Cermet potentiometers

Cermet potentiometers of course are not new, but are still relatively unknown. The reason they are used in the Model A is simple: they are sonically clearly superior to any other potentiometer type.

Still, they (apart form being somewhat fragile) have one shortcoming too: low level inter-channel miss-tracking. I am not aware if this was discussed previously anywhere, so more information on this follows.

As opposed to the conductive plastic potentiometers, which two channels are nicely balanced at all levels, cermets “officially” have two kinds of inter-channel tolerances: the relative and absolute one. Typical relative tolerance of 2% is acceptable (conductive plastic pots are similar here), however typical absolute tolerance of 3 Ohm is a potential problem, as it becomes a major contributor at lowest levels, taking into account that we are operating here with relatively low impedances, namely with series resistance below 1 kOhm.

Also, dealing with cermet pots for some time, and measuring many samples, I have found another, “unofficial” and potentially more important cause of inter-channel miss-tracking. Due to manufacturing issues, cermet “audio taper” is not really logarithmic, but is the approximation made of two or three segments of differently changing resistivity, which simulates required logarithmic curve. The problem is, two channels often do not cross the “borders” between segments fully simultaneously, so the resistance of one channel changes there more rapidly than that of the other. This again happens at relatively low levels, somewhere between 5 and 20 Ohm (Vishay), making the pot possibly useless at and below this point.

So, with such a potentiometer used as a shunt element, and 750 – 820 Ohm series resistance (please see previous article on Model A volume control), we can usually have 30 – 40 dB usable volume control range. In most cases such a range is in fact sufficient, however different systems have different gains and efficiencies, and customers’ requirements are generally different as well. As a result, and adopting classic 60 dB attenuation requirement for volume control, this low level inter-channel miss-tracking may leave us with 20 – 30 dB of the unknown part of loudness equation.

Now, there is no universal solution to prevent this inter-channel miss-tracking to come into the actual listening level. For the customers it is usually too complicated to intervene into the gain of the system, however there is a simple fix to move this point down, and it is to add some series resistance in front of such a volume control. (Of course, such a fix can be applied to the shunt potentiometer only.) So, by using say 1.5 kOhm instead of 750 Ohm (or simply by adding 750 Ohm – miniature resistor can be put even into the RCA connector), this point will drop down by roughly 6 dB.

It can look like inconvenience, and added resistance may compromise a bit the performance of the potentiometer itself (remember, we want low impedance), but things can be ultimately set to operate smoothly.

And reward of cermet potentiometer sonic properties is high enough. No other volume control sounds that natural and full-bodied. And in my view, it is only digitally controlled resistive networks (sometimes mistakenly referred to as “digital potentiometers”) that can come close to the performance of such a volume control setup.



Related topic:

Model A volume control

4 Replies

  1. 12-07-2016

    Interesting & thank you for sharing this with us. I would have two questions: any type of potentiometer you would recommend? (Bourns model 95 or Vishay P11 / P11L) Problem is unfortunately to source these in 1K with audio (logaritmic) taper…
    My second question (and solution to the sourcing problem): did you try a linear pot using a law faking resistor with same results?

    Best regards,

  2. 16-07-2016

    Hello Berny,

    Cermet pot availability might be the hardest part here. However you do not really need 1 k, you can use 5 k, or even higher potentiometer values with say 820 Ohm series resistance. Of course, the higher the pot value, the less convenient volume adjustment will be, but so long as you are setting this volume control for your own system only, I believe you can tweak it nicely.

    In most cases linear potentiometer will be too rough though, and law faking resistor won’t be of much use in this case.


  3. 21-07-2016

    Cermet pots from SFERNICE has a good reputation in France in the DIY communauty in France! It’s an industrial part with a good reliability and tolerance !

    If I remmember , it can be sourced at selectronic.com

    Maybe Worth to try this brand too, but I don’t know the Z range of choice but for sure they have both linear and logaritmic pots.

  4. 21-07-2016

    Ah just seen SFERNICE is now a brand of Vishay…

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