[sdiy] Moog filter self-FM questions

[Simon Brouwer]

cheater cheater schreef:
> Simon,
> Thanks for the writeup. Interesting thoughts. Would you be able to
> point to a resource that explains how the principle you're mentioning
> works exactly (hopefully with a derivation of the formula)? My
> googling for formulas didn't bring up a lot.
>
> In your analysis, I think what you're looking at (tail current) is the
> current in a long-tailed pair after common-mode rejection, right?
>   
I was referring to Q14 / Q15. That's a long tailed pair.
> But notice that this isn't exactly a long-tailed pair. In an LTP the
> emitters are tied together (DC coupled) while in the moog ladder
> filter the emitters are only AC coupled. This is, as I understand, to
> prevent common-mode rejection of the control DC(*).
Well, no. The capacitors C4..C7 do the actual filtering, in combination 
with the dynamic resistance of the emitters connected to them.
(see http://www.analogzone.com/col_0503.doc for an explanation of 
dynamic resistance)
So you can see the ladder filter as a buffered string of RC filters, the 
R's in which (and thus their cutoff frequency) can be varied by changing 
the emitter currents.

>  In that case, I
> think your analysis could be a little bit off - what do you think? I'm
> no good with electronics, just worried if we're looking at this
> through the right lens.
>   
Actually, having another look at the circuit, I see I did make a mistake.

I had not considered that the left and right side of the ladder (split 
in the middle of C4..C7) can not be regarded as independent if you're 
looking at large signals. The R in the RC is a combination of the 
dynamic resistances of the left and right emitters, and while one goes 
up, this is mostly canceled out by the other going down. So the effect 
of the modulation will be rather more subtle than I supposed.

> So I guess my question should be: in the collector current of one side
> of the differential pair on a stage of the moog filter what is the
> max/min control DC and max/min audio AC in amperes? (phew that's long)
>   
Actually the absolute values don't matter, what matters is the relative 
imbalance in the left and right currents.
> I might be thick, but I'm having a bit of a problem understanding your
> analysis; on the one hand because you don't seem to be relating it to
> the control current (but it's quite probable I don't understand
> electronics well enough to see it indirectly), and on the other hand
> because it just doesn't *seem* like the audio modulates the filter so
> much.. 
Agreed, the effect will be much more subtle.

(...)
>
> BTW, I totally don't understand why the base in the differential pairs
> gets the control current too...
>   
That's normal transistor operation. You can regard the base-emitter 
junction as an ordinary semiconductor diode (the emitter current has the 
same exponential relation to the base-emitter voltage as a diode), the 
big difference being the presence of a collector, which pulls away 
almost all of the current that would go into/from the base (hence the 
name collector).


-- 
Vriendelijke groet, 
Simon Brouwer.