I find a low pass filter on the speaker line cures any receiver's high
frequency bleed through. There was one published in QST a year or two
ago. Mine uses a 70 volt to voice transformer to raise the impedance.
Actually I switch the impedance and capacitances to get different cut
off frequencies with the same inductors. Then have a second transformer
to get back to speaker impedance. With no active devices in the filter
(7th order cheybychev low pass) there is no detectable noise added by
the filter.
Its not true that a twin-tee notch filter in the feedback will cause
greater reduction in the noise at the notch frequency. Quite the
contrary, a twin-tee notch filter in the feedback makes a bandpass
amplifier. At least 4 decades ago, maybe 6, General Radio based an audio
spectrum analyzer (manual type) on a twin-tee feedback network and an
amplifier.
Oh, it takes three R and three C to make a twin tee network.
73, Jerry, K0CQ
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