Audio Notch Meter
A variable notch filter with both high and low pass filters.
At first glance this circuit looks fairly complex, but when broken down,can be divided into high pass and low pass filter sections
followed by a summing amplifier with a gain of around 20 times. Supply rail voltage is +/- 9V DC. The controls may also be adjusted
for use as a band stop (notch) filter or band pass filter.