dbx 3BX III, Service Manual

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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
a time 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.