[sdiy] Re: moog high pass

[Theo]

A subtractive filter is always the complement of the filter it is based on.
In case of a moogladder LP, with no resonance you get HP.
But if you turn up the resonance the ladder becomes more and more like a BP.
So the subtractive filter becomes more and more like a notch.
Indeed quite different from a normal HP with resonance.

What you say about stereo use seems a bit doubt full.
One of the popular uses of subtractive filters is in active-crossover
filters for loudspeakers.
Think highend bi- / tri-amped systems or active studio monitors.

The phase & frequency  response of the subtractive filter depends on the
"normal" filter that it is based on.
So if the LP functions of your "normal" left and right filter are not
matched then the HP functions don't match either.

Theo


----- Original Message -----
From: Karl Dalen <karldalen at yahoo.se>
To: Magnus Danielson <cfmd at swipnet.se>
Cc: <synth-diy at dropmix.xs4all.nl>
Sent: Thursday, July 31, 2003 2:07 AM
Subject: Re: [sdiy] Re: moog high pass


--- Magnus Danielson <cfmd at swipnet.se> skrev:
>Have you now figured out why I have spent years pushing this thing
> infront of me and saying "another day"?

Absolutely! :-) It will take me 2 years to figure out that
math you just have drawn!

> Yes, and if you have a highpass filter with the cutoff set low, all of
> the signal will pass through (this is why it is called passband)
> >essentially  unmodified. Ever played a MS-20?

> So what is wrong?

Resonance, and this topology when used in stereo (two filters)
especially if run with a real HP or LP, i have tested it this
evening,the result was, stereo image either gets sort of cramped-
squeezed or phase bent "sort of", difficult to explain! I also
compared the phase canceling design with a real 24db HP filter
and it sure sounded different, not bad but different, like a
HP with a notch or phasor stage in it!!

But the biggest issue is resonance, i think Juergen also mentioned it,
as i said before, imagine high Q in this topology, odd and strange
things will happen! Not bad but different!

> In my point of view, it will not give a "correct" HP-filter, but
> probably a sufficiently good one to be useful musically. The trick
> has been used  before and with success, so there is nothing
>terribly wrong with it.

Nothing is wrong with it i just wanted a real sharp HP
with true resonance, otherwise i thank you for the splendid
math and explanation (i will save that!!)And many thanks to
you other people who put your time in for my question.

Thanks folks it was enlightening!

Reg
KD

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