[sdiy] Moog filter self-FM questions

[Simon Brouwer]

Hi,

If  you take Rick's schematic 
(http://dropmix.xs4all.nl/rick/Emusic/Moog/moogvcf_schematic.gif) as a 
reference, you
can see that a 2.5V p/p input voltage is attenuated to 62.5 mV p/p at 
the input of differential pair Q14-Q15. So at a peak in the signal, the 
differential voltage
will be 31 mV.

As the differential output current of such a differential pair is given 
as Ic = Ie*tanh(Vd/(2*Vt)), with Vt=26 mV and Vd=31 mV, Ic=0.6 * Ie.
That means the current in one transistor goes from 0.5*Ie to 0.8*Ie, and 
in the other from 0.5*Ie to 0.2*Ie, where Ie is the tail current.

That's 60% modulation of the corner frequency (up in one leg, and down 
in the other, which however does not simply cancel out). Obviously the 
Moog filter can not be regarded as a straightforward lowpass filter as 
it has some interesting nonlinearities going on near the corner frequency.

Best regards
Simon




cheater cheater schreef:
> Hi guys,
> I have a question about the control current on the transistors in the
> moog ladder filter.
>
> First I'll explain how I see the filter, this is how Richard Atkinson
> explained this to me (thanks Richard!) - I'm not that great with
> electronics so I might be screwing up something he told me.
>
> I understand the moog ladder is (basically) 2x butterworth filter,
> connected in parallel in differential mode, with the resistors
> replaced with transistors; the transistors change resistance based on
> the DC current across them; that same current contains an AC signal
> that is the audio. Because the whole thing is differential, once the
> signals are mixed together, the DC current which is common mode gets
> rejected whereas the audio which is flipped on the left side gets
> amplified (via normal differential operation).
>
> The key thing is that on each side of the ladder the signal and the
> control current are the same signal. In this case the audio modulates
> the cutoff to some extent too; on one side of the ladder it modulates
> it positively, on the other negatively. The question is to what extent
> this happens.
>
> I understand that this is different depending on the implementation
> (again pointed out by Richard); so when talking about this let's
> mention what synth we mean and if it's modular or custom-made then
> also what specific transistor model and manufacturer.
>
> Questions: 1. what is the swing of the (common-mode) DC control
> current? What is the maximum and minimum?
> 2. what is the swing of the (differential-mode) current that
> represents audio input?
> 3. if audio is patchable into the cutoff control, what's the swing of
> the current control that can be created this way?
> 4. are the ladder sides biased by some DC that's always present?
> 5. what's the DC injected at the minimum and maximum cutoff setting?
>
>
> BTW, when you mix down the filter sides they will actually be
> (somewhat) different, as I understand it; to the extent to which the
> audio is actually FMing the filter.
>
> Thanks a lot
> D.
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>
>   


-- 
Vriendelijke groet, 
Simon Brouwer. 

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