Treble 'Bleed' / By-Pass Circuits to Change Log Pot Taper towards Linear

jaymenon

If I use a capacitor and a resistor in parallel (rather than series), I understand that it can influence the taper of a potentiometer.

That would be a desirable thing, since I would like to change my 500kΩ audio taper pots (Les Paul) to behave in a more linear fashion.

Calculation:
500kΩ audio taper pot
So at the mid-point ('5' on the dial)
Resistance would be 10% or 50kΩ (450kΩ on the other side of the pot)

At the mid-point (at '5')
The 330kΩ resistor is in parallel with the side that has 450kΩ resistance
Effective pot resistance would be 198kΩ (302kΩ on the other side of the pot)
(1/R = 1/450 + 1/330)

So the taper would be somewhat closer to linear...

I guess a 450kΩ resistor will mathematically make the pot linear. But would that compromise the efficacy of the Treble Bleed circuit?

What say you?

stylesforfree

You might lose some volume when rolling the treble back, I think that's the general compromise with a treble bleed circuit, but you probably know this already.

Your maths seems to be on point in regards to the effective pot resistance, I would say that's as close to linear as you'll get while mostly negating any unwanted side effects.

ICBM

Your maths isn’t right - you need a far larger modification than that. Ignoring for a minute that the pot is most likely audio taper (30% log) not true log (10%)…

A log pot has 10% of its resistance in the lower part of the track at half turn - ie 50K, for a 500K pot. A linear has 50% - so to turn a log into a linear, you need to reduce the upper part of the track to 50K as well. This would need a 56K resistor in parallel with the remaining 450K - the problem with that is that the total resistance to ground is now only 100K at half turn, which will dull the tone a lot.

Now, the good news - for a 30% audio taper, it’s 150K at half turn, so you need 262K in parallel with the remaining 350K, which is a much less drastic change in load, only down to 300K at halfway.

I’m sure it’s not a coincidence that my favourite treble pass values are 680pF in parallel with 220K - that’s not far off a linear taper for the low-end, but with a treble boost to offset the slightly duller tone as well as the cable capacitance.

Funkfingers

If you want linear taper pots, just buy ******* linear taper pots!

jaymenon

Funkfingers said:

If you want linear taper pots, just buy ******* linear taper pots!

Yes- but I'll have to take out the log pots from my Les Paul (with all the push-pull wiring faff), install linear ones and re-wire.

Installing a treble bleed (if it would have worked) would have involved much more 'superficial' surgery

ICBM

Funkfingers said:

If you want linear taper pots, just buy ******* linear taper pots!

Actually, I don’t - I want pots that *sound* like linear to my ear, but that isn’t the same thing… actual linear pots still dull the tone too quickly, and if you put treble-pass caps on them something sounds ‘off’, like they get brighter before they get quieter. So an audio pot with a resistor and cap is usually better. (Depending on the guitar.)

It’s a bit like those ancient Greek temple columns which have a curved profile, intentionally so that they look straight when you’re standing at the bottom of them .

jaymenon said:

Installing a treble bleed (if it would have worked) would have involved much more 'superficial' surgery

It will work. Try 220K and 680pF… you can thank me later .

jaymenon

In parallel or Kinman style Series @ICBM ?

My aim is to have a wider and easier range of ‘blend tones’ in the middle position between 5 and 10 on one pickup (the other pickup being on 10). 

ICBM

Parallel.

jaymenon

Will a 270k resistor to just as well (in stock at Casa Jay )  or do you prefer a 220 K?

ICBM

Probably just as good really - there will be a difference, but fairly minor. 270K is actually closer to the theoretically correct value!

jaymenon

ICBM said:

Probably just as good really - there will be a difference, but fairly minor. 270K is actually closer to the theoretically correct value!

Done. Bingo! Great stuff @ICBM

The taper has changed very subtly - and the treble bleed / pass works really nicely…

…and now I’m getting a nice range of ‘blend’ tones between 7 and 10 on one pot, the other one remaining on 10. (Previously it was between 9 and 10).