dbx 163, 164 User manual

dbx
Models 163 and 164
Instruction Manual

WARNING
This triangle,
which appears on your
component, alerts you to
the presence of uninsulated
dangerous voltage inside
the enclosure ~voltage
)that may be sufficient to
constitute arisk of shock.
)PREVENT FIRE OR SHOCK HAZARD,
DO NOT EXPOSE THIS COMPONENT
TO RAIN OR MOISTURE.
/* \CAUTION
;«SK0F&GCnKSHOCK
VoDMororei
.
CAUTION
To Further Reduce the Risk
of Shock, Do Not Remove
the Cover or Back. There Are
No User-Serviceable Parts
Inside; Refer All Servicing
to Qualified Personnel.
T-his triangle
also appears on your
component, and it alerts
you to important operating
and mcuntenance instruc-
tions in the accompanying
literature.

INSPECTION AND INSTALLATTDW
Ycvt unit W3.S carefully packed at the factory in acarton designed to
protect it. Nevertheless, we recommend examining both carton and contents for
any signs of damage that may have occurred during shipping. If there is
evidence of damage, don*t destroy the carton or any of the packing material,
and notify your dbx dealer immediately.
In any case it is agood idea to save the carton and packing materials
should you ever need to ship your unit in the future.
In addition to a unit and this instruction manual, the carton should con-
tain hookup cables with RCA phono (pin) plugs and awarranty/ registration
card.
"dbx” and "Over Easy" are registered trademarks of dbx Inc., Newton, Mass., USA

CONTENTS
Introduction 1
More on features and performance 3
Applications 4
Mono or stereo? 5
Other dbx compressor/ limiters 5
The compressor as tool 6
Performance curves facing page
Brief operating instructions 7
Installation 8
Signal connections 9
Input &output impedances and terminations ..9-10
Operation 10
Compressor/limiters: areview 12
More applications 13
Specifications 17
Schematics 19-20
Warranty and factory service 21
Important information for users in the UK ....22

INTRODUCTION
The dbx Model 163 and 164 compressor/ limiter feature dbx's Over Easy
compression. Over Easy refers to a"soft-knee" compression characteristic
that provides virtually inaudible gain control at ratios up to infinite
compression. Other unique dbx circuit designs such as true-rms-level
detection, feedforward gain reduction, and automatically variable attack
and release times provide the 163 with audibly superior performance.
The Model 164 is astereo-tracking version of the 163. working with a
pair of channels as opposed to the single (mono) channel of the 163 In all
other respects the 163 and 164 are identical, and throughout this manual we
make reference only to the 163. for the sake of convenience. The 164
accomplishes its stereo tracking by feeding the signal in each channel to a
detector, which computes the true-rms sum of the two channel
fnThlr. forward to activate the gain-change circuitry
for both channels, ensuring an accurate and constant image.
Anyone who has ever tried to fine-tune astudio-type compressor/limiter
wxll appreciate the Model 163 's ease of operation. Asingle front-panel
COMPRESSION knob simultaneously adjusts the threshold of compression and the
overall system gain, thereby maintaining essentially constant output level
Just slide the knob to get any desired amount of compression. This ability to
H’’Tr, ^program- with asingle control is the reason
cne Model 163 nas become known as "THE ONE-KNOB SQUEEZER." (While the 163 is
factory-set for nominal -10 dB line levels, the rear-panel controls enable the
user to select any convenient nominal level from -20 to +10 dB. with 0dB =
0.775 Vrms.)

2
The 163 features proprietary dbx true-rms detection. Most
compressor /limiters use some form of peak detection, with fast-response
characteristics that can have adisturbing effect on program material.
level detection closely simulates the response of the human ear.
Even at 15 to 20 dB of compression, the gain-changing action of the Model 163
is highly listenable and natural-sounding.
Because of its feedforward topology, the dbx 163 compressor/limiter can
achieve effectively infinite compression with complete stability and low
distortion. The dbx approach is quite unlike gain reduction in aconventional
compressor/limiter. Traditional gain reduction is accomplished by sensing
signal level at the device's output and applying acorrection signal via a
feedback loop. At progressively higher compression ratios, the feedback loop's
gain reduction increases, distortion increases, and eventually instability or
oscillation occurs. To avoid this problem, many conventional units restrict the
maximum amount of feedback, thus restricting the maximum compression to some
lower ratio, such as 10:1 or 20:1. Free of the instability of excessive loop
gain, the dbx 163 therefore can provide effectively infinite compression.
In addition to increasing both the stability and the available range of
compression, dbx's feedforward circuitry and rms-level detector combine to make
it possible for the attack time to "track" the rire of the signal envelope. In
conventional compressor/ limiters ,the attack time depends on feedback-loop gain,
which means it must be continually readjusted for best results at different
compression ratios. The operation of the 163 is greatly simplified because the
attack time is controlled solely by the rms detector and varies automatically
with the amount of level change in the program material (the shape of the envelope)

3
No manual attack-time adjustments are required. The release rate —not time —
IS fixed by the detector characteristics at asmooth-sounding 120 dB/second.
These unique circuit characteristics preserve the natural quality of any given
sound
.
The 163 's most significant advance in preserving natural sound quality is
provided by acontinuously variable compression ratio which increases gradually
from 1:1 toward infinity as the input level rises above the compression threshold.
With aconventional hard-knee compression curve, one typically hears normal
program dynamics until the input level rises above threshold; then the gain is
suddenly reduced by afixed ratio and, depending on the nature of the program,
the listener senses a"holding back." With the Model 163 ’s Over Easy characteris-
tics, the ratio of gain reduction increases gradually as the input rises farther
above threshold. Thus there is no sudden change and no feeling of holding back,
just anatural-sounding program that avoids undesirably high output levels.
More on Features &Performance
The Model 163 has a12-LED display that indicates the instantaneous amount
of gain change from 2dB to 30 dB. Input and output connections are made via
unbalanced RCA phono jacks. Maximum rated input level is +17 dB (5.5 V), although
the unit will- accept inputs of up to +24 dB (12.3 V) without hard clipping
(a moderate rise in distortion occurs as the input is overdriven) .Maximum output
level is +18 dB (6.2 V) into 2k-ohm or higher impedance loads.
The standard Model 163 comes with walnut sides, aluminum front and back
panels, and vinyl-clad steel top and bottom panels. The unit is extremely compact,
measuring just 9" wide, 1-7/8" high, and 6-7/16" deep and weighirig 2% pounds.

4
APPLICATIONS
dbx noise-reduction systems now make it possible to process programs
with 100 dB or greater dynamic range. In some applications, however, it is
necessary to curtail dynamic range. While dbx noise-reduction systems are used
in the studio and in record production to maintain wide dynamic range, this
range often must be restricted to optimize the- sound for broadcast and home
playback. The restriction, of dynamic range is usually done with acompressor
in the final stages of recording or record mastering. In the broadcast field,
where many radio stations compete for an audience, it is felt that ahigh average
audio level can make abig difference to ratings. Compression is used to attain
high average levels. However, unless ahigh-quality compressor/limiter is used,
distortion and unwanted audible side effects may degrade the sound so badly that
the advantages of high average level are overcome and audience share is lost.
The model 163 does an excellent job of reducing dynamic range in both recording
and broadcast applications.
The dynamic range of alive musical performance can be even greater than
100 dB. Ahigh-quality compressor/limiter, used judiciously, can restrict the
dynamic range, raise the average level, help the operator avoid thermal damage
to speakers, and improve the sound quality overall. Because the 163 sounds so
natural, it won't displease performers, and thus is especially suited to sound-
reinforcement applications. Compressor/ limiters have been used for years to
provide special "studio" effects with drums and bass guitar as well as to control
"wandering" vocal levels. The 163 serves as an ideal multi-purpose performer in
the small studio. See page 13 for more application ideas.

5
Mono or Stereo?
The 163 uses precision or matched components in critical circuits, where
they make adifference to performance. In non-critical circuits, normal-tolerance
components that yield excellent audio quality are used. If one intended to make
acompressor/limiter for stereo operation without image shifts, one would have to
use closely matched or high- tolerance components throughout, which would greatly
increase the cost of the unit without providing any added benefits for mono
operation.
For those applications where stereo operation is necessary and the
performance of the 163 is desired, dbx has designed the companion Model 164, as
noted. It is atrue stereo-tracking version of the 163, featuring matched rms
detectors in each channel. This "dual-detector" system provides the 164 with
total freedom from the errors experienced by cotiventional "stereo-strapped"
compressor/limiters when faced with phase error between left and right channels.
Other dbx Compressor/Limiters
dbx also offers the Model 160X single-channel compressor/ limiter ,with an
actively balanced input and unbalanced output for use in systems requiring higher
signal levels and output-drive capability, as well as the top-of-the-line 165A.
The 165A lets the user control attack and release parameters, and has aseparate
PeakStop peak clipper to stop transients absolutely at auser-determined level.
These units also feature dbx's rms-detection and feedforward circuitry and
Over Easy characteristics.

The Compressor as Tool
PropexLy set up, a. compressor is auseful device. On. the facing page
are anumber of comparison curves for Over Easy and conventional compression
settings, giving input vs. output levels (gain-change characteristics).
Note what happens as one approaches infinite compression —the output is
gently but absolutely limited. Above the block diagram at the lower right
are compression curves for the dbx model 160X, including its negative-
compression characteristic.
With a2:1 compression ratio, a2dB increase in input level (above
threshold) causes the output to rise only 1dB. With a4:1 ratio, a4dB increase
at the input results in a1dB increase at the output. With a10:1 ratio, a10 dB
increase at the input results in a1dB increase at the output; and so forth.
Therefore, the amount of musical alteration depends on the compression ratio used
as well as the threshold setting. Because the Model 163 gradually increases its
compression ratio as the signal level increases above threshold, musical alteration
is inoffensive, even when the threshold is lov/ and the maximum amount of compression
quite high.
Acompressor/ limiter ,like any tool, can be misused. In fact, many people
refuse to use compression even when it would improve the audio quality because
they have heard compression being misused all too often. One of the nicer aspects
of the dbx 163 is that it*s easy to use and difficult to abuse.

BRIEF OPERATING INSTRUCTIONS (see facing page)
LED DISPLAY. These 12 light-emitting diodes display the amount of
instantaneous gain range (dB of compression). The first 6LEDs are calibrated
to display 2dB steps from -2 dB to -12 dB of gain change. The other 6LEDs
are calibrated to display 3dB steps from -15 dB to -30 dB gain change. The
more gain change, the more LEDs turn ON. Note that while the display has a
maximum range of 30 dB, much more gain reduction is available.
COMPRESSION. This control varies the threshold of compression from
+4 dB (1.23 Vrms) to -36 dB (12 mV rms) when moved from left to right (tov?ard
"MORE"). As the threshold is lowered, the overall gain of the system is increased,
to maintain aconstant nominal output level. The compression characteristic is the
same at any threshold setting., gradually increasing from 1:1 to effectively
infinite (126:1) compression as the input level increases above the threshold.
The dynamics of signals below threshold remain unaltered. Refer to page 6ff. for
the Over Easy compression characteristic. (The unit can be considered to be
limiting when the signal level is high enough above threshold to incur compression
ratios of 10:1 or higher.)
NOMINAL OPERATING LEVEL. This recessed, screwdriver-adjustable control and
adjacent switch set the nominal operating level of the Model 163. Two fixed
levels may be switch-selected, -10 dB and +4 dB. In addition, the switch may be
set at "variable" and the adjacent control then provides alevel-adjustment range
of from -20 to +10 dB. Most professional installations will require a+4 dB
setting; semi-pro or hi-fi equipment may operate better at settings of -10 to
-20 dB. To calibrate this setting for the system in which the 163 is being used.

8
run asignal through the 163 at standard operating level and slide the 163 *s
COMPRESSION control back and forth between 1/4 and 3/4 of maximum while
monitoring the output over speakers or headphones .The NOMINAL OPERATING LEVEL
setting is correct when no apparent change in the output level takes place as
the COMPRESSION knob is moved back and forth.
INSTALLATION
The standard package has side panels of oiled solid walnut. Adhesive
rubber feet are included and may be attached if desired.
Packaging is available so that one or two Model 163s may be rack-mounted
in asingle 1-3/4” panel space. Under these options, dbx models RM-18-1 (for a
single unit) and RM-18-2 (for two units), the wood panels are removed and brackets
are provided for installation in astandard 19” equipment rack.
In any placement of the Model 163, standing alone or rack-mounted, avoid
placing the unit above any heat source or strong electromagnetic fields, such as
those generated by power amplifiers or vacuum-tube equipment.
AC POWER. In the US, connect the Model 163 to a117 Vac, 50 or 60 Hz power
source only. For elsewhere, the correct information is printed on the rear panel
next to the power cable’s entry hole. The unit may be damaged if it’s not
connected to the proper power source. If your unit has aswitch for two different
power sources, be sure it is set correctly.

9
SIGNAL CONNECTIONS. The 163 *s inputs and outputs are unbalanced, and
terminate in RCA phono (pin) jacks. Make the signal connections as shown in
the illustrations on the opposite page, depending on your circuit *s characteristics
(To use a164 for mono operation, employ aY-plug to divide the signal
and send it to both 164 inputs, then connect either of the 164 *s outputs to
your terminating unit. We suggest that you connect the signal to both inputs
so that the level-detecting circuitry, which expects information in both
channels, isn’t fooled into inaccurate calibration by its absence in one. Note
that you must never connect two different mono program sources to the 164 *s two
channels; doing so will cause the circuitry to read both signals as one stereo
program and create unwanted interactions of compression in the two programs .
)
INPUT IMPEDANCE &TERMINATIONS. There is sometimes amisunderstanding
about the nature of matching and bridging inputs ,the use of terminating
resistors, and the relationship between actual input impedance and nominal
source impedance. Most electronic outputs work well when "terminated" by an
input (connected to an input) having the same or higher actual impedance.
Outputs are usually overloaded when terminated by an impedance that is lower
than the source impedance. When the input impedance is nearly the same impedance
as the source, it is known as a "matching" input. When an input is 10 times the
source impedance or more, the input is considered to be a"bridging” input.
The dbx Model 163 has an actual input impedance of 25,000 ohms (a ”high-Z"
input, "Z" being the symbol for impedance). This makes the dbx input suitable
for use with virtually any nominal source impedance, low or high. The dbx input
will bridge 150-ohm or 600-ohm (low-Z) lines and will match 10,000-ohm or greater

Connecting the 163 in aBalanced Circuit

10
impedance (high-Z) lines. Terminating resistors are not needed for the dbx
unit to operate correctly; however, they may be called for to satisfy the output
requirements of the preceding output. Consult that unit’s instruction manual.
OUTPUT IMPEDANCE &TERMINATION. The 163 ’s output jacks will properly feed
inputs of any device whose input impedance is 2k-ohms or greater. Note that
connection to true 600-ohm lines will cause loss of bass response in the 163
.
Some devices labeled as having 600-ohm operation can be used with the 163 if their
actual input impedance is at least 2k-ohms .The 163 outputs will properly feed
the high-Z inputs of mixers, preamps, power amps, and/or tape recorders, but if
you are in doubt about the true load impedance, consult the specifications (or the
manufacturer) of the unit you are feeding.
OPERATION
COMPRESSION ADJUSTMENT. Set COMPRESSION at minimum (slider all the way
left). Apply 'average-level program material to the input. The LED display
should be off. (With sufficiently high input levels, the LED display will light
up regardless of the COMPRESSION slider’s position.) Increase the COMPRESSION
by moving the knob to the right (toward MORE) until the first one or two LEDs
begin to flicker on program peaks. At this setting, no more than afew dB of
compression are obtained, and the ratio remains low. Further advancement of
the COMPRESSION control toward the right does two things: (1) it starts
compression at alower point relative to the maximum input level, and (2) it
applies more compression to the higher -level portions of the input. The
instantanous compression (in dB) is indicated by the LED display; more LEDs
ON means that more of the program is being compressed. It is suggested that

11
the unit be set for only as much compression as required.
NOMINAL-OPERATING-LEVEL ADJUSTMENT. The Model 163 comes set for unity
gain at the nominal operating level of -10 dB, meaning a-10 dB input will
appear as a-10 dB output when the COMPRESSION control is pushed past the
1/4 point. As the COMPRESSION is increased, the gain of the system is increased
by acorresponding amount, decibel for decibel, so that the average output level
remains the same —-10 dB. If it is desirable to change the nominal output
level, this can be done with "the rear-panel NOMINAL OPERATING LEVEL switch and
control
.
The Model 163 *s Over Easy compression is very forgiving of peak overshoots,
and audible distortion is difficult to create with the unit. However, audio
signals often have peaks that are 20 dB. above VU-meter readings (VU meters indicate
average levels). Even when compressed at a2:1 ratio, such peaks can still reach
10 dB above VU- indicated levels. To avoid distortion in the most critical
applications, use an average input level that is well below the peak +24 dB input
level (i.e., average between -20 and +8 dB) .
LED -DISPLAY INTERPRETATION. If the LEDs are not illuminated ,_ the input
program is below the threshold of compression, so little or no dynamic alteration
is occurring. As the input program level approaches the compression threshold,
one or more LEDs will turn ON: the higher above threshold, the more compression
and the more LEDs turn ON. The actual amount of compression can be read out
directly, as labeled above the LEDs. (The first 6LEDs each represent 2dB of
compression, and the next 6represent 3dB each.) When the entire LED display
is ON, the unit is generating 30 dB or more of compression.

COMPRESSOR/LIMITERS: AREVIEW
COMPRESSOR: Avariable-gain amplifier whose gain decreases as the
signal at its input increases through aset threshold.
LIMITER: Acompressor with ahigh compression ratio; the high ratio
maintains essentially constant output level despite any increase in input
level above the threshold.
OVER EASY COMPRESSOR/LIMITER: Acompressor in which the compression
ratio increases as the input signal rises through the set threshold. Thus,
for signals hear the threshold, the unit functions as alow -ratio compressor,
gently reducing the amplifier gain. For signals well above threshold, the
unit functions as alimiter, substantially reducing gain and preventing the
output level from rising regardless of further increases in input level.
Because the Over Easy compression "comes on" gradually, limiting is far less
audible than with conventional compressor/ limiters
.
COMPRESSION RATIO: The ratio, in dB, of input-level changes (above
threshold) to output- level change. Acompressor whose output level changes
1dB for a2dB input-level change has a2:1 compression ratio.
THRESHOLD: The level at which compression begins. In dbx's Over Easy
compressors, we define the threshold as the level at which the compressor
generates 6dB of gain reduction. The dbx Model 163 compressor/ limiter has
an adjustable threshold (set by the COMPRESSION control) .If the input level
is above the set threshold, the unit functions as acompressor or as alimiter,
depending on how far above threshold the input-signal level rises.

13
MORE APPLICATIONS
CHANNEL PATCHED. By placing the 163 in the "pre-fader loop" or between
the "patch points" of achannel in an audio console, aspecific instrument or
vocal can be processed.
Vocals
.
When asinger or speaker has atendency to move closer to and
farther away from the microphone, the 163 can smooth out the level variations
which would otherwise occur. Large amounts of compression on alead vocal will
cause it to cut through the mix and give it agreater feeling of intimacy due
to the increased gain provided for subtle vocal inflections
.
Instruments
.
The 163 can be used to even out an instrumental level. For
example, an electric bass will frequently have dead spots, which produce softer
notes than those in other places on the finger board. The 163 will smooth out
the level variations due to either the nature of the bass or the action of the
musician.
In general, larger amounts of compression produce afatter sound. This
is typically used to increase punch or kick drums or to alter the wash of a
cymbal for increased impact.
MIX PATCHED. By placing a163 in the submaster or "group" points of an
audio console, entire groups of instrimients or vocals can be processed. This
provides the mixer with greater control over the level, for example, of ahorn
section relative to other parts of the mix. This is important because there