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

brianw brianw at audiobanshee.com
Wed Mar 6 01:31:12 CET 2024

It's very common to use a single supply and handle all signals with a DC offset internal to the circuit. This just requires biasing the op-amps to "virtual ground" - which is half of the supply voltage (or somewhere around there).

I glanced at page 35 of that schematic, and the output op-amps on the right of the schematic have a series capacitor. This series capacitor will remove the DC output and present an AC signal beyond that point. The waveforms are probably all biased with a center around 6V instead of 0V.

The drawback of this approach - besides requiring a lot more capacitors on the inputs (if any) and outputs - is that the frequency response is not flat all the way down to 0 Hz. It's usually not even flat down to 20 Hz, which we can hear. However, since nearly all "affordable" electronics do this, we're not really complaining about the sound.


p.s. If you can find a copy of Craig Anderton's "Electronic Projects for Musicians," that might be a good starting point for someone like yourself who is interested in analog electronics for audio. That book was my starting point - apart from an uncle and grandfather who taught basic electronics - and it covers guitar effects, filters, and the kinds of circuits you might see in a synth.

On Mar 5, 2024, at 3:36 PM, hannes.janetzek wrote:
> 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. 
> Regards
> Hannes

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