Voltage Controlled Amplifier (exponential)

The VCA module controls the amplitude of a synthesizer signal. The control voltage normally comes from an ADSR or LFO module.

The VCA can be controlled via two voltage curves simultaneously and processes two signal inputs. For the second input the voltage control can be disabled via a switch and the input signal passes the vca with maximum amplification. For the first input the input level and the reaction sensibility level concerning the control voltage can be attenuated by pots.
Besides that the output level of the synthesizer signal follows the envelope.

The schema is part of the book "Formant Pro MSS 2000" by H.J. Helmstedt.

Sound example:
VCA controlled by an ADSR.

Reproduction hints:
The schema in the book contains an error concerning the control voltage exponentiator: The transistors are drawn as NPN types; but PNPs have to be used.
As this circuit needs only a few components I rerouted the layouts and placed three VCAs on one eurocard.
Please send questions or remarks to:
Carsten Toensmann