WP-20 MODULE DESCRIPTIONS
VOLTAGE CONTROLLED FILTER (VCF) - (See figure 4)
IC2B and associated components from the mix-
ing and filtering section of the WP-20. The noise
signal is dropped across R47 (Noise Volume Con-
trol), picked off at the wiper and fed to the in-
verting input of IC2B via C19 and R27. Similarly,
the VCO output is dropped across R46 (VCO Volume
Pot), picked off at the wiper and is also fed to
the inverting input of 1C2B via C20 and R26.
C13, C14, R25 and Q3 located in the feedback-
loop of IC2B perform the voltage-variable filter-
ing function. Control voltage dropped across R38
is picked off at the wiper and current is fed
through R28 to the base of Q3. As current through
R28 increases the emitter to collector resis-
tance decreases. This causes the pole frequency
of the filter circuit to go up passing higher har-
monics of the signal being fed in to the output.
Damped oscillation inherent in the filter circuit
also adds color to the sound of the waveform of
the signal being fed in. When S14 is closed, C15
(470PF) is connected from the input of IC2B to
it's output. This adds more damping to the cir-
cuit, reducing it's resonance.
VOLTAGE CONTROLLED AMPLIFIER (VCA)-(See figure 5)
The input signal to the Voltage Controlled
Amplifier is fed into R24 and dropped on R23.
This signal is really riding on the DC output
bias of IC2B. It can pass from Q2's collector to
emitter, through C5 and R48 into buffer amp IC2D
if a current is being fed to Q2's base through
R22 and R7. S8 selects either the LFO or A.D.
outputs to modulate the amplitude of the signal
being fed into IC2D when R39 is turned fully
clockwise (marked 'Modulation' on front panel)
Turning R39 fully counter-clockwise (toward 9
volts) , turns Q2 steadily on. The output of IC2D
is dropped across R45 (Output Level Pot), picked
off at the wiper and fed through C18 to the out-
put jack.
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