[sdiy] CV inputs with bipolar "amount" knobs

[Tom Wiltshire]

On 25 Jul 2010, at 03:06, cheater cheater wrote:

> On Sun, Jul 25, 2010 at 02:47, Mattias Rickardsson <mr at analogue.org> wrote:
>> Interesting topic.
>> 
>> Here's Roman Sowa's version of the buffer/inverter solution, with a
>> shunting resistor at the output giving a "bipolar fake log" response.
>> The log ("audio taper") response is good for two reasons: our general
>> nonlinear preferred feeling of signal level pots; and the widening of
>> the null point since the pot curve is less steep in the middle.
>> 
>>  http://www.sowa.synth.net/modular/inverter.gif
> 
> But if you want the curve to be less steep around the center you want
> an exponential or even better an nth power response such as x^3. A
> logarithmic response makes it more difficult to zero in on the center.
> The curve needs to be steeper on the middle so that the function can
> cover the whole range while the curve is flatter at the extremes. This
> way it lessens the precision at the center while improving the
> precision around the extremes. Have you actually tried it? If it
> behaves the way you describe, maybe the response isn't actually
> logarithmic at all.

I think you're talking at cross-purposes. This stems from the usual confusion over what "log" means when you're talking about pots. A "log" pot is supposed to give an even response when used as an audio level control. This means it having a dB/angle response, at least in theory. Whether this is called "exponential" or "log" is something that some people seem to do one way, and other people the other. Those two are the flipsides of the same coin, so I guess it doesn't matter, but it does sometimes lead to confusion.

Either way, if you want an easy null in the centre, you want something that looks like x^3.

T.