[sdiy] advice on mixer/summer circuits

[Tom Wiltshire]

On 28 Jul 2008, at 04:09, John Mahoney wrote:

> At 06:20 PM 7/27/2008, Dan Snazelle wrote:
>
>> very cool
>>
>> odd it looks like that circuit uses a summer (inverting) but never  
>> inverts a second time before the 2044...so that does that mean all  
>> the CVS go in upside down?
> ...
>
> Does the SSM 2044 respond inversely to frequency CV inputs? The  
> datasheet circuit inverts only once, also. Check the datasheet,  
> which Paul graciously serves here:
>         http://www.synthtech.com/cems.html

The datasheet seems to suggest that the Fc Frequency control input is  
itself a summing node, so perhaps inputs are re-inverted by the chip.

It's actually fairly sensible of them (SSM) to do it this way, since  
you're pretty much bound to have a CV mixer in front of that pin, and  
it's bound to invert the CVs, so having an inverting Freq CV input  
saves you an op-amp.

Regards,
Tom