The EVU (Expansion Voice Unit) is, in principle, a Wave 2.3 without keyboard. It was created as a precursor to the Wave 2.3 to add more voices to the PPG System. In addition to that, it was an improvement over the Wave 2.2 that was sold at that time, since it could play 8 sounds at the same time and already implemented 12-bit technology.
As a specialty, each of the 8 voices can be adjusted separately in loudness and panorama position.
One of my current projects is to disassemble and analyze operating system V6 of the Wave 2.2, 2.3, and V3 of the EVU. Should anybody be interested in helping with the project or have old PPG Assembler listings, please contact me.
I got my EVU in a non-working state and had to invest a lot of time to get it working. Here's a little picture from the repair period - EVU in "Debug Mode" :-)
Additionally, the EVU I got was one of an earlier series - it still had an I/O-82-Board without MIDI and operating system V1. To make up for that, I added MIDI and upgraded it to V3.