[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.

If you use a standard one-opamp, inverting/non-inverting, differential-amp
configuration, then it is very easy to do this.  You can do it two ways:

1.  With resistors:

Connect two equal resistors between the centre tap and each end tap of the
pot.  With no resistors, the gain response is linear.  This response gets
flatter in the centre and steeper at the ends (like the tan function between
pi/2 and 3*pi/2) as the resistors get smaller with respect to the pot
resistance.  For example, with a 100-k pot, two 47k resistors will give a
nicely flattened response.  For an even flatter response, use smaller
resistors.

2.  With a trimmer:

Instead of the two resistors, connect a trimmer across the pot, with each
tap of the trimmer connected to each corresponding tap of the pot.  Again, a
100-k trimmer in parallel with a 100-k pot will give a nicely tangential
response.  Presuming the trimmer is set exactly in the centre, this is
equivalent to using two equal resistors.  However, in this case the trimmer
can be adjusted to trim the centre point of the pot.  This is especially
useful for establishing a true zero point in the centre of centre-detent
pots (as Mark Verbos pointed out on Monday).

In either case, for impedance matching, the input and feedback resistors on
the opamp's negative terminal should be reduced to the parallel resistance
of half the pot and one of the resistors.  Hence, if a 100-k pot and two
49.9-k resistors (or a 100-k trimmer) are used, then the input and feedback
resistors should each be 24.9k (I think).