Symetrix 425 User manual

SymetrixSymetrix
425
Dual Compressor / Limiter
Revision 2.0, 10/29/93
Part number: 530201
Subject to change at our whim, and without notice.
Copyright 1992–1993 by Symetrix Inc. All rights Reserved.
Batteries not included. Void where taxed or prohibited. Ground ain’t ground!
Owner’s Manual
Signal Processing at it’s Best!

Table of Contents
1. Introduction...........................................................................................1-1
1.1 About this manual...........................................................................................1-1
1.2 Operator Safety Summary..............................................................................1-2
1.2.1 Equipment Markings....................................................................................1-2
1.2.2. Terms .........................................................................................................1-2
1.3. Other Safety Information ...............................................................................1-3
2. Dynamics Processing Tutorial and Functional Basics.....................2-1
2.1 Dynamics Processing Basics..........................................................................2-1
2.1.1 Compressors and Limiters...........................................................................2-1
2.1.2 Expanders and Gates..................................................................................2-2
2.1.3 Ratio............................................................................................................2-2
2.1.4 Gain vs Output ............................................................................................2-2
2.1.5 Attack Time..................................................................................................2-2
2.1.6 Release Time ..............................................................................................2-3
2.1.7 Threshold Setting ........................................................................................2-3
2.1.8 Interpreting the Displays..............................................................................2-3
2.2 Using the Sidechain........................................................................................2-3
3. Technical Tutorial................................................................................3-1
3.1. Matching Levels vs Matching Impedances.....................................................3-1
3.2. Signal Levels..................................................................................................3-2
3.3. I/O Impedances..............................................................................................3-2
3.4. Polarity Convention ........................................................................................3-2
3.5. Input and Output Connections........................................................................3-3
4. Front Panel Overview..........................................................................4-1
5. Rear Panel Overview...........................................................................5-1
6. Fast First Time Setup ...........................................................................6-1
6.1 Connections ...................................................................................................6-1
6.2 Settings ..........................................................................................................6-1
6.3 Initial Setup.....................................................................................................6-2
6.4 Refining Your Settings....................................................................................6-2
6.5 Stereo Coupling..............................................................................................6-2
7. Using the 425.........................................................................................7-1
7.1 Block Diagram................................................................................................7-1
7.2 Installation......................................................................................................7-1
7.3 The 425 as a Compressor..............................................................................7-1
7.4 The 425 as a Ducker......................................................................................7-2
7.5 The 425 as a Limiter.......................................................................................7-2
7.6 The 425 as an Expander................................................................................7-3
7.7 Using the Expander and Compressor Simultaneously ...................................7-3
i

8. Applications.........................................................................................8-1
8.1 Vocal Level Smoothing...................................................................................8-1
8.2 Removing Noise from Vocal Tracks................................................................8-1
8.3 Constant Level Paging ...................................................................................8-1
8.4 Paging with Ducking.......................................................................................8-2
8.5 Stage Monitors ...............................................................................................8-2
8.6 Keyed Bass....................................................................................................8-3
8.7 Sibilance Control ............................................................................................8-3
9. Troubleshooting Chart........................................................................9-1
10. 425 Dual Compressor Limiter Limited Warranty..............................10-1
11. Repair Information...............................................................................11-1
11.1. Return Authorization....................................................................................11-1
11.2. In-Warranty Repairs.....................................................................................11-1
11.3. Out-of-Warranty Repairs..............................................................................11-1
12. Specifications......................................................................................12-1
13. PCB Layouts and Schematics............................................................13-1
13.1 Troubleshooting Hints...................................................................................13-1
13.2 Additional Reading .......................................................................................13-1
Appendix A. Architects and Engineers Specification............................A-1
Appendix B. Disassembly Instructions ..................................................B-1
ii

IN
+10
GAIN (dB)
+20
0
BYPASS
BYPASS
+12.5 +5 +15
+52
RATIO (X:1)
-10
10SLOW 1
-2.51.3 2.5
THRESHOLD (dBu)
THRESHOLD (dBu)
-10
FAST
BYPASS
-40
SLOW
-25 +10
-20
DUAL STEREO
STEREO
LINK THRESHOLD (dBu)
BYPASS
+5
DUAL MONO
-5 -30
RELEASE
FAST
IN
GAIN (dB)
+10
+20BYPASS 0
BYPASS
+12.5 +5 +15
+52
RATIO (X:1)
10 -101 SLOW
-2.5
1.3 2.5
THRESHOLD (dBu)
-10
THRESHOLD (dBu)
BYPASS FAST
SLOW -40
+10-25
-20
THRESHOLD (dBu)
BYPASS
+5
-5 -30
RELEASE
FAST
RELEASE RELEASE
OUT
0VU CLIP-10-20-3
LIMIT
-6-9-12-2
COMPRESSOR
-4-6-10-12 -8-25 -15-6-12 -2-18
EXPANDEROUT
0VU CLIP-20-3
LIMIT
-10-6-9-12-2
COMPRESSOR
-8 -6-10-12-25 -15-2-6-12-18
EXPANDER
-4
1. Introduction
The Symetrix 425 is a dual channel compressor/limiter/expander. The two channels may be
operated in dual-mono mode, or in stereo mode. In stereo mode, both channels receive
identical control signals, which minimizes image shifts caused by unequal gains in the two
channels.
At Symetrix, we feel that the 425 represents the distillation of our 15 years of signal
processing experience into a product that is at once versatile and easy to use without having
important controls removed in the interest of simplicity. The 425 is unique because its
processor sections are always in-line. There is no selector switch to select between expander,
compressor, or limiter. Now you can expand the low-level parts of a signal to improve its
signal-to-noise ratio, then apply compression to add punch and density and still have the
security of a peak limiter for overload protection. We call this approach to total level control
IDP or Integrated Dynamics Processing.
IDP also means powerful, streamlined controls that make the 425 easy to learn and quick to
set up, even with three types of processing in one rack space. Each section has the controls
that you need to respond to any audio situation fast. Individual LED meters show you what's
going on inside each section.
The 425 is Listed by Underwriters Laboratories Inc. (UL). Samples of this product have been
evaluated by UL and meet the applicable UL Standards for Safety.
In the final analysis, Integrated Dynamics Processing means clean, quiet sound that meets
professional demands in any situation. High-quality components and a minimalist signal path
make the 425 exceptionally transparent.
1.1 About this manual
We recommend that you read this manual from cover to cover. Somewhere between the
confines of the two covers you should find the answers to most (98%) of your questions, both
technical as well as musical.
If you're in a hurry (like most of us), or if you really don't believe that someone could write a
decent owners manual that you can read and understand, then do us both a favor and read
section 6, "Fast First Time Setup." This section will help you get connected, tell you what the
knobs do, and send you on your way.
This manual contains the following sections:
Chapter 1. Introduction: introduces the 425 and this manual.
Chapter 2. Using the 425: describes how to use the 425. Covers installation, signal levels,
input and output impedances, and physical features.
Chapter 3. Technical Tutorial: a basic and not-so-basic discussion of signal levels, input and
output impedances and connection polarity.
Chapter 4. Front Panel Overview: a brief look at the controls and switches located on the front
panel of the 425.
Chapter 5. Rear Panel Overview: a brief look at the rear panel of the 425.
Chapter 6. Fast First Time Setup: is the setion to read if you can't wait.
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Chapter 7. Using the 425: describes the use of the 425 in detail.
Chapter 8. Applications: gives some hints for using the 425 in a variety of situations and
provides a short introduction to dynamics processing..
Chapter 9. Troubleshooting: tells what to do when the 425 doesn't work.
Chapter 10. Limited Warranty: describes the 425's limited warranty.
Chapter 11. Service Information: tells how to get your 425 repaired, how to obtain parts, and
how to contact the factory.
Chapter 12. Specifications: lists the technical specifications of the 425's performance.
Chapter 13. Schematics: contains the printed circuit board layout(s) and schematic diagrams.
Appendix A. Appendix A: contains the Architects and Engineer's specifications.
Appendix B. Appendix B: contains disassembly instructions.
1.2 Operator Safety Summary
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; they do not appear in this summary.
The notational conventions used in this manual and on the equipment itself are described in
the following paragraphs.
1.2.1 Equipment Markings
S’adresser a un reparateur compétent.
Il ne se trouve a l’interieur aucune piece pourvant entre reparée l’usager.
No user serviceable parts inside. Refer servicing to qualified service personnel.
NE PAS OUVRIR
RISQUE DE CHOC ELECTRIQUE
ATTENTION:
CAUTION
DO NOT OPEN
RISK OF ELECTRIC SHOCK
The lightning flash with arowhead 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
p
ersons.
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 appliance (i.e.
this manual).
Caution
To prevent electric shock, do not use the polarized plug supplied with this appliance
with any extension cord, receptacle, or other outlet unless the blades can be fully
inserted to prevent blade exposure.
Rev 2.0, 10/29/93
1-2

1.2.2. Terms
Several notational conventions are used in this manual. Some paragraphs may use Note,
Caution, or Warning as a heading. These headings have the following meaning:
Convention Description
Caution Identifies information that, if not heeded, may cause
damage to the 425 or other equipment in your system.
NOTE Identifies information that needs extra emphasis. A
Note generally supplies extra information to help you
use the 425 better.
Warning Identifies information that, if ignored, may be
hazardous to your health or that of others.
In addition, certain typefaces and capitalization are used to identify certain words. These
situations are:
Convention Meaning
CAPITALS Controls, switches or other markings on the chassis.
Boldface Strong emphasis.
1.3. Other Safety Information
Power Source This product is intended to operate from a power source
that does not apply more than 250V rms 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
Grounding The chassis of this product is grounded through the
grounding conductor of the power cord. To avoid
electric shock, plug the power cord into a properly
wired receptacle before making any connections to the
product. A protective ground connection, by way of the
grounding conductor in the power 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.
Proper Power Cord Use only the power cord and connector specified for the
product and your operating locale.
Use only a cord that is in good condition.
Rev 2.0, 10/29/93 1-3

Proper Fuse The fuse is mounted internally and is not considered
user serviceable. The fuseholder accepts American sized
fuses (1/4 in dia.) or European sized fuses (5mm dia).
For 117 VAC operation, the correct value is 1/4A,
250VAC, fast blowing (bussman type AGC)
For 230 VAC operation, the correct value is 1/8A,
250VAC, slow blowing (Bussman type MDL or GDC.
Operating Location Do not operate this equipment under any of the
following conditions: explosive atmospheres, in wet
locations, in inclement weather, improper or unknown
AC mains voltage, or if improperly fused.
Stay Out of the Box To avoid personal injury (or worse), do not remove the
product covers or panels. Do not operate the product
without the covers and panels properly installed.
User-serviceable parts There are no user serviceable parts inside the 425. In
case of failure, refer all servicing to the factory.
Rev 2.0, 10/29/93
1-4

2. Dynamics Processing Tutorial and Functional Basics
The 425 brings together three widely-used signal processors: a downward expander, a
compressor, and a peak-limiter. It's important to note that although the 425 is three
processors in one box, it contains one-third less circuitry than there would be if each
processor was in its own box. The reason is simple: there is only one VCA (voltage-controlled
amplifier) in each channel of the 425. Smart analog computer signal processing circuitry (the
sidechain by any other name) combines the three control signals from the downward
expander, compressor, and limiter to control the one VCA. There is no compromise involved in
doing this and the signal passes through two fewer VCAs.
This part of the 425 tells how to use your 425. If you are new to signal processors, read on; the
remainder of this section is a primer on the three basic parts of the 425.
2.1 Dynamics Processing Basics
Audio signals possess several basic properties: amplitude or volume (measured in volts or dB),
frequency or pitch (measured in Hertz), duration (measured in hours:minutes:seconds) and
waveform (described graphically, like sine, square, triangle, pulse). Complex signals like
musical sounds are made up of simpler waveforms such as sine waves, mixed in the proper
proportions.
Signal processors allow you to manipulate various parameters of an audio signal. Equalizers
change the amount of amplification given to different frequencies (a perfect amplifier amplifies
all frequencies by the same amount). Dynamics processors change the dynamic range of audio
signals.
The dynamic range of an audio signal is the difference between its loudest and softest
moments. For audio equipment, this is the difference between the noise floor (residual noise
output, with no input signal) and peak clipping (the point at which the output clips or
distorts). A hypothetical black-box having a noise floor of -90 dBu and a maximum peak
output level of +24 dBu would have a dynamic range of 114 dB (+24 minus -90). Audio storage
devices like tape machines have worse limitations, a typical professional analog two-track tape
machine may have a dynamic range of 65 to 70 dB.
If you've used an analog tape recorder before, then you are already familiar with the problem of
setting recording level. Record too hot and you get distortion; record too cold and get noise in
return. Many musical instruments have dynamic ranges that exceed that of most tape
recorders. So, how do we squeeze a 80 or 90 dB signal into a 60 or 70 dB window?
The answer lies in a common audio signal processor: the compressor.
2.1.1 Compressors and Limiters
A compressor or limiter monitors the level or amplitude of a signal and reduces the amplitude
according to a rule whenever the signal level exceeds a predetermined level. The
predetermined level is known as the threshold level and is usually set by a front panel control.
The rule by which the compressor lowers the level is the compressor's compression ratio and
this parameter is also usually set via a front panel control.
Compression ratio refers to the ratio of a change at the input versus the change at the output
of the device. Thus, if we apply a signal that changes 10 dB to the input of a hypothetical
compressor, and measure a 2 dB change in the output signal, that compressor would have a
compression ratio of 10:2, or 5:1 (reduce the fraction). Different compression ratios have
different uses. Use lower ratios (6:1 or less) for level control, intermediate ratios (8:1 to 12:1)
for leveling (making the signal level more or less constant), and higher ratios for limiting
(putting an absolute ceiling on the signal level).
Limiters are nothing more than compressors, but being possessed of much higher
compression ratios (20:1 or higher). Limiters are typically used to stop occasional peaks which
2-1
Rev 2.0, 10/29/93

would have otherwise caused overload or distortion. Typically you set a limiter (via its
threshold control) so that it "stays out of the way" until a peak comes along.
2.1.2 Expanders and Gates
While a compressor or limiter reduces the dynamic range of a signal by reducing its level once
it has exceeded a threshold level, an expander does the opposite (well, almost). The easiest
way to visualize an expander is to think of it making loud signals louder. This is fine, except
that in the real world, you run into the limitations of processors and amplifiers after the
expander going into terminal overload.
The solution is to make soft signals softer, or downward expansion. This is what the 425's
downward expander does. When the signal level falls below the level set by the threshold
control, the expander reduces the gain by the amount dictated by its expansion ratio. Thus,
for a below-threshold signal, a 10 dB output change results from a 5 dB change in the input
signal, if the expansion ratio is 1:2.
A gate is similar to an expander except that its ratio is much higher; thus the action is more
like a switch once the signal falls below threshold. Some expander applications for the 425
may be quite similar to a gate, like tending a lone announce microphone, but the 425's
expander isn't well suited to typical gate applications like removing leakage from drum
microphones.
2.1.3 Ratio
The compression ratio of the 425 tells how much the output changes for a change in the
input. A linear amplifier (like a simple preamp) has a ratio of 1:1 because a change of 1 dB at
its input results in a 1 dB change at its output. A compressor alters the input/output
relationship by its compression ratio. Thus a 20:1 ratio means that a 20 dB change at the
input results in a 1 dB change at the output. In other words, a very audible change at the
input (20 dB) turns into a barely discernible change at the output (1 dB).
Compressors are not the only devices to have an input/output ratio. Any device that is
capable of changing the input/output relationship can be said to have a ratio. Thus
expanders, gates, compressors and limiters all fit this category.
An expander maginifies output changes for a given input change. Thus, once the input signal
falls below threshold, the expander changes the output by the amount of the ratio. The 425's
downward expander has an expansion ratio of 1:2, which means that an input signal that gets
5 dB quieter turns into a 10 dB quieter output change.
A gate can be looked at as an expander with an infinite expansion ratio. Thus the slightest
change in the input signal, above threshold, turns the gate full-on.
2.1.4 Gain vs Output
The gain control allows compensating for signal level lost to compression. As an example, try
setting the 425 for a 4:1 ratio. Now adjust the THRESHOLD control for 10 dB of gain reduction
as read on the compressor's gain reduction display. The output level should be significantly
lower than what it was. You supply the additional gain (make-up gain) by adjusting the GAIN
control until the input and output signal levels match.
2.1.5 Attack Time
The attack time represents the amount of time that a compressor (or limiter, or expander, or
gate) needs to react to an input change. You might ask, why not just make it lightning fast?
Because you may sometimes want to let occasional peaks through, which helps maintain the
dynamic characteristics of the input signal. Usually, what we're after is control of the input
signal, not total homogenization.
Rev 2.0, 10/29/93
2-2

2.1.6 Release Time
Most dynamics processing equipment has a knob marked release on the panel. This refers to
release time, and affects the length of time required for the gain to recover to the no-signal
state.
For compressors, the no-signal point is unity gain and this applies to any signal whose level is
below threshold. For expanders the no-signal point is the gain reduction set by the threshold
control under no-signal conditions and this applies to any signal whose level is above
threshold. In this case, the release time control governs how long it takes for the expander to
reduce the gain when the signal disappears.
The release time control allows tailoring the compressor's recovery time to the program
material. Generally, peak limiting is associated with short release times and compression or
leveling associated with longer release times.
The 425's compressor release circuitry has a unique dual-release time feature designed to
make life easier. The dual-release feature makes the release time partially program dependent,
giving you the best of both worlds: fast release for short-duration peaks and a longer release
time for longer-duration peaks. The release time control adjusts the speed of the longer of the
two.
2.1.7 Threshold Setting
The threshold control sets the audio signal level where the compressor/expander/limiter
begins working. In the case of the compressor or limiter, the processor begins working once
the signal has exceeded the threshold level. For the expander, it begins working once the
signal has fallen below the threshold level.
For any of the three processors, the threshold control setting also determines the degree or
amount of gain reduction. Thus, for the compressor, rotating the control counter-clockwise
(towards -40) results in increasing amounts of compression. For the expander, counter-
clockwise rotation raises the level that the signal must exceed to pass through the expander
untouched. This has the effect of "shutting off" the signal once you reach and then pass the
threshold level.
For most compressor applications, moderate amounts of gain reduction are all that is
required, 3-9 dB at the most. If you are using the compressor to minimize level changes of a
wide range of program material (automatic level control), then higher amounts of gain
reduction are needed; the amount of gain reduction corresponds to the range of change that
you can respond to (slower release times are also indicated, too).
2.1.8 Interpreting the Displays
The 425 has many displays; one per processing section. The three displays associated with the
expander, compressor and limiter indicate a parameter called gain reduction. Simply stated,
the gain reduction indication shows how far the gain or amplification was reduced from unity.
Another way of looking at this is: if the gain reduction display says 10 dB of gain reduction,
switching the unit to bypass will result in a 10 dB increase in the output level.
The output display indicates output level, in VU (0 VU = +4 dBm = 1.23V RMS measured at
the balanced outputs). For most applications, just make certain that you never see the CLIP
LED illuminate.
2.2 Using the Sidechain
The sidechain is a patch point in the control circuit of a dynamic range processor, which
provides access to the part of the circuitry that tells the VCA what to do. The 425's sidechain
is routed through a TRS jack located on the rear panel that provides both a send and return
via the same jack. The sidechain connection affects all three processors in the 425.
2-3
Rev 2.0, 10/29/93