[sdiy] CV inputs with bipolar "amount" knobs

[Ingo Debus]

Am 19.07.2010 um 18:18 schrieb Harry Bissell:

> Advantages: the 'null' point depends on the quality of the pot  
> alone. In theory you could
> get an 'infinite' null (zero).  The differential solution can only  
> be as good as the opamp...
> probably 80db max. Could be worse depending on resistor matching  
> (the 80db usually assumes
> .1% resistors... can you set the pot that well ?)

What do you mean with the quality of the null point? Is this an AC or  
a DC thing?

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.

If it's an AC thing: once you found a position of the pot where the  
fundamental is completely turned off, higher harmonics are passing  
though. This can happen with the inverter/buffer solution too. The  
inverter's frequency response can differ from that of the buffer (if  
there actually is a buffer, you could feed the input signal directly  
into the pot instead).
You can probably adjust this with a gimmick capacitor ;-)

Ingo