Re: ZX81 audio.
Posted: Fri Feb 17, 2023 11:43 am
Seems by chance that also specdrum was using A5! 
I honestly can’t remember what Zeddy Interfaces/expansions use A5. Just like the ZX Spectrum, all ‘simple’ I/O addresses are already used.
A5 is used by the Kempston joystick interface (designed for the ZX Spectrum, but the early versions fitted with ZX81 size edge-connectors) do work on a ZX81.
Somewhere in the hardware section, I did post the I/O addresses of all interfaces and expansions known to me at that time. But that was some years ago. I will link to it when I find it.
Mark