[sdiy] Re: moog high pass

[Theo]

After toying with a half way working Voyager and missing a HP mode on the
filters.
I actually asked mr Moog if there was a reason why the simple subtraction
trick would not work on a moog ladder.
His answer was there was none and that he had build subtractive HP filters
using a moog ladder LP before.
Next question was, that if adding a HP function was that simple why the
Voyager didn't have one.
He replied that he didn't see the need for a HP in the Voyager,
but also mentioned that more pp had complained about missing a HP and was
thinking about it.

Theo



From: Karl Dalen <karldalen at yahoo.se>


I suggest that it wont work, because you got one LP
filtered signal phase comparing/canceling a inverted input signal!!
When the cutoff point are moved close to zero (or even before that)
what you have is a straigt inverted "inputsignal" at the output!!

KD

 --- Gavin & Delia <elmystico at earthlink.net> skrev: > sum the output of
the low pass with the inverted input signal, through
> an op
> amp is fine, the low frequencies phase cancel and you're left with the
> highs.
> ----------
> >From:
> >Date: Sat, Jul 26, 2003, 8:16 PM
> >
>
> >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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