Single op amp voltage controlled filter

[Fancy Lime]

Hi everyone,

I'm trying to find out if there is a way to build a filter section for an auto wah from just one op amp and one photoresistor (no I cannot use more than one op amp in the filter section for this particular experiment and no OTA, I could use a transistor instead of the photoresistor although I would prefer using the NSL7053 that was sold to me as a NSL-32 for extra style points  ). Ideally a resonant low-pass with a way to control the resonance, but a band pass would do if it has to. R.G. mentions multiple feedback topology wahs in his wah article but despite quite some exploratory calculations I was not able to find out how to design this such that the center frequency can be moved without completely changing the Q. Or if that is possible, for that matter. Q changes with center frequency, which is not bad per se as long as the effect is not too extreme. Can someone name a circuit that used MFB topology for this purpose, as a starting point?

Then I've also been looking at the EHX Doctor Q and Mutron Micro V to see if they might be adaptable for this purpose. Both look kind of like MFB filters, maybe, that can be switched between low-pass and band pass (or maybe not?) but have some extra pieces stuck on which I cannot make heads or tails of. I really hope it's just because I'm tired. Can someone enlighten me as to what these filters do and how they do it? Am I getting this right that the CA3080 is used as a variable low pass in the Micro V? The Micro V does not sound super impressive but the Doctor Q may be a suitable base for something interesting, but I have no Idea what's going on there. Damn it, shouldn't be hard to understand, should it?

Thanks,
Andy

[Kipper4]

Some of the Morley optical wah wah use mfb filters.

Many tweeks to do too.

[anotherjim]

Escobedo Q&D filter. One op-amp. Low pass with resonance pot. Voltage controlled. No LDR. Trick of using it is to make sure input signal is small - it has high gain to work.

Similar concept in the Phucgnosis...

[Fancy Lime]

Rich, Jim, thanks! Good starting points.

In the meantime, I have stumbled across this:
http://www.experimentalistsanonymous.com/diy/Schematics/Filters%20Wahs%20and%20VCFs/Sallen%20Key%202P%20Filters.pdf

Voltage across diodes as control of their resistance. Same principle as in a diode bridge compressor. The whole thing looks a bit archaic in this implementation, but the idea is tempting. Never seen this used in a wah context, wondering why? Allows Sallen-Key low and high pass, plus passive band pass before buffer. A T-notch option can easily be added by connecting high and low pass at the same time. The cap values likely need adjusting for that. I'm thinking it should be possible to drive the diode bias current with a Mutron III type sidechain, no? Anyone ever tried this type of thing?

Cheers,
Andy

EDIT: The high-pass is not Sallen-Key topology, actually; I wrote that a bit misleadingly. The high pass is a passive first order filter in front of the buffer, like the band pass.

[Transmogrifox]

Another take on the Sallen-Key type of topology:
http://cackleberrypines.net/transmogrifox/TransmogriNotes/skwah/index.html

Down the page is an op-amp implementation example.  The variable resistance element can be implemented with FETs, LDR, diodes, etc.

[Scruffie]

Have a look at the Mike Matthews Soul Kiss.

[Kipper4]

Wasn't ray rings cream wah a sallen  key filter ?

[R.G.]

Twin T - see Geofex.

[KarenColumbo]

http://sound.whsites.net/project63.htm

[Fancy Lime]

Hi folks,

thanks for all the input!

After simulating a variety of different circuits and building a bunch on breadboard for real life testing, this is what I found so far:

The diode controlled circuit I found on experimentalistsanonymous and wondered why no one uses that particular topology: Turns out there are reasons (who'd've thought?). First of all it is quite noisy. Not super bad but much worse than others. Diodes in their "voltage controlled resistor" region seem to give off more noise than I had anticipated when the signal is quiet. Well, now that I know that it seems kind of obvious given that they are setup to amplify any thermal noise from the voltage control into the signal chain. But I did not know that before, so thats a lesson learned, I guess. Also the control is a bit twitchy. The control current naturally interacts with the capacitors when it changes, so balancing all the resistances and capacitances just right is crucial, which makes it a daunting task to design this in a way that is both consistent and flexible in terms of filter sweep and Q variability. So, as always, not as simple as it looked at first.

The Twin-T, Bridged-T and MFB band pass variants all seem to have the problem of either sweep dependent Q or sweep dependent gain. This can be designed to be advantageous by making it so that the highest Q is in the human voice register. Again, very delicate to design in a way that has flexible Q and sounds good in all settings. For guitar use I'm with you on those all the way but this is not what I'm after here, so this will be a project for some other time. Seems to make more musical sense to me in a pedal wah than in an envelope controlled filter. But I've been meaning to build a proximity controlled super flat expression pedal one of these days anyway, so that may be a good match. If memory serves that is the combination in the Zvex proximity wah.

For bass, I seem to always come back to resonant low pass filters. I had originally considered to pair a sweeping band pass with a parallel static low pass but its not really the same. The fat bubbling of a true low pass is just so addictive. Unfortunately there seem so be no way to build a resonant low pass with only one control element, unless I missed something. MFB low pass filters can be made to do that but again at the expense of flexibility because Q changes with the sweep and the range of the sweep becomes highly interactive with the Q setting. So I bit the bullet and agreed with myself to two control elements, namely NSL7053 optical isolators. They have an enormous range from 1kΩ to >20MΩ and at least the two that I have are very consistent with one another. So it seems that this is the higher quality albeit much costlier choice over home-made LED-LDR combinations. I used those as resistors in a run-of-the-mill Sallen-Key low pass with switchable caps adjusting the Q. Sounds great, very "organic" and very low noise. I originally meant to use rail-bootstrapping on this to add more headroom but I cannot get it to clip in any setting as it is (maximum Q tested was 10), so I'll have to see if that is even necessary. The simplicity of the filter tempts me to keep this whole thing as simple as possible. We'll see, that temptation has quickly worn off before...

I'll keep you posted,
Andy

EDIT: I just found some info about the elusive "NSL-7053" that was sold to me as an NSL-32. There seems to be no data sheet and Silonex reportedly does not know them, so I was thinking cheap knock off. But I found one site selling them that claims that NSL-7053 is the name of the housing, whereas the part name is NSL-32, which is not mentioned on the unit. Seems a bit fishy still but I thought I'd share it for what it's worth. Anyway, I measured light and dark resistance and the ones I have exceed the Silonex specs, so good enough for me, I guess.

[R.G.]

It is possible to divide circuits into two kinds: designed for best control, and what could be called "lucky find" for want of a better term.

For a controllable filter, especially one that can be swept, it is quite difficult to beat the various versions and relatives of the state variable filter. You get highpass, lowpass, bandpass, and notch. Q is both fixed and separately controllable, as is gain and center frequency. You can also get wide sweep range, which can be quite difficult with other types.

For fixed frequencies, others are considerably simpler. Even the Vox inductor wah filter has issues. The frequency sweeps as the square root of the change in the apparent capacitance of the virtual capacitor. Ditto gyrator-based things. Wahs have been made with multiple bandpass, Twin T, and other variants. But for getting maximal control, go to the state variable. Note that a quad opamp package makes this a one-IC solution.