[sdiy] CV mixer trouble

Harry Bissell harrybissell at wowway.com
Wed May 16 03:33:15 CEST 2012


trimming the input resistor will work, but only for that input. If you have only one precision CV source, its a viable approach. Otherwise making all the summing resistors match (maybe 0.1%) is the usual way...

H^) harry


----- Original Message -----
From: Tom Wiltshire <tom at electricdruid.net>
To: Neil Johnson <neil.johnson97 at ntlworld.com>
Cc: synthdiy diy <synth-diy at dropmix.xs4all.nl>
Sent: Tue, 15 May 2012 18:02:18 -0400 (EDT)
Subject: Re: [sdiy] CV mixer trouble

Thanks everyone. Lots of good ideas and many different approaches.

I was able to get it working as Neil suggested below, by making the input resistor the variable part rather than the feedback resistor. I swapped R6 for PR2+R22 and bingo! The interaction disappears and the whole thing suddenly becomes *a lot* easier to tune.

Tom


On 15 May 2012, at 21:53, Neil Johnson wrote:

> Hi Tom,
> 
>> 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.
> 
> As Veronica says, your "1v/oct" trimmer adjusts the gain of your summer, which will affect *both* inputs (CV and offset).
> 
> If you just want to adjust the CV gain then I suggest getting rid of PR2 and making R6 your adjustable "1v/oct" trimmer.  You might want to fiddle with some of the values, and DO NOT use the supply rails directly for references - heck, even a zener would be better than all the crap coming in from the supply rails.
> 
> Neil
> -- 
> Modules and more :: www.cesyg.com
> Homepage :: www.njohnson.co.uk ::

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