Wah frequency cap control question

[johnadon]

First, apologies if this topic has been covered before. I tried a variety of search term combos but couldn't turn up anything 100% relevant. Every time I've posted in the past I've discovered a bazillion threads relating to the subjects after I've already posted, and I imagine this will turn out the same way.

I just wired together my second wah circuit to the Maestro Boomerang specs. I left the frequency range filter cap out to play around with a variety of cap controls. Long story short, I forgot and soldered my first experiment directly in, and I left so little space on my board that I'm reluctant to unsolder it.

What I have soldered in is a control similar to the input cap blend as found on some of Joe Gagan's circuits, e.g. the EZ Face. (Thanks to Mark Hammer for pointing me towards those wild and amazing circuits!) It works even better than I expected, with a nice continuously adjustable center frequency. I was going to try two other things, but since I have the first toy soldered in, I'm wondering if anyone else has tried either of the following and if so how well they worked:

1. Using a cap blend in which the caps are tied to the outer lugs of a pot, and with the output coming from the middle lug toward Q2.

2. Using a Big Muff style tone control in place of the frequency cap.

Knowing myself, I'll probably end up desoldering to tinker more but I thought I'd post to see if anyone's got any opinions before I start mucking up all my solder joints.

Thanks much to everyone who posts in these forums. What a friendly environment for learning how to make an endless variety of noises to scare neighbors with!

-John

[johnadon]

Sorry.. Looked at my old post and it was John Lyons who pointed me toward Joe Gagan's circuits. Thanks to John, but also thanks to Mark for all the wonderful info!

[johnadon]

Hm... Now that I think about it I think the first one would do essentially the same thing as what I've already got wired in - and with some possible drawbacks. I'm not sure what would happen in the middle of the pot sweep.

Also, I'm not sure using a Big Muff style tone control will do what I was expecting, and if it did work it might have a lot of unusable extremes.

I'm going to try them anyway because I'm compulsive. I'll post my results in case anyone stumbles upon this thread in the future.

By the way I'm not sure I was clear about which capacitor I was referring to in my first post. I was referring to the one that controls the center and range of the sweep, e.g. the one that ties directly to the emitter of Q2 and for which many people substitue a 6 position rotary with different cap values. I was just looking for something that takes up less room than the rotary and gave me another knob to twiddle with.

[johnadon]

And the verdict is in. Using a muff style control in place of the frequency range cap is a bit too extreme even when differing combos of caps are used. May be useful at some point for a special effect. Much of the range was useless. The other approach, tying caps directly to outer lugs of pot, didn't produce results substantially different from how I had it wired in in the first place.

I'd definitely strongly recommend the cap blend control as an alternative to the rotary switch selectable caps often used in wahs, especially since the control takes up less room and gives a continuously variable wah range rather than having to select fixed frequency caps going to Q2.

Thanks again to all for the great info in the forums!

[johnadon]

Not trying to turn this into a blog here, but I thought I'd better post some further findings in case anyone stumbles upon my ramblings in this thread.

For some reason the center frequency control described above does not work nearly as well with the standard Vox wah circuit. As you dial the center frequency down in the Vox circuit, the wah pot value (the actual rating of the pot) has to go down correspondingly to maintain a good volume level across its throw. Not sure why, but I think the bottom frequency of the sweep goes below what the rest of the circuit wants to let through resulting in a volume loss at the bottom. I'm a relative newbie to these types of circuits so that's just speculation. I'm sure there are components one could tweak to make it work, but since the control works so well in the Maestro Boomerang circuit I'm probably not going to fiddle around with it any further in the Vox circuit.

[ACS]

Nice thread!  I'm hoping to do a wah soon, so this could be very useful...

Do you think the volume change issue could be the reason this is often done with a rotary switch? ie you can wire the right combination of resistor with the cap at each point, to keep the volume constant?  Just a thought....

[johnadon]

I hope this isn't too late. I'm beginning to have a bit of uncertainty about whether or not this is working as a continuously variable center frequency/range control. It definitely does move the top available frequency down at the same time as the bottom frequency - the range moves in other words. However, this only works in the Maestro Boomerang circuit and not the Vox circuit, and I have no idea why. Perhaps someone else can enlighten us.

I did find a thread on another site where a guy suggests putting in the same type of control as what I've put in, but he apparently never tried it. I think if you google 'Pimp My Wah' you will find it. He refers to it as a bass control, which is what I already know it is normally. I have no idea why it seems to work as a variable center frequency control in the Boomerang circuit.

I'm haven't really had a problem with volume as you mentioned, however there is a dead spot in the middle of the control where the wah doesn't do much.

So much to learn!

[johnadon]

Oh, and also since the whole point of having the inductor in the circuit is to make a 'variable capacitor' as RG Keen mentions in his wah pedal, I have a difficult time believing that this simple 'bass control' could  change the value of capacitance so simply.

Please, please if someone smarter and more experienced could explain some of these things, I would sure appreciate it. I've been fascinated and confused by this simplest or controls.

I'm a little bit mystified as to why the real capacitance doesn't normally change in the configuration described above when it's used as an input capacitor control as in Gagan's circuits. And why does it sort of act like a variable capacitor in the wah center cap spot (but only in the Boomerang circuit)? Normally it's just a bass rolloff (which is what it does in the Vox.

Begging for knowledge!

[johnadon]

Oh! The volume control problem was actually the wah sweep control that I had to adjust just to be perfectly clear in case I wasn't. (I'm good that way.)  I was a bit misleading I think. This problem occurred strictly in the Vox wah circuit and I strongly suspect you are correct.

[Paul Marossy]

since the whole point of having the inductor in the circuit is to make a 'variable capacitor' as RG Keen mentions in his wah pedal

Well, that is part of it. But its other main function is to make it a resonant circuit. Take out the inductor, and you no longer have a resonant circuit. The classic inductor wah is based on capacitive reactance. The wah patent document explains how it all works.