[sdiy] Re: moog high pass

[Magnus Danielson]

From: Karl Dalen <karldalen at yahoo.se>
Subject: Re: [sdiy] Re: moog high pass
Date: Thu, 31 Jul 2003 02:07:03 +0200 (CEST)

>  --- 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!

... and that was my short-hand math expose. Ask anyone here... they'l swear by
it! (They usually swear like drunk seamen who can't get up on their legs when
they see my formulas, so it should be a fairly easy thing for them to do...)

> > 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!

Yes, but I also said it wasn't really the same either... if you want that you
have to work more on it.

As for the high-Q values, things DOES happend if you don't compensate for the
feedback-term and you need to mix the feedback term in too or else you are in
a mess.

The DC-gain shifts as Q raises. This comes natural due to how the topology
work.

For a perferct variant you need input, all intermediary voltages and output
voltages in perfect mixing.

> > 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!

Don't get struck by the lightning! Your enligthenment may be your last!
(Much lightning in Sweden lately)

Hmm... maybe I should make a full-blown exercise on this one...

Cheers,
Magnus