[diy] moog VCOs

[Tony Allgood]

Hi all,

I am about to layout the minimoog VCO onto a PCB. I have built two
prototypes, one which was the original all transistor VCO, and the other
is the later version with the op-amps.

The first type I didn't have much success with. Its frequency range was
limited, and the amount of matched pairs it needed was prohibitive. Even
when I replaced some of the discrete op-amps with modern equivalents, I
still had the problem of a not too good expo function. The timing pulse
of that schmitt trigger, that controls the charge-up, was sufficiently
long enough to cause HF error. Also the saw wave generated by this VCO
was very much a standard shape, although it had a slight flat top due to
the charge up transistor. I didn't see the change in gradient in the
discharge portion, that others had reported. This may be caused by the
waveshapers. These I didn't look at.

I therefore pushed on with the later version of the VCO. This is much
better. With a 2210 matched pair in the expo, and a 1K temp co for
thermal stability, it is producing very good results. However, there are
two questions I would like to ask:

What the heck is a E112 FET? Is it the same as a J112? I have tried it
with a BF244 and a 2N3219. The former produces a higher value of saw
peak. The latter doesn't turn-on hard enough to charge the cap up to the
+5V reference. I guess I need a FET with a lower channel resistance.

Are there any differences in using the expo transistors the other way
round? That is; in most of my previous expos, I sink the VCO/VCFs
current in the transistor that has the control voltage applied. This is
the same as the Formant and the ASM-1 I think. But the minimoog VCO uses
the second one, with the op-amp's feedback going to the first. At first
glance, I cannot see any advantage in doing this, but am I missing
something?

Any help gratefully received.

Regards,

Tony Allgood  Penrith, Cumbria, UK

Walsh synthesiser, SuperLadder, TB303 clone and rack mounted filter

http://www.techrepairs.freeserve.co.uk/projects.htm