Low leakage

[user 37518]

Mica is a solid mineral, different mechanical properties than a thin plastic film. I think mica capacitors are usually enclosed in an epoxy molding. Could be better seal than the typical plastic film construction, but I'm not sure about that.

Yes, Mica is a solid mineral, but the way it is used in caps is as a film.

From Webster:

film: a thin skin or membranous covering, an exceedingly thin layer.

But I do get your point.

 

[Newmarket]

As is usual in life things have multiple factors going on. JR's comments are of course perfectly correct but there are other factors at play, like how smooth and 'clean' the potentiometer track really is. whether there is any DC voltage present on either the incoming signal or the destination the wiper is feeding. Then how much 'disturbance' is permissible. For example if a high gain amplifier follows the pot (anywhere in the following chain) then any disturbance may be noticed. As you may be changing the 'source impedance' of the amplifier the wiper feeds into you may be altering the noise contribution from the resistance of the pot if there is a lot of gain being applied.

+1 It's useful to differentiate between "electrical" and "mechanical" noise from a pot. The best quality CP pot' will still cause noise on a circuit if there is DC on it where modulating that DC level causes noise. It will probably sound like "rustling". OTOH problems with the track / wiper interface will cause what I'll term as scratchiness / pops etc. This is what you are getting if noise increases with age / wear. Multi-Finger wipers on eg faders can help. Not perfect though

 

[saint gillis]

Minimizing Johnson noise is good, but you have to apply engineering principles, don't just say "noise bad" and that is it.

So for you, with a unity gain stage, it's more beneficial to use 1µ film cap with 220K volume pot, than 22µ electrolytic with 10K volume pot

 

[user 37518]

So for you, with a unity gain stage, it's more beneficial to use 1µ film cap with 220K volume pot, than 22µ electrolytic with 10K volume pot

In terms of leakage, which is what we are discussing here, yes. And maybe (although generally not) also in terms of distortion under certain circumstances, because a non-inverting stage will produce higher levels of common-mode distortion as the resitance goes up. In terms of noise and DC offset, no, but, as I said, the circuit you provided will not present a disturbing amount of noise. And, if you replace those NE5532s with a FET input op-amp, you can use a 1u cap and a 200k pot and there will be no real problems in terms of noise current and DC off-set.

With a FET op-amp in the circuit you provided, noise contribution will be dominated by op-amp noise voltage of the last stage; a quick estimation with the circuit you provided, and using a 200k pot and a 200k resistor, resistor noise translates into roughly 182 nV up to 20 kHz. The opamp, considering, say, 8nV/rtHz, will provide something like 1.1 uV due to op-amp voltage noise; this last term is the one that dominates since it is much larger than the resistor noise. The total noise output will be around 1.1 uV over the entire band, not considering 1/f noise, which will add more noise at low frequencies, lets round that number to 1.5 uV of noise, that's around -114 dBu of noise at the output. We would also have to add the noise of the first op-amp, but that noise contribution is at its maximum when the pot is all the way up. In a worst-case scenario, the noise contribution of the first op-amp would only raise the noise floor to around -111 dBu.