[sdiy] Ladder Filter - dark side

[rob]

Hi List
Problem solved.....error in my schematic. Back from the dark side...
I had tied the differential amp into the base of top transistor in the 
ladder rather than emitter.
Thanks Christian, this list is a great resource!
And the Roland SH3A LPF clone is complete and sounds great!

Regards - Rob


ChristianH wrote:
> 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
>
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