[sdiy] SH-3A: so it's Pwm, but ...

[>>>marjan<<<]

> What I did figure out so far:
> PWM yes, ordinary PWM no.
> 

I'd say it's mix of various Roland's ccts

> There's a fairly ordinary triangle VC-LFO (slightly
> unsymmetrical wave, but just a little), and a comparator.

classic two opamp LFO with FET instead pot

> Between the comparator and the LFO, a LPF.
> So the higher the "chorus rate", the lower the effective
> LFO amplitude. Also, the waveform is almost sine-shaped
> for high rate, soft triangle for medium rate, and almost
> triangle for low rate.
> 

Yes, but isn't that characteristic for Roland real choruses,
take a look at Juno6 service manual, lowest rate is 0.4Hz
with 20Vpp triangle, and highest is 8Hz sine wave 2.6Vpp
OK Juno came later, but it seems Roland 'discovered' what
style 'chorus' they like so they stick with the principle?

> This is a quite sensible choice already, but there might
> be more. There's a "Bias" circuit built around another
> opamp and a transistor, adjustable with two trimpots,
> which apparently adds an offset to whatever signal is used
> on the audio side to feed the comparator.
> Unfortunately, I can't see what feeds the bias generator,
> nor how the audio signal is processed before it runs into

My guess it that it's fed with CV (look same line goes to
VCF CV summer without pot, what else could it be?)
tripots set the amount of CV and other is eh, hm, ref DC
point or something. Sadly it doesn't control LFO speed,
rather input level of 8' signal to comparator via trans
set as attenuator. Higher pitch - higher Cv - trans opens 
more - signal attanuates more.
Hm some sort of another Roland happy thinking. Is that some
kinda acoustic lookalike or what? Higher freqs produce 
lower volume for this pwm/chorus, fill here why ...

-- 



marjan


me    : Marjan Urekar
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