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VMQ Series User Manual P/N 821504 Rev-D
Section 3: Tecnical Information
General Mechanical Observations
Elimination of cabinet and component sympathetic vibrations (resonances) requires considerable attention to
mechanical details. Because of this, and the servere physical constraints of the speaker's acoustic enclosures,
seemingly minor changes to any of the mechanical details of the unit will frequently seriously impair its acoustic
performance. If mechanical work on the unit is absolutely necessary, be sure to make adequate notes to permit accurate
reassembly. Unfortunately, the unusual and wholly proprietary method of magnetic shielding is usually degraded
somewhat by ANY disassembly of the unit, except removal of the rear panel. The fuse for channels 1 and 2 is locted
along the back edge of the main circuit board near the power jack; it may be replaced by removing only the rear panel.
Almost any other maintenance or repair will require removal of the cover.
Audio Circuit Description
Input Section
The balanced input is buffered by a differential input of the audio signal to dual op amp ICs 1and 2. The output of
ICs 1 and 2 goes to the input of the quad VCA (U7) after receiving a slight treble boost. The output also goes to a
quad, high-gain rectifier (IC8), used to drive the LED activity indicators, which indicate the presence of an input signal
in each channel.
Volume Controls
Volume controls are implemented through a quad VCA chip (U7). The control potentiometer for each VCA is buffered
by IC4 and IC5. Section A and B of the dual IC each buffer one volume control and an associated mute switch.
Clockwise rotation of the volume pot decreases the wiper voltage causing the IC4/5 output to rise, which decreases
the audio level of the VCA. The mute switch performs this function instantly by taking the IC4/5 inverting input directly
to ground.
Limiting Circuitry
The output of the four VCA's are summed into IC6 and IC7 where the audio waveform's conversion to voltage takes
place, and the rectified output from each of these stages is the input to a corresponding non-inverting section of IC4
and IC5. These non-inverting sections develop limiting control voltages for each VCA section. A voltage above 2
volts across D17, D18, D19, and D20 will cause a lower gain in the corresponding section of the VCA.
Headphone
The output of the four VCA's (IC6 and IC7) is also the input circuitry for the headphone amplifier as well as the input
of the speaker amplifiers, through IC8. The signal passes through U8, a quad CMOS switch. The control input is
normally high (above 10 volts) on its way to the speaker amplifier; as well as going to the summing resistor pack
SP17, and capacitors C97 and C98 for use with an optional woofer. Channels 1 and 2, which are the outputs of IC6,
A and B, are also summed to the input of IC9, which drives the left headphone channel output. IC7, A and B, is summed
to the input of IC10 for the right headphone channel. Jumping JP13 will cause all signals to be summed to both
amplifiers, thus causing the entire signal to become monaural in the headset.
Headphone Output/Speaker Muting
While headphones are not plugged in, the output terminals are open circuit and the op amp DC bias supplied by R7
and R8 keeps pins 2 and 3 of H1 high. Plugging headphones in provides a shunt path to ground for this DC bias through
the headphones, which turns off transistors Q1 and Q2. This sets pins 9, 10, and 11 on analog switch U8 high, which
switches the signals that normally go to the speaker amps to AC ground (bias) instead. The RC networks of C9 and
C10/R9-R11 filter any high-level low frequency transients, which might otherwise affect Q1 and Q2. Note that for
this sensing circuit to work, the headphones MUST have a DC resistance not greater than about 1K ohm.
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© 2006 Wohler Technologies Inc. ALL rights reserved
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