[sdiy] as3340 PW/tuning issue

[Brian Willoughby]

It does not make sense. At least not unless something is drastically wrong.

Note Oakley's solution below to build a triangle to pulse circuit external to the chip using a comparator or op-amp. Since that works outside the chip, I don’t see why it should affect pitch if implemented internally, unless the comparator pulls so much current that it causes the supply pin to drop.

Ideally, the comparator should draw no current from the triangle oscillator (and there shouldn’t even need to be any feedback for comparator operation). The PWM CV input should be totally isolated from the triangle core. The pulse output should also be completely independent - except for the shared power. In other words, the pitch of the triangle core should not vary at all when the PWM CV is driving the far input of a comparator.

I suppose the AS3340 either has a really poor power supply routing internally, or the PWM circuit is not a simple, isolated comparator design.

Brian


On Jan 2, 2020, at 7:49 PM, ColinMuirDorward <colindorward at gmail.com> wrote:
> That's right, Brian, any pitch change will happen across all the waveforms. This makes sense, since all waveforms are derived from the single triangle core.
> When checking for this error, I always monitor the sawtooth while applying PWM, since it's easier to hear the pitch change without the pwming getting in the way.
> Does that make sense?
> Cheers,
> Colin
> 
> On Thu, Jan 2, 2020, 7:35 PM Brian Willoughby, <brianw at audiobanshee.com> wrote:
>> I didn’t pay close enough attention the first time through. I assumed that people were listening to the Square a.k.a. Pulse waveform output when adjusting Pulse Width, since that’s what it’s intended for.
>> 
>> Tom, you said you were listening to the Ramp wave alone on Oscillator A, with LFO routed to PW on Oscillator A. That shouldn’t have any audible effect at all.
>> 
>> Are folks saying that the pitch of non-Pulse non-Square waveforms drift when Pulse Width modulation is added on the appropriate input pin? That would be quite unexpected.
>> 
>> I recreated the quick test on my Pro-One, and I hear what sounds like a sort of frequency modulation when the Triangle LFO is directed through the Wheel to Oscillator A PW and the shape selector for Square engaged. But there are psychoacoustic effects in play, because even amplitude modulation can be perceived as pitch modulation.
>> 
>> In any event, some clarification would be educational … or at least a link to those Muff’s threads.
> 
> 
>> 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
>> 
>> 
>> 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
>>