After the amp switches on, can you mute it by connecting pin 4 to AGND?
I will try this when I'm back home wednesday.
...the LA4625 datasheet shows 2 diodes in series between transistor base and ground.
If one of these diodes failed short, then it would be difficult to generate enough Vbe voltage to turn on the transistor and unmute the amp.
Interesting thought, that may be the case. As you stated unfortunately no simple way to determine the integrity of this diodes without decapping the IC or having a fully functional reference IC. When I have tested the IC outside the circuit, perhaps I can exclude this or take it into consideration.
It's a standard analog input amplifier, just disable that part of the circuit and wire in a cheap amplifier module to replace its function.
Yes, probably that's what it will come down to. But until then, above all it's the curiosity about why the circuit behaves the way it does. Like an itch you can't reach.
Circuit ground is connected to mains through small capacitors in the PSU and roughly follows mains voltage, but can't source much current because of the high impedance of those capacitors. (Its AC voltage quickly falls down when loaded with some low resistance/impedance to real earth).
When you connect some circuit node to earth, it will be subjected to a fairly high AC voltage limited to very low current..
Thank you for your detailed explanation magic, but unfortunately I don't quiet understand. I think it maybe my fault by missleadingly expressing myself. When I said that the circuit works when I connect pin4 to mains earth, I meant protective earth, not neutral.
The PSU is 12V DC and is Class II without connection to protective earth. This behaviour even occurs when I use an additional isolation transformer.
You stated that when I connect some circuit node to earth to it will be subjected to a fairly high AC voltage limited to very low current. I don't understand how that happens, when the keyboard is floating. How does the current flow when there isn't a closed circuit? It would be great if you would explain.