A voltage controlled music signal filter circuit is shown where an input attenuator (R10 and R11) limits the signal amplitude presented to the FETs to about 0.1 volt p-p at O VU to ensure low distortion.
Output amplifier A7 makes up exactly for this loss. An op amp having external frequency compensation was used here so that this relatively high—gain stage could be tailored for flat response to 15 kHz (a 741 could be used, but would roll off slightly above 10 kHz). Resistors R16 and R17 attenuate the output signal by an amount equal to the gain, so that this amplifier doubles as the unity gain buffer required for filter operation. The highest cutoff frequency is dictated by minimum FET resistance and capacitors C1 and C2. The latter should have values in a ratio of about 3:1 to produce the desired Butter- worth response.
Output amplifier A7 makes up exactly for this loss. An op amp having external frequency compensation was used here so that this relatively high—gain stage could be tailored for flat response to 15 kHz (a 741 could be used, but would roll off slightly above 10 kHz). Resistors R16 and R17 attenuate the output signal by an amount equal to the gain, so that this amplifier doubles as the unity gain buffer required for filter operation. The highest cutoff frequency is dictated by minimum FET resistance and capacitors C1 and C2. The latter should have values in a ratio of about 3:1 to produce the desired Butter- worth response.
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