What.size.pot.to.use?

[VolksWilliam]

Ok. I have a general understanding of how pots work and how to use them. What I don't know is what size to use. I see everywhere from 1k to 500k in schematics and don't know why.

I'm modding my ehx mole to not cut as much highs. I don't want to completely remove the cap that is acting as a low pass  because then it would basically be a clean boost. I want to put the cap between the wiper and one side of the pot to create a high cut knob. But what size should I use? And why? Just so I don't have to ask this again on my next mod.

[Thecomedian]

You'll want to learn some general circuit theory.

go ahead and google steelwheelsdown voltage divider and what is a transistor.

A pot is a simple voltage divider. http://www.seymourduncan.com/blog/tips-and-tricks/250k-pots-versus-500k-pots-going-deeper-into-the-subject/

If you get a simulator or you have an oscillator and a breadboard to build a fuzz, yo can see that switching the size of the output pot, the "volume", will have a large effect on the shape of the sine wave at various frequencies. It can also lower the voltage or "amplitude" of the wave, which is the "information" that you want to pass on to the amplifier. At the same time, Voltage and amps are related. If you raise resistance, Voltage prior to the resistance increases, however, amps throughout the circuit decrease, and voltage will be lower after that high resistance. Since

It's kind of like a river with dams of various height through it.

[thelonious]

Can you link to the schematic? Also, can you clarify your question? It sounds like you want to know what pot and cap values to use in the mole circuit to add a variable high cut. Is that right?

[VolksWilliam]

Can you link to the schematic? Also, can you clarify your question? It sounds like you want to know what pot and cap values to use in the mole circuit to add a variable high cut. Is that right?

Ok. I have a mole already. I don't like it because it cuts the highs way too much. I've figured out the cap that does most of the high cut. But bypassing it lets too much highs pass and its basically a clean boost. So I want to put that cap on a pot and be able to dial in how much low pass I have.

[garcho]

Then you need a variable resistor, not a potentiometer. Luckily, for us, they come in the same package.
A variable resistor is one terminal and the wiper, leave the third terminal open or solder it to the wiper. In this configuration, the pot is acting like a resistor that changes with the turn of a knob. It will go from 0 Ohms to its max rating. If you don't have a breadboard to try this out, go with a 500k or 100k, to be safe. You might not use the entire turn for dialing in the right sound, but you'll have the whole range of possibilities available.
Solder it in series between the signal and cap.

[Jdansti]

I think Volks' question is how does the pot "size" (resistance rating) affect tone.

In general, the larger the pot value, the brighter your effect will sound. The best thing if you're working on a mod or a new circuit, is to get several different value pots and try them. As for treble bleed caps, read this:  http://ratcliffe.co.za/articles/volumepot2.shtml


It will take some trial and error.  The final answer will be how it sounds to you. 

[VolksWilliam]

Then you need a variable resistor, not a potentiometer. Luckily, for us, they come in the same package.
A variable resistor is one terminal and the wiper, leave the third terminal open or solder it to the wiper. In this configuration, the pot is acting like a resistor that changes with the turn of a knob. It will go from 0 Ohms to its max rating. If you don't have a breadboard to try this out, go with a 500k or 100k, to be safe. You might not use the entire turn for dialing in the right sound, but you'll have the whole range of possibilities available.
Solder it in series between the signal and cap.

Cool. That was my original plan. I guess I didn't explain it very well int original post. I was just unsure on the value of the pot. Why so some pedals use pots as low as 5k?

[thelonious]

Why so some pedals use pots as low as 5k?

If you're asking about why they use a 5K treble bleed/low pass pot, maybe they use 5K because a bigger value didn't make enough audible difference in that circuit---or they didn't like the way it sounded past a certain point on the pot. So they put in a 5K to get fine adjustment within the range that sounded good to them. It's always a balance between versatility and simplicity---they could have used a 500k pot and gotten more versatility (because you can theoretically get all the values of a 5k pot on a 500k pot), but it would have made most of the usable range "bunch up" at one end of the adjustment range (like garcho said), and most of the end users would have complained that the knob is way too touchy at one end and doesn't do anything for the rest of the range. So... to avoid all that, they just put in a 5K pot. Or there was not enough range of adjustment, so they put in a 100k pot. Like Jdansti said, it's all about how it sounds to you.

Also, this is just a guess, but I'm wondering if the usable range of adjustment might depend partially on what frequency the low-pass filter is centered on. If it's centered at 10,000Hz vs 800Hz... that's a big difference in the amount of signal that actually goes through that cap, and you might need a larger range of adjustment if it's passing a lot of the signal to ground (as it would at 800Hz).

[GGBB]

In general, the larger the pot value, the brighter your effect will sound.

I always though it was the other way around.  From "The Technology of the Fuzz Face":

The value of the output level control has been the subject of some debate. Eric Johnson supposedly favors 100K over the stock 500K. This could have some effect, as the 100K acts as a load on the collector resistor. More importantly, it cuts more lows out by it's interaction with the output coupling cap, and is less subject to stray capacitive loading cutting highs than the 500K; a 100K should sound somewhat brighter.

I know that with guitars, a larger pot is brighter sounding, but I've understood this to be due to pickup loading and the resulting change in the frequency and size of the resonant peak.

Is it only with output level controls like the fuzz face where lower is brighter?

[Jdansti]

In general, the larger the pot value, the brighter your effect will sound.

I always though it was the other way around.  From "The Technology of the Fuzz Face":

The value of the output level control has been the subject of some debate. Eric Johnson supposedly favors 100K over the stock 500K. This could have some effect, as the 100K acts as a load on the collector resistor. More importantly, it cuts more lows out by it's interaction with the output coupling cap, and is less subject to stray capacitive loading cutting highs than the 500K; a 100K should sound somewhat brighter.

I know that with guitars, a larger pot is brighter sounding, but I've understood this to be due to pickup loading and the resulting change in the frequency and size of the resonant peak.

Is it only with output level controls like the fuzz face where lower is brighter?

Good points. All the more reason to get the breadboard out and try a few different pot values and see how they sound with the particular circuit he's working on.

[garcho]

You could always throw a few different cap sizes on a switch, say a $3 1P12T from Small Bear, and not add another resistor to the mix. That way you could always have the original dialed in too, just for craps and giggles.

[VolksWilliam]

You could always throw a few different cap sizes on a switch, say a $3 1P12T from Small Bear, and not add another resistor to the mix. That way you could always have the original dialed in too, just for craps and giggles.

That could be cool. Have the original sound. No cap at all. And 3 or 4 steps in between.