I tried hooking up my MIDI2VC+ to the old bread boarded synthesizer and it sounded terrible. Basically you can hear a lot of noise that turns on exactly when the USB device is configured and data is transferred on the USB bus. The thing is, I was a bit foolish and routed the I2C-lines very close to the USB data lines. My bet is that crosstalk is leaking into the analogue parts through the I2C bus. I'll look into this and my first remedy will be to patch the I2C bus so it doesn't go so close to the USB bus,and perhaps also shielding it from the environment.