[sdiy] Capacitors in Moog ladder filter

[Antti Huovilainen]

On Tue, 27 Apr 2004 jhaible at debitel.net wrote:

> It's true that the bias voltage will not change much (log
> function of the pn junctions!), but for that very reason

I meant the voltage in the resistor string, not the base-emitter voltage.
For ideal transistors, the I/V transfer function of each transistor pair
can be shown to be Ic*tanh(Vc/(2*Vt)). I'd tell more, but I'm waiting for
word on the acceptance of a paper (end of may or so) that deals with the
circuit.

> quite different. For instance, the old Moog Modular VCLPF (904, wasn't
> it?) has a rotary switch to choose 3 sets of capacitors, and the filter
> will sound slightly different with each setting (cutoff correction
> by applying a CV assumed).

Can you provide some audio examples of the differences (same sound but
different modes or compared with "modern" moog filter)?

> all these little detailt - but IMO it means you must analyse all the
> tiny contributions of special properties - like choice of capacitor
> values, beta of transistors, architecture and component parameters of
> the differential amplifier - and find a way to emulate

Can you shed more light on what effects are important and where?
And can they be simulated with Spice?

Antti

Give a man a fire, and he'll be warm that day,
Set him alight and he'll be warm for the rest of his life