Update to the Technology of Wahs

[R.G.]

One of the forum members who name I don't know - he uses only "Panterafanatic" - PM'ed me about the Rfb resistor in the standard Vox wah circuit. He suggested it acted like a tone control in a guitar. Sure enough, it does, but in a somewhat quirky way.

I went back and messed with Rfb a bit, varying it from 750 ohms (half of the stock value) to 6K (4x the stock value).

What I found was that it's major effect is cutting bass out of the frequencies below the resonance. In this it is something like a guitar tone control, but it is not shunting treble to ground, it's lowering the input impedance at Q1 base. This, in concert with the input cap affects primarily the bass response below the resonance.
It works entirely outside the feedback loop which makes the resonance, but does not interfere with the resonance. Now that I think about it, that makes sense as the impedance of the resonant circuit gets quite high at resonance.

It has a side effect on overall gain, acting to raise/lower response at and above the resonance by about 2db, and another effect on the frequency of the resonance, a few Hz, but nothing to write home about.

So you can tune how much bass is left in your wah sound by making Cin big (perhaps one of those nice 1uF films) and adjusting Rfb from 1.5K up and down.

[panterafanatic]

i'm jared btw

edit: i'd say it acts more like when you use the 2nd tone control on a strat for a bass cut rather than treble. as i said in my reply to the pm

[Paul Marossy]

Huh, interesting finding! It's amazing how much there really is to such a simple circuit like that. It's like almost every component affects all the others to some degree.

[Gus]

http://www.google.com/patents?id=pd1UAAAAEBAJ&printsec=abstract&zoom=4#PPA1,M1

4th page, paragraph just under column 4,  last three sentences in the paragraph concerning resistor number 36.

[Paul Marossy]

http://www.google.com/patents?id=pd1UAAAAEBAJ&printsec=abstract&zoom=4#PPA1,M1

4th page, paragraph just under column 4,  last three sentences in the paragraph concerning resistor number 36.

Oh yeah, I've seen that before. I've read those patent docs several times. I didn't make the connection until you pointed that out, though.

[Gus]

R.G. gave it some more detail the about 2db part and using a larger value cap

[panterafanatic]

hey guys. i was given a link on a different sit about he values of the resistors and such, but does anyone have info on the rocktron blackcat wah? i'd like to test it out myself.

[space_ryerson]

Interesting. I've been meaning to do a re-read of 'The Technology of Wahs' lately. Thanks RG!

[rchavez]

One of the forum members who name I don't know - he uses only "Panterafanatic" - PM'ed me about the Rfb resistor in the standard Vox wah circuit. He suggested it acted like a tone control in a guitar. Sure enough, it does, but in a somewhat quirky way.

I went back and messed with Rfb a bit, varying it from 750 ohms (half of the stock value) to 6K (4x the stock value).

What I found was that it's major effect is cutting bass out of the frequencies below the resonance. In this it is something like a guitar tone control, but it is not shunting treble to ground, it's lowering the input impedance at Q1 base. This, in concert with the input cap affects primarily the bass response below the resonance.
It works entirely outside the feedback loop which makes the resonance, but does not interfere with the resonance. Now that I think about it, that makes sense as the impedance of the resonant circuit gets quite high at resonance.

It has a side effect on overall gain, acting to raise/lower response at and above the resonance by about 2db, and another effect on the frequency of the resonance, a few Hz, but nothing to write home about.

So you can tune how much bass is left in your wah sound by making Cin big (perhaps one of those nice 1uF films) and adjusting Rfb from 1.5K up and down.

I have  5k pot for this on mine. It's pretty darn dark when up all the way but my input cap is .01uf.

[R.G.]

i'm jared btw

edit: i'd say it acts more like when you use the 2nd tone control on a strat for a bass cut rather than treble. as i said in my reply to the pm

Hi, Jared. Sorry I missed your pm reply before posting. I'm not familiar with that use on a strat.

[BAARON]

i'm jared btw

edit: i'd say it acts more like when you use the 2nd tone control on a strat for a bass cut rather than treble. as i said in my reply to the pm

Hi, Jared. Sorry I missed your pm reply before posting. I'm not familiar with that use on a strat.

G&L Legacy strats do it all the time.  I think a lot of Reverend guitars do too now.

[panterafanatic]

i'm jared btw

edit: i'd say it acts more like when you use the 2nd tone control on a strat for a bass cut rather than treble. as i said in my reply to the pm

Hi, Jared. Sorry I missed your pm reply before posting. I'm not familiar with that use on a strat.

G&L Legacy strats do it all the time.  I think a lot of Reverend guitars do too now.

its a pretty easy mod. put a cap between your input lug and a smallish resistor (parallel i believe, your choice on both) and attach them to the wiper lug which you use as an output.

[RedHouse]

Aka  "Treble-Bleed" circuit

[panterafanatic]

no dude. you put the cap to connect the two lugs. that way the treble doesnt bleed off to the ground, so its more pronounced.

[spaceace76]

no dude. you put the cap to connect the two lugs. that way the treble doesnt bleed off to the ground, so its more pronounced.

It's called "treble bleed" because it prevents the treble from bleeding off, like when using the standard wiring. You're both talking about the same thing

[John Lyons]

More acurately it should be called the "Treble Bypass".
What happens is that the cap bleeds treble to the output, bypassing the volume control.
The bigger the cap the more treble bypasses the pot when the pot is
turned down some.
When you turn down the vol pot some of the signal passes
straight through the cap bypassing the volume control. Since the cap is
small, only the highs are passed which gives a brighter sound.
The trick is to find a cap that balances with your normal (pot fully on)
sound. So when you turn down the vol pot the frequency balance sounds
natural. A resistor in series with the cap lessens the effect and creates a
shelf type filter.
With a straight cap across the hot and wiper lugs anywhere from 180p
to 470p works well for a strat. If I remember correctly a .001 and 150 ohms resistor
in series is also popular.

John

[ashcat_lt]

Except, that's not the same thing as the G&L style bass-cut knob.  I think the G&L is generally wired like this:

That said, the folks at GuitarNutz did some tests a while back and determined you can get more bass cut by wiring it like the treble-bled volume pot, with one lug grounded, or at least partially so (through a resistor).