Volume Pot Values

[Nick C.]

It seams to happen with a number of distortion pedals and amps. As I turn the volume up it goes from dull at low settings and then bamm all of a sudden it sounds great but now it's too loud. I was thinking could a change in pot value or taper or the addition of a fixed resistor help the situation? The volume pot is a voltage divider, now if I have a large value pot at low settings there will be a lot of resistance in series with the signal and a smaller amount to ground.  Is the large series resistance messing with the output impedance such that a smaller value pot would work better? Right now I'm toying with a BossTbone Fuzz with a 100A pot.

[XXISouthpaw]

I'm pretty sure Log pots are typically used for volume as our ears are naturally logarithmic, although depending on the circuit the value of the potentiometer may be too much, which might explain why the volume is all bundled at the end. From what schematics I've seen the Tbone uses a 100k log so you should be fine using that.

[askwho69]

i've been wondering this question too.. how do we know what value to use as volume control in dirt pedals? because in guitar says using 500k adds highs and 250k lowered it.. so how about in dirt pedals?

[Nick C.]

Here's another part of the puzzle, a 100k volume pot set at 20% should give the same voltage out as a 1M or 10k set at 20%, but I know there is more going on such as current and impedance. If I change my 100k pot to a 10k there should be more current flowing thru the pot (I=V/R), but would more current be going to the amp or to ground? Or is the circuit going to try to supply 10x more current which could have a big impact upstream? I guess it could also depend on the freq.

[askwho69]

i'll wait for the answers too

[aron]

> because in guitar says using 500k adds highs and 250k lowered

OK you guys went and did it again.... it's the dirty word impedance.

http://www.diystompboxes.com/smfforum/index.php?topic=14019.0

Then check this out - scroll down a little:

http://www.diystompboxes.com/wiki/index.php?title=Frequently_Asked_Questions_(DIY_FAQ)#MEASUREMENTS

[Goodrat]

It's the pickups that care about the volume pot, not really the amp.
The amp will have a high enough input impedance to accept 10K or 100K etc. and not make much difference.
I would guess if a pedal or guitar output impedance is higher than 100K, it may put more noise on the guitar cable.
That is my first reaction anyway.

[Nick C.]

I read the links Aron posted and a new one,

http://www.diystompboxes.com/smfforum/index.php?topic=102646.0

I'm concluding (at least for me):
- This is a complicated subject.
- We have to consider what comes before and after the pot.
- A lower volume pot on the output can be better for the amp if we have the current going into it.
- There are compromises and some trial and error might be called for.

So, for my case where with a too loud pedal that is dull at low volume settings, a lower value pot might help. This may be why I've heard that Eric Johnson like a 100k volume pot in a Fuzzface.

[askwho69]

> because in guitar says using 500k adds highs and 250k lowered

OK you guys went and did it again.... it's the dirty word impedance.

http://www.diystompboxes.com/smfforum/index.php?topic=14019.0

Then check this out - scroll down a little:

http://www.diystompboxes.com/wiki/index.php?title=Frequently_Asked_Questions_(DIY_FAQ)#MEASUREMENTS

Thanks aron

[Goodrat]

On a distortion pedal, try a 250K linear with a 50K from wiper to ground.  This will simulate a log with better parts and smoother range. Most log pots are not manufactured as well as expected. Correct me if I'm wrong, but I tried this with good results.

[Thecomedian]

Log pots are built with geometrically created values of different resistance in order to keep down the cost, instead of creating nice curved shape resistance material.

Different mfg make those resistance values for each "leg" different.

I'd scan one of my books to give a picture, but the simple phrase is "as high impedance as possible, maximum voltage transfer, minimum current, middle value, equal volt/current, low value impedance maximum current transfer, minimum voltage".

The circuits inside a box or a guitar depend on a certain level of current flowing around the output area. If there's less resistance to ground, there's more ability for current to go that way rather than be deflected into the output loop trying to find ground. The reason highs get louder with a larger ohmic value pot is because resistance has more effect on high frequencies than on low frequencies. Remember that even if you cant hear the guitar or keyboard in a club, you'll still here the wump wump of bass coming through that brick wall.

Caps are the opposite. they have more effect on lows than highs in terms of resisting passage of frequencies.

This stuff is why a high pass filter (passes highs along to the output more than lows) consists of a resistor to ground, while a low pass consists cap to ground

With a much larger resistance, as you increase the volume, you're "getting back" a lot more mid-high freqs, which makes it louder.

You may notice that with all volume knobs, you lose highs more than lows as you turn it down.

There are "sweet spots" of resistance values to have for pots.