[sdiy] Pin 14 on the 3340

[Oakley Sound]

Roman > My guess is that's for protecting that sensitive entry from 
picking up RF interferences, which when rectified on this transistor 
might lead to considerable pitch shift.

I was thinking the same. Odd that most don't use it though.

What's bizarre is that Oberheim didn't put any local power supply 
decoupling near the 3340s, yet they added the pin 14 decoupling.

KD > Crumar Spirit two 3340 has cap to gnd on pin14...

I always forget about the Crumar Spirit. Such an unusual synth. Always 
worth upgrading the stock DAC from a DAC08 to a better grade one if you 
have one.

 > Roland has a capacitor between 14 and 15 in the Jupiter 6.

Yes, I saw that and wondered about it. Never seen that before. I wonder 
if it actually does add some HF roll off - but is 15 to 14 a negative 
feedback pathway?

Here's what Alex at ALFA said when I asked him about needing to decouple 
pin 14 on the AS3340.

Alex > Yes, it is good practice. But it must be in optimal range. Maybe 
100nF too big (22nF, 33nF, 47nF).

There is a reason to my question other than curiosity. I have an OB-Sx 
here that needed the full CEM chip replacement (and indeed pretty much a 
full rebuild). Some AS3340 seemed to have pitch instability (when fitted 
in place of the CEM3340) at certain pulse widths. But it was only some 
AS3340s and only when the voice card was in positions 5 and 6 in the 
OB-Sx. These are the boards furthest from the power supply so 0V 
impedance is higher. Removing the decoupling on pin 14 helped with the 
instability but didn't remove it entirely. And each time I'd put my 
scope probe on both the linear input pin and triangle wave output pin of 
the AS3340 the instability would stop. Talk about frustrating.

I had already changed the output loading on the pulse output to 52K as 
the AS3340 requires. Additional decoupling on the -5V lines to the 3340 
did help. Clearly the AS3340 appears to be more prone to pitch 
instability than the CEM3340. It doesn't help that the OB-Sx voice cards 
are pretty badly laid out for 0V with the tracks going all over the place.

Need to play around some more with this I think.

Tony

www.oakleysound.com