[sdiy] Synthacon filter

[Yves Usson]

Hi Karl !

The synthacon filters is based on a paper by Nyle Steiner that you can 
find at
my sdiy site ( 
http://www-timc.imag.fr/Yves.Usson/personnel/SDIY/archives/index.html ).

In this paper it is said that the input level for low resonance
is 0dBm (that is roughly 2Vpp) and should be limited to -10dBm for
higher resonance otherwise you'll get a lot of distorsion. It's a 
feature
of Sallen & Key filters, therefore if you don't want distorsion at high 
resonance
you have to limit your input level !

If you want to use 100k resonance pot you will have to change the 
values of
of the 1k resistor in series with the pot to 100k, and of the 2k2 Q3's 
emitter
resistor to 220k. As a matter of fact this filter is of the Sallen & Key
kind and in this architecture the resonance is depending on the gain
of the NPN-PNP stage, with a gain of one there is no resonance.
The circuit will start oscillating for a gain of 3. This is why you have
to modify the values of these two resistors if you wish to use a 100k 
pot. Still I
am not sure that with such values the NPN-PNP will behave linearly !!!!
Another solution is to use your 100k pot with a 2k resistor in parallel
this way the total resistance is 1k96 (that is the maximum gain won't 
exceed 4).

I hope this helps !

> Hi all, so, i've built Ken Stone's Synthacon filter. I
> managed to smash it onto a 3x8cm breadboard. The
> filter's working great, though it distorts on high
> resonance levels. Can the distortion be a cause of
> both input signal (i've got something like 5pp now)
> and the LEVEL resistor found on the feedback of the
> opamp (got 100k now)? Also, i'd like to use a 100k pot
> instead of 1K as resonance pot, anyone has an idea of
> a small mod?
>
> thanks
>
> /Karl
>
> Höstrusk och grå moln - köp en resa till solen på Yahoo! Resor på 
> adressen http://se.docs.yahoo.com/travel/index.html
>
>
-------------------------------------------------------------
Yves Usson

Lab. TIMC UMR5525 CNRS
Institut d'Ingéniérie et de l'Information de Santé (IN3S)
Faculté de Médecine
Domaine de la Merci
38706 La Tronche cedex

email : yves.usson at imag.fr, yves.usson at ujf-grenoble.fr

tel.  (+33) (0)456 520 074
fax.  (+33) (0)456 520 022