[sdiy] Roland System 700 VCF

[Stewart Pye]

I just realised I probably was way off...

Since the input of each stage is going into the non inverting input of 
the OTA and the OTA then goes to an inverting integrator, it is negative 
feedback through the 22K resistor. However I believe that when using the 
SSM 2164 followed by an inverting stage, the output is not inverted. If 
this is true I'd have to either add an inverting op amp stage for the 
feedback, or add a differential amp in front of the VCA.

Regards,
Stewart.


Stewart Pye wrote:
> Hi,
>
> I've been looking at the System 700 VCF schematics with the thought of 
> designing a circuit based on that but using the SSM2164.
> Sys700 Schematic here....   http://fa.utfs.org/diy/roland700/703e_a3.jpg
>
> It's a 4 pole OTA into integrator design. The output of each 
> integrator is fed back via a 22k resistor to the non inverting input 
> of the OTA for that stage. Does this mean that for the filter to NOT 
> oscillate it needs overall negative feedback from the output to the 
> input? I think this is being done in the resonance control in the 
> Roland circuit...
>
> I'd appreciate it if someone could clarify this for me. I may be way 
> off....
>
> Regards,
> Stewart.
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