PLAN B analog blog

This issue came up again on another blog.  My response to that post is
important and something anyone considering purchasing a Model 15
should consider:

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.  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 transistor pair) - it's hard to center into what's called the
'linear zone' of the expo converter's response curve.  That's what
gives you the stellar 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.  

This was my original goal.  To introduce a modern day equivalent to
the Buchla 258 and I believe I succeeded. 

- P