Experimenting With a Wah Circuit

[Rob Strand]

Saturating the core with AC saturates symmetrically where as apply a DC field saturates symmetrical.   There's quite a few aspects which aren't the same.

Saturating with a DC field is sometimes used to vary the inductance since the small signal permeability is reduced near saturation.

There was a pedal called the Schaller Rotor Sound.  It had 18 (yes 18)  inductors on ferrite cores.   Each inductor had two windings:  one for AC and one for a "DC" current.   The idea was "DC" winding was modulated in order to modulate the inductance.   The pedal was a phaser/leslie type of thing.
(and yes it used a current source not a voltage source for the "DC" winding.)

Here's some pics.   Click on the pic to scroll through the images.  The second or third image shows the inductors:
https://www.reddit.com/r/vintageguitarpedals/comments/1982a4j/schaller_rotor_sound_1969_analoge_leslie/?rdt=64841

[Elektrojänis]

For the unmodifed wah the 22k collector load is loaded down by the *ac* coupled 100k pot resistance.  That reduces the gain.   Reducing the gain will prevent the oscillation and make it behave like the original.   When you add the buffer the gain is higher, that will affect how the wah behaves (different Miller effect multiplying factor).   So I guess in order to make it act like the original you need to add a bit more load to the 22k.    Your low emitter resistor on the buffer does that to some degree so maybe a small reduction of the 22k will help.   All under the proviso that the bias isn't changed too much.

The odd thing (at first) was that it really didn't start oscillating even with the buffer until I added the 15 kohm resistor from wiper of the pot to ground. This loads the previous stage down a lot more in the heel down position. The oscillation that caused was at the resonant frequency. That was simply solved by changing Cout to 10µF. (1µF was actually ok too.)

The low end of sweep went from about 410Hz without the buffer to about 380Hz with the buffer. This I see as a direct result of the gain increase. However, I'm not too worried in making it sound just as the original. The current goal is to make the sweep wider, and being able to tune the taper of a linear pot with that load resistor so it feels nice. The goal may change though. And I will probably try to tweak the sound in way too many ways.

I need to remember that low emitter resistor on the buffer. I think I tried other values too before almost giving up with the oscillation. That was probably the last value on the breadboard before I found out the Cout thing.

When you have a buffer driving the output lead it's a good idea to add a series resistor to the output lead as a measure against HF oscillation with the capacitive cable load.

I have to remember this on the final build. Many sources say this is a must with opamps, but it totally makes sense that the same goes with discrete transistor buffers too.

[/quote]
"anotherjim" had some HF oscillation issues adding buffers to a wah.   FWIW in the unmodified wah the cable capacitance does load down 22k collector resistance a small amount.   That could prevent some HF oscillations.  It can also mellow out the sound or affect the wah response shape - just how much needs a closer look.   You can add a 470pF across the 22k.[/quote]

You mean this one: https://www.diystompboxes.com/smfforum/index.php?topic=127808.msg

I've had that one too. I don't really have anything to measure or prove it as I only have an ordinary multi meter and I use computer with a regular audio interface for a scope, but the crackle at one end of the pot rotation seems to be a sure sign. I've used 10pF or 100pf from Q2 base to ground and it seems to tame it. when it happends. At least it stops the crackle. I tried small caps at other positions and I'm not sure anymore how I settled to that.

There was a pedal called the Schaller Rotor Sound.  It had 18 (yes 18)  inductors on ferrite cores.   Each inductor had two windings:  one for AC and one for a "DC" current.   The idea was "DC" winding was modulated in order to modulate the inductance.   The pedal was a phaser/leslie type of thing.
(and yes it used a current source not a voltage source for the "DC" winding.)

Here's some pics.   Click on the pic to scroll through the images.  The second or third image shows the inductors:
https://www.reddit.com/r/vintageguitarpedals/comments/1982a4j/schaller_rotor_sound_1969_analoge_leslie/?rdt=64841

Well that's an interesting design for sure... I will be looking for soundsamples of that.