Psuedo-Log-Taper Pots

[John Speth]

I looked into this about a year ago.  Here are a couple of ASCII-matics for your use:

simulation of a lot pot:

        o vi
        |
        >
        <
        > rh
        <
        |
  wiper +-----------+----o vo
        |           |
        >           >
        < rl        < rx
        >           >
        <           <
        |           |
        +-----------+
        |
       gnd

simulation of a reverse lot pot:

        o vi
        |
        +-----------+
        |           |
        >           >
        <           <
        > rh        > rx
        <           <
        |           |
  wiper +-----------+----o vo
        |
        >
        < rl
        >
        <
        |
       gnd

I even went so far as to write a little C program that generates the raw data vo/vi curve for various values of resistors and pots.  Then I pasted the output into Excel, graphed it, and compared it against a true log curve.  (Excell is greate way to do little "what if" scenarios.)  I found there is a best ratio of pot resistance to extra resistor value but you never get a true log.  The bonus is that you can tune the curve for a response more to your liking.

If anybody wants this C program I wrote, let me know and I can email you a copy.

John Speth
Object Engineering, Inc.
johns at oei.com

-----Original Message-----
From:	We Are As1 [SMTP:WeAreAs1 at aol.com]
Sent:	Wednesday, April 15, 1998 3:47 PM
To:	synth-diy at mailhost.bpa.nl
Subject:	Psuedo-Log-Taper Pots

Hello list members,

I seem to remember hearing something about a trick which can be done to get a
pseudo-log-taper response out of a linear potentiometer.  I think it was done
by adding a resistor or two in parallel with the pot.  Has anybody ever tried
this?  Exactly how should it be wired, and how should the resistor values be
selected?  My applications for this:  one would be a three-terminal
attentuator (for audio), and the other would be using the pot as a two-
terminal variable resistor (such as the speed/freq. control in a typical two-
opamp square/triangle LFO circuit).

I imagine that if this trick works, it could also be used to affect a psudo-
reverse-log-taper response, just by wiring it backwards, no?

Thanks,
Michael Bacich