[patrick-turner]

Phil mentioned :-

"The notch filter typically consists of two cascaded stages, neither of which
is very deep so there is adequate bandwidth to the combined notch to cope
with minor drifts."

Electronics World produced a cascaded notch filter which I was tempted to try, but which seemed unecessary with tube circuits which rarely ever achieve 0.001% THD with power amps. I found the LC bridged T notch I have works well enough, and this, with a following x10 amp with a BPF for 1.4kHz to 14kHz seems to work OK and low levels of THD can be displayed on the CRO without excessive amplitude change due to filter or oscillator instability.

The idea of a wien bridge for notch filter also appeared in EW articles.

This all goes to show there are more ways than one to examine THD.

Patrick Turner.