moog hp

[Joachim Verghese]

Hi,

> Can you offer any suggestions for a transistor matching procedure that
> will match NPN and PNP pairs?  Will the Moog matching technique work?
> (as published in the Minimoog service manual)

I've also built a phaser using the Moog HPF circuit modified for
all-pass response. This was a six-stage circuit with op-amp
buffers.

The transistors do indeed need to be selected to avoid large
CV feedthrough, but it's not very easy to match NPNs to PNPs.
The PNPs I had (ZTX214C, if memory serves) seemed to require
a significantly higher Vbe for a given Ic, compared to the
NPNs (ZTX109C).

What I did instead was that I took six matched PNPs and six
matched NPNs, and added an offset trim for the CV that drives
the PNPs. This way, the mismatch between the NPNs and PNPs could
be compensated for using a single trimmer.

I didn't end up with a feedthrough-free circuit, though, because
as has been pointed out before, transistors really need to be
matched at more than one point on the Ic/Vbe transfer curve.

Fortunately, phasers are typically modulated by relatively slow
signals, so a little CV feedthrough is quite harmless.

Also, like Osamu, I had to limit the CV range and capacitively
decouple the CV lines for stability.

-joachim