[sdiy] as3340 PW/tuning issue
oakleysound at btinternet.com
Thu Dec 19 10:41:30 CET 2019
> ... "So as a conclusion the goal is to have on pin3 as close as
possible to -5V and the PWM to tune error will not manifest itself."
I did not find this to be the case since I always run any 3340 circuit
from a precision 5V reference. The solution for me was to get the
CEM3340 rev G. The CEM3340 has the detune too, it's just that it is
quite small. I can't remember exactly, but I think it's in the region of
+/-1 cent. It would almost certainly be there on the SH-101.
Alfa's solution is to use the newer AS3340-HYB. The other thing to do is
make your own triangle or saw to pulse wave convertor from an op-amp or
comparator external to the chip.
When repairing an OB-Sx I found that the AS3340 did not work well as a
sub for the original CEM3340. Even with the various resistor changes
required to get the AS3340 to work properly, the VCOs in last three
voice card positions on the motherboard displayed a very audible pitch
instability at pulse widths at less than 45%. Clearly, there was some
layout issue but although I tried all sorts of things (including bus bar
0V and liberal dousing of capacitance) it wouldn't solve the problem.
Again, the solution was to replace all the VCOs with CEM3340 G.
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