303 clone filter and VCO

[Fraser, Colin J]

> -----Original Message-----
> From: Stewart Pye [mailto:stew at uq.net.au]
> Sent: 02 November 1998 09:57
> To: synth-diy at mailhost.bpa.nl
> Subject: re:303 clone filter and VCO
> 
> 
> I have to agree with Paul. 
> From what I can see (from the schematic) there are four 
> capacitors in the
> 303 ladder filter, making it a 4 pole, *but* the bottom 
> transistor is of
> smaller value, therefore giving an 18dB/octave slope until that last
> capacitor comes into effect, after which it will be 24dB/octave.
> Does this sound logical?
> 

The biggest difference is the use of diodes as dynamic resistances in the
303 filter as opposed to transistors in the Moog design.
The output of each pole in the Moog filter is 'buffered' before it enters
the next pole.
Each stage has high input impedance and low output impedance.
With the diode filter each stage has high input and output impedance.
Consequently, there is a complex interaction between the poles in the 303
filter.

I'm sure someone else can provide a more technical explanation ;-)

Colin f