PLAN B analog blog

My opinion (doesn't make it right, just makes it mine):

What Hex said was fine.  He was frustrated because he had problems
with his Model 15's tracking, which he called 'intonation'.  Basically
he would set it up, cal it and then it would go out a few minutes
later.  In order for me to comment on this I'd have to see the VCO to
determine what the problem was.  I never received the unit back, so I
can't verify or discredit his results. 

More on M15 tracking in a second.

In reply to his initial comment, Dan Levey (ReWire) went ahead and did
our cal procedure on his M15 and then recorded a sound file which
plays six octaves.  To that, Hex put it through a tuner and found
variances, the worst of which was 4 cents off dead on, with most only
1 cent off.  One actually was spot on.

To his results, I was elated.  This type of resolution is really good
for an analog VCO. Hex however made a comparison to a out of tune
singer.  This really isn't fair, at best a gross exaggeration given
that most people can't hear something out of tune until it's in the 20
cent range. 5 cents is about the peak of human ability, and this is
considered the best of the best of those with perfect pitch.  

His second comparison to an MFB VCO was also a bit unfair in that it's
digital. 

OK fine no harm done, no fowl, right?  The bottom line is my VCO will
not nail tracking perfectly...but none will.  Some will do better than
others (read: ours) and there are reasons for this, most of which have
to do with the incorporation of a Moog's octave switch which limits
the range of the VCO to within what's called the 'linear region' but
is able to offset that to higher or lower registers.  In this way, the
circuit is responsible for tracking a much smaller distance, but it
can move that gap higher and lower over the audio band.  If you want a
VCO which demands keyboard tracking, this is the type you should stick
to because this is the only config which will give you large-range
keyboard tracking.

So why didn't I do that?

The Expo converter circuit used in the M15 is about 80-90% responsible
for it's sound - it's reaction to audio bandwidth FM. It's a discrete
driver - no op amps in the expo circuit whatsoever. Sounds great, but
tracking is tougher than using other methods. 6.5 to 7 octaves, no
problem. 10? It'll never happen. Maybe for a day. Most VCO's drive
their expo with either an op amp or an transconductance amp such as
the 13700. I can do this and tracking improves - but the instrument
loses it's sound and I am not willing to make that trade off.
Unfortunately, even when matching the discrete semi's in the expo
drive circuit (not the expo itself, that's always been very tightly
matched via a Linear Systems matched transistor pair) - it's hard to
center into what's called the 'linear zone' of the expo converter's
response. That's what gives you the steller tracking.

If your type music requires a large tracking range - you may want to
look into one of the other Eurorack VCOs on the market. There are many
Excellent ones to choose from. Actually, the Aries VCO is about the
best in the world for tracking if you can find one. If you want the
Buchla 200 sound, then either pay $2k for a 258, or get a Model 15
which tracks a lot better than the 258 ever did. That's what the M15
was designed to do.

OK, that's why.  

Over the course of this company's existence, I'm proud to say that I
do look into people's comments and I have made changes to circuits in
reply.  The M15's sync, the soon-to-be M10's retriggering capability
to name a few.  So I tell you what...let me look into this one.  If
there's a way to improve the tracking without sacrificing the audio
qualities and without changing the design of the expo converter
circuit as it is I'll gladly do that.  It may be as simple as
replacing the zener diode in the expo circuit with a 6.9 volt voltage
reference and making a few resistor value changes to compensate for
that delta (7.5 volts to 6.9).

Added to the list and I'll update you all as I've had an op to test
it. In the meanwhile, I hope this helps to clear things up. The M15 is
a great VCO...for what it does.  Like anything else, it's a crap VCO
for things it can't do.  It is however what it is...!

- P

P.S.  Matrix: please don't post, thanks!