[sdiy] Re: moog high pass

[Ethan Duni]

Except for the inversion, isn't that exactly what you'd expect out of a
highpass filter with a cutoff of 0? Lots of filter topologies do exactly
this to obtain a highpass (state variable for example). I've certainly used
this trick lots of times to get my sampler to do highpass filtering. You can
also construct bandpass/bandstop filters this way by taking the difference
of the outputs of two LP filters with different cutoff's.

Ethan

----- Original Message -----
From: "Karl Dalen" <karldalen at yahoo.se>
To: <synth-diy at dropmix.xs4all.nl>
Sent: Tuesday, July 29, 2003 5:18 PM
Subject: Re: [sdiy] Re: moog high pass


> 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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