Markings on Potentiometers ('A,B,C,L,F')

[David G4EBT]

In the 'Parts Wanted' section I noticed a thread about a replacement pot, which prompted me to provide to info that might help as to the markings 'A' or 'B' on pots, which are not to be relied upon. Without measuring the resistance of a pot at various points, or by reference to the circuit, the makings 'A' or 'B' on pots give little assurance as to what law they actually are. True, as often as not, the chances are that a volume control will have a log track, and a tone control linear, but not necessarily 100%.

The problem is there isn’t one standard practice of marking pots – it depends in which country the pots are made, and who makes them.

Here are some examples of notations:

Linear:

Asia ‘B’
America ‘B’
Europe ‘A’
Vishay ‘A’

Log:

Asia ‘A’
Europe ‘C’
America ‘A’
Vishay ‘L’

Anti-log (AKA inverse log/reverse log):

Europe ‘F’
America ‘C’
Vishay ‘F’

Thus, depending on which country their pots are sourced from, the same supplier may use both conventions. For example, to take the 20mm Omeg pots sold by Maplin, linear are marked ‘A’, and log are marked ‘B’. However, the smaller 16mm diam pots of different manufacture that Maplin sell have the opposite markings – linear are ‘B’ and log are ‘A’. (The catalogue does make that clear).

Incidentally, almost all of the so called ‘log’ pots that we encounter aren’t actually true ‘log’ pots – they’re what are sometimes known as ‘commercial log’ because they're made down to a price. Basically they use two sections of linear track - one section of a much lower value than the other - which are joined. Hence, instead of getting a smooth log law track, we get two straight lines with a kink to roughly mimic a true log taper. You see this if you attach a paper dial marked in degrees of rotation and plot the resistance versus rotation on a graph, say every 10 degrees. (Or maybe not - life's too short!).

Hope the info might prove useful.

[60 oldjohn]

Hi David, I think the Maplin A / B types confusion may catch a lot of people out.

Incidentally, almost all of the so called ‘log’ pots that we encounter aren’t actually true ‘log’ pots – they’re what are sometimes known as ‘commercial log’ because they're made down to a price. Basically they use two sections of linear track - one section of a much lower value than the other - which are joined. Hence, instead of getting a smooth log law track, we get two straight lines with a kink to roughly mimic a true log taper.

In use I very much doubt anyone could tell by ear alone. I have used Lin pots for volume when I was out of Log pot. I just put a resistor about 0.25% value of the track from the slider to the earthy end of the circuit. eg 10k Lin with 2.2k and I have been unable to tell it was not the proper pot.

John.

[McMurdo]

There's also 'D' which like a log but the gradual bit is even more so and the steep bit even steeper! Often used on audio equipment 'gain' controls where you need to have the line and mic (or phono) range on the same pot.

[Omegaman]

Has anyone come across an 'E' taper?
I have a Rane (US) active crossover that has 4 gang 100KE pots in.
I was curious as to what they may be but can't find anything on them.

[Andrew B]

There is yet another type known as "S" law, it appears linear in the middle part of the travel, logarithmic on the final 10% of travel on both ends. By the way, anyone know which "log" they use Log10 or log"e". ?

[Alistair D]

S taper pots are sometimes used in wah wah pedals. I have also seen ICAR taper mentioned in the same application. Not sure if S and ICAR tapers are the same or even related.

Al

[Andrew B]

I found this page. http://www.potentiometers.com/potcomFAQ.cfm?FAQID=29