Dbx 168A Service Manual

Page 1

MODEL 1'68A

_~.____.__ _________-_ - ' '

STUDIO DYNAMICS
uPROCES-SOR

I

OPERATION/SERVICE

Page 2

CAUTION

RISK OF ELECTRIC SHOCK msoue DE CNOC
DO NOT

ELECTRIGUE
NE PAS OUVRlR

DO NOT EXPOSE TO RAIN OR MOISTURE

OPEN

CAUTION: To REDUCE THE RlSK OF ELECTRICAL SHOCK, DO NOT REMOVE COVER (OR BACK).
NO USER SERVICEABLE PARTS lNSlDE. REFER SERVICING TO QUALIFlED SERVICE PERSONNEL.

WARNING: TO REDUCE THE RISK OF FIRE 0R ELECTRICAL SHOCK, Do NOT EXPOSE THIS APPLIANCE TO
RAIN 0R MOISTURE.

This symbol. wherever it appears, This symbol, wherever it appears.

alerts you to the presence of uninsulated alerts you to important operating

dangerous voltage Inside the enclosure - voltage ' and maintenance instructions in

that may be sufficient to constitute a risk of shock the accompanying literature.
Head the manual

This Manual is part number 1B-2039~B

© Copyright 1995 by dbx Professional Products

dbx Professional Products
8760 South Sandy Parkway, Sandy, UT 64070
Telephone (801) 568-7660 Fax (801) 56&7662

Page 3

:

CAUTION

RISK OF ELECTRIC SHOCK

ATTENTI 0 N1 alsousos crloc ELECTRIOUE - NE PAS OUVRIR

WAR II I N8: TO REDUCE THE melt or FIRE on ELECTRIC
SHOCK no not exposarms comment To RAIN on MOISTURE

The symbols shown at left are fntemetionally accepted symbols that warn ct
potential hazards with electrical products. The lightning flash with arrowpolnt in
an equilateral trnngle means that there are dangerous voltages present within
the unit The exclamation point in an equilateral triangle indicates that it is nee.
essary forthe usertc refer to the owners manual.

These symbols warn that there are no user serviceable pans inside the unit.
Do not open the unlt. Do not attempt to service the unit yourself. Refer all ser-
vlcing to qualified personnel. Opening the chassis for any reason wlll void the
manufacturers warranty. Do not get the unit wet. ll liquid ls spilled on the unit,

during storms to prevent damage.

shut It Off immediately and lake It to a dealerfor service. Dlsconhecr the.unlt _: ;.-

K. AN U WA NING

A moulded mains plug that has been cut off from the cord is unsale. Discard the
mains plug I! I suitable disposal facility. NEVER UNDER ANY CIRCUM-
STANCES SHOULD YOU INSERT A DAMAGED OR OUT MAINS PLUG INTO
A 13 AMP POWER SOCKET. Do not use the mains plug without the fuse cover
in place. Replacement ruse covers can be obtained from your local retailer.
Replacement fuses are 13 amps and MUST be ASTA approved to 381362.

LITHIUM BATTEBY WABNINQ

CAUTIONI

Thls product contains a lithium battery There is danger of explosion if battery is

' replaced. Replace only with an Eveready CR 2032 or equivalent.
Make sure the battery is installed wlm the correct polarity. Discard used batten

' ies awarding to manufacturer's Instructions.

ADVAFISELI

thiurrbatterl - Eksplosidnsfere. Ved utsldtting benyttes ltun batten sum erbe-

lelt IV epperaltabrikarrten. Bruld batten returneree apparatleveranderen

ADVARSELI

thhiun'banerf - Elcpldelonclere no lojlagtlg handtcrlng. Udsklftnlng ma kun

tke med batten av earnrne febrllrat cg type. Lever det brugle batten tilbage til

Iavetal'lmren.

VAROITUSI

Paristo voi Idlehlaa. lbs to on wheellisesti nsennettu. Vaihda pansto einoas-

taan lanevalmistajan suosluelernun ryyppin. Hevlta kaytetty paristo valmislajan

ol'ljeiden nulaieesti.

VARNINGI

Emloslonsfara vid lelaktigt panorama Arrvlnd earnma battentyp ellar en ekvi-

velerIt typ sorn rekornmenoem av lop-rerulrvenurren. Kassara anvant batten

enllgt labrllramene instrukocr-I.

§AFETY INSTRUCTION§ IEQRQPEANI

NOTICE FOR CUSTOMERS IF YOUR UNIT IS EOUIPPED WITH A POWER
CORD.

WARNING: THIS APPLIANCE MUST BE EARTHED.
The cores in the mains lead are coloured in accordance with the following code:
GREEN and YELLOW - Earth

BLUE ~ Neutral BROWN - Live

I As colours of the cores in the mains lead of this appliance may not correspond

with the coloured markings identifying the terminals in yDUr ping. proceed as Iolv
lows:

- The core which is coloured green and yellow must be connected to the
terminal in the plug marked with the latter E, or with the earth symbol. or
coloured green. or green and yellow.

- The core which is coloured blue must be connected to the terminal
marked N or coloured black.

- The core which is coloured brown must be connected to the terminal
marked L or coloured red .

The power cord is terminated in a 0557/7 plug (Continental Europe). The
green/yellow wire is connected directly to the unit's chassis. If you need to
change the plug, and if you are qualified to do so, referto the table below.

CONDUCTOR WIRE COLOR
Normal Alt
L LIVE BROWN BLACK
N NEUTRAL BLUE WHITE
E EARTH GND GREEN/YEL GREEN

WARNING: If the ground is detected. certain fault conditions in the unlt or in the
system to which It Is connected can result In full line voltage between chassis
and earth ground. Severe injury or death can then result If the ctrassis and
earth ground are touched simultaneously

THE FCC REGULATION WARNING

This equprnent generates and uses radio frequency energy, and If not installed
and used property. that is. in ether emordance with the manufacturers Instruc-
tions, may muse interlerences to radio and television reception. It has been
type tested and found to comply with the limits for a class B oomputlng device
in accordance with the speclflcanons tn Subpan J of Part 15 of FCC .
Rules.which are designed to provide reasonable protection against such inter-
ference in e resldenfiel Installation.

However. there is no guarantee that Interference will not ocwr in a particular
installation. It this equipment does muse Interterence to radio or television

- Reorlent the recetvlng antenna.
Move the equipment away from the receiver.

IMPORTANT!
WW

WATER AND MOISTURE: Appliance should not be used near water (eg nu:
a bathtub. washbowl. kitchen sink. laundry tub. in a wet basement. or near a
swimming pool. etc). Care should be taken so that objects do not fall and to
uids are not spilled into the enclosure through openings.

POWER SOURCES: The appliance should be connected to a power empty
only of the type described In the operating instrunions or as marked on the
appliance.

GROUNDING OR POLARIZATION: Precautions should be taken so mat the
grounding or polarinticn means of an appliance is not defeated.

POWER GORD PROTECTION: Power supply cards should be routed so that
they are not likely to be walked on or pinched by frame placed upon or against
them. paying paniwlar attention to cords at plugs, commence receptacles.
and the point where they exit from the appliance.

SERVICING: The user should not attempt to service the appliance beyond that
described In the operating instructlons. All other servicing should be relerred to
dualitied service personnel.

FUSING: If your unit is equipped with a fuse receptacle. replace with only
same type fuse. Reler to replacement text on the unit for correct fuse type.

reception (can be determined by turning the equipment oft end on), the user ls encouraged to try to correct the interference by one or more of the following measures:

- Relocate the equipment with respect to the receiver.
- Plug the equipment into a different millet so that equipment and rocelver are on different branch circuits.

ll necessary. the user should consult the dealer or an emenenced radio/television technician for additional suggestions. The user may find the following booklet pre-
pared by the Federal Communicatiors Commission helpful: How to Identity and Resolve Radio-TV Interference Problems. This booklet is available from the US
deemment Printing Office. Washington 0.6. 20402. Stock No. one-000003454.

Page 7

Ll

OPERATING CONTROLS db: paopsssmmiu. PRODUCTS

Front Panel
54m near/mow (us; DE-ESSING (.15) pm amt/never ms} 4 M A s r s n
30 27 24 21 15 15 I] 9 5 3 GAINHIJLD I 5 '30 -27 -24 27 ~18 ~15 -IZ -5 -3 I]
609003 @630 O 00 0000000000 0 O O
5 BYPASS srsnra BYPASS
COUPLE
l a i
a ll] Low HIGHID 0 10 O O O
omsrrv GAIN you) DE-ESSINE SPECTRAL ENIM CEMENT aurpur 1 ' fl-

Note: The following controls are available tor both channels of the 168A Studio Dynamics Processor (except for the
STEREO COUPLE button - see below).

GAIN REDUCTION (dB) Meter: This meter shows the true peak gain reduction in dB. It the LED at the far right
of the meter lights, no further gain reduction is available. When the STEREO COUPLE button is pressed In, the
Channel 2 GAIN REDUCTION meter tracks the Channel 1 GAIN REDUCTION meter.

DRIVE Control: The DRIVE control determines the amount of gain reduction by adjusting the level of the audio be-
ing input into the Compressor.

DENSITY Control: Use this control to speed up or slow down the program-dependent release time (in order to
match the processing to the type of audio being processed). The scale is arbitrary because the release time is
automatically varied according to the nature of the program material (to minimize audible compressioninduced
side effects). Higher settings (towards 10) are faster.

GAIN HOLD Control and GAIN HOLD LED: The GAIN HOLD acts as a "silence gate," determining the input
level below which the Compressor Gain circuitry is "frozen" to prevent noise rush-up during pauses and low-level
program material.

The GAIN HOLD LED lights when the input level falls below those set by the GAIN HOLD control. This indicates
that the Compressor's Gain control circuitry is "frozen."

DEESSING Control and lldB) and 6(dB) LEDs: Use this control to set the sensitivity of the De-Esser as a per-
centage of the average program level atthe 168A's input. This means that the De-Esser tracks the input level so
the amount oi de-essing remains constant with changes in input level.

The first segment at the DeEsser meter (green LED, labeled 1) lights when the De-Esser has caused 1dB ol'gain
reduction; the second segment (red LED, labeled 6) lights when SdB of gain reduction occurs. It is normal for both
segments of the meter to light when you are using a substantial amount of de-essing, but when the red segment
lights, you should listen careiully to make sure that you are not over-de-essing the program material ,

LOW SPECTRAL ENHANCEMENT Control: The 168As LOW SPECTRAL ENHANCEMENT simultaneously
applies an BOHz boost and 250Hz cut to the program. This lets you apply substantial bass boost without making
the program sound muddy or boomy due to excessive mid-bass boost.

HIGH SPECTRAL ENHANCEMENT Control: This control determines the amount of High-Frequency Spectra|<
Enhanced signal added to the input signal. Spectral Enhancement is a form of dynamic phase and amplitude
equalization. Continuous analysis of the input signal intelligently and automatically determines the amount of
equalization necessary at each moment to achieve detailed, defined audio that is never shrill or over-sibilant.

OUTPUT Control and PEAK OUTPUT LEVEL (dB) Meter: The OUTPUT control sets the level at the bal-
anced line output.

Page 25

SPECIFICATIONS db: FROFESSlONALPRODUCTS

De-Esser

Characteristic: The De-Esser performs quick. wideband gain reduction whenever the ratio of power above approxi-
mately 6kHz to the total signal power exceeds a threshold set by the user with the DE-ESSING control.

Distortion: The De-Esser uses the same VCA as the Compressor and therefore adds no significant distortion to the
system specification. *

Release Time: Approximately SOms, program-dependent.
Interchannel Coupling: Each channels De-Esser operates independently at all times (the use of last release
times precludes disturbances of the stereo images stability)

~' "4.. * ~ ,3, t. . ¥ ..

' '_' min;

High Frequency Spectral Enhancer

Characteristic: The HF Spectral Enhancer is a program-controlled shelving equalizer. its control circuit monitors
the relative high frequency power content of the program and dynamically adjusts the equalization to make this
high frequency content more consistent at the HF Spectral Enhancers output,

Maximum HF Boost: Approximately 15dB.

Total Harmonic Distortion: The HF Spectral Enhancer will add no more than 0.05% THD to the program material
present at its input. (Because it receives the output of the Compressor andDe-Esser, it will amplify any harmonics
generated in this earlier circuitry as would any other HF equalizer.)

Noise: The noise floor of the HF Enhancer circuit itself at maximum boost is more than 106dB below its output clip-
ping level, and is determined by the dbx 2155 VGA that it uses as a variable-gain element. (Because it receives

the output of the Compressor and De-Esser, it will amplify any noise generated in this earlier circuitry as would
any other HF equalizer.) ' 3

Low Frequency Spectral Enhancer

Characteristic: The LF Spectral Enhancer is a fourth-order equalizer with a bell-shaped boost at BDHz and a bell-
shaped cut at 250Hz. The ratio between the boost and cut (in dB) is approximately 2:1. The LOW SPECTRAL EN-
HANCEMENT control determines the absolute amount of boost and cut, 0 to +12dB. (see Figure 2.)

Noise and Distortion: The noise and distortion introduced by the LF Spectral Enhancer are well below noise and
distortion introduced by the rest of the 168As circuitry.

Physical

Dimensions: 19 (48.3 cm) wide, 103/5 (26.4 cm) deep, 13/4 (4.5 cm) high.
Operating Temperature Range: 32e113°F (ii-45°C).

Humidity: 0-95% RH, non-condensing. 1

Power Requirements: 115/230 volts AC i10%, 50-60Hz. 16VA. IEC-standard detachable mains cord. EMI-sup-
pressed.

Fuse: Use a 1/2-amp SAG 250V Slow Blow tuse tor 115V operation; use a ia-amp (250mA) "T'-type fuse for 230V V
operation.

Warranty
Two Year, Parts and Labor: Subject to the limitations set forth in dbxs Standard Warranty Agreement.

Page 28

1 68A

CIRCUIT DESCRIPTION

IC102 stays roughly constant. So the more gain that the DRIVE oontrol attempts to
force lC102 to produce, the more gain reduction is produced by the Compressor. Thus
the DRIVE control determines the amount of gain reduction in the Compressor.

Through current source 0102. control VR105 determines the average amoum of
current removed from the timing circuit (and thus the release speed). Pulling 0102s
collector to -15V (through |Ci10-A) diverts the release current from the timing circuit,
defeating the release process and freezing the gain.

OverEasy circuit lC105-C, IC105-D receives the output of the timing circuit and
generates a soft-knee compression characteristic by progressively increasing its gain
as its input increases. The circuit is arranged so that 20dB of gain reduction in the pilot
compressor circuit becomes 10dB gain reduction at the output otthe OverEasy circuit.
Beyond this, the compression ratio becomes approximately 4.5:1. CR105 generates ,
the increasing gain as it conducts more and more heavily with increasing input.
CR106 and associated components provide temperature compensation.

IC104-B sums the output of the OverEasy circuit with fixed offset voltages from the
DRIVE and OUTPUT controls. The output of lCt 04-3 is summed with the output of
the De-Esser (lC1DS-C) and applied to the gain control port of 10101. The output of
ICtO4-B also feeds the STEREO COUPLE switch. When the unit is in stereo mode,
the output of lot 04-B thus determines the gain of both Channels 1 and 2 (other than
de-essing; the De-Essers remain independent because the De-Esser control voltage
is summed with the output of lCt 04-8 directly at the gain control port of |C101).

The gain-holding gate is activated when the output of IC110-A is negative, and
defeated when it is open (the outputs of quad comparator IC110 are open-collector).
The gate circuit gets its information aboutthe input level from the output of broadband
rms detector lC106. lCi10-A and lC110-B receive the output of lot 06. GAIN HOLD
control VR107 sets the reference voltage on lCi 10-A and lCt10-B, determining the
threshold (referred to the 168As audio input level) where gating occurs. ICt to-As
output defeats the release process by diverting release current from the timing circuit.
lC110-B drives the GAIN HOLD indicator circuit.

To save current, the Channel 1 GAIN HOLD, Channel 2 GAIN HOLD, and MASTER
lamps are in series. lCt 10-D turns the Channel 1 GAIN HOLD lamp On and Off by
diverting its current to the -15V supply. When the channels operate independently.
IC111-C turns the Channel 2 GAIN HOLD lamp On and Off by shorting it out with
0103. When the 168A is in stereo mode, the output of lCt 1 1-0 is forced to the -15V
rail by R180 and R181 . turning 01 03 Off. In stereo mode, the collector 010104 goes
to ground, permitting lCi10«C and lC110-D to switch identically. Thus |Ct 1 O-C
causes both the Channel 1 GAIN HOLD and Channel 2 GAIN HOLD lamps to switch
simultaneously, following the Channel 1 hold circuit.

Current source 0503. R542, R543, R544, CR506 applies a constant current to the
series-connected diodes in LED assembly CR4. A string of ten comparators lCSOB-C
to lC$10-D receives the dB-Iinear gain reduction voltage. The comparators are biased
with DC reference voltages (by voltage divider string R511, R513-R521, R546) from
0.9 to 9.0VDC in 0.9V steps. Each 0.9V step corresponds to adB additional gain
reduction. When the gain reduction is less than adB, the gain reduction voltage is less
than 0.9V and all the outputs of all comparators are active negative, shunting current
from the seriesconnected LED string. As the gain reduction increases. the output of
each comparator in turn becomes open collector and current is permitted to flow in
the LED string until it encounters a comparator whose output is still active negative,
at which point the active comparator shunts the current away from the rest of the
LEDs.

A 50 or SOHz dither" signal is injected into the reference voltage string through 0527,
R512 (connected to the power transformer secondary). This AC signal modulates the
turn-on threshold of each comparator. causing the LEDs to appear to smoothly fade