# Which circuit for variable band-stop?

#### stygian

Joined May 8, 2009
8
Hello, new member, first time poster. I found a thread like this while searching but I have more questions.

I wish to add a mid-range scooping function to the tone knobs on my guitar (currently there are 3 250k pots, one for volume and the other two have caps on them for rudimentary tone control, but I dont like them.) First I looked into the Twin/bridged-t but I'm wondering if perhaps a resonant filter would be better? I am looking for variable Q and variable notch frequency.

I believe in the resonant type design, a variable capacitor should achieve the frequency adjustment, but where would the pot go to vary the Q, without affecting the frequencies I wish to leave be? And in the bridged-T (see http://www.geofex.com/Article_Folders/EQs/paramet.htm at the very bottom) would the two R's vary the center frequency?

I'm still rather new at this, been reading books from the library for 2 or 3 weeks (later today I am going to practice my vector maths.) Apparently varying the ratios in the Twin-T circuit has a negative effect on the Q (or do the C's vary the frequency and the R's vary the Q?)

I would really appreciate any input on this. Thanks.

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#### CDRIVE

Joined Jul 1, 2008
2,219
Apparently varying the ratios in the Twin-T circuit has a negative effect on the Q (or do the C's vary the frequency and the R's vary the Q?)
You're a quick study, as you have it correct.

For a Parallel Resonant circuit, Q is max when:
(1) The Resistance across the tank is maximum.
(2) When the resistance within the tank (inductor Ω) = minimum.

For a Series Resonant circuit, Q is max when:
(1) Series resistance is minimum.
(2) Parallel resistance is max.

The resistance of the inductor (in either case) is treated as a resistance in series with the inductor.

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#### Darren Holdstock

Joined Feb 10, 2009
262
Have a look at figure 1, which is based on a Reg Williamson graphic equaliser circuit from 1973. The heart of the circuit is the op-amp with all the pots hanging off it. For the sake of analysis, just look at one pot - the wiper is taken to ground via a tuned circuit, which in the Williamson original would have been a series LCR network. The LCR circuit resonates at a particular frequency and with a particular Q, and the pot enables cut or boost depending on position (the pot set mid-way gives a flat response).

The Williamson circuit is now very popular for graphic equaliser circuits, except that active filter elements are preferred over inductors. The Rod Elliott circuit mostly uses high-pass Sallen and Key filters as resonant elements*, but other filter types and gyrators are also popular. These work as the input impedance of an active filter is also a function of frequency.

A proper parametric EQ would also do the job, and more, but these can be a bit complex.

* I'm not sure about the wisdom of using S&K HP filters for the mid-band elements as the frequencies above fc will shelve. I'd prefer to use bandpass filter elements, with maybe a HP element for the top frequency band only.

[Edit - I should have made this a bit clearer: The Williamson circuit is very handy here as it means you're not limited to bandstop filters - bandpass filters can be used too, just swap the outer terminals of the pot over.]

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#### stygian

Joined May 8, 2009
8
I was actually aiming to replace the standard passive circuitry inside my guitar, but I really want to keep it passive, as I dont relish destringing and taking apart the thing to change batteries. A thought has occured to me though. I was considering using a small inductor (few mH) and a variable cap. Then I realized I have inductors of several H as the pickups, and I dont want to alter their resonance, I just want to modify the the tones that come from them (replacing the basic stratocaster tone controls.) Can someone point me in the right direction, where to look, how to figure that out?

#### Darren Holdstock

Joined Feb 10, 2009
262
Destringing a guitar is nothing compared to trying to implement a workable passive notch filter based around Strat pickup circuitry. If nothing else, you have no control over the output impedance of the filter, as this will be dependent on the other Strat volume and tone control settings and the input impedance of the amp.

If you want to go for it, the first thing you need to do is sketch out the pickup circuit. You'll need to destring your guitar to do this...