The filter produces three outputs: high-pass, bandpass, and low-pass. Frequency is linearly proportional to the gain of the two integrators. Two CA3080s, (IC2, 4) provide the variable gain, the resonant frequency being proportional to the current ^. Using 741 op amps for IC3a control range of 100 to 1, (resonant frequency) can be obtained.If CA3140s are used instead of 741s then this range can be extended to nearly 10,000 to 1.
Audio Variable Filter Circuit Diagram
