Potentiometer Values

jaymenon

On one Les Paul, I have pot values that are around the 450K mark - and fitting 500K+ volume pots will arguably make the instrument sound a little brighter...

However, in discussion with a friend I understand:

A 500k pot can have an actual value that can be anything between 440K and 560K.  

You can’t add frequencies that aren’t there by changing the values of pots.  What you can alter is the resonant peak of a circuit and the roll off point at which high frequencies can drain to earth (low pass filter). 

When you change the value of a volume pot what you are doing is altering the peak resonant frequency of the combination of volume pot and pickup by changing the combined impedance.  This is expressed as the reciprocal of the reciprocal sum.  So R = 1/(1/r1 + 1/r2).  For a 8k pickup and a 250k pot the global impedance is 7.75k  for a 500k pot this is 7.87k.  This impedance combines with capacitance in the tone control to produce the peak resonance factor.  The maths is much more complicated than shown here, but illustrates why it’s orders of magnitude that matter, not nitpicking values.  

Re: the difference between the global impedance for pots that are closer in value, say between 450 and 500k.  The global impedance for a 450k pot is 7.86. That’s just .01k difference.  Given that pots and pickups can vary in impedance by as much as 10 - 20%; and also the fact that our perceptual threshold for change is much wider than the changes that can be effected by minor shifts in component values, we may well be experiencing merely a perceptual bias filtered by our own expectations - again exacerbated by the fact that the comparisons we are making are not direct, but filtered by memory...

So perhaps fretting over the values of pots this close together is a bit pointless?

sixstringsupplies

jaymenon said:

On one Les Paul, I have pot values that are around the 450K mark - and fitting 500K+ volume pots will arguably make the instrument sound a little brighter...

However, in discussion with a friend I understand:

A 500k pot can have an actual value that can be anything between 440K and 560K.  

You can’t add frequencies that aren’t there by changing the values of pots.  What you can alter is the resonant peak of a circuit and the roll off point at which high frequencies can drain to earth (low pass filter). 

When you change the value of a volume pot what you are doing is altering the peak resonant frequency of the combination of volume pot and pickup by changing the combined impedance.  This is expressed as the reciprocal of the reciprocal sum.  So R = 1/(1/r1 + 1/r2).  For a 8k pickup and a 250k pot the global impedance is 7.75k  for a 500k pot this is 7.87k.  This impedance combines with capacitance in the tone control to produce the peak resonance factor.  The maths is much more complicated than shown here, but illustrates why it’s orders of magnitude that matter, not nitpicking values.  

Re: the difference between the global impedance for pots that are closer in value, say between 450 and 500k.  The global impedance for a 450k pot is 7.86. That’s just .01k difference.  Given that pots and pickups can vary in impedance by as much as 10 - 20%; and also the fact that our perceptual threshold for change is much wider than the changes that can be effected by minor shifts in component values, we may well be experiencing merely a perceptual bias filtered by our own expectations - again exacerbated by the fact that the comparisons we are making are not direct, but filtered by memory...

So perhaps fretting over the values of pots this close together is a bit pointless?

Fretting over pot values is absolutely pointless. I get emails every week from people asking for my opinion on the perfect pot values for “<enter pickup manufacturer here>”and “<enter style of playing here>” Most pots are made to a tolerance of 20% for the simple reason that the human ear is not capable of detecting the audible difference between a 400k pot and a 500k pot.

you’ll see a lot of guitar parts suppliers (myself included i might add) selling tighter tolerance pots, be it 10%, 8% 7%!!) but you’re paying extra for no benefit, except peace of mind I guess, or a more favourable taper curve.

ill be honest and I’ll say that I have a matched pots option - link here: https://www.sixstringsupplies.co.uk/product/matched-cts-tvt-pots

i haven’t posted the link as promotion, I’ve posted to highlight something - if you read the product description in full, you’ll notice I state a wee disclaimer at the end:

“Please note - we do not believe matching pots makes any audible difference to your tone whatsoever. Pots are made to a tolerance of 10%-20% for a good reason - the human ear will not detect any audible difference.”

i charge £1 extra per pot and that’s for trawling through a big box of them and measuring them all on the multi meter. 

its amazing how well they sell. Regardless of your knowledge or what you believe, there’s a market for tight tolerance pots/matched pots - many people have read something somewhere online about it so they try and source right pot values for their perfect build/tone. I’m simply supplying demand. 

Having said that, the pots I order are made to a tolerance of 10%, but the reality is, out of a box of 500 or so 500k pots, the vast vast majority will measure between 495k and 520k. 

As a general question, when you talk about pot value tolerances, do you mean (let’s take a 20% tolerance) to be:

400k-600k (20% of 500k being 100k)

or

a range of 20%  (ie 450k-550k)

ICBM

What sixstringsupplies said.

I did some direct-switching testing on this, using resistors to simulate changes in the pot value, and the result was that the smallest difference that was audible was at about 25% under the marked value, and 33% over.

That does mean that you could potentially hear a difference between pots at the extremes of the tolerance range at +/- 20%, eg a 400K and a 600K (both nominally 500K), but it would have to be that extreme.

You won’t hear any difference between a normal 500K pot and a 550K, *especially* as one of the selling points of the 550Ks is tighter tolerance, so you won’t get one as high as 600K.

The quality and length of your guitar cable - which directly changes the capacitance - will make a bigger difference anyway.