[sdiy] CV inputs with bipolar "amount" knobs

[David G. Dixon]

> 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
> 
> The output impedance is a bit problematic though, and the shunting pot
> also draws a lot of current near the pot's ends. Any improvements out
> there? Or any other " bipolar fake log" solutions? :-)

I don't know about "bipolar fake log" solutions, and Roman's technique seems
a bit opamp-intensive.  However, I just rebuilt my 2164 4P LPF with an
inverting/non-inverting CV input based around a single opamp, using Harry
Bissell's suggestion of impedance matching at the inputs, and it works
great!  I never realized how cool it is to have such a CV input on a filter,
but it really adds a lot.  Put in a sawtooth wave, and the filter goes
"tiew-tiew-tiew" when the knob is positive, and "ooup-ooup-ooup" when the
knob is negative.  Plus, it isn't too hard to find the middle, but it would
be easier with a centre-detent pot.  I just wonder whether such a pot would
actually have to be trimmed, or whether using all 1% resistors of the same
value around the opamp would be sufficient (they could even be
hand-matched).