Wah pot values and there effect?

[Mark Abbott]

I'm wondering what difference tonally is there between the 100K and 200K pots sold by Small Bear electronics?

Thanks

Yours Sincerely

Mark Abbott

[FlyingZ]

I recently added the option of none, a 33K, or an 82K resistor in parallel with the 100K pot in my wah to complement various styles with great success. I can't help you with the new pots.

Smaller then 100k values worked best for me.

[slideman82]

For me too! I like smaller pot values better. The Maestro Boomerang had a 25k pot.

[Gus]

The wha pot is can be looked at as a volume control.

The emitter follower stage input resistance loads the Wha pot(volume control) changing it's taper therefore changing the input signal level therefore changing the sweep.

http://www.aronnelson.com/gallery/main.php/v/gus/wha1.gif.html?g2_imageViewsIndex=1

This was drawn to reduce the loading of the pot and remove the mixing from the stock 470K EF bias in some Whas

There is also the output resistance of the first gain stage and what happens with different loading of gain stage with different volume control values.

[Gus]

Read and study this it explains a lot about the wha.

http://www.google.com/patents?id=pd1UAAAAEBAJ&printsec=drawing&zoom=4#v=onepage&q=&f=false

Someone built and posted clips of the circuit I shared(at a different forum) of a different blend of fragments for a wha the bootstrapped part is my little addition..

[Gus]

Is it because I am not R.G. or Mark H. I don't get a response?

I just posted what you need to know to understand the wha pot.  It is simple stuff.
Use science

Go to R.G.'s site or others and read about loading of pots and how the taper changes with loading

Google etc. EFs and calculate what the EF stages input resistance is

You also have the RC's  the cap to and from the wha pot.

Google etc transistor gain stages what happens to the gain of the stage as the output is loaded, Hint, the pot value and what it is connected to the pot wiper are in parallel with the collector resistor.  The collector resistor is part of what sets the gain.

A lower value wha pot both loads the gain stage AND is a lower source resistance to the 2nd stage EF(emitter follower)

Say you like a certain wha pot.  Remove it measure the wiper to ends resistance at different degrees in it's rotation.

Wha pot does not have enough sweep up the gain of the first stage, to much sweep lower the gain.  As you adjust the gain you might need to adjust the sweep cap value to move the center of the sweep.

[frank_p]

Gus, I think the Mr. asking the question just didn't came back to the forum since he posted: the question was his lasts writings...
Be patient...

[Mark Abbott]

I didn't reply as I didn't receive any further notifications. I came back to this site as I was going to cut and paste my question elsewhere. I don't know why I only received one reply notification?

Thanks for your reply by the way. I haven't read it yet, So I will reply to that a bit later.

Yours Sincerely

Mark Abbott

[noelgrassy]

Gus,
Thank you for both links! I play hell when searching the US Patent Office. I think I'll reread their suggested
search methods. That patent you posted downloaded without a hitch.
It seems in most threads where you've responded the information you provide is exactly what has been
requested or you pose suggestions or questions that serve to increase our understanding by magnitudes.
You also have the admirable gift of avoiding verbose and long winded responses which also aids our
(eventual) understanding. Thanks again for an awful lot of damn good stuff at a decent price,

Noel Grassy. 

[Paul Marossy]

Personally, I don't notice much of a difference between a 100K and a 200K pot in the CryBaby wah circuit. And in the Maestro Boomerang, I haven't noticed much difference between a 25K and a 50K pot. Some people say a 100K also works fine in that circuit.

I have come to the conclusion that the pot value isn't terribly important - the taper of the pot is most significant. The most interesting experiment I did was to stick a 100K reverse log pot in one of my vintage Maestro Boomerangs. That had an interesting sound to it.

[Gus]

Paul

Think about this, why not DESIGN the taper using a lin pot.

Take my gift of the bootstapped EF for a wha and add a tapering resistor from the wiper to the "top" of the pot  or "bottom" and taper the pot the way you like.

Use a lower collector value resistor and adjust the emitter resistor for the gain you want.   I would design the first gain stage with a 4.7K collector for about 1ma (4.5VDC/4.7K is about 1ma)

Use a 100K and taper the pot with a resistor(100k to 22K etc at 22K you might want a lower output resistance try a 2.2K collector resistor if tapered wiper to top).  If the EF has a very high input resistance It should not load the pot.  If you use a 200K(220K to 47K) etc.

I have an other idea for the EF stage and first stage.

10-14-2009 GS

[Paul Marossy]

Yeah, that's true. You could design your own taper. I'm not too picky about the taper, either, though. I just play wah pedals I have the way they are. I'm used to each one of them, it's like changing a pair of shoes. They're different shoes, but still feel familiar.

[matt239]

Wah pot values. I see this is a little old, but if anyone is watching:
- All factors considered, can we retain a bit more output level with a higher value pot?
It would seem that way to me, but I could be missing something..
I don't know if it would be enough to be noticeable.?

I have liked the "sound" and "feel" with 100k and a parallel resistor to ground, (about 100k, I think.. It's been a while..)
- mellower with a good sweep,
- but I'm thinking I could use a larger value pot, and a larger  sweep cap, and possibly some tapering resistors, to get a similar sound and feel, with a little more output level.?
- Some of you may remember I was working on building a totally clean wah, so a bit lower gain from Q1.  I was pretty successful, but it could use just a bit more output level..

[thermionix]

- All factors considered, can we retain a bit more output level with a higher value pot?

No, not that I know of.  Maybe a different route, I added a clone of the FoxRox output buffer to my wah, it has a trimmer to set the overall volume, and it can go a little louder than the wah stock without buffer.

[GibsonGM]

I just lowered the input resistor on mine to give it a little bit more kick.  Caveat - if you lower it too much you can decrease input impedance and cause a "tone suck" situation.   I didn't experience that at all (forget what value I went with, think original was 68k...maybe 47k or something, experiment).

[matt239]

Thanks.
A post - wah output boost is an obvious fix, but requires an add on board.. Since mine was very close to where I wanted it, I was looking for a way to preserve a little level, post Q1, without a boost.
- I have a board with input and output buffers, but the output buffer doesn't have gain, or an easy way to convert it to a gain stage. (I may well add a booster, but I'm still curious about a 470k wah pot..)

Lowering the input resistor, or increasing Q1 gain defeat my purpose, which was to build a clean, no distortion wah.
Distortion in a wah can be a good thing, (growl..) just not my purpose here.

It would have been helpful if I had posted a link to my thread on that:

http://www.diystompboxes.com/smfforum/index.php?topic=112274.msg1035642#msg1035642

[GibsonGM]

I see now you've been having trouble with a hot signal...generally, the input resistor trick will work, but seems you've already 'splatted' doing that...

Could you potentially modify the output buffer to become a 'buffer with greater than unity gain'...?

[matt239]

Yes. It would be logically very simple to give the output buffer gain. It would just be physically tricky on this board.
- I could just stick an AMZ Minibooster or mosfet boost after it...

I got to about 90 percent of where I was trying to go with that wah, then I got very busy with work and haven't been able to revisit it.

Maybe we should bounce back over to that topic to continue discussing how to make a low distortion wah?
- There are a hundred factors..
- but this thread is about wah pot values.. 😉

[thermionix]

After reading through this thread several times, it's still not clear to me what a "there effect" is.  Is that what the Presence knob controls?

[Transmogrifox]

The effect of a wah pot:
It does more than only changing gain.  It changes the feedback circuit LOOP GAIN. 

Increasing loop gain (higher pot value):
Wah will sweep lower, and Q keeps increasing.  This will increase quack and distortion in the extreme heel position.

Decreasing loop gain (lower pot value):
Reduces the sweep range and the low-end Q is also lower.  Decreasing the pot value will mellow the wah in the heel position.

In the TOE position the pot value only changes the forward gain on the input and parallel LC tank as there is very little (ideally none) feedback in this state. 

Another way to look at is in terms of how this changes the "contour" of the wah:
Larger pot value --> wah is more "heel-heavy" (that is, output level increases as the wah sweeps down, and more Q in the heel position)
Smaller pot value --> wah tends more "toe-heavy" (that is, output level decreases as the wah sweeps down)

Of course in the grand scheme of things the pot value has a subtle effect based on the range of values typically used/available.  To me it's "jus' that little sumpin' "   I would argue a very similar effect can be had by tweaking the emitter resistor on the first BJT stage.

I just lowered the input resistor on mine to give it a little bit more kick. 

A second caveat is this resistor also plays with the Q of the pot, so it may give it some added "kick" but at a lower Q.  I'm not certain at which point the lower Q becomes a wash against the extra gain, but I suspect the "tone-sucking" problem happens first.