Twin-T filters in series SOLVED

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

by Zandbass
December 13, 2018

2 Answers

Answer by mikerogerswsm

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.

ACCEPTED +1 vote
by mikerogerswsm
December 13, 2018

by mikerogerswsm
December 13, 2018

Answer by Zandbass

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.

+1 vote
by Zandbass
December 13, 2018

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


by mikerogerswsm
December 13, 2018

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 and these values gives me a narrow deep notch for these freq's with convenient values.

by Zandbass
December 18, 2018

Interesting. Thanks for that.

by mikerogerswsm
December 18, 2018

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?

by Zandbass
December 19, 2018

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.

by mikerogerswsm
December 19, 2018

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