[sdiy] Ladder Filter - dark side

[ChristianH]

On Mon, 22 Feb 2010 22:27:01 +0000 rob <rob at emulatorarchive.com> wrote:

> 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?
> Is it the CV control signal being too strong?
> I have to admit the transistors (beyond the base pair) in the ladder are 
> not matched.

Could this be a symmetry problem between the ladder rungs, maybe the CV
affects both sides differently? Might show up as some signal at the diff
amp.

"There is no dark side. As a matter of fact, it's all dark"

Christian