[sdiy] CV mixer trouble

[Veronica Merryfield]

If I read you and the schematic right, your offset is always going to be negative (-4.7v to -7.1v). The gain is always going to affect the offset since the gain will change the offset and the span. You are probably best off by setting the offset to a known value (DVM at PR1/R23), say -5v. Then set the gain to get the output you want. Check by putting a couple of known note CV inputs.

It is easier to have an offset designed to give you zero since zero times the gain is still zero.



On 2012-05-15, at 1:00 PM, Tom Wiltshire wrote:

> 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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