Panpots

Voltage Controlled Panner circuit

Voltage Controlled Panner circuit
(c) G. Forrest Cook 1994

Description:
This circuit is useful for audio effects and may be used as
a standard plug-in module for voltage controlled synthesizers.
The circuit has one control voltage input, one audio input and
two audio outputs.  The control voltage input can be supplied
with a continuously variable voltage from 0V to 10V, 0V pans
the signal  to the left output, 5V pans the signal to the center,
and 10V pans the signal to the right output.

Theory of operation:
The control voltage is fed into the first half of a 1458 op-amp,
this stage inverts the signal and sets the offset and gain for the
right channel gain control circuit.  This signal is then fed into
the second half of the 1458 op-amp which inverts the voltage again
and generates the signal for the left gain control circuit.  The
10K trimmer adjusts the Left channel gain, set it for equal gain
from both channels with 5V on the control input.  Each audio stage
is identical, only one is shown in the schematic.  The pair of
2N3904 transistors form a differential pair that provides voltage
controlled gain.  The two transistors should be matched, see the
note on the schematic.  A pair of transistor sockets may save you
a lot of soldering.  The gain control stage is fed into half of
an NE5532 low noise audio op-amp for further amplification and
buffering.

Construction:
I recommend building this circuit on perforated circuit board
or make a PC board if you have the tools.  Keep the input lead
lengths short and provide a filtered +/- 15V power source.

Use:
Connect the input to an audio signal source, typically an analog
synthesizer.  Connect the outputs to a stereo amplifier.  Apply
various wave-shapes to the control voltage input, listen to the
results.  Sine waves provide a nice control signal and I have
experimented with a microprocessor controlled digital to analog
converter (DAC) that can generate arbitrary wave-shapes with
interesting results.  If you have a modular synthesizer (lucky
you), an interesting thing to do is send a triggered modulation
wave to the panner, try using the wave that controls the VCF
for starters, one can get fancy and send a triggered amplitude
modulated sine wave to the panner for a great effect.