What filter topology is this?

[composition4]

Hi all

Some time ago I came up with an inverting filter I either invented or more likely copied from somewhere, but I can't make out what topology it is. Any ideas?

[antonis]

Mid-hump filter..

Look at Boss SD-1 Tone control..
(actually, a TubeScreamer one with the addition of a cap..)

P.S.
https://stompboxelectronics.com/2023/02/16/making-sense-of-the-boss-sd-1-c6-mod/
https://www.marshallforum.com/threads/boss-sd-1.86554/

[Rob Strand]

I'd call it a Wien based circuit.

This one set to full boost,
https://www.diystompboxes.com/smfforum/index.php?topic=95483.0

[antonis]

Easily turned into oscillator..
(by "accidentally" shorting input to GND or VREF ..)

[ElectricDruid]

I'd call it a Wien based circuit.

+1 agree. That R4/C3/C2/R3 network is a wien bridge like you'd see in many oscillators.

They do turn up in tone control circuits occasionally, but it's not particularly common in my experience. The Boss MT-2's sweepable mid control is one example that comes to mind:



Note this uses an unusual dual C pot (Rev Log) to try and make the sweep feel more even. For a fixed version like yours, this isn't an issue. I think your circuit looks good for cocked wah type sounds.

[Rob Strand]

Easily turned into oscillator..
(by "accidentally" shorting input to GND or VREF ..)

The Wien Oscillator needs a gain of x3 to oscillate but the 2x10k's fix the gain to x2 which keeps it out of oscillation.   As you increase the opamp gain, the peak gain increases, and when you get near gain of x3 the boost peak becomes infinite and that's when you get an oscillator.

The George Giblet Parametric has unequal cap values to set the Q & gain so the oscillation take off point is at a different gain.

If you try to get too much boost/cut for a given cap ratio on these circuits there will be a boost/cut pot setting that oscillates or goes wonky.  You need to consider part tolerance in how close you push it.   The George Giblet Parametric has a got enough safety margin but I remember one of the commercial Ibanez or Boss circuits which used this kind of equalizer had a wonky zone around 9dB, probably the 9dB boost.  (With equal cap values 9dB to 10dB is a gain of 3.)

They do turn up in tone control circuits occasionally, but it's not particularly common in my experience.

The single opamp version like the George Giblet Parametric  is quite common on bass preamps because of the low opamp count, good sounding Q as well.

[antonis]

The Wien Oscillator needs a gain of x3 to oscillate

(always tend to forget 0.333 feedback fraction..)

[composition4]

Wow... of course it's a Wien filter. I actually got the idea to use it from your MT-2 analysis on your web page Tom, I remember now! I think the article on ESP gave me a few more ideas and info about it, it's all coming back to me now.

Thanks all!

[ElectricDruid]

They do turn up in tone control circuits occasionally, but it's not particularly common in my experience.

The single opamp version like the George Giblet Parametric  is quite common on bass preamps because of the low opamp count, good sounding Q as well.

Fair enough. I don't read many bass amp schematics, I guess. It's a neat little circuit, for sure.