Having staggered to the end of this project in 2021, there were a few niggling problems -
1) The Mk1 was going to have a battery hatch on the back, and a class D amplifier module built in to a cavity on the back. The chosen amp module (Ebay) turned out to be very noisy, so I decided to replace it with a larger module hanging on the back, which preventing easy access to change the battery. This was the basis of the Mk2.
2) The Mk2 had a simple preamp containing a gain stage and an HPF, but it produced an annoying click as the compo was powered down.
3) The Mk2 was limited to battery power only, unless connected to an external mains 12-14v power source.
The Mk3 is a rebuild intended to fix those issues and add some further benefits.
David