variable-Q twin-T notch filter

[Paul Perry (Frostwave)]

Someone (?? Mark H?) mentioned variable-q twin-T notch filters, here's some info:
http://sound.westhost.com/project52.htm

[R.G.]

Also see the bootstrapped Twin T at GEO:

http://geofex.com/Article_Folders/EQs/paramet.htm

[Tim Escobedo]

I've played around with bootstrapped Twin T notch filters, in my usual hack style, and found they can easily "go weird" depending on the RC values and how stiff the bootstrapped signal is. The notch filter can easily go into oscillation.

I've wondered if anyone's ever tried to make a pseudo phase shift type effect using Twin T filters. Besides my feeble effort, that is. I think it might be worthwhile.

[B Tremblay]

I was recently experimenting with a modified Phozer that got much closer to the original design goal than the released version.

Unfortunately, in the process I ended up violating the runoffgroove.com Prime Directive of simplicity.  I used the gain stages of the Purple Peaker, added a twin to the existing T-filter, and used the Tremulus Lune LFO (based on the recommendation of Constantin Necrasov) to sweep it all.

I built my modified Purple Peaker with a daughter board for the twin-T filter and I was hoping that the LFO and filters could be a separate daughter board.  It now has an awfully high parts-count, so a real phaser is probably a better investment.

[Nasse]

:shock: Many years ago I made a simple cap or inductor meter from a mag. Unfortunately I don´t have the schem anymore but it was wonderful and simple circuit. Basically it was a Wheatstone bridge, another side was a linear potentiometer, and the other side was a known cap and unknown cap in series. An earphone (I used a cheap crystal earpiece) was connected between the pot wiper and the junction of the caps under test. Another side of the wheatstone bridge was earthed, and the other was connected to simple oscillator. Now you just adjusted the pot until the sound disappeared from the earpiece. You could calculate the unknown cap value relative to resistor values at opposite side of viper.

But it was just fine to notice wheatstone bridge works with AC signal too, not just DC. And it was magigal IMHO how you could make a certain freq sound totally disappear just by tuning a pot. I never listened audio (music) signal trough the circuit... maybe I should do it some day...

EDITED :x  :oops: OK I tried it but now I believe such wheatstone arrangement attenuates all freqs equally...