that some here do nstream
there ill 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, hen used ithout the pad.
Including the 15dB pad, the total range is 75dB, ith a minimum overall gain of 5dB.
Any mic output level is easy to match ith the 302's stepless gain control, regardless if it is
the -60dBV to -80dBV levels common ith 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 belo clipping.)
Both source level and preamp gain are directly related to the noise floor, as ell as to head-
room at the output. The key to using the 302 at its optimum performance level is to:
1. Use the maximum allo able source level.
2. Use only as much gain as necessary.
The combination of the highest possible input levels ith the lo est possible gain al ays
results in minimum noise and maximum headroom.
The PAD push-buttons on the 302 reduce the input level at each channel by 15dB. Use these pads
hen a very high output mic is used or hen the incoming signal is so hot that the clip indicator is
lighted ith 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 ider 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 lo est
end of their range may be too coarse if the pads are not engaged. There is a trade-off here,
ho ever: 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 iring from the XLR connector
pins 2 and 3 is reversed. This allo s for correction of mis ired cables, and for polarity match-
ing of various manufacturers microphones.
Powering for Condenser Microphones
All condenser microphones require some kind of electrical po er. This po er may be supplied
by internal batteries, an external po er supply that is connected to the microphone by a
special multi ire cable, or through a standard microphone cable by phantom or T system
po ering. Phantom po ering and T system po ering are incompatible systems.
Phantom po er derives its name from its invisibility to audio signals, even though the micro-
phone cable carries both phantom po er (as direct current) and audio signals (as alternating
current). Specifically, the term phantom po er means a positive DC voltage sent to the microphone
on both audio leads, through current-limiting resistors hich also serve to isolate the audio leads

302
8
from one another. The front panel phantom po er button applies phantom po er via pins 2 and 3
of the mic input XLR connectors.
The phantom po er technique uses the t o signal conductors in a standard balanced mic cable to
deliver the po er required by the microphone, eliminating the need for internal batteries or an
external po er supply. Because the voltage is applied equally to both sides of a floating balanced
circuit, no current flo s 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 po er and
that ribbon mics cannot be plugged into an input that is phantom po ered. For the most part, these
are myths that gre 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 ill make
contact at exactly the same time. It is possible that a damaging current could flo through
the mic for a brief moment under these conditions. Ho ever, this is a connector problem,
not a problem ith the mic itself or phantom po er in general.
2. In the past, it as a common practice to ground the center tap of the mic s output
transformer. Ho ever, this practice should be avoided in phantom po ered systems. The
solution: Locate the center tap and cut the connection bet een it and pin 1 of the XLR
connector.
3. If the mic s output transformer has developed leakage, the microphone may become noisy
hen phantom po er is turned on. Crackling, sputtering or even humming noises may
occur. The leakage, not the po er, is the problem. The solutions are:
A. Turn off the phantom po er.
B. Put a 1:1 lo -impedance transformer bet een the mic and the input.
C. Get the mic repaired.
The dominant remote po ering system in use today is the phantom po er system, hich is
compatible ith both condenser and non-condenser microphones (dynamics, ribbons, etc.). If your
microphone's specifications sheet claims that it requires phantom po er, the Symetrix 302 can
po er it.
The technical requirements for operation and/or compatibility are:
1. The microphone must have a balanced, lo -impedance output.
2. The balanced output must be floating ith 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 po ering:
DO: Verify that your microphone can be phantom po ered (if it is a condenser mic).
Ensure that your microphone s output is lo impedance, balanced and floating. This is
especially important for ribbon mics.
Turn the phantom po er off hen connecting vintage ribbon microphones.
Mute your monitor speakers or headphones hen turning the phantom po er on or off.
If you don t, there ill be a loud, nasty pop.
Mute your monitor speakers or headphones henever you plug in or unplug a phantom
po ered microphone. If you don t, there ill be a loud, nasty pop.
DON'T:Plug in an A-B po ered microphone ithout a suitable adapter.
Worry about your dynamic or ribbon microphones, as long as they are ired 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 ith 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 po ered).