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Hi, Here are two Twin-T circuits in series. Center frequencies are roughly 270 Hz and 2,7 K.

My questions: - Will these two filters influence each other, or can they be considered to be functioning independently? - Can this circuit be simplified?

Thnx in advance, Mark

The second filter will load the output of the first. Ideally there should be a unity gain buffer between them. An acceptable solution would be to put the second one first, because it is ten times lower impedance and the loading will be minimal. I assume conventional signal flow left to right.

To sharpen the notches, add a unity gain buffer after each filter and feed back one tenth of the output to the 'ground' of that filter.

http://fourier.eng.hmc.edu/e84/lectures/ActiveFilters/node4.html

Thanks for your suggestions. I'm aware that buffers will improve the circuit, but I'm also trying to keep the circuit as simple as possible. There is gain loss, but not enough to pose a problem. It's part of a small on-board mixer/preamp for an acoustic instrument. It will be used to suppress (acoustic) feedback-prone frequencies.

When you say the output of the first filter will be loaded, how does that translate into what I'll hear? (Sorry, I'm more of a musician than an electronics guy). I will try the reverse order first.

Looking a bit closer at your circuit, the component values are not what I'd expect.

See: https://www.radio-electronics.com/info/circuits/rc_notch_filter/twin_t_notch_filter.php

That's correct. I'm aware of the standard of the resistor and cap going to ground being half the value of the other two. But I calculated the values with here http://sim.okawa-denshi.jp/en/TwinTCRkeisan.htm and these values gives me a narrow deep notch for these freq's with convenient values.

Interesting. Thanks for that.

You're welcome :) Still wondering about this: When you say the output of the first filter will be loaded, what does that actually mean technically and sonically?

Any design maths is based on zero impedance drive and infinite impedance load. Other conditions will cause errors. The errors might be notch frequency or notch depth. This is because the complex load or drive impedance must be added to the in circuit impedance.