[sdiy] as3340 PW/tuning issue

Mattias Rickardsson mr at analogue.org
Thu Dec 19 13:06:51 CET 2019

I remember talking to Dieter Doepfer about this back in the day... Ah, here
it is:


At 17:52 2003-02-02, Dieter Doepfer wrote:
> > -----Ursprüngliche Nachricht-----
> > Von: Mattias Rickardsson
> > Gesendet am: Mittwoch, 29. Januar 2003 11:10
> > An: hardware at doepfer.de; technik at doepfer.de
> > Betreff: A-111 problem? (PW affects Frequency)
> >
> > Hi,
> > I have a problem with several of my A-111 High-End VCOs:
> >
> > The pulsewidth (both PW knob and PWM input+knob) affects the frequency
> > of the oscillator. I.e., when turning the PW knob or PWM modulating with
> > a slow LFO, the frequency makes a clearly audible change.
> >...
>Dear Mattias,
>I forward your inquiry to our A-111 specialist Matthias Marass
>(mailto:keyboards at doepfer.de). He is responsible
>for the final tests and repairs
>of the A-111. He will answer you directly.

At 15:36 2003-06-11, Döpfer Musikelektronik - Keyboardservice wrote:
Dear Mr. Rickardsson,

the influence of the external and internal PW
control voltage to the oscillator frequency is a
problem of nearly 80..90% of the CEM3340 we ever
used. Unfortunately there is no chance to repair
this problem. The only way would be a strict
selection of the CEM3340 circuits.
Best regards,
Matthias Marass
keyboards at doepfer.de

Den tors 19 dec. 2019 10:42Oakley Sound via Synth-diy <
synth-diy at synth-diy.org> skrev:

>  > ... "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.
> Tony
> www.oakleysound.com
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