[sdiy] CV inputs with bipolar "amount" knobs

[Mattias Rickardsson]

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

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? :-)

Speaking of null point problems caused by different behaviour of the
buffer and the inverter - would it be better to use two inverters
connected in series instead, and send their two outputs to the pot?
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?

To conclude, I'm also curious about what Mark Verbos just asked,
regarding Buchla's bipolar solution. The 259 has several of them, for
instance.

/mr

On 20 July 2010 00:08, mark verbos <mverbos at earthlink.net> wrote:
>
> On Jul 19, 2010, at 5:50 PM, David G. Dixon wrote:
>
>>> If it's a DC thing: as I understand it the only problem is, the null
>>> point is not exactly in the middle of the pot travel. Crude way to
>>> take care of this: mount the knob on the pot shaft so that the
>>> marking on the knob is in middle position at the null point.
>>
>> I believe Harry was talking about centre-detent pots, in which case you
>> can't really choose the centre independently.  However, if you use 1%
>> resistors and a decent opamp, I can't imagine this should be a problem.  I
>> mean, after all, if you need absolutely zero CV to your circuit, unplug
>> the
>> damn patch cord!  If that's not good enough for you, then a 1k trimmer off
>> one end of the pot will do the job; which end depends on where the "true
>> centre" is relative to the detent.
>
>
> A trim pot in parallel with the pot is easier.
>
> BTW. is there any reason to have the pot after the opamps rather than
> sending the signal through a resistor to the wiper and then one side to an
> inverter and the other side into the summer like Buchla does?
>
> Mark
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