[sdiy] Moog Ladder in HP mode!

[Karl Dalen]

Hi!

Maybee someone has a clue to this!

I have been experimenting with the 24db ladder filter and wonder,
has any one any good ideas for getting a HP response out of that
filter in simple way? Has anyone taken a look under the Moog Voyager
hood to se how Moog did it?

I have tested a trick which i found at the web somewhere (cant remember
where) it involves to send the input signal to the top left emitter of the
first transistor pair (top) through one capacitor, this works to some
degree but as it seams it alter the output impedance the extra capacitance
so it then shift the last pole of the filter and this affects the ability
to resonance , (one can do a differential input and send it to both
emitters aswell) it still oscillates but has quite difficulty to do so
and as it seams the resonance disappear at low frequency, which i guess
is the natural behavior after one have alter the phase shifts of the last
pole !? The responce is somwhat like of a HP but i think it sounds
somewhat strange! I might be completely wrong in my assuming but anywhay! 
I wonder would it be a insane thing to use two OTA's feeding the emitters
through series resistors to the emitters so they see just a current
instead of a capacitive load? Has anyone tried to use the base of the
top trannys as HP inputs? Would that work?

Thanks!

Regards
KD

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