Cermet vs Conductive Plastic

[JMan]

I’m looking at a circuit that I have on the bench that has two potentiometers on the control panel (read: they’ll get adjusted frequently) that are both seeing DC voltages. In my limited experience, when I’ve encountered this situation the recommendation has typically been to use conductive plastic pots to help extend their life and avoid them getting scratchy. Currently there are cermet pots in these positions instead. I could get conductive plastic versions of these same pots (readily available), but I’m wondering if it’s worth the effort. I know little-to-nothing about cermet. How does it fare with DC?

 

[Newmarket]

No need to change them unless they are faulty. Cermet is better for heat dissipation but that's not usually a consideration for small signal audio. It has good wear characteristics. Conductive Plastic is less good with heat but also has good wear.
Carbon has good thermal characteristics but wears more quickly.
All this dependent on manufacturing quality as always.
None have any real dependency on DC. "Scratchiness" is due to either fader wear - multi finger wipers can help here - or DC change associated with eg an opamp. This not dependent on the resistive track material.

 

[57sputnik]

The temperature coefficient of conductive plastic pots is much higher than cermet.

The S159 series ( S159 Series - 5/8 inch Square Modular Potentiometer ) have a 1000 ppm tempco in conductive plastic; cermet is 150 ppm.

If you're setting precise DC voltages the cermet would be a better choice.

 

[abbey road d enfer]

I’m looking at a circuit that I have on the bench that has two potentiometers on the control panel (read: they’ll get adjusted frequently) that are both seeing DC voltages. In my limited experience, when I’ve encountered this situation the recommendation has typically been to use conductive plastic pots to help extend their life and avoid them getting scratchy. Currently there are cermet pots in these positions instead. I could get conductive plastic versions of these same pots (readily available), but I’m wondering if it’s worth the effort. I know little-to-nothing about cermet. How does it fare with DC?

It depends very much on how scratchiness translates in terms of audible effects. For example the typical dbx compressors use mostly DC through the potentiometers but the voltages are used in such a way that there's almost no audible effect.
Now, if signal is passed in potentiometers, cermet are the worst, very scratchy. It's the price to pay for reliability.

 

[JMan]

It depends very much on how scratchiness translates in terms of audible effects. For example the typical dbx compressors use mostly DC through the potentiometers but the voltages are used in such a way that there's almost no audible effect.
Now, if signal is passed in potentiometers, cermet are the worst, very scratchy. It's the price to pay for reliability.

Interesting to know! One is a threshold control (so, as far as I can tell, just passing CV) while the other is a gain control, so I’ll probably swap out the gain control in that case.

It’s a recently built unit, so the current pots haven’t actually developed any issues as of yet. I’m just thinking about longevity. I don’t mind paying for a pot or two, even if they’re a little pricey, to sidestep avoidable issues.

 

[abbey road d enfer]

Caution! I forgot to mention that in carbon pots, the track wears out first, before the wiper usually. In Cermets, it's often the contrary.
A long time ago, I used cermet pots where scratchiness was reduced because they used a carbon wiper. As a result, the guaranteed number of rotations was not as high as some competitors.
If I were you, I would keep the old pots when you replace them.

 

[FIX]

Not very many companies make carbon pots anymore for audio, the new thing is polished carbon, and they last between 10,000 and 100,000 rotations.

 

[JMan]

Definitely didn't know much about cermet previously. A little internet research as well as this short old thread helped me get my head around it better. I think I'll swap out for conductive plastic since that seems better suited to the application in question. The only annoying thing is that the CP version of my current pots (which I'll go with due to physical spacing needs) also has a different shaft diameter, so now I get to think about my knob solutions. Anyone ever tried a shaft adapter with a collet knob?

 

[abbey road d enfer]

Definitely didn't know much about cermet previously. A little internet research as well as this short old thread helped me get my head around it better. I think I'll swap out for conductive plastic since that seems better suited to the application in question. The only annoying thing is that the CP version of my current pots (which I'll go with due to physical spacing needs) also has a different shaft diameter, so now I get to think about my knob solutions. Anyone ever tried a shaft adapter with a collet knob?

From what diameter to what?
The general idea with collet knobs is that you can have several shaft diameters for each knob.
I have used shaft adapters for 4mm pots and 6mm set-screw knobs. Glued them with neoprene glue.

 

[abbey road d enfer]

Not very many companies make carbon pots anymore for audio, the new thing is polished carbon, and they last between 10,000 and 100,000 rotations.

Do you kave a reference/brand? Googling "polished-carbon potentiometer" gave nothing.

 

[JMan]

From what diameter to what?
The general idea with collet knobs is that you can have several shaft diameters for each knob.
I have used shaft adapters for 4mm pots and 6mm set-screw knobs. Glued them with neoprene glue.

The new shaft diameters will be 1/8" and the knobs are twice that. Huh, I don't know why I had never thought of gluing the the adapter in place!

 

[Newmarket]

Caution! I forgot to mention that in carbon pots, the track wears out first, before the wiper usually. In Cermets, it's often the contrary.
A long time ago, I used cermet pots where scratchiness was reduced because they used a carbon wiper.

Good point. I was aware that cermet is a 'hard wearing' material but the wiper has a hard time then I guess the end result isn't so good.
Thinking about it - where I've used cermet it's usually been in trimmer applications to set DC bias etc. where the tempco was important given that the fixed resistors were something like 10/15 ppm.
I'm curious about "polished carbon" as opposed to just "carbon" but I see you've already asked that question.

 

[FIX]

Do you kave a reference/brand? Googling "polished-carbon potentiometer" gave nothing.

Alps, Noble, Many US companies have them make pots, I set up a meeting Between Bourns and P&G many years ago, Bourns were amazed at the technology, and may have switches over. You can't google much because they are mostly trade secrets, but if the carbon element is whiney, it's polished.

 

[Newmarket]

Alps, Noble, Many US companies have them make pots, I set up a meeting Between Bourns and P&G many years ago, Bourns were amazed at the technology, and may have switches over. You can't google much because they are mostly trade secrets, but if the carbon element is whiney, it's polished.

Whiney ? Shiny ?

 

[abbey road d enfer]

Alps, Noble, Many US companies have them make pots, I set up a meeting Between Bourns and P&G many years ago, Bourns were amazed at the technology, and may have switches over.

Am I correct supposing you use them in your own builds? What make/model are they?

 

[zamproject]

TKD offer this kind of material for some of their fader/pot product range.
So far I only use CP from them, so can't say about the material look and performance.
Data sheet give more or less the same about mechanical performance, but CP have better accuracy

 

[abbey road d enfer]

I must be dumb, because I don't see the option for polished carbon in either Bourns or Alps catalogues.

 

[FIX]

They don't offer any options...

 

[FIX]

Am I correct supposing you use them in your own builds? What make/model are they?

Alps faders and alps/noble pots