dbx 166 User manual

dbx
Model 166
Professional
Dynamics Processor
ATTENTION
IF YOU BEGIN WITHOUT READING THIS
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 CATE 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 wilt stop the audio from
passing. Note that signals BELOW the Cate threshold
are gated; signals ABOVE the compression threshold
are compressed. If ifs 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 aprotective 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 agood 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 amodel 166 and this owner's manual, the carton should contain awar-
ranty/registration card. Please fill it out and send it to us.
The chassis has integral brackets (rack "eau-s") for mounting into astandard 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 PREVENT FIRE OR SHOCK HAZARD,
DO NOT EXPOSE THIS COMPONENT
TO RAIN OR MOISTURE.
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.
This triangle
also appears on your
component, and it alerts
you to important operating
and maintenance instruc-
tions in this accompanying
literature.
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)
Rear panel (connections)
Typical hookups (plug and cable wiring)
Application notes (gating, compression, limiting, sidechaln). ..
.
Block diagram
Schematic
Warranty and factory service
SPECIFICATIONS
Frequency response
20 Hz-20 kHz +0.5 dB
THD (total harmonic distortion)
0.2% at maximum
compression, 1kHz, 0dBv
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, singie-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: 0to +21 dBv
Attack times
Compressor (program-dependent):
15 ms for 10 dB,
5ms for 20 dB,
3ms for 30 dB
Gate:
2ms for 28 dB (70% of return to unity gain)
Release times
Compressor:
8ms for 1dB,
80 ms for 10 dB,
400 ms for 50 dB (125 dB/s rate)
Notes
1) Speciiicetiofts ere subject to change.
2) AU voltages are rms <root*mean-square).
3) 0dBv is defined as 0*773 Vregardless of load impedance. Subtract 2.2
from the dBv figure to convert to dBV <Ue.j referred to 1V). When the
load impedance is 600 ohms, this particular dBv is also known as "dBrn.**
4) Noise figures are for 20 Hz-20 kHz.
5) Measured in the infinite-compression region of the dbx OverBasy curve,
attack time is the time required to reduce the signal by 63% of the level
increase above threshold, while release time is the time required to
restore gam to 90% of the level decrease below threshold.
Gate, slow: 100 ms for 1dB
fast: 100 ms for 100 dB
Maximum compression
Greater than 60 dB
Power requirements
90-135 V(120V model),
200-260 V (240V model),
50-60 Hz; 15 W
Dimensions
l-3/4"h x 19"w X8"d
2
2
4
7
13
14
15

REAR, Channel 2(1 is identical)
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1AUDIOilN, OUT. These two S^arcuit phone jacks are the Input and Output. As
marked, these jacks are the standard tip/plus, ring/minus, and sieeve/ground.
2SIDECHAIN;IN. This jack provides adirect input (optional) to the rms-detector
circuitry. It is used for connecting asignal processor for altermg 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 1(2 is identical)
3GATE:THRESHOLD and RELease RATErFAST, SLOW. The 166 gate is abelow-
threshold attenuator with two release rates. The Threshold kr>ob 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 RATE 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 10dB/s. In the Fast position, the rate is
very fast (1000 dB/s), useful 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 edlow all of the
transient at the beginning of anote to come through.
4OVEREASY COMPRESSOR:THRESHOLD
and RATIO. The Threshold knob sets the
point at which dbx's well-known Over-
Easy circuit begins to compress the dyna-
mic remge of the signal. The calibration
numbers refer to the middle of the Over-
Easy curve (see right); at an Infinity:!
ratio, they denote the point where there's
approximately 6dB of compression. The
OverEasy sound is musical and unobtru-
sive, without the "held-back" feeling of
other compressors.
-2-
Input

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., a4:1 setting
means that for a4-dB increase in input level the output will increase by only 1dB.
At Infinity:!, the output will remain at aconstant level irrespective of input dyna-
mics (as long as the input is above threshold) —in other words, full limiting.
5GAIN REDUCTION LEDs. These show how much the signal is being attenuated by
the Gate, the Compressor, or both.
6PEAKSTOP LEVEL knob. This sets the maximum peak-output level. The control is
calibrated to the rms value of asine wave just being clipp^. 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.
7SIDECHAIN 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 aconsole/mixer or tape recorder. Note
that PeakStop is in the sidechain signal path when Sidechain Monitor is selected.
8OUTPUT GAIN. This knob controls the overall gain of the 166, from -20 to
+20 dB. It is independent of all other controls although, as mentioned, it does
come before the ^akStop circuit.
9BYPASS switch and LED. Pushing this in provides ahardwire bypass for the 166,
connecting input to output (fully balanced if so wired) even in the absence of ac
power.
10 STEREO COUPLE switch and LED. Pushing this in turns the 166 from adual-mono
unit with two identical, independent sets of controls into astereo unit. Except for
Sidechain Monitor and Bypass, Channel 2's ccxitrols are overriden by Channel I's in
the Stereo Couple mode. Note that the detection circuitry senses the true rms
levels of the combined signal, so it is unaffected by phase shifts (or other discre-
pancies) between the channels. Among other benefits, this enables stereo compres-
sion without loss of imaging stability.
11VOLTAGE 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 asuitable
adaptor plug.
-3-

TYPICAL HOOKUPS
All 166 connections are made through tfie rear phone jacks; our figures in this sec-
tim 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 signai inputs are balanced electronically with differential ampli-
fiers. They won't unbalance abalanced source, and they may be used with unbaianced
sources as well. Here's ageneral-purpose diagram:
SIGNAL FLOW —
>
For maximum hum rejection, avoid common grounding at the 166's input and ouput.
The best starting point Is to ground the shield of the input Ce^le and the source device
(ieaving it unconnected to tiie 166) and to ground the shield of the output cable to the
ground of the 166 (leaving it unconnected at the receiving device).
Abaianced 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 aslngle'^onductor Yielded
cable with the center conductor carrying the signal and the shield at ground potential.
Figure 1shows the ccmnection of baianced signal sources to either audio input
jack. Note that for this connection a3-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.
AT TMI9 Mir
H*. OCVB.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 a2-cir-
cuit ("mono") phone plug is used. Since this plug has acontinuous 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.
Figure 2a: Unbalanced sources, dual^:onductor shielded cable
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 ashield, the ground connection
actually being made with one of the center ccmductors.
RiNe AT THI9 e>«
If oevCLOM
TO 1M
AUDIO
JACK
Figure 3a: Unbalanced sources, dual-conductor shielded cable, stereo plug
CCMUCrCM AT THIS DC
If HJU OCVCLORS
TO 1W
AUDIO IKWT
JACK
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. Abie to drive 600-ohm or greater loads to
+21 dBv, these stages are suitable for connection to most studio equipment, balanced or
not.
Figure 4shows the balanced connection of the set of output terminals to balanced
inputs. Note again that each output has signal tip/ring/sleeve ([+!t [-3, and ground), like
the balanced inputs. The outputs are connected directly to the inputs in Bypass mode, so
abalanced 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
FRCM tl
OJTPiJTS 11 -
AT THIS OO
IF HUM OeVCLOPS
Figure 4: Baianced ins and outs, dual-conductor shielded cable, stereo plug
Figures 3a and bshow the output connections f(X‘ unbalanced loads with 3- and
2-circuit plugs. As with the inputs, a2-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.
cte:
Figure 5a: Unbalanced loads, single-conducts shielded stereo plug
noM
OUTWTS
Figure 5b: Same, mono plug
-5-

Figures 6a and bshow 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.
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A
COrtCCT »1I£LD
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AT THia
IF HUU
Figure 6a: Unbalanced loads, dual-conductor shielded cable, stereo plug
FAOM 164
OUTR/T8
CC64CCT
AT THIS D€
IF nu OCVCLOP9
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LOAD
Figure 6b: Same, mono plug
Sidechain hookups
The sidechain inputs are unbalanced. They’ll work with balanced and unbalanced
sources, but will unbalance abalanced output.
Figure 7a shows the connection of an unbalanced signal source to either sidechain
input jack; a2-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 c<mnection of abalanced source to either sidechain input. Most
balanced sources will work without the dotted connection between the (-) output and
grourKi (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 a3-cir-
cuit plug must be employed.
A
TO It*
SlOeOlAIN
Figure 7a: Unbalanced source
TO 164
SIOCCmiN
Figure 7b: Balanced source

ABOUT GATING
Noise reduction
The basic purpose oi anoise gate is to remove unwanted background sounds in the
spaces between desired foreground sounds. Note that there has to be some real distance
in level between the unwanted and wanted material —at least afew dB —in order for
the 166's gate to "get its foot in the door.V 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 agate is to tighten or "dry up" drum sounds. As
with most dynamics processing, it's ideal if there is asep>arate mike on each drum and
cymbal (or group oi 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 defin^.
When you don't have erK>ugh mikes (or 166es0 to cover each drum, then group them:
snare and center toms, side toms, bass, cymbals. The idea is to get as close as p>ossible to
only one mike on at atime so only one sound is picked up, instead of several.
Another common use for agate is in vocal recording. Especially after compression,
the noises picked up by a mike an inch away from asinger's mouth can be very obtrusive.
Try the 166 in its slow relecise mode to gate out these noises. Other applications include
keeping live drum tracks from "contaminating" an acoustic-piano track, and general solv-
ing of other sorts of leakage problems.
Chanf^g 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 tfie quality of instrumental ambience and reverb. As the sound decays
after an instrument stops, its reverberation level will fall through the 166'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, a166 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, a166 can be used to clean
up noisy feeds Oand lines, ENG audio) if it's placed at the output. Wind or air-condition-
ing noise during aremote 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 acompressor is to reduce the dynamic range of aprogram and give
you control over its dynamics. The i66's Ratio and Threshold controls can produce a wide
variety of dynamic-range-reduction effects, from gentie taming of overaii dynamics to
iimiting of peaks to squashing all dynamics.
For example, at low compression ratios, avery low Threshold setting can be used to
reduce gently the overall dynamic range of aprogram. 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 abass guitar, snare, and vocals. Moderate
settings typically are used during mixdowns and for leveling the program in abroadcast.
The 166's OverEasy circuit prevents compression at high ratios from sounding too
unnatural. This is because as the signal rises above threshold, the ratio changes graJ^
uaily, from 1:1 (no compression) to that set by the front-panel knob. You can put this
feature to especially g<^ use in those situations when you need protection from exces-
sive F>eak levels but desire gentle compression on most of the program. By setting the
threshold at amoderate level and using a moderately high compression ratio (6:1, 10:1,
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 OverEasy 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, aglance will confirm what your ears tell you —
that things are going okay, or that there's alittle too much or too little. Your 166 can
achieve desirable effects with proper settings derived from experience; whoi 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 alow threshold setting to begin to smooth these out. With OverEasy,
ratios up to 10:1 can be used here to good effect.
Variations in instrument levels
To achieve smooth electric^ass 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 amixed 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 amix
Since r^ucing dynamic range can increase the average signal level and meter
readings, asingle track can be brought up out of amix by boosting its level slightly and
applying compression. It's also possible to separate certain vocals and instruments from
an ciiready mixed program by using the sldechain; see p. 10.
-g-