[sdiy] Re: moog high pass

[Gavin & Delia]

you put the LP out of phase, the input signal in phase, when the cutoff is
at zero you get the input signal which is exactly what you get out of a high
pass with a low cut off,  high pass, low cutoff = all pass
----------
>From: 
>Date: Tue, Jul 29, 2003, 8:18 PM
>

>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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>
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