General => Open Discussion => Topic started by: PhiloB on March 06, 2014, 04:20:04 PM

Title: Pot Taper Question
Post by: PhiloB on March 06, 2014, 04:20:04 PM
I have a basic understanding of the different tapers.  I have a Ross Distortion that I need to replace a pot on.  The distortion pot is a 500K reverse audio taper.  Would it be a big deal to replace it with a linear?  My mind tells me a linear would make more sense.  But, I've been confused about a lot of things pedal related;)
Wouldn't reverse audio put a lot of the distortion at the end of the pots rotation, making the majority of the rotation (beginning) fine tuning the more overdriven end?
Title: Re: Pot Taper Question
Post by: jimilee on March 06, 2014, 04:33:16 PM
Yes on all accounts
Title: Re: Pot Taper Question
Post by: PhiloB on March 06, 2014, 04:47:14 PM
Thanks Jimi.  My next question would be why a builder would choose one value pot over another.  For instance 500k vs 50k.  I think I know the answer but would like confirmation.
Title: Re: Pot Taper Question
Post by: jimilee on March 06, 2014, 06:16:24 PM
Same as a resistor really. More headroom overall. Say with a 5k pot vs 10k you have more room to turn the pot. I know what I'm trying to say, do you?
Title: Re: Pot Taper Question
Post by: davent on March 06, 2014, 06:19:55 PM
You could also put in a 500ka and swap the outside leads, maximizes the distortion at the opposite end of the dial but keeps the sonic mayhem correctly apportioned across the travel of the pot.

Title: Re: Pot Taper Question
Post by: chromesphere on March 06, 2014, 06:21:22 PM
here's my non-calculator explanation :D Take it with a grain of salt. The Ross Distortion looks pretty much like the Distortion plus and the distortion pot would control the gain of the op-amp.  You can increase this usually without any negative side effects but if you go too high the op-amp will become, I think the word is 'unstable' and it just doesn't sound very good.  I found 1m in the lovepedal eternity was too much gain and cut it back to 500k which is the top end of what that circuit seemed to be able to handle comfortably.  increasing from 100k to 500k did much more distortion available in the LPE.  The ross distortion has the gain pot in the high pass filter of the opamp circuit which is a little different configuration to the LPE but I would expect it would work similarly to the LPE.

EDIT: oh yeah good point Davent.  Unlike the LPE changing the value of the pot in this circuit may actual effect its frequency response as in the dist+
Title: Re: Pot Taper Question
Post by: PhiloB on March 06, 2014, 09:35:32 PM
Thanks guys.  So lug 1 becomes lug 3 and vice versa?  Very helpful info.
Title: Re: Pot Taper Question
Post by: midwayfair on March 06, 2014, 10:06:21 PM
Yes on all accounts

Whoa there. Not necessarily. And in fact, if we're talking about this circuit ...


the answer is very likely to be that a reverse audio is the correct taper.

R4 sets the limit on the gain. R4 + the gain pot's value divided by the 1M in the feedback loop will give you the total gain, so it goes from just under 2x (1,000,000/504,700) up to 1,000,000/4,700 = 200x.

Smaller resistances increase the gain, and as the resistance to ground approaches 0, a smaller change in resistance is required to increase the gain. It's a logarithmic relationship. You can see this by setting the resistance at, say, half on a linear pot:
1,000,000/250,000 = 4x (we only have twice as much gain with half a turn!)

And then 3/4:
1,000,000/125,000 = 8X (see how we have TWICE as much gain with only 1/4 turn?

... 15/16:
1,000,000/31,250 = 32X

Anyway, I think you get the idea. The smaller the resistance becomes, the more quickly the gain scales up.

A linear would probably bunch up the gain a lot, and an audio taper will be a very poor choice.

The gain pot in the fuzz face is similar, and most people agree that a 1KC is the correct choice there.

Unless you actually disliked the taper in the pedal to begin with, use the reverse audio again.
Title: Re: Pot Taper Question
Post by: PhiloB on March 06, 2014, 10:44:57 PM
Well, I have a lot to learn.  Jon, I'll be ordering the stock value/type.  I didn't have any gripes about the distortion pot.
A buddy gave me the pedal.  I didn't play it much.  Kinda fuzzy distortion.  It is the black version.  I modded it with a toggle with the the Tan version diodes.  Also put in a 'modern' DC plug and true bypass with LED.  When I drilled the hole for the toggle some metal fell in the pot and messed it up.  It's what I get for being lazy and not removing the board completely.