Does volume pot value really matter?

[Bishop Vogue]

Found a lot of discussions regarding 250k and 500k for your guitar's vol pot but nothing for pedal builds.  I recently built a pedal and put in a 500 ohm pot that I have had laying around forever.  Seems to do the same job as a 100k.  The circuit I built seems pretty quiet, but it doesn't seem logical that the pot value would affect maximum volume.   Err... right?  Any advice well appreciated!

[Brisance]

The pot defines the output impedance pretty much.

[PRR]

> 500 ohm pot ... Seems to do the same job as a 100k.

Draw the WHOLE circuit!

What feeds that pot?

There are many sorts of "pedals".

Some circuits come out as high impedance. A 50K collector/drain resistor is common. If you load 50K with 100K, the drop is small. If you load 50K with 500 Ohms, the drop is huge.

OTOH, a chip opamp will drive 2K bigtime with clean ease, and usually 500 Ohms to guitar-cord level with unimportant extra distortion. 

[mcknib]

The difference in resistance as far as I understand it with a simple volume control would just mean the range of the pot would act differently so when you turn it fully clockwise or up i.e. to 0 ohms there would be no resistance between lug 3 signal in and lug 2 signal out allowing the full signal to flow and when you turn it the other way i.e. down you'd have less resistance between 2 and 1 ground allowing more signal to be dumped to ground.

I don't think it would affect the volume as such because the volume for the circuit would always be the same anyway i.e it's not going to get any louder because the pot just controls how much signal gets out and how much is dumped to ground so you're just adjusting the resistance to one side and increasing it to the other and visa versa with a different value pot it just means that it will take longer than a 100K pot to get to full resistance or least resistance. If you just imagine a 100K at 50% rotation should read around 50K  and you guessed it a 500K around 250K.

It doesn't really matter because both can get to 0 and 100K ohms. Having said that I always stick to the taper and value for any pedals I'm making. Gain controls etc are a different matter depending on how they are wired and what to etc.

With guitars it's to do with treble loss / pickup type etc as you've probably read

EDIT Just noticed with Mike's informative answer it's a 500R pot I thought you'd used a 500K pot oops sorry better read my values correctly next time!

[GibsonGM]

It's about what Brisance and PRR said, and yes, it's important.  The output pot works with the output impedance of whatever is driving it.  Yes, a 500 ohm pot WILL turn down your signal (as long as 500 R is enough to silence it), but it's also probably not the right load for the device driving it (chip opamp excepted, as explained).

If your transistor has an output impedance of 60K, and you use a 50K pot, you are using the ideal impedance (resistance...) at the output.   And as stated, 100K will work ok too, not that picky.  That's why 100K is so common.

VERY mis-matched impedances begin to lose high frequencies...you are basically throwing part of your signal away.  That's why using the right output transformer for a tube circuit (or radio...) is important, among other reasons.  It's also why buffers exist. They transform impedances.   Using 500 ohms where 50K is better means your "leaking" portions of your signal to ground that you shouldn't.    If you don't notice anything "bad", that simply means you got lucky.

[petemoore]

  Impedance calculating does involve the value of most pots labeled 'volume'.
  In the guitar it involves the rest of the guitar circuit, often what the guitar plugs into 'matters' also.
  If it is a 10k volume pedal pot, 100k volume pot wired together when the guitar cable connects them, the volume pedal pot 10k value will dwarf the 100k value, calculations put the 2 seriesed volume control pots combined value approximately around 9k [most 10k pots actual value is <10k] when both are set atw up. Raising the R to ground in a case like this will probably allow more treble proportionately and more overall signal pass...and likely 'matter'.

[Bishop Vogue]

Sorry I didn't post the schem - didn't realize it would matter!  Here it is:



Thanks all for your responses.

[Brisance]

that's not a schematic 

[Bishop Vogue]

Oh for cripes' sake why did I bother even asking?

[slacker]

To be fair even without a schematic you can see that the output is going through a 47k resistor, this forms a voltage divider with the pot setting the maximum output volume, so using a 500 Ohm pot instead of a 10k will drop the maximum volume to about 1/20th of what it should be. It will work fine other than that, if it's loud enough for you stick with the 500 Ohm.  

[GibsonGM]

+1  ^^  What he said.

I think it really is one of those situations where "It MIGHT matter".  If it sounds weird and you used an off-value pot, that might be why.
Or maybe you'll notice nothing at all....

[Bishop Vogue]

Thanks for the good input guys.  I ended up replacing it with a 10K pot and, as slacker said, the volume shot through the roof.  Definitely learned something on this build.

[PRR]

> Oh for cripes' sake why did I bother even asking?

Alternatively, if you use 500K instead of the 10K (not 100K) called-for, and there's a big resistor like 47K in series, you might ask if you could reduce the 47K in the same proportion.

10K = 10,000

10,000/500 = 20:1

47K/20 = 2.3K

Now the poor amp faces about 2.8K load. Can it handle it?

The LM386 can handle 8 Ohm loads. Not a problem.

Series caps must pass bass. The apparent design was 47uFd into that 57K. Frequencies above 0.03Hz will pass fine. The new design might be 20:1 higher. Frequencies above 0.6 Hz will pass. As we only care about 82Hz, maybe 41Hz, this is still fine.

We might wonder why it was scaled for 0.03 Hz. The 47uFd was taken from a loudspeaker app-note. 16 Ohms against 47uFd is 200Hz, which is good-enough for a pocket radio. This value was not re-considered when converted to a guitar-cord device of MUCH higher impedance. A smaller part might be cheaper but in this range the difference is teeny, less than the value of the mental-effort to re-compute. (And certainly not worth ripping-out to replace with a still-over-large 1uFd electro, or a more-costly 0.1uFd film.)