[sdiy] Ladder Filter - dark side

[Tim Stinchcombe]

> Having just built a Roland version of the Moog classic 
> transistor ladder 
> low pass filter....I have a problem or a unique feature..
> 
> The frequency cutoff works fine, turning it down cuts off the 
> output signal. But turning it even lower goes past complete 
> cut off and then starts to 
> increase the output but to a much lower signal level.
> The beyond cutoff signal is 10x times lower in amplitude, 
> same as the  
> input frequency, but a sharper wavform.
> 
> Any ideas on what causes this behaviour?

Coupling caps before the filter, and in the feedback loop, can cause some
weird effects. I might be able to offer an explanation, but would need to
see the exact circuit you used - do you have a link to it?

Tim 
__________________________________________________________
Tim Stinchcombe 

Cheltenham, Glos, UK
email: tim102 at tstinchcombe.freeserve.co.uk
www.timstinchcombe.co.uk