[sdiy] SSM2164 VCF Problem

[Neil Johnson]

Stewart,

> I've breadboarded a 2164 based 4 pole VCF and it has a problem...
>
> When the cutoff frequency is low (2164 output current low) there is  
> a 76Hz sine wave on the output which increases in level (but  
> frequency stays the same) as the cutoff frequency is reduced. If  
> there is a signal going through the filter is appears to wobble,  
> but you can see the sine wave if there is no input to the filter.

How does your Q circuit work?

> I noticed this even when I first tried a single low pass section.  
> It's just a 2164 into a TL072 integrator with  270pF cap, and a 30k  
> feedback resistor back to the 2164 input.
>
> Does anyone have any idea what's going on here?

Do you have the compensation network on the inputs?

Neil
--
http://www.njohnson.co.uk