[sdiy] Free SSM2044, SSM2056 chips for pickup and one question about single supply VCAs

hannes.janetzek at gmail.com hannes.janetzek at gmail.com
Wed Mar 6 23:34:48 CET 2024

Am Mi., 6. März 2024 um 15:05 Uhr schrieb René Schmitz <synth at schmitzbits.de

> Am 06.03.2024 um 00:36 schrieb hannes.janetzek--- via Synth-diy:
> > Hello - now that I have your attention I would like to bring up the
> > sad topic of the numerous synthesizers covering in 80s home-organs
> > that are daily thrown in the trash by their inheritors.
> > Ok maybe this is a specific southern Germany problem where it appears
> > every second home had a Hohner organ which outlived their role as
> > status symbol by now.. In the US these model were sold as Wurlitzer
> > Omni 3000-6500 - not sure if these are regularly in classified for
> > pickup as well.
> >
> > My interest in these devices is mainly in their MEG sound-generator
> > for which I'm currently writing an interface library. As this starts
> > to work out nicely I wanted to throw in the filter boards and was very
> > surprised to see that all the VCAs and most of the mixing is powered
> > with ground to 12V.
> > Has anyone an idea what could be the reasoning or advantage of this
> > approach? (I'm very much a beginner on the analog electronics side)
> > The schematic is on page 35 and all boards I've seen use the BA6110
> > version:
> >
> https://www.synthxl.com/wp-content/uploads/2020/08/Hohner-Symphonie-D98-Service-Manual.pdf
> > Page 24-29 for the sound-generator - if you are curious.
> The reason is that the Iabc (the control current) is created with a
> resistor from a voltage referenced to GND.
> The OTAs sinks this current into the negative supply terminal.
> If you'd run them from +-12V then you need extra circuitry to translate
> the voltage.
Oh, yes! I recall scratching my head where the control current goes while
trying to build the Ambika SMR filter with spare BA6110s.

> (If you want to read a bit more on OTAs there is an article at my
> homepage, link below.)
yup, I found your collection while working on this filter. It will probably
take a few more attempts for me to really grasp how these are working.

Thanks everyone for the hints! The D98 filter-boards are probably good
enough as-is for the first iteration of my Hohner polysynth. Figuring out
how to properly calibrate the sound-generator DACs and timing for the
integrators should be the next task before polishing the low end.


> Best,
>   René
> --
> synth at schmitzbits.de
> http://schmitzbits.de
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