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

[jh.]

> > 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?

Yes, it's pretty much standard for chorus, phasing, flanging, etc.
But it's the first time I've seen it built into a PWM circuit.
"First time" means me noticing it, of course. The SH-3 surely
predates a lot of FX boxes.

> 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 ...

Normally you want either the PWM rate or the PWM depth
to be reduced for lower notes on the keyboard.
Obviously there in no scaling of the Rate. Maybe there's a scaling
of Depth. PWM depth should be increased with higher key CV
voltage ...

JH.