One of those things that I've been wondering for a while:
Do garden variety carbon pots generate appreciable (or at least measurable) amounts of distortion? What about Alps RK09 or fancier types?
A carbon pot is, after all, sort of a carbon film or carbon composition resistor, which are not known for the very lowest temperature coefficient. Now that doesn't matter much as long as the part warms up evenly over the entire length of the track (as in most application, it is the ratio of resistances that counts), but track width on log pots would have to vary, and so I would presume things wouldn't be quite so even then. This in turn may then cause distortion depending on wiper setting.
In many cases pot values are high enough for dissipation inside them to be minimal, but when pushing for lower noise it would be useful to know whether there is any other limit to power other than the thermal one in the datasheet.
I can't imagine noone has ever looked into this, but there is nothing much to be found.
Do garden variety carbon pots generate appreciable (or at least measurable) amounts of distortion? What about Alps RK09 or fancier types?
A carbon pot is, after all, sort of a carbon film or carbon composition resistor, which are not known for the very lowest temperature coefficient. Now that doesn't matter much as long as the part warms up evenly over the entire length of the track (as in most application, it is the ratio of resistances that counts), but track width on log pots would have to vary, and so I would presume things wouldn't be quite so even then. This in turn may then cause distortion depending on wiper setting.
In many cases pot values are high enough for dissipation inside them to be minimal, but when pushing for lower noise it would be useful to know whether there is any other limit to power other than the thermal one in the datasheet.
I can't imagine noone has ever looked into this, but there is nothing much to be found.
There may be a slight difference in microphonic behaviour between loose carbon granules next to a thin diaghram and carbon particles embedded in a fired clay matrix.
It is likely that carbon pots generate some distortion. It is also likely that this will be very small, given the small voltages usually used. My guess is that the best option is to ensure that the load driven from the wiper has fairly high impedance and is AC-coupled.
It is likely that carbon pots generate some distortion. It is also likely that this will be very small, given the small voltages usually used. My guess is that the best option is to ensure that the load driven from the wiper has fairly high impedance and is AC-coupled.
I've seen older, cheaper pots where the wiper resistance has got high and any mechanical vibration modulates it. Caused some really weird distortion in my bass. 🙂
Yes, the wiper-track interface is usually the weak point in a pot - hence the need to use a high impedance load.
And is why putting a pot in the negative feedback(MF A1 etc) or allowing dc to flow through the wiper is a bad idea
Tolerence to DC is another weak point.
I believe DC bias (leaky coupling caps etc) causes carbon pots to go noisey/crackly, when rotated especially, but also when left in any particular position.
I am not sure of the mechanism, but I suspect electrolysis is the root cause.
In pots that I have opened, the contact grease has gone copper green and stiff/waxy, and a hard film on the carbon track is present/formed.
On some vintage gear I have had to clean/polish the carbon track with Brasso, and then cleansed all contaminants/grease etc with solvent before re-lubricating.
In these cases, a measurable leakage DC bias was present, usually on the wiper.
Dan.
This is a known effect, yet still we see lots of circuits (both DIY and commercial) which apply small DC currents to pot wipers. People seem so frightened by capacitors that they prefer having to replace their pots from time to time. Others try to avoid pots by using switches - then discover to their horror that caps are even more necessary with switches!
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