Wah pot taper tweaking/my brain hurts

[John Lyons]

Ok, I have built the colorsound inductorless wah with a few mods to change the Q and the range of the treadle throw  (frequency that the full down position is at...)

My problem is that the usefull sounds bunch up around the end of the pot rotation. A reverse log taper will help with this I believe...No?

Here's what I've come up with:
100K linear pot wired as a rheostat/variable resistor
CW/hot/ lug 3 connected by jumper to wiper
25K resistor connected to lug 1 and wiper.

Is this the correct way to set up for reverse log taper in a rheostat style arangement?

And the larger the resistor the more reverse log it becomes?

I'm just getting my head around the tapering thing...Bear with me

Thanks for any tips. I've read all the posts I can find about tapering  and the Secret life of pots article at GEO.

john

[John Lyons]

No advise out there? Bump!

[Ethan]

According to RG Keen, that is the correct.  It has NOT worked for me though.  In fact, I think I tried every different combination and R values and could not get a reverse log rheostat.  I do not think it can be done..
-ethan

[Fret Wire]

Most wah's used a log pot. Have you tried a plain old 100k log pot? If you're going to make a 100k reverse log pot out of a linear pot, you'll  have to start with a greater value lin pot than your target value rev. log pot.

[John Lyons]

I started with the plain old 100K pot but the sweet spot comes in the last 10% of the rotation...or less... I want to widen that sweet spot so I can have a little more in the end range of the sweep.

Right now I'm using a 100L linear pot with a 50k ish resisrotr across an outside and wiper lug. As said above. But this gets me in the 30K range of total resistance... As fret wire said...Too low.. Although it sounds fine and almost right.

I'd just like to confirm this is the theory and then try a larger pot, maybe 250K and then taper it back to around 100K and with a reverse log taper. Basically I'd just like to slow down the sweep at the end of the pots rotation.

Thanks for the posts

John

[John Lyons]

Well...other than the fact that I need some confirmation on this (see first post above) This would seem to make a good addition to the simple mods and tips page, no?

If someone could make a few simple illustrations of the correct way to wire a linear pot to taper it to a log or reverse log pot that would help out some folks I think. The GEO article is very good at the details but it's still a bit hazy on the hookup of a rheostat arrangement. Maybe just a rundown of tapering in the first place (graphically):

Changing the pot value
Changing the taper, Log reverse log etc.
Adding resistors to the outer lugs and the affect of each on slope



John

[Paul Marossy]

If you're going to make a 100k reverse log pot out of a linear pot, you'll have to start with a greater value lin pot than your target value rev. log pot.

As in a 250K pot?

[Fret Wire]

Yup. Sometimes, making a log pot out of a lin pot thats only slightly larger in value requires large value resistors on one side.
http://www.diystompboxes.com/analogalchemy/emh/emh.html

A 250k lin pot would need  321.4k and 10.8k resistors to make a 100k log. A 500k lin pot would need  140.6k and 10.4k resistors to make a 100k log. pot.

A 1M lin pot would need 4.5M and 56k resistors to make a 500k log. A 2M pot would need 818k and 53k resistors to make a 500k log. Flip around the two values the calculator gives and you have reverse log.

http://diystompboxes.com/sboxforum/viewtopic.php?t=35702

[Paul Marossy]

What if I wanted to make a 25K log pot out of a 100K log pot? Would I just solder a 33K in parallel with it? It seems like it wouldn't be quite that simple because it would mess with the taper, wouldn't it?

[Fret Wire]

I'm stumped as well on log to log conversions. It would mess with the taper, though. Guitarists who try to make exact value 250k or 500k log pots out of larger ones usually find the taper is quite off after.

[Paul Marossy]

Hmm.. maybe it's not really possible because of the log taper?

[Fret Wire]

I guess it doesn't really matter. Since lin pots are more common, they're perfect to use for log pots.

[Paul Marossy]

I guess it doesn't really matter. Since lin pots are more common, they're perfect to use for log pots.

Yeah, that's true. It's just something I wonder about.  :wink:

[filterazonatie]

If you're having trouble finding a reverse log pot, why not dermine the amount of resistance where the current pot becomes useful (e.g. 90k), then replace the pot with a fixed resistance of that value (90k) and a smaller pot for the useful range (the remaining 10k, in this example)

Wouldn't that do the trick?  

[Paul Perry (Frostwave)]

In any case, the common "log" pot isn't really a log pot at all, it's just an approximation (made from two linear sections of different resistive material). Sometimes you can even feel the 'bump' where the two bits of track material meet.
This whole pot business is another argument in favor of using an optical method, of course! You can spend all winter snipping away at the optical shutter until you get the curve "just right" :wink: