[sdiy] Moog filter self-FM questions

[cheater cheater]

Simon,
thanks a lot for your email. Unfortunately some more explanation will
be necessary, I'm not smart enough yet to understand what you mean :-)
Some questions below..

On Sun, Mar 28, 2010 at 20:59, Simon Brouwer <simon.oo.o at xs4all.nl> wrote:
> 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.

OK, yeah. But at that point the common-mode control current will have
been rejected, right?

>>
>> 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.

OK. Yes, I know they're similar to the capacitors in a butterworth
filter, I was just looking at them differently for a second to try to
understand what you'd meant. After all they *are* a bit like a
long-tail pair, aren't they?..

> (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.

yep. I'll give the doc a read, looks interesting, thanks for the read.

>>  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.

I am afraid I don't follow.

For one thing, are you saying that there is only one set of four
stages of RC filter? That means, are you saying there are only four
'poles' there?
This would conflict with what I understand, which is that there are
eight RC stages, set up into two four-stage RC filters, in a
differential topology.

>> 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.

It matters to me.. I like to work with specific values.

>>
>> 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).

OK, but functionally, what does the signal coming into the base do?

Thanks a lot
D.