The electronics day: DC supply and loss detection experiments
I found in practice that my dc supply detection scheme does not work, since, as expected in the first comment, operation amplifier will follow the non-inverting input signal and produce the output signal which will have effectively the same DC component. I also was not able to test the theory with the single-supply operational amplifiers, since it happend that op-amp chips do not support this: they only have two pins for the power supply and this is actually enough.
I will not use relays to switch off the loudspeakers (since I basically do not have the components), instead I will implement a simple enough addition (or actually change the relay control circuit from the above link) to turn off the mute current supply transistor, which will switch LM3886 into the silence mode when there is no power supply or it is not yet stabilized, so that amplifier output ‘feels’ that.
Also found a very interesting note on how this things are usually implemented. The first, AC supply turns off very quickly compared to DC one (likely because of the output filtering capacitors), so every loss-of-the-supply detection scheme should be attached there (I will not implement some cabinet stackable solution, but instead attach it to the rectifying bridge). Contrary turning-on detection scheme usually is not implemented, and instead a mute timer is used. I’m not ready to design it out of the head, but expect that it is somehow related to the capacitor charge and the fact that diodes (or transistor) eat 0.6 V of the attached potential, so it could be possible to attach a transistor with the appropiate voltage divider, so that when capacitor is charged enough, transistor would be opened thus turning off the mute function (or allowing the current to flow into the amplifier). Charging time could be specified by the appropriate RC-circuit (and its 2*Pi*f relation). Since it is not really needed for the LM3886, I will not play with it now.