[sdiy] CV mixer trouble

Richie Burnett rburnett at richieburnett.co.uk
Tue May 15 22:34:05 CEST 2012

Don't you just need to adjust the scaling pot PR2 first and locktight it, 
then null out any offset using the PR1 pot?

If you do it the other way round then adjusting the scaling pot PR2 will 
alter the gain applied to the offset voltage from PR1.

I think it may also be a good idea to have a resistor in series with the 
non-inverting input to the op-amp to balance the effects of input bias 


----- Original Message ----- 
From: "Tom Wiltshire" <tom at electricdruid.net>
To: "synthdiy diy" <synth-diy at dropmix.xs4all.nl>
Sent: Tuesday, May 15, 2012 9:00 PM
Subject: [sdiy] CV mixer trouble

> Hi All,
> I'm having  spot of bother with an oscillator's CV mixer. The circuit is:
> http://www.electricdruid.net/images/CVMixer.png
> I've got a Note CV coming in to one end of R6, 0-5V, and I'm expecting to 
> get 5V-0V out of the other end. The Offset trimmer adjusts the -5V offset 
> that gets fed to the mixer. Seems simple enough on paper.
> The problem I'm seeing is that changing the 1V/Oct trimmer seems to alter 
> the offset as much as the scale. If I start with a low C and then try the 
> octave above, I find perhaps a D. So I tweak the 1V/Oct trimmer until I 
> have a C at the top. But when I go back to the low C, it's now a B. If I 
> was trying this with several volts of CV (say a C at 4V and one an octave 
> higher at 5V) then I'd understand why the bottom shifts too, but I'm 
> seeing it at the lowest C on my ProOne (my CV source) which is only 0.15V. 
> It should be 0V, but there's an offset on the ProOne's CV output - not 
> that it should matter, I ought to be able to trim it out too.
> Initially I had a lot of problems with the Offset trimmer because I had a 
> 100K trimmer in there and the 100K input impedance wasn't enough for it to 
> behave at all. Could the (still fairly high) combined 60K of the Offset 
> trimmer cause this?
> Sorry this is so confused, but if I knew better what the problem is, I 
> might have sorted it out by now. Perhaps that's my answer. I was hoping 
> someone could provide a idea which (even if not entirely relevant!) might 
> let me see what I'm doing wrong.
> Thanks,
> Tom
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