TB303 vco pt2

[Tony Allgood]

Hi all,

Built the TB vco today. Worked first time. Yeah. Much more fun than
simulations. I used BC550s as the pair in the SCR with a 2N5322 as the
pnp. This is a medium power type which I just had hanging around. The
voltage output across the timing cap was a perfect sawtooth, no flat bit
at all. Rise time was very quick. I will measure it tomorrow when I get
some time. However, once I put the FET in, differences were evident. I
used a BF244 and then a 2N3819. Both n-type JFETs. Firstly, the flat
part of the sawtooth output was now obvious. The time of the flat was
frequency dependent, but then again it would because the FET follower
could follow the input. I always thought there will always be a
noticeable Vgs between input and output in a source follower. Surely if
the sawtooth is at 12V, the supply, the FET output will not be able to
follow.  What type of FET are Roland using? Is this 2SK30 not a JFET? Or
am I completely missing the point here? Anyhow, the upshot of this is
that the sawtooth from the follower is down on output amplitude, and
thus the square convertor is generating a very nice pulse.

Tony Allgood, Cumbria, UK (who always used op-amp followers in the past)

Rack mounted Moog VCF module. Details to be found at...

http://aupe.phys.andrews.edu/diy_archive/schematics/effects/filter.html