[sdiy] Capacitors in Moog ladder filter

[Tim Heffield]

Hi Tim S.,

Thanks for the information.  I suspected that it probably changed the
dynamics of the filter as well as the cutoff frequency.  I must say though,
I have built several of these filters with the 0.1 uF and the 0.27uF caps
and other than the freq shift I could not really hear a difference.  I guess
you would have to run a Bode plot on both cases to really analyze the
differences.  Thanks for the offer on the transfer function but I don't
think I am ready to get that involved right now.

Regards,

Tim H.



-----Original Message-----
From: Tim Stinchcombe [mailto:tim102 at tstinchcombe.freeserve.co.uk]
Sent: Monday, April 26, 2004 5:06 AM
To: 'Tim Heffield'; synth-diy at dropmix.xs4all.nl
Subject: RE: [sdiy] Capacitors in Moog ladder filter


Hi Tim,

> I have a question about the four caps in the Moog ladder
> filter.  Looking through all my old documents and on the web
> I see values that range from 0.022uF to 0.1uF for the the
> four caps that create the poles.  It seems to me that this
> would just raise or lower the resonant freq for a given CV.
> Does anyone know if there is more to it than this?  I noticed
> that the Mini-Moog filter started out with 0.068uF and
> someone changed it to 0.027uF...was the cutoff frequency to
> low?  EFM seems to use 0.1uF on all of his filters.

Well the answer to your first question is 'yes' and 'no': 'no' on the basis
that you are right, simply altering the cap values will alter the cut-off
frequency for a given frequency CV; 'yes' on the grounds that to
characterize the whole circuit is quite tricky. From my calcs, the cut-off
frequency is:

f=I/(8*Pi*C*Vt)

where Vt is the 'thermal voltage' (0.025V), and I is the current drawn from
the bottom of the ladder. This current is the output of an exponential
converter, where the current is exponentially related to the frequency CV
input; the range of current from this will probably depend on the
transistors used in, and the biasing of, the ladder. Hence the values of the
caps used depends on the design of the whole thing. I have a note detailing
the derivation of the transfer function of the ladder (which I can send if
you are interested: it doesn't cover the expo converter), but the maths is
not for the faint-hearted!

Tim
__________________________________________________________
Tim Stinchcombe

Cheltenham, Glos, UK
email: tim102 at tstinchcombe.freeserve.co.uk