dbx 3BX III User manual

Model 3BX III
Three-Band
Dynamic-Range Expander
with Impact Restoration
SERVICE MANUAL
All dbx products are manufactured under patents in the US and abroad, and on all
dbx circuit designs dbx holds copyright in one or more of the following years:
1979-’85. “dbx” is a registered trademark of dbx, Newton, Mass. USA.

CONTENTS
SPECIFICATIONS (performance minimums)............................ 4
CIRCUIT DESCRIPTION.............................................. 1
SCHEMATIC CONVENTIONS .......................................... 1
SIGNAL PATH .................................................... 1
CONTROL-VOLTAGE (CV) PATH ...................................... 3
High-Band CV Path ............................................ 5
Low-Band CV Path ............................................. 7
Mid-Band CV Path ............................................. 7
LED DISPLAYS ................................................... 7
POWER SUPPLY and MUTE GENERATOR ................................ 8
ALIGNMENT PROCEDURE.............................................. 9
1. INSTRUMENTS REQUIRED ........................................ 9
2. INSPECTION and WARMUP ....................................... 9
3. POWER-SUPPLY TESTS .......................................... 9
4. CONTROL-CIRCUITRY TESTS ..................................... 9
5. VCA-SYMMETRY ADJUSTMENTS ................................... 12
6. VCA-GAIN ADJUSTMENTS ....................................... 13
7. PERFORMANCE TESTS .......................................... 14
MODEL 3BX III PRINTED CIRCUIT BOARD ASSEMBLY.................... 20
MODEL 3BX III MAIN ASSEMBLY..................................... 25

SPECIFICATIONS (performance minimums)
Expansion
To 50% increase,
maximum 12 dB upward
and 20 dB downward
Impact restoration
To +12 dB (upward
only), program-
dependent
Frequency response +0.5 dB 20 Hz –
20 kHz,
any setting
Dynamic range 107 dB
Total harmonic distortion (THD), no
expansion 0.15%
Intermodulation distortion (IMD) IHF or
SMPTE 0.1%, any setting
Equivalent input noise –90 dBV
Attack rates Program-dependent,
optimized for each band
Release rates
Linear expander program
-dependent, optimized;
impact restorer
adjustable
Maximum input and output 7 V
Notes
1) Specifications are subject to change without notice.
2) All data are for 20 Hz-20 kHz unless otherwise specified; line inputs
are driven by a source impedance of 1 k-ohms and outputs are loaded by
10 k-ohms in parallel with 1000 pF; all voltages are rms (root-mean-
square).
3) Dynamic range is defined as the difference between the maximum rms
output signal and A-weighted noise. All noise figures are A-weighted.
4) Frequency response figures are for pink noise (or music).
5) SMPTE IMD is measured with 60 Hz and 7 kHz mixed 4:1; IHF (difference-
tone) IMD is measured with 19 kHz and 20 kHz mixed 1:1; output 1 V.
6) Inputs and outputs have identical polarity.
7) All dbx home products are designed to be used with components whose
output impedance is less than or equal to 5 k-ohms. All units are
designed to drive loads of at least 5 k-ohms in parallel with 1000 pF or
less.

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CIRCUIT DESCRIPTION
SCHEMATIC CONVENTIONS
The 3BX III is a stereo unit, with two independent, identical signal
paths. We will refer here to the left channel only (channel 1). Almost
all of its components are identified by designations ending in L (e.g.,
C705L, R708L); the right channel’s component designations generally end
in R; and components common to the channels generally end in numbers
(e.g., C803, R801). Furthermore, components are coded according to their
function within the system. The 700 series indicates the signal path or
detector path (the detector section is common to both channels, so 700
components ending in a number are in this area, while those ending in L
or R are in the signal path). The 800 series indicates the power supply.
Finally, the L## series indicates the LED display (e.g., RL21, UL05).
SIGNAL PATH
Refer to Fig. 1, a block diagram of the signal path (note that L, R
suffixes are not shown and that test points [TPs] are for both channels,
left first.) Audio input signals first are buffered by 1/2-U701L and its
associated circuitry. The output of 1/2-U701L is TP1 (TP2, right
channel). The signal there should be identical to that at the input
except for a small attenuation (-0.83 dB) and a roll-off at 175 kHz.
Figure 1, Signal Path

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This signal is applied to a 4.21 kHz, two-pole high-pass filter (2/2-
U702L), a 210 Hz, two-pole low-pass filter (1/2-U702L), and a summing
stage (2/2-U701L). The summer subtracts the outputs of the high-pass and
low-pass filters from the output of the buffer, forming a band-pass
filter with cut-offs at about 4.21 kHz and 210 Hz. Its roll-off will be
6dB/octave (with some peaking at each corner frequency), while that of
the high-pass or low-pass filters is 12 dB/octave. Fig. 2 shows the
frequency responses of these filters. Note that in their pass-bands the
high-pass and low-pass filters are unity (0 dB) gain.
Figure 2, Low, Mid, and high Filters, 10 dB/division
The outputs of each filter are applied to the inputs of voltage-
controlled amplifiers (VCAs), one each for the low (U706L), mid (U705L),
and high (U704L) ranges. The gain of each VCA is independently
controlled by a voltage derived from the rms-level detector for each band
and the front-panel control settings. This circuitry is described in the
Control-Voltage Path section, next page.
The VCA’s gain in decibels is proportional to the voltage at pin 3 of
each IC. Ideally the gain is 0 dB when the voltage is 0.00 mV; gain
trims in the control-voltage (CV) path (see section 6 in the Alignment
section) allow for a small adjustment. When pin 3 is positive, the gain
in dB is negative (less than unity); when pin 3 is negative, the gain in
dB is positive (greater than unity). Also under ideal conditions,
variations in pin 3 voltage will not cause any variations in the dc level
at the output of a VCA, but in the 3BX III, symmetry adjustments (VR701L
for the low band, VR702L for the mid, VR703L for the high) are provided
to compensate for non-ideal performance. (See the Alignment Procedure,
sections 5.1 through 5.6, p.12 and following.) For a detailed
explanation of this part, refer to the booklet on the VCA IC.
The outputs of all three VCAs are connected to a single summer stage,
2/2-U703 (in the right channel this stage is 1/2-U703). This op-amp
converts the VCA-current output to a voltage signal and recombines the
three bands into one. Note that the signal at pin 6 of U703 (pin 8 of
the VCAs) is a current, not a voltage, which means that there won’t be
very much voltage at pin 6 of U703 even if the VCA is working properly.
By the way, a relatively large signal at this pin (more than 10-20 mV)
usually indicates a fault with U703.
The output of 2/2-U703 goes through an RC-coupling stage (R731L, C776L,
R732L) before connecting to the FET-based muting circuit (Q802 in the
left channel, Q801 in the right channel). These FETs are turned on for a
short time whenever power is applied or turned off, to attenuate the
output during power-up and power-down transients.

Model 3BX III Service Manual
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Finally, the signal passes to the switching circuitry. The switching
allows the 3BX III signal path (a) to come either before or after a tape
deck plugged into the tape jacks of the 3BX III, (b) to receive its
signal from either the tape deck or the source, and (c) to bypass the
circuitry.
CONTROL-VOLTAGE (CV) PATH
The 3BX III has two main purposes: first, to make loud signals louder
while making soft signals softer (upward and downward expansion), and
second, to emphasize musical transients (impact restoration). The three
rms detectors translate the signal level in each of the three bands into
voltages (at dc or nearly so) that indicate how loud the input signal is
in each of these bands. These voltages can then be processed and used to
control the gain of the VCAs in the 3BX III to accomplish its two
purposes. Choosing the correct time constants for the rms detectors in
the first place is critical to doing this job right. Then the rms-
detector signals must be processed to find the transients and produce
signals suitable for increasing the music’s impact. All of this
detecting and processing takes place in the CV-path section of the
3BX III.
Refer to Fig. 3, a block diagram of the CV path. The audio signals at
the outputs of each of the signal-path filters are connected to the
inputs of three rms-level detectors, one for each band (U707 is the low-
band detector, U708 the mid-band, U709 the high-band). For each band,
the left- and right-channel signals are summed at pin 1 of the rms-
detector IC. For a detailed explanation of this part, refer to the
booklet on the rms-detector IC.
Figure 3, Control-Voltage Path

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The output of these ICs (pin 7) is a dc voltage proportional to the dB
signal level at the output of the filter that feeds them. TP9 is the
low-band output, TP11 the mid-band, and TP13 the high-band. Table 1
shows the voltage and frequency required (simultaneously at the L- and R-
channel inputs) to cause (ideally) 0.00 mV at pin 7 of each rms-detector
IC. The exact calibration of these voltages is not critical, since
adjustments are provided in the stage following the rms detector.
Input Voltage Input Frequency Rms IC Test Point Approx.
Voltage
227 mV 100 Hz U707 TP9 0.00 mV
88 mV 1 kHz U708 TP11 0.00 mV
69 mV 10 kHz U709 TP13 0.00 mV
TABLE 1
Input levels and frequencies (both channels driven)
for 0.00 mV at the rms output
Along with the dc voltage is a small amount of ripple. Ideally, it will
be at twice the frequency of the input, with no fundamental. Trim pots
(VR704 in the low band. VR706 in the mid. and VR708 in the high) are
provided to allow this waveform to be adjusted for perfect symmetry (see
the Alignment Procedure, sections 4.1-4.3, p.9 and following). For
larger input signals the rms-IC output is positive, and negative for
smaller input signals; it varies by 6 mV for each decibel of input-level
change.
The time-constant of the rms detector is dependent on the capacitance
connected to pin 6. The larger the capacitor, the slower the time
constant. The mid-band and high-band rms-detector ICs have small
capacitors connected directly between pin 6 and ground (C731 in the mid
band and C741 in the high band). Also connected to pin 6 is a larger
capacitor (C720, C729, C740), which connects to an op-amp. This
capacitor and the circuit to which it connects form a “nonlinear
capacitor” with an equivalent “value” that changes with the signal
conditions. If there are problems in verifying the expansion timing (see
alignment procedure, section 7.2), check that this circuit is operating
correctly by probing the output of the associated op-amp (pin 1 of U710
in the low band, of U711 in the mid band, and of U722 in the high band).
The output of each op-amp should be a sinusoidal wave at twice the
frequency of the input signal. See Table 2 for appropriate frequencies
and levels.
Input Voltage Input Frequency Op-amp Pin
Number
Approx
Voltage (p-p)
1.0 v 50 Hz U710 1 60 mV
1.0 v 100 Hz U711 1 540 mV
1.0 v 100 Hz U722 1 510 mV
TABLE 2
Test conditions for the nonlinear-capacitor circuit

Model 3BX III Service Manual
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The rms-detector outputs connect to inverting buffer stages (2/2-U710,
2/2-U711, and 2/2-U722) with gains of 9. At these buffers, individual dc
voltages from trim pots VR705, VR707 and VR709 are added to the rms-
detector output voltages, and a single dc voltage from the Transition-
Level control (VR710 on the front panel) is added to all three stages.
The trim pots allow the outputs of the rms detectors to be calibrated to
specific references (see the Alignment Procedure, sections 4.1, 4.2 and
4.3), and the front-panel control allows the entire system’s unity-gain
point (no upward or downward expansion) to be adjusted by the customer to
match the levels in his or her stereo system.
TP10 is the low-band rms-buffer output, TP12 the mid-band, and TP14 the
high-band. The CV path now splits in two, differently for the high- and
low-bands from the mid-band. First the high band…
High-Band CV Path
TP14 connects directly to the high-band-expansion control (VR715) on the
front panel. This is one section of a three-gang pot (the other sections
are labelled VR713 and VR711) which controls the amount of rms-detector
signal that eventually reaches pin 3 of the high-band VCA IC. Setting
the pot for more expansion causes more of this signal to be allowed
through. At 50% expansion, a 100 mV change at the rms output (TP13)
causes a negative 50 mV change at pin 3 of the VCA (TP7). When the
signal at TP14 is positive, the gain of the high-band VCA will be either
negative in dB or 0, depending on the position of the expansion control.
When TP14 is negative, the gain will be positive or 0. The wiper of the
expansion control connects to the positive input of a summer stage
2/2-U719) whose output is sent on to the VCA.
TP14 also connects to the impact detector (U720 and associated
circuitry), which differentiates the rms-detector-output waveform and
clips off the negative-going portions of it. Its output looks like a
sharp positive-going spike every time a sudden increase in input-signal
level takes place. This positive-going spike will cause the gain of the
high-band VCA to increase (how much it increases depends on the setting
of the Impact-Restoration control). Note that because the impact
detector clips off the negative portions of the control signal, the
impact restorer never causes negative gain (unlike the Expansion
section). See Fig. 4, next page, for typical waveforms in the impact-
restoration part of the CV path (note that column ‘a’ is low, ‘b’ is mid,
‘c’ is high). The output of the impact detector connects to a time-
constant circuit (1/2-U717, 2/2-U717), which stretches this spike out for
atime determined by the setting of the rear Impact-Release-Rate control
(VR723).
The output of the time-constant circuit is directly connected to the
impact-level control (VR716) on the front panel. This, too, is one
section of a three-gang pot (the other sections are labelled VR714 and
VR712), and it controls the amount of impact-control signal that reaches
pin 3 of the high-band VCA. The wiper of the pot connects to the impact-
disable circuit (Q703, driven from 1/2-U721), which turns off the impact-
control signal at low signal levels. This prevents record-surface noise
and other small signals from being raised in volume by the impact-
restoration circuitry.

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All photos:
All front controls @ maximum;
Rear trim (Impact Release Rate) centred
a&bphotos: c photos;
300 Hz tone-burst @ 316 mV rms (447 mV peak),
128 cycles on, 172 cycles off, 0.2 s/div
3kHz tone-burst @ 316 mV rms (447 mV peak),
128 cycles on, 172 cycles off, 20 ms/div
4.a.1 4.b.1 4.c.1
Top: Low-pass filter output, U702L,
pin 1, or U702R, pin 7, 0.2 V/div;
Top: Mid-band summer output, U701L,
pin 7, or U701R, pin 1, 0.5 V/div;
Top: High-pass filter output, U702L,
pin 7, or U702R pin 1, 0.2 V/div;
Bottom: Low-band rms-detector output,
U707, pin 7 (TP9), 0.1 V/div.
Bottom: Mid-band rms-detector output,
U708, pin 7 (TP11), 50 mV/div.
Bottom: High-band rms-detector output,
U709, pin 7 (TP13), 0.1 V/div.
4.a.2 4.b.2 4.c.2
Top: Low-band rms-buffer output,
U710, pin 7 (TP10), 1 V/div;
Top: Mid-band rms-buffer output,
U711, pin 7 (TP12), 1 V/div;
Top: High-band rms-buffer output,
U722, pin 7 (TP14), 1 V/div;
Bottom: Low-band impact-detector
output, U713, pin 5, 0.2 V/div.
Bottom: Mid-band impact-detector
output, U714, pin 3, 0.5 V/div.
Bottom: High-band impact-detector
output, U717, pin 5, 0.2 V/div.
4.a.3 4.b.3 4.c.3
Top: Output, low-band impact time-
constants, U713, pin 1, 1 V/div;
Top: Output. mid-band impact time-
constants, U715, pin 7, 1 V/div;
Top: Output, high-band impact time-
constants, U717, pin 1, 0.5 V/div;
Bottom: Low-band VCA control voltage,
U706L or R, pin 3 (TP3 or 4), 0.1 V/div.
Bottom: Mid-band VCA control voltage,
U705L or R, pin 3 (TP5 or 6), 0.1 V/div.
Bottom: High-band VCA control voltage,
U704L or R, pin 3 (TP7 or 8), 0.1 V/div.
4.a & b.4 4.c.4
Left or right output, 5 V/div Left or right output, 0.5 V/div
Figure 4