gripes and more on cheap sampling

Magnus Danielson magnus at analogue.org
Wed Jan 28 23:44:22 CET 1998


>>>>> "ES" == Entropy Struct <entropy at thecia.net> writes:

 ES> Basically the only problem I have left is that the VCO's are still dead. I
 ES> checked the placement of the JFET and 3904's.. no go. I have the
 ES> FET properly placed and the 3904's right with the flat sides facing the
 ES> MAT-2 square.. There's a bit of low current running through the outputs,
 ES> but nothing more. Adjusting the trimpots doesn't help. 1 bad VCO I can
 ES> understand, but *3*? I have to be doing something fundementally wrong
 ES> here. It's the only "mystery" about this synth that's still bugging me...
 ES> and also, well, keeping it from being a usable synthesizer. If anybody
 ES> has ideas about what could be wrong I would *really* appreciate it. 

Hmm... this sounds like something that hit me...

If your VCO's get hung at +15V you will probably find some very high
noise there as well (you should not be able to hear it). If this is
the case I would suspect that the slow down cap of 18 pf sitting
across the LM 311 is too low. Hook on a few more pf (say 10 or 18 to
start with) and your VCO may just start oscillating on you. The 18 pf
there is very close to the limit for some JFETs and this causes the
JFET not to get enougth time to empty the cap and ith that hitting the
floor properly.

I must say that this problem was quite unsolvable without an scope and
quite some time of schematic analysis.

I have tried to make some notes about this specific problem on the web
page, but a more detailed description has not emerged since I have
been both lazy, bizzy and just forgot about it all...

Hope it helps...

Cheers,
Magnus



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