Emulating log pots with lin pots...your experiences

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Silvas

Well-known member
Joined
Aug 15, 2006
Messages
727
Location
Chia, Colombia
I know that´s possible to emulate a log pot or a rev log pot somewhat decently with a lin pot and resistors....i haven´t tested this and would like to know how it worked, from people who has done it in practice ...and nice ways to do it (yes i searched!). What i want to do is -emulate- a rev log behavior for an idea i´m working on.


Many thanks!
 
Hey Silvas - have you tried this from Sneakthief?
http://www.groupdiy.com/index.php?topic=19789&postdays=0&postorder=asc&start=330
 
[quote author="Silvas"]I know that´s possible to emulate a log pot or a rev log pot somewhat decently with a lin pot and resistors....i haven´t tested this and would like to know how it worked, from people who has done it in practice ...and nice ways to do it (yes i searched!). What i want to do is -emulate- a rev log behavior for an idea i´m working on.


Many thanks![/quote]

This is one of those things that looks good on paper but doesn't work as well in practice. The problem, at least for large scale production repeatability is that the bulk resistance of typical potentiometer resistive elements is 20%, so the result of loading them down with a smaller value to pull the taper can be a little too variable in practice.

Potentiometers are most predictable when used ratiometrically and lightly loaded so the bulk resistance is not a source of errors.

Note: reverse log taper pots are common in some low noise mic preamp topologies so for DIY use, consider buying repair parts from a mixer manufacturer's service department. I recall at my old day job we tooled a special pot that had excellent low ohms hop off resistance at one end and 25k total resistance at the other. It required extra process steps as a single screened resist wouldn't deliver that well behaved of a taper.

JR
 
Search.

This ONLY even PARTIALLY works in a 'kinda-sorta' a little-bit-better-than-linear-but-not-really-log' way, when you're using them in a THREE-TERMINAL application... i.e. when one terminal is connected to ground, and the other two are input and output for example.

You CANNOT do this with a two-terminal 'variable resistance' application; it doesn't work. -This seems to be where people are most interested in it: making reverse-log pots (which are otherwise nigh-impossible to source) for mic preamp gain controls (a la SSL 9k) or parametric EQ sweep controls...

In these applications, it CANNOT work.

Do a search; a full explanation has been typed out several times before, and it's pretty long-winded, but the fundamental concept is simple... If you're looking for a 2-terminal application however, it doesn't work.... and that's all you need to know if you don't want to read further.

Keith
 

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