The filter produces three outputs: high-pass, bandpass, and
low-pass. Frequency is linearly proportional to the gain of the two
integrators. Two CA3080's, (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 CA3140's are used instead of 741's then this range can be extended to nearly 10,000 to 1.
Audio Variable Filter Circuit Diagram
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