Help with a notch filter

[stallik]

I have a need for a filter to lower levels at 110hz. It seems that a twin T circuit may help and I can get the R/C values via an online calculator


What I'm unsure about is how to control the level of this cut. Can I simply use a pot to bleed some of the signal into this circuit or should this be part of an active layout?

[diffeq]

Voltage divider has variable series resistance element, so it will interact with filter. Much like tone control. Buffering it will insure that filter only "sees" a set resistance at its input.

[PRR]

A plain twin-Tee has a wide range of reduced level, and an infinite null. Is that really what you want?

A bootstrapped twin-Tee can give a narrow infinite null, say to reduce 110Hz power-line hum. Of course a narrow null is tough to tune.

Do you really just want a general dip?

Are you adverse to op-amps?

[stallik]

Thanks for the replies.
What I really want is is to dip 110hz. Don't know exactly how much by but I'm estimating around 6db. This is to try to control feedback only on the A string of  a nylon strung guitar fitted with an under saddle piezo. It only has to work in one location so i didn't think it necessary to have something tuneable. perhaps a parametric EQ would be a better idea?

[Rob Strand]

What about a bridge-T?
Less parts, can set depth, probably more narrow notch.

[stallik]

Hmm. I have to temporarily shelve this one. My new speaker cab may have a hump at 110hz. If this is the case, I'd rather get this sorted than fix the symptoms.
So far however, this has made me realise just how little I know. I'm heading up the learning curve and may have to ask some stupid questions shortly.

[Transmogrifox]

On the Twin Tee, you can tie the grounded end of "R" and "2C" together and connect to a pot to vary the resistance between nominally grounded node and ground.  When the pot gets close to the value of "R", then it will make the notch shift, but then it will be wide enough you may not notice.

I think a fixed-position BMP-style tonestack might be worth consideration if a passive solution is desired.  Jack Orman's "body" control is what you would use to control the depth of the notch.  LTSpice might be the best tool for evaluating this option.

If you need a more narrow and better controlled notch, then an active filter is probably the best bet.  Just tune one band of a typical graphic EQ circuit to 110 Hz and Q (bandwidth) desired and then fix it where you need it (or put a pot on it if you will need to adjust the depth on the fly).