[sdiy] Pin 14 on the 3340
tom at electricdruid.net
Wed Jul 18 12:28:57 CEST 2018
Thanks for sharing your experiences with this. I sometimes get emails from customers asking me questions about the implementation of the AS3340 in various old synths, and I don’t do repair work (except on my own synths, I suppose!), so I’ve got little to offer. Finding out what the real-world problems are is very useful.
AS3340 seems to be a good chip (and for the price, an amazing one), but perhaps still a little bit quirky. I’m hoping we’ll see some revisions in the future. We tend to forget the new CEMs are Rev.G, so the vintage ones underwent six revisions. Lord knows what the early ones were like.
Synth & Stompbox DIY
> On 18 Jul 2018, at 09:57, Oakley Sound <oakleysound at btinternet.com> wrote:
> 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.
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