Voltage Controlled Panner

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. Each gain control stage is fed into an NE5532 low noise audio op-amp that is wired as a differential amplifier.
Voltage Controlled Panner - schematic

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. Taking the circuit further, it should be possible to replace the two VCA circuits in this panner with a National Semiconductor LM13700 dual transconductance amplifier IC wired as two voltage controlled amplifiers. See the LM13700 applications notes for details.

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