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AUDIO CIRCUIT DESCRIPTION
Input Section (Audio)
The balanced input is buffered by a somewhat unconventional cascade differential input stage of Sections A and B of IC1
(left ch.) and IC2 (right ch.). H1 provides a point where alternative signals from certain options may be input, and buffered
output from the primary input of IC1 and IC2 are available.
Volume Controls
Unlike our earlier circuits, volume controls for the ATSC series units are implemented with VCAs. The primary advantages
of this method are a reduction in possible signal degradation from having to physically route the audio signals through the
volume/balance controls. This is especially true for the inevitable wiper contact degradation in the pot itself, which no longer
results in scratchy noise in the audio.
Quad VCA, Sections A and B, effect either volume-balance or A volume-B volume control functions, depending on which
headers the pots are plugged into, and the setting of jumpers on JP3. An increasing voltage above a DC refence level on the
control terminal of each VCA section increases attenuation of that section. For volume-balance operation, the volume control
pot for both channels is connected to H3. Its wiper voltage is applied directly to the control terminal for channel B, and via
a jumper on JP3 to channel A's VCA. The balance control is connected to VCA, Sections C and D, which are also used for
overall limiters for the side speakers. In a unit configured for A volume - B volume operation, pins 1 and 2 of JP3 are
disconnected so that the "primary vc" pot is connected only to VCA, Section B. The other pot is connected to H4 instead of
H5, so it will directly control VCA, Section A. By disconnecting pins 3 and 4 of JP3, the balance control circuitry is configured
for minimum quiescent attenuation in VCA, Sections C and D.
Headphone Output
While headphones are not plugged in, the output terminals are open circuit and the op amp DC bias supplied by R70 and R71
keeps pins 2 and 3 of H9 high. Plugging headphones in provides a shunt path to ground for this DC through the headphones,
which turns off transistor Q11. This sets pins 9, 10, and 11 on analog switch IC16 high, which switches the signals that normally
go to the speaker amps to AC ground (bias) instead. The RC networks of C49 and C50 / R74 through R76 filter any high-
level low frequency transients which might otherwise affect Q11. Note that for this sensing circuit to work, the headphones
MUST have a DC resistance not greater than about 1K ohm.
Equalization and Side Speaker Outputs
Most of the unit's overall equalization occurs at IC8 and IC9. For the side speakers, there is an additional upper HF shelf boost
from the peaking network just after the input stage at IC8. There is a small amount bass summing at R101 and R102, which
is directly after the main EQ stage. The side speaker amplifiers are conventional IC power amps.
Low Frequency Section
The low frequencies are summed at IC11; the output is on pin 1. R80 and R81 (on pin sockets) adjust the bass output level,
but this also adjusts the bass limiter threshold relative to the unit's input, and therefore, also the amount of bass limiting. R82
and C53 set this stage's low pass filter rolloff point, the first of several low pass filters.
The low frequency (LF) section has it own limiter circuit controlled by a Vactec type opto- isolator. The LF signal is applied
to the base of Q13 for positive excursions, Q14 for negative. At low signal levels, there is not enough output from the transistors
to turn the LED in the Vactec on. When the signal is great enough at the bases of the transistors, they supply current to the
LED. This decreases the resistance of the Vactec's photocell, increasing the attenuation of signal at IC17b. IC20b provides
additional HF rolloff for the active crossover function. The bass level after the limiter stage is set by R115 (on pin sockets).
Power amps IC19 and IC21 are arranged in a differential input push-pull output. NOTE: There are NO coupling capacitors
on the power amp outputs; care must be used to avoid shorting EITHER speaker terminal to chassis, so that destructive DC
currents are not accidentally passed through the woofer!
© 2006 Wohler Technologies Inc. ALL rights reserved
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