dbx

Model 166

Professional
Dynamics Processor

ATTENTION
IF YOU BEGIN WITHOU1 READING TH IS
MANUAL, YOUR NEW 166 MAY SEEM TO BE NOT
WORKING PROPERLY.

NO AUDIO
If you're not getting any audio out of the 166, check
the GATE knob. Is the Threshold off? If not, turn it off
(full counterclockwise).

If the Threshold isn't off, the circuit will attenuate

the signal 40 dB, which will stop the audio from
passing. Note that signals BELOW the Gate threshold
are gated; signals ABOVE the compression threshold
are compressed. If it's vice versa, nothing happens.
Refer to the manual.

NO COMPRESSION
Did you plug into the Sidechain input? Take it out.
Patch in ONLY when you have sidechain applications
(again, refer to the manual). Also check to make sure
the Sidechain Monitor button isn't pushed in.
 INSPECTION and INSTALLATION
Your unit was carefully packed at the factory in a protective carton. Nonetheless,
be sure to examine the unit and the carton for any signs of damage that may have
occurred during shipping. If there is such evidence, don't destroy the carton or packing
material, and notify your dealer immediately.

It's a good idea in any case to save the carton and packing should you ever need to
ship the unit.

In the event of initial problems, first contact your dealer; your unit was thoroughly
inspected and tested at the factory.

In addition to a model 166 and this owner's manual, the carton should contain a war-
ranty /registration card. Please fill it out and send it to us.

The chassis has integral brackets (rack "ears") for mounting into a standard equip-
ment rack (19" or 48.3 cm wide). No special cooling or ventilation is required in any
installation; other components may be stacked above or below the unit provided they
don't generate excessive heat.

WARNING

TO PRE VENT FIRE OR SHOCK HAZARD,

DO NOT EXPOSE THIS COMPONENT
TO RAIN OR MOISTURE.

This triangle, This triangle

which appears on your CAUTION also appears on your
component, alerts you to llllt CFEUCnlCINIICI( component, and it alerts
IIOIIOTCIIBI
the presence of uninsulated you to important operating
dangerous voltage inside and maintenance instruc-
the enclosure - voltage tions in this accompanying
that may be sufficient to literature.
constitute a risk of shock.

CAUTION
To Reduce Further the Risk
of Shock, Do Not Remove
the Cover or Back. There Are
No User-Serviceable Parts
Inside; Refer All Servicing
to Qualified Personnel.

"dbx," "OverEasy," and "PeakStop" are all registered trademarks of

dbx Professional Products. San Leandro CA.
CONTENTS
Front panel (operation). • • • • • • • • • • • • • • • • • • • • • • • • • • • • • 2
Rear panel (connections). • • • • • • • • • • • • • • • • • • • • • • • • • • • • 2
Typical hookups (plug and cable wiring) • • • • • • • • • • • • • • • • • • 4
Application notes (gating, compression, limiting, sidechain). • • • 7
Block diagram . • . • . . • . . . . . . . . . • • . • . . . . • • • • • • . . . . . . 13
Schematic • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • 14
Warranty and factory service. • • • • • • • • • • • • • • • • • • • • • • • • • 15

SPECIFICATIONS
Frequency response
20 Hz-20 kHz +0.5 dB
THO (total harmonic distortion)
0.2% at maximum
compression, 1 kHz, 0 dBv
Equivalent input noise
-85 dBv unweighted
Maximum input
+24 dBv
.Maximum output
+21 dBv
Input impedance
25 k-ohms differential,
18.5 k-ohms unbalanced
Detector: 6.8 k-ohms, unbalanced
Output impedance
Low, single-ended, for driving 600 ohms or greater
Output gain
-20 to+ 20 dB
Threshold range
Compressor: -40 to +20 dBv
Gate: +10 to -60 dBv
PeakStop: 0 to +21 dBv
Attack times
Compressor (program-dependent):
15 ms for 10 dB,
5 ms for 20 dB,
3 ms for 30 dB
Gate:
2 ms for 28 dB {70% of return to unity gain)
Release times
Compressor:
8 ms for l dB,
80 ms for 10 dB,
400 ms for 50 dB (125 dB/s rate)
Gate, slow: l 00 ms for l dB
fast: l 00 ms for l 00 dB
Maximum compression
Notes
Greater
. than 60 dB I) Specifications are subject to change •
Power requirements 2) All voltages are rms (root-mean-square).
3) 0 dBv is defined as 0.775 V regardless of load impedance. Subtract 2.2
90-135 V ( 120V model), from the dBv figure to convert to dBV (i.e., referred to I V). When the
200-260 V (240V model), load impedance is 600 ohms, this particular dBv is also known as "dBm."
4) Noise figures are for 20 Hz-20 kHz.
50-60 Hz; 15 W 5) Measured in the infinite-compression region of the dbx OverEasy curve,
attack time is the time required to reduce the signal by 63 % of the level
Dimensions increase above threshold, while release time is the time required to
l-3/4"h x l 9"w x 8"d restore gain to 90% of the level decrease below threshold.
 REAR, Channel 2 (1 is identical)
.
IN_ ._.
AU.
.,._. , • NC)
foll-~D
_._
CIIIICUfT

-,.,o,. .- u.,._
::11.-...- +:.@
011 -

MIOM Ol"Tt4

@
~
.. """"'
~HAIN

@....:
t +:.@ @
1
""""' -@....:t
......... dbx·
-·- 7
0
- ~-
..,11>4
---- ..-..-
a.'79a.-
,-
-
a,~
..-,,

··--,
"""·...........
,_,'
? - -
.. •.,...,
OUT IN *- "' OUT ON -~·
o...,.._•_.,,·-COio•
--~"""° --

1
cb 11

1 AUDIO:IN, OUT. These two 3-circuit phone jacks are the Input and Output. As
marked, these jacks are the standard tip/plus, ring/minus, and sleeve/ground.

2 SIDECHAIN:IN. This jack provides a direct input (optional) to the rms-detector

circuitry. It is used for connecting a signal processor for altering the dynamic
response of the 166. An example would be an equalizer to make the compression or
the gating frequency-sensitive (see Sidechain Monitor, below). This input uses a
2-circuit phone jack: tip is plus, sleeve ground.

FRONT, Channel l (2 is identical)

0 1 r- OA Te:--,
~~' qc"°!
r-: OVER ~y I dbX PCN<STOP
i OUTPUT
i 60
,--
.. CU.TE.-, r;
=6-':C:'N:"°!
0\/VI £A5Y -,
188
PCAKST OP OUTPUT
i • 0

6
0 • 0~ 6GAIN O 0~ 0 6GAIN
... -•• ~ ~ ••.l't\.·~·
rr==,i O V. GAIN A[DUCT ION
·.l't\.·· ·••
u rr--,J
..•
ll"""'\l
~
IF"il
- -•• - ~
r;===,i O
··~ ·~·
(>AON AU.J C:T OON
-~ I ..•
··11"""'11 ..•
IF""il
OW •10 tb::::::il •:80 t.1 CG1 0 C) 0 0 0 0 0 0 0 IIO 1.--=1 •IO •IO ~ 6===!J Of" •10 lb::::::::B ...0 40 1,1 • 0 0 0 0 0 0 0 0 0 ~ .. ... l6=!J
~ ,o.,o .._ ~ O... ,..,.tO » - 1S 10
a 6 6 t I •
~
•
1!11
"'9.ASS S'Tt:Re-0 ~ 0.0 M. ,...... Ch.0 ""' TtO :» - I S 10
• 6 • t I •
~
•
DVPA.$5

0 • M ff - ~ 'TOIi - • Mff • ••»• ITC.

0
7

3 GATE:THRESHOLD and RELease RATE:FAST, SLOW. The 166 gate is a below-

threshold attenuator with two release rates. The Threshold knob sets the level
below which the 166 gates - attenuates - the signal. The LED lights whenever this
takes place. As marked, the threshold ranges from +10 to below -60 dB; Off, of
course, inactivates the gate. The amount of attenuation is set at 40 dB.

The RELease RA TE switch controls how fast the signal gets attenuated. In the
Slow position (in), the rate is useful for general-purpose gating of noise behind
vocals and acoustic instruments - about 10 dB/s. In the Fast position, the rate is
very fast (1000 dB/ s), use ful for tightening up the sound of percussion (e.g., kick or
snare drum) and drying up leakage from other instruments into percussion tracks.

The attack rate of the gate (which controls how fast the signal is restored after
being attenuated) is internally set to be very fast - fast enough to allow all of the
transient at the beginning of a note to come through.
...
4 OVEREASY COMPRESSOR:THRESHOLD ;B-
and RA TIO. The Threshold knob sets the 0::s
f
- I/ l,/
L,,

ltl

• I

point at whic h dbx's well-known Over- ~/

Easy c ir c uit begins to compress the dyna- /

mic range of the signal. The calibration
number s refer to the middle of the Over- /
Easy curve (see right); at an Infinity:! /
ratio, they denote the point where there' s
appro~imately 6 dB of compression. The I .!.
Over Easy sound is musical and unobtru-
sive, without the "held-back" feeling of
H- ......
.._.
I

other compressor s. / _,_,

~ 1"' .'
-2- I/
Input
0 1 ,-- GA
TC--, r ,J;,.VO;t;;,.-, db ~OP ~ i- GAff-, •c~Al5"~ -, ~JJ_OP ~~ a 0
-~ ~-
± 1_, ,. I
../1"\_. ~ ...
A ~..~t -if.( I -,. I
0
-if.( •.~·· .•. X .~ ·• • ,: ·~·· .•. 188 °.~ ·• .• ./1"\_.,.
\:_J
°" . -v- • 10
fi"""'il
u.-., - - "' 1
°" IN ,-,cT
o o o o o o o o1
IQN
o
- \:.J
'====!I -. ...
11'"9
~
r==il
~
\:_/--V
ow . ,0
IF",!
~ --.:i ••
' , .,
GAIN
O O O O o O o o
, o
rr==i1 \:.J fi"""9l
AEDUC TI ON
lbb -ao •• "===!)
-,,;;=:.410 ~ e'G'JI MT IO
• • ,a ,.- • • a •
~
• 9Y'PASS m o T .........O
•
-. WZ 01..0 "" T"° » - ,a ·~ · 4
a 1
• •oOCCI-¥>--
"°"'TOI
• 8'1'1) A 5S

0
ft -ft •
0

The Ratio knob controls how much the signal will be compressed once it's well
above the threshold, in the straight-line section of the OverEasy curve. The ratio
is the change in input level divided by the change in output level, e.g., a 4: 1 setting
means that for a 4-dB increase in input level the output will increase by only 1 dB.
At Infinity:!, the output will remain at a constant level irrespective of input dyna-
mics (as long as the input is above threshold) - in other words, full limiting.

5 GAIN REDUCTION LEDs. These show how much the signal is being attenuated by
the Gate, the Compressor, or both.

6 PEAKSTOP LEVEL knob. This sets the maximum peak-output level. The control is
calibrated to the rms value of a sine wave just being clipped. The LED lights when-
ever this clipping - soft at the start, becoming hard as necessary for absolute
protection - occurs. See the note on page 9.

This is the last circuit the signal goes through, so it always controls the maxi-
mum output regardless of any other control - including Output Gain.

7 SIDECHAIN MONITOR switch and LED. Pushing this in connects the Sidechain
input directly to the Audio Out, for monitoring the sidechain signal during setup.
Be careful not to push this button inadvertently - it affects what you (and every-
body else) will hear, or what's going to a console/mixer or tape recorder. Note
that PeakStop is in the sidechain signal path when Sidechain Monitor is sele cted.

8 OUTPUT ·GAIN. This knob controls the overall gain of the 166, from -20 t o
+20 dB. It is independent of all other controls although, as mentioned, it does
come before the PeakStop circuit.

9 BYPASS switch and LED. Pushing this in provides a hardwire bypass for the 166,
connecting input to output (fully balanced if so wired) even in the absen ce of ac
power.

10 STEREO COUPLE switch and LED. Pushing this in turns the 166 from a dual-mono
unit with two identical, independent sets of controls into a stereo unit . Except for
Sidechain Monitor and Bypass, Channel 2's controls are overri den by Channe l 1' s in
the Stereo Couple mode. Note that the detection ci rcuitry senses the true rm s
levels of the combined signal, so it is unaffected by pha se shift s (or oth er disc re-
pancies) between the channels. Among other benefits, this enables stereo compres -
sion without loss of imaging stability.

11 VOLTAGE SWITCH. This must be properly set for your AC voltage. be sure to check
before plugging in and powering up. For 220V operation you will need a suitable
adap tor plug.

-3-
TYPICAL HOOKUPS
All 166 connections are made through the rear phone jacks; our figures in this sec-
tion show the wiring of plugs and cables to make these connections. Again, tip is plus (or
high, or hot), ring is minus (or low), and sleeve is ground or shield (or earth).

Audio inputs and balanced and unbalanced sources

Your unit's two signal inputs are balanced electronically with differential ampli-
fiers. They won't unbalance a balanced source, and they may be used with unbalanced
sources as well. Here's a general-purpose diagram:
11'FUT OUTPUT
(±) SHIELD (±)
~->-++---H--<>-~+ +~-o---1--4----1-+---<>-_;::_~
/"E-----<1--\_-t_l-_-_-_-_-~~
.....
-1--<>--1 166

e / ---0---l,__•
__ •~-o-.....J -7Je
~ CCN-ECT IF CCN-ECT IF /?I
'of HM DEVELOPS HM DEVELOPS ~

--->
SIGNAL FLOW
For maximum hum rejection, avoid common grounding at the l 66's input and ouput.
The best starting point is to ground the shield of the input cable and the source device
(leaving it unconnected to the 166) and to ground the shield of the output cable to the
ground of the 166 (leaving it unconnected at the receiving device).

A balanced line is defined as two-conductor shielded cable with each of the two
center conductors carrying the signal but of opposite polarity and equal but opposite
potential difference from ground. An unbalanced line is a single-conductor shielded
cable with the center conductor carrying the signal and the shield at ground potential.

Figure 1 shows the connection of balanced signal sources to either audio input
jack. Note that for this connection a 3-circuit phone plug is necessary, as is dual-
conductor shielded cable. Sometimes the plug is called a "stereo" phone plug because it's
often used for such stereo circuits as headphones.

+ -
vT s R T
TO 166
y •
vR
s n II AIX>IOI I\FUT
JACK
...r: ----
CCN-£ CT SHIELD
AT THIS~ IF
tf..M DEVEL.OPS

Figure 1: Balanced sources

Figures 2a and 2b show an unbalanced source connected to these inputs. In 2a, a

3-circuit phone plug is used (and the ring is connected to the shield)., while in 2b a 2-cir-
cuit ("mono") phone plug is used. Since this plug has a continuous sleeve (where the 3-cir-
cuit plug has both sleeve and ring), the plug automatically shorts the sleeve to the ring at
the 166 input.
+ -+-+---- ----- --v,..._T~--1
vR
S R
n II
T
TO 166
Al.l>IO 11\FUT
...1..._--4Y,__
_____ _.1__ .c.·-----1 JAO(
s

Figure 2a: Unbalanced sources, dual-conductor shielded cable

+ -1---+-- --------....1',..;T:,__---I s T
TO 166
Al.l>IO 11\FUT
JAO(
..L----------Llo....-------1 s

Figure 2b: Same, single-conductor cable

-4-
 In situations with high radio-frequency interference (RFI) but unbalanced sources,
wiring as shown in 3a and 3b may be of use. These call for 2-conductor shielded cable
with 3-circuit or 2-circuit plugs and use the shield as only a shield, the ground connection
actually being made with one of the center conductors •

+ - ...,T
s R T
FROM
lNIALANCEI) ..r: T y
..,.R
n nv') TO 166
AU) I O I ""'-IT
SOURCE -- I • JAO<
CONECT SHIEI.D ~ s
RING AT THIS EH>
IF HlM CEVELOPS

Figure 3a: Unbalanced sources, dual-conductor shielded cable, stereo plug

+ - ...,T s T
nv TO 166
Al.1>10 l""'-IT
T y •
---- I s
JAO<
eot.NECT SHIELD ~ CENTER
c:a-llUCTOR AT THIS EH>
IF HlM CEVELOPS

Figure 3b: Same, mono plug

Audio outputs and balanced and unbalanced loads

The two audio outputs are driven by unbalanced single-ended line amplifiers when-
ever the Bypass switches are not depressed. Able to drive 600-ohm or greater loads to
+21 dBv, these stages are suitable for connection to most studio equipment, balanced or
not.

Figure 4 shows the balanced connection of the set of output terminals to balanced
inputs. Note again that each output has signal tip /ring/sleeve ([ +], [ -], and ground), like
the balanced inputs. The outputs are connected directly to the inputs in Bypass mode, so
a balanced input remains balanced at the output when the unit is bypassed. The minus
and the ground terminals of each output are internally connected whenever the unit is
not bypassed.
T R s T~ - +
FROM 166 < vn n Rv
y
8AI..ANCa)
LOAD
OUTPUTS
s
•
CO!"'ECT
------ ....a...
SHIELD -
AT THIS EH>
IF HlM OEVELOPS

Figure 4: Balanced ins and outs, dual-conductor shielded cable, stereo plug

Figures .5a and b show the output connections for unbalanced loads with 3- and
2-circuit plugs. As with the inputs, a 2-circuit plug (5b) will contact the ring (-) contact
in the jack, which is perfectly okay for unbalanced loads; note that in Bypass, this will
unbalance balanced sources, because the jack shorts the ring(-) contact to ground.

FROM 166
~___:_T~R------
--- ----i~-+ \NW.ANCEi)
OUTPUTS LOAD

L--J
f------'-"----------+---d:-
s

Figure .5a: Unbalanced loads, single-conductor shielded stereo plug

-
T
< -n
s T...,

• y + I.NIALANCED
LOAD
s

Figure 5b: Same, rnono plug

-5-
 Figures 6a and b show the use of 2-conductor shielded cable and either 3- or 2-cir-
cuit plugs with unbalanced loads; like 3a and b, this often is useful for combatting RFI
with unbalanced loads.
T R s Tv ,..,
FROM 166
OUTPUTS c ~nn Rv
• y I

s ----'
~

CQ.NECT SHIELD
AT THIS El-"
IF 1-«.M DEVEL.OPS

Figure 6a: Unbalanced loads, dual-conductor shielded cable, stereo plug

s Tv ,,...
FROM 166
OUTPUTS <
T
II
1- A y ___
, I
+
:J.....
LteAL.ANCED
LOAD
s eot,t£CT SH IEI.D
AT THIS EN>
IF 1-«.M DEVEL.OPS

Figure 6b: Same, mono plug

Sidechain hookups
The sidechain inputs are unbalanced. They'll work with balanced and unbalanced
sources, but will unbalance a balanced output.

Figure 7a shows the connection of an unbalanced signal source to either sidechain

input jack; a 2-circuit plug is used since there are only two connections to make. If a
3-circuit plug is used, either leave the ring unconnected or connect it to the shield.

Figure 7b shows the connection of a balanced source to either sidechain input. Most
balanced sources will work without the dotted connection between the (-) output and
ground (this is true for "active-balanced" outputs and "ground-referenced" outputs). How-
ever, some sources require the dotted connections shown - "transformer-isolated" bal-
anced outputs. We recommend making this connection only if necessary for your instal-
lation, because some active balanced and ground-referenced outputs may be damaged by
doing so. Note that a 2-circuit plug is used here; follow the instructions above if a 3-cir-
cuit plug must be employed.
~

+ r vT s
11
T
TO 166
SIOE:0-iAIN
..L • l"-"UT
s

Figure 7a: Unbalanced source

,..,
+ '(
vT s
II
T
TO 166
SIOE:0-iAIN
..L A l"-"UT
j
s
-=---------~"'~
I

ONLY IF NECESSAAY THIS eot-t.ECTION

<SEE TEXT>

Figure 7b: Balanced source

-6-
ABOUT GATING

Noise reduction
The basic purpose of a noise gate is to remove unwanted background sounds in the
spaces between desired fore ground sounds. Note that there has to be some real distance
in level between the unwanted and wanted material - at least a few dB -- in order for
the 166's gate to "get its foot in the door.~' If levels are too close (e.g., because of earlier
compression, or because the mikes were closer to the unwanted than to the wanted
sound), the gating efforts of the 166 will be for naught.

One of the more common uses for a gate is to tighten or "dry up" drum sounds. As
with most dynamics processing, it's ideal if there is a separate mike on each drum and
cymbal (or group of cymbals) and each is individually gated. Each mike is auditioned
separately and the gate threshold adjusted to eliminate the sound of all but the desired
drum. (Hint: start with the threshold very low, so all the sounds come through, then
increase it until only the desired drum is left.) It's likely that you'll find the Fast release
setting best for most drums, while slow usually is better for cymbals and some toms.
When things are adjusted correctly, each drum will sound tight, punchy, and dry --
detailed and defined.

When you don't have enough mikes (or 166es!) to cover each drum, then group them:
snare and center toms, side toms, bass, cymbals. The idea is to get as close as possible to
only one mike on at a time so only one sound is picked up, instead of several.

Another common use for a gate is in vocal recording. Especially after compression,
the noises picked up by a mike an inch away from a singer's mouth can be very obtrusive.
Try the 166 in its slow release mode to gate out these noises. Other applications include
keeping live drum tracks from "contaminating" ~n acoustic-piano track, and general solv-
ing of other sorts of leakage problems.

Changing sound quality

There's more to gating than just keeping out unwanted background noises: you can
use the 166 gates to change sonic character. This is because gates can be used to reduce
or otherwise alter the quality of instrumental ambience and reverb. As the sound decays
after an instrument stops, its reverberation level will fall through the l 66's Threshold
setting, below which it can be made to die out more or less quickly - in any case faster
than the natural sound. Experiment with changing the "tail" of the sound; the fast setting
will nearly eliminate reverb.

In other situations, a 166 can be used to prevent or reduce leakage among mikes in
sound reinforcement or during panel discussions. Simply set the Threshold control below
the level of the music or speech. Similarly, in broadcasting, a 166 can be used to clean
up noisy feeds (land lines, ENG audio) if it's placed at the output. Wind or air-condition-
ing noise during a remote interview can be controlled as well: proper 166 gate attenua-
tion can keep noise during pauses from being either loud (obtrusive-sounding) or too soft
(peculiar-sounding) - or modulated.

The attack time of the 166 gate is short, so the complete transient at the beginning
of a sound comes through. We've already touched on the differences between the Fast
and Slow release rates, but never hesitate to experiment to get the best results for a
particular situation.

-7-
ABOUT COMPRESSION

General
The purpose of a compressor is to reduce the dynamic range of a program and give
you control over its dynamics. The 166's Ratio and Threshold controls can produce a wide
variety of dynamic-range-reduction effects, from gentle taming of overall dyna.mics to
limiting of . peaks to squashing all dynamics.

For example, at low compression ratios, a very low Threshold setting can be used to
reduce gently the overall dynamic range of a program. Higher ratios with low thresholds
will provide leveling for instruments and vocals. High thresholds generally are used for
limiting program levels overall. Ratios of 6:1 and higher effectively prevent outputs
levels from much exceeding the threshold (assuming the Output Gain is set to 0).

Note that compression of the entire program (produced by low thresholds) tends to
sound less natural at high ratios. Ratios of perhaps 4:1 and lower affect dynamics to a
lesser degree, and are often used to tighten up a bass guitar; snare, and vocals. Moderate
settings typically are used during mixdowns and for leveling the program in a broadcast.

The l 66's OverEasy circuit prevents compression at high ratios from sounding too
unnatural. This is because as the signal rises above threshold, the ratio changes grad-
ually, from l :1 (no compression) to :that set by the front-panel knob. You can put this
feature to especially good use in those situations when you need protection from exces-
sive peak levels but desire gentle compression on most of the program. By setting the
threshold at a moderate level and using .a moderately high compression ratio (6:1, l 0:1,
etc.), you will provide limiting for signals well above threshold and gentle compression
(much less than that set) for signals at or below threshold. See the Over Easy curve, p. 2,
to understand how this works.

It's always useful to watch the gain-reduction LEDs to see the amount of proc-
essing taking place. With practice, a glance will confirm what your ears tell you -
that things are going okay, or that there's a little too much or too little. Your 166 can
achieve desirable effects with proper settings derived from experience; when it's used
too liberally, the unusual results may be suited to special effects only.

Here are some specific situations.

Variations in mike levels

As the distance between vocalist and microphone changes, signal levels change.
Start at 2:1 with a low threshold setting to begin to smooth these out. With OverEasy,
ratios up to l 0:1 can be used here to good effect.

Variations in instrument levels

To achieve smooth electric-bass sound, start at about 4:1. Strings and horns like-
wise benefit, and strings will have their "sustain" increased. Note that large amounts of
compression are usually more audible in a mixed stereo program; if the separate tracks
were compressed before mixing to create the program, compression is much less notice-
able.

Raising the signal out of a mix

Since reducing dynamic range can increase the average signal level and meter
readings, a single track can be brought up out of a mix by boosting its level slightly and
applying compression. It's also possible to separate certain vocals and instruments from
an already mixed program by using the sidechain; see p. l O.

-8-
Preventin ta saturation
A high threshold but below tape saturation) and high compression ratio will cause
the compressor to reduce gain in a controlled manner before the tape overloads and
distorts.

Speaker protection and acoustical distribution

Compression will keep excessive levels from damaging drivers in sound-reinforce-
ment systems. Limiting also enhances intelligibility by letting low-level material be
reproduced throughout the house at higher volumes; in a performance, this increases
intimacy, as whispers become clear at virtually every seat. Our OverEasy characteristic
permits high amounts of compression (e.g., 10:1) to be used without vocalists or musicians
feeling choked back - and with high average levels maintained without speaker damage
due to heat buildup in the drivers. As a rule, to give the best protection, your 166 should
be as close as possible in the signal path to the power amp. For maximum SPLs, large
sound-reinforcement systems frequently have a separate compressor on each output of
the electronic crossover(s).

Where the 166 will be expected to allow virtually no level change unless an emer-
gency (wildly excessive levels) arises, set the ratio to Infinity:! and the Threshold to the
highest level. OverEasy will never act in the fast, unpleasant manner of a typical "hard-
knee" compressor, but it will give a measure of real protection. See the next section,
too.

ABOUT LIMITING and SOFT. CLIPPING

PeakStop allows you to control the maximum peak levels at the output of the 166
irrespective of any other control. As mentioned, it comes after the compression and
other circuitry, including the output gain, so it lets an absolute limit be put on the peak-
to-peak excursions of the output. PeakStop works instantaneously; you'll be able to apply
moderate amounts of compression and still be independently protected from large trans-
ients, other short-term overloads, and broadcast overmodulation.

PeakStop consists of a sophisticated voltage-controlled clipper that produces a

minimum of audible distortion. It rounds the corners of a peak rather than cutting it off
sharply, as the term "clipping" implies. By making a signal's leading and trailing edges
curves instead of sharp corners, it reduces the amount of higher-order, offensive-sounding
harmonics that conventional clipping causes. The level at which PeakStop is activated is
adjustable from Oto +20 dBv. Note that small signal excursions above the set value of
PeakStop are possible, to allow the rounding to take place. Therefore, for any applica-
tions where you must not exceed a given ceiling, set the PeakStop control 1-2 dB below
it to be sure.

The PeakStop LED flashes whenever peaks attempt to exceed the PeakStop level
and get reduced in amplitude. To disable the function altogether, simply set the control
to +20 (which is the maximum output level of the 166 anyway).

In use, the PeakStop function can prevent an amplifier from being driven into hard
clipping, where it may lose control over the speaker system. PeakStop is a smooth, well-
controlled clipper whose behavior is sonically similar to the gentleness of OverEasy com-
pression; its clipping is much preferable to a power amp's. As noted, control of speaker
overexcursion, of broadcast overmodulation, and of harsh electronics clipping are all
applications; with PeakStop and OverEasy ·, you have the best of both worlds: virtually
inaudible rms compression and peak protection downstream, at the end.

-9-
 Normally, the control is set to just under the peak clipping level of the equipment
downstream. This way, clipping will be softer and controlled within the 166. The LED
should light occasionally, on peaks only; if the LED lights often, of course, reduce the
compression threshold and/or increase the ratio, to allow less peak signal level at the
output.

SIDECHAIN APPLICATIONS

In all the following situations, it's the Sidechain Monitor function that will save you
time and trouble, letting you adjust the preceding processor(s) and instantly confirm that
things are (or aren't) going the way you want them to.

General hints
It's possible to separate certain vocals and instruments from a mix by making the
compression fre uenc -sensitive. With an equalizer inserted into the Sidechain input
(but not in the audio path, the EQ settings do not shift the timbre or equalize the audio
signal; they merely alter the threshold of the compressor as a function of frequency.

In such an arrangement, frequencies that are boosted on the equalizer will be sup-
pressed in the audio signal. The converse may also be used, of course: dipping the equal-
izer on a particular band prevents any sound with dominant energy in the affected regis-
ter from compressing so much, because the 166 will detect less need for compression.

For example, if you want to suppress an overly loud bass drum, boost the equal-
izer's response below about 150 Hz. This will make the compressor reduce gain whenever
energy in this region is detected. Furthermore, raising the threshold will cause this to
happen only on very loud kicks. To put it generally, a relatively high threshold setting
can prevent most sounds from being affected while solo and very loud sounds within that
frequency range are held back. (Of course, when compression does occur, the entire
program level is affected.) Depending again on the threshold setting, frequencies outside
that range will not cause compression.

During the recording of cymbals and toms, a compressor with an EQ in the side-
chain path can help prevent tape saturation. The equalizer can be adjusted for a boost
peaking at about 5 kHz, causing the cymbal to be compressed on a very loud crash and
stopping saturation of the tape at higher frequencies, where there's less headroom. How-
ever, gentle tapping with a stick or cymbal brushing will not be held back. And the tom-
tom likewise, being a lower-frequency instrument and better-tolerated by the tape, has
less need for compression. EQ in the sidechain circ .uit can make the compressor not as
readily triggered by a loud tom-tom beat as by an equally loud cymbal crash.

De-essing
In the absence of a de-esser, small amounts of high-frequency (6-10 kHz) boost in
the sidechain path frequently will help in the processing of vocals that may have been
brightly equalized beforehand or that suffer from prominent sibilance ("ess" sounds).

Speaker protection
If a •single compressor is to be used with a multi-way speaker system (after EQ,
before the crossover), you are faced with the problem of keeping the entire system level
down below the point of destruction of the most sensitive component. If midranges are
frequently blown, for example, the whole system must be run at a lower SPL or addi-
tional mids must be added. By the insertion of an equalizer in the sidechain of the 166,
it can be made sensitive to the frequencies the midranges handle, permitting the entire
PA system to be run at higher average levels and dropped back only when damaging
signals are present.

-10-
Broadcasting
A preemphasis filter network pla c ed in the sidechain of a 166 proces sing preemph-
asized audio permits higher average signal levels to be run within the headroom limit s of
the broadcast chain.

Filtering
Narrow-band ("notch") filtering for rumble, feedback, equipment noise (e.g.,
cameras) may be put in the sidechain to make the compressor less sensitive to such
problems.

Anticipatory compression
If you feed the program directly into the sidechain and send the audio signal
through a delay before the 166 audio input, the 166 can "anticipate" the need for gain
change. With experimentation, the effect can become that of "zero" atta c k time at a
given frequency. Additional delay beyond this "zero" time will then cause the co m-
pressor to finish reducing the gain before the leading edge of the loud passage even
enters the 166, which will cause the program material preceding the loud pass a ge to be
suppressed. The 166 will then begin to recover from compression (it will release, in
other words) before the loud passage has dropped back down toward the set threshold.
This will cause the output level to surge as the note(s) should be decaying. Suc h a spe c ial
effect sounds similar to the dynamic-envelope inversion you may be fa miliar with fr om
reverse tape playback. See diagram.
166

SOURCE DELAY AUDIO

INPUT
AUDIO OUTPUT
OUTPUT
SIDECHAIN
INPUT

Keyed gating
Controlling the gating of one signal by another perm its perfe c tly in- sync h pla yin g
and overdubbing among individual instruments or precise soni c a ugmentation - "fa tten-
ing" - of a weak solo. An example of the former would be synchroni z ing bass guitar an d
drum; an example of the latter would be using the drum signal to key an oscillat or which
is set an an appropriate frequency to "tune" and "punch up" the drum sound. See dia gram.
As noted, the Release Rate switch is helpful for determining the sound of the final
produ c t. :,o 1-1z-vARv To
"TI.M" DA\.M
OR BASS GUITAR
166
0__; MIXER
AUDIO 'FAr
osc INPUT / I(ICK-oR\.M
AUDIO
"WF.AK" SIDECHAIN
OUTPUT ,L TRACK
OR KICK-oR\.M
TRACKWITH
SYNCI-ED BASS
I( ICK-oR\.M INPUT
TRACK

Selective gating
You can also do frequency-sensitive gatin g, letting you tune t he re sponse of the
gating action. If you're gating a kick drum, for example, in a tr ack with lots of le a kag e,
you c an tune in to the frequency of the kic k with an EQ and the gat e will respond only t o
the drum. Again, see diagram below.
166

SOURCE AUOIO
INPUT
AUDIO OUTPUT
OUTPUT
EO SIDECHAIN
INPUT
FILTER Tu-El
TO K ICK-oR\.M
FRE0..ENCY

-11-
STEREO DYNAMICS PROCESSING

Stereo coupling is useful for all applications where two channels must be com-
pressed and the left /right perspective must remain the same. Examples include the L
and R overhead mikes on a drum kit or piano, a stereo submix of a vocal ensemble, the
feed from an X-Y or other pair of mikes in a classical recording, a complete stereo mix,
etc. All of our earlier comments about compression and gating apply here; the difference
is that when the Stereo Couple switch is pushed, the gain changes in the two channels
will be identical.

In this mode, the signals at each rms detector are combined (so the true rms sum
can be sensed) and controlled. As mentioned, all functions of Channel 1 control both
channels except for Sidechain and Bypass.

Use this mode whenever imaging must remain stable.

NOTES

-12-
BLOCK DIAGRAM
I Ii
> '
-
' -q
I
b-il1

is/\ "w~
• ~
'
•= - ..~.
•
• -•
0

-"'> •;
i:. '
i
..
..,
C

-
N
..,
~1½
l= b-
-- ·i--
I ....
gl__ ,
D
...
'
I I!!•
al!
5
L.....,,,

f-0• - ii
lw
!I
"'
":z:
z
C
•
_....
I I ..
u
--.. ---
:I
15 Ii..
..__
--
/\
Ii '
- 'I 11

-.
O

1-
' 1
ci-jl
•
'a
-!
l!~
-•
•

..
lL lL [_
0
"' [t>--
•
l
..
"'
0- ...
- ....
&
I
I
II
>

-
-q
i• ' b-ill

ii/\ 'w
"
• ..
,

. .._,
•:, a

,e: .-- ' .,

..~.
•
-
•0
' 'i
-"'"'.. :.;
..I f:; 0

-
..,
a
-1
;=
'

!
·1-~...'
;, I ~
,.g .
i
'
• •..
W

I"
~
..•
g

-
:,c
cw
W•
JC
we
z"'
z
-1
•
b-
ll
!:•
-0
....
•
C
C

iu C
-- i
..
ll 15 ••
-•
..
--
cc

'\
'!
-~ '
'-
:1, ' 'I II
'
"-IS
., ..
--
:!l!!

-13-
 SCHEMATIC
:r w ! 0 u CD ([
,-- -----''---------'---- -·----'------·- - - -'- . ------~-------~-------
. :- N
_JI
, • fat
:. : :• !:_.
~
::l
t•f
: .:,
....
-.: ....
,~ ia i
:~ ~ •• •
.' :
&.
ii
j
: :
-
7 t~ I • ,. ' • II . . '¥ .~ ! -
• a !
•
ii:. !
• • • F.
'- - •'-' '•
~fl • _· ~} . •• !. I !i • . II '!. ; ;
=t.
'!. ...
=i~
• • ~ !,
<-=<
i:
.. ,
..
~:~
•
•r •so
T" l' • ... • ~ ~ • •· • • ; • -;r-1"" •
,~ ~t '!--' I • 1!...~:-:J'+--------- - - - ""+--' •
u
~=
'½';·' ;I !I .---.----.
.r -•
z •

t· • ~p
iii . .
• ; .
!. ~~r
r.::= •'......
£. ! r;.~

L..--_-""
~T
•r. 'it-.:...
I

-.,..;a.-
- ...... C
I'

.t, ,;.
i,

~
,. ; "ti
.
>
l ! .. ~ i
-S'l'-< . !
-,•
i
l. P'i>.
t"t{
I'
,'! ~
.
!'-
.re•
• . ~J
,, •• , • ~i)
•
....
........
....;;;i..
...........

••
ii:
..

l
..·.......;a.
~--.....i--~ ...,__
....
_a.;;...--,
-i • !
t 'ii ... • i ii \ , ·
• ; !
;: ·~·~=-~===;::~::j:
~~!~
! ! . ,. - ;: I l!,
~ : a

N ~----.
~t-,

::.~,r~~,r
t+--:J

l- ~i-~·i.;
: '-"----i '

, t, .• .. i ..._._1
~5i.,+- _ _,._:~: ·: ...
'
----• :--•-
--
••11>0
- ll-I•-•,_•
-•· • •-
r-;:::==:;--1
· L.. . '" 11, 't '-1---,

: i !::~·
.% . I., I •
-,4_
• '
~,-~!
t -1

•· J
......
• •
~~,l! • ""
r.Ii
j~
i•
~'. I ; ~
I .._
ii
.f ~.,... _ __,
1- ·
-., T I I ~·
-•••·
_,.,, --·
..,_-•---
.. ...... '=re--

- . -·--
1 Iii •• ..! •. - •
• a a · ~~., r. • I I I ...,. _ _..,,,,, - ·· • • SHOISIAJV
1 l'-i . . .;.i , :• 1,·
• I; • i ::-! ~ a • rl
•
1
• ~-••• I . : . t. f 1
.1' , '. ., ;_
. .. l! ,,_! !,l•'-T~'.
'• I
?1
1----, .....;f.+----,
r :!.
,..._---,
~P
Y• t----
r
..... -+-'-
i
~r-
--1-'
~P
i ~ ill
..
. ...
!
: ..i, :r. , . !
~
•
' ,' ,i'-l•._ __ i...,.
• ....:;;J!;.,._·.._·:i;
'!'- __ _.._ ____ _,~ h -t • ·'
' ;
n n
.
. ~ ' .. -•1
~t !.•
\• I
1 •

..
'.
-
"' ! !,
. ~ . it!
! L
•' .
•

.,.._-""-~....
! I
•
; H l
• •
'-i•
.
..J UJ • • ' ~
t
~ UJ > ~
z ([ ';i
z ..J ;• '
([ (/) •! •
:r -
u ·I
•c-t,
.
.' !,
ji
i •
~

;
\
.•..:
..: :!
<K il. .t
! • '. -i...
-
I ! ii . ., '.
;

t--t:'-t., -4
! !I

• r-
. I
..:l;,' .
:i'' I

~~· ,; Ii ...
.u.
!
•

i
,.
.. I i,
.

- i-- .r-;::_ .....

-.-.-:;;;
•,,......
~-11 ~' !j
I l=
,!f,!".~ 1
a, I!" : 1
~ I _I}
,_•..,,,.-• •• !- "!-•' -J I ii- J,,-:, I-+--~ I
' ~ · ;: , ! . j '. • •'-"I
' ,, , I i ~ i
I7 :: .. .:
,; '
1~ -
~ .
r

,mi.
•.
...
.
~ ~

,,
;
! ~-• ·· I
• • J !
::l
~
.
. i~l;:.. L..,---
+-- ---''----i ...__- +-' ! ~
! i L--;
,
4 s
S!
•
• l--------.'-i• ri 7 .,
!
,~,
.. !'-.
C
. . ..... ' ~
·=--
• •
' i ..,
iill •
. '•
•
- ~
~--,-

I
~
I
~·,
.......~
of•
i
I
;
,,
Vo

•• .
i~i
.
!
~
i,
. ~ ' .. : ,if :• .it
:1ir
~
! '; ' L •••••••••• .J
I ' . : ·1-.!!--! ' :1 1r,- • t--•- ..a-<'.
,.._ .
.., - '-<
'l!
p
.
.
•I
:" 1 ,·tt.:v-~··~
':-~~~
''
'
'

.;,,• ~: ~ - • - ' . •
. !; '''
''
''
. i i
? ii l ' !
!~ i
.,
L;,
'
•• 'L' ••••••••••

~-
. ~-
:r!: 1!:
. • ' _'
..J a:
I r
WW
z,-
ZC/l
([
' l I

:r :,:
([
,[
u- ,c .,, -14-
a,
!
.
i I.
WARR ANTY and FACTORY SER VICE

All dbx products are covered by a limited warranty (warranties for produ c ts pur-
chased outside the USA are valid only in the country of purchase and the USA). For
details, consult your warranty /registration card or your dealer /distributor.

dbx Customer Service will help you use your new product. For answers to que stions
and information beyond what's in this manual, write to:

dbx
1525 Alvarado Street
San Leandro, CA 94577
Att: Customer Service
'
You may call 415/351-3500 between 9:30 and 4:30 Pacific time (USA).
The Facsimile No. is 415/351-0500.

Should problems arise, consult your dealer or distributor. If it becomes necesary to

have your equipment serviced at the factory, repack the unit, including a note with a
description of the problem, your name, address, and phone, and the date of purchase, and
send the unit freight prepaid to the above street address, marking it Attn: Repair s.

- 15-