Moog 904A lowpass filter Question

[jh.]

>I have looked at other moog filters and they are mostly different.
>
>Memorymoog = .027
>Sonic six = .068(tranny) .1 (diode)
>Minimoog = .068
>Multimoog = .033
>Taurus I = .1
>
>Wouldn't these different values in the ladder make them all sound
different?

Yes and no - depending on how you look at it.

(1) different cap value = different cutoff frequency (for the same control
current)

(2) offset the control current so that you get the same cutoff frequency -
would
give exactly the same behaviour in theory with "ideal" transistors.
(Transistors act as current controlled resistors - amongst other functions -
here, so just adjust the resistance to match the altered capacitance.)
So, "no" difference.

(3) In real world, transistors are not ideal, so there would be *some*
difference from the ladder running at different current alone. But I
reckon this is just a theoretical difference - you would not hear it.

(4) The ladder is loaded by a buffer amp. This impedance does
slightly interfere with the ladder's behaviour (a constant and basically
linear impedance in parallel to the ladder's variable and nonlinear
impedance) - and that effect would be slightly different for different
ladder currents (see (2)) but constant load. Again, we're speaking
of *minor* differences, as long as the caps are in a reasonable range.
(Would be a different story with caps in the uF or pF range.)

(5) Are there and Moog filters that look the same, apart from the
cap values ? Apparently Moog used slightly different buffer amps
for each VCF they made, and these will most likely affect the sound
way more than all the above points. Same goes for different VCAs.
You rarely run a VCF without a VCA (at least a Moog VCF which
hasn't the best CV feedthru), and the VCAs in Moog synths are
as different as the various VCF buffer amps.

JH.