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

[René Schmitz]

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.

(If you want to read a bit more on OTAs there is an article at my 
homepage, link below.)


Best,

  René


--
synth at schmitzbits.de
http://schmitzbits.de