Variable filter

  
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.
Variable filter  - schematic

If CA3140's are used instead of 741's then this range can be extended to nearly 10,000 to 1.




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