[sdiy] CV inputs with bipolar "amount" knobs

[cheater cheater]

On Sun, Jul 25, 2010 at 12:24, Mattias Rickardsson <mr at analogue.org> wrote:
> On 25 July 2010 04:06, cheater cheater <cheater00 at gmail.com> wrote:
>>
>> 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.
>
> Yes, a "log" pot is exponential. :-)
>
>>> Perhaps the limitations of the first inverter such as slew rate and HF
>>> roll-off would not occur much in the second inverter, removing
>>> problems with spikes etc passing through in the null point?
>>
>> No, their frequency responses would accumulate. In the end a simple
>> model of the situation looks like this: every partial of the signal...
>
> Perhaps I shouldn't have mentioned frequencies, I was thinking
> primarily of slew rate. If "too quick" changes are slewed out in the
> first inverter, they can pass the second inverter quite unaltered. The
> positive and negative versions of the signal taken after the first and
> second inverter would then be more alike than if taking them before
> and after the first inverter.

Yes, but with many worse op amps you're going to still get a roll off,
aren't you? I mean: is the slew rate not the only thing that puts a
limit on the signal coming through the op amp, but there's a bit of
roll-off either way?

I'm specifically talking about supersonic frequency content, which can
easily creep in and create a bias in the signal which will still be
affecting the way our module affects audible frequencies (easy
example: ring mod). But yes, I think the argument with slew rate
limiting is very good when the signal only has content in which the op
amp behaves real well. So maybe a small fixed filter band limiter
before the op amps could be a good idea?

Regarding the x^n response: you can also just use x^2. It'll be better
(the central dead zone will still be significant while not taking up
as much range of the knob). Of course the branch for x<0 will have to
be flipped, but that comes naturally in the circuit we're talking
about I understand.

Cheers,
D.