[sdiy] CV inputs with bipolar "amount" knobs

[Harry Bissell]

Common mode rejection is an AC and a DC thing... it depends on what
annoys you more (if you care).

DC would be seen as an inablility to return to a reference pitch in a VCO,
for instance. It might always be sharp, or flat a little bit.

AS would be modulation that cannot be turned completely off.

The Chris MacDonald "MiniModular" used the common mode method, and was a little
touchy on the null point (but it did work)

H^) harry


----- Original Message -----
From: Ingo Debus <igg.debus at t-online.de>
To: Synth DIY <synth-diy at dropmix.xs4all.nl>
Sent: Mon, 19 Jul 2010 16:41:54 -0400 (EDT)
Subject: Re: [sdiy] CV inputs with bipolar "amount" knobs


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
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-- 
Harry Bissell & Nora Abdullah 4eva