Will the On/Off switch be disabled or in On position all the time? Cause I guess it's not a push button that latches a relay.
Regarding the power for the MCU etc, don't tap into the secondary side of the transformer, add a small USB charger or similar on the incoming side instead.
Btw, what model is the amp?
The pushbutton: the contacts of the existing switch of the amp are bridged and a new pushbutton is connected to the microcontroller.
The relay is not an electric latching relay, but mechanical self-locking (Finder 26-2-08.012.0000)
The microcontroller has a separate power supply, which is completely isolated from the amp. I have considered a small USB-charger, but the relay needs 12 V to operate, so I will use a 12 V power supply and a step down buck converter to get the necessary 5 or 3,3 volts for the microcontroller
It is a Luxman R-1050, the link to the service manual and specs can be found in post #5 . I have several of these vintage amps, but this one is the beafiest. If I can work out an upgrade kit for this one, I would also build it for two other amps.
I understand voltage divider and analoge voltage. It was pointed out that a voltage divider is not the most reliable solution and logic level shifter could be a better option.
But, as it seems, a voltage divider may be the obvious solution to connect the ACS 712 to an ESP32
Is the amplifier a differential output type or transformer output or single ended/gnd referenced output?
Have you a schematic of the output stage of the Luxman.
Vintage amplifier these days could mean 5 to 10 years or older.
ie a flux capacitor model as against a class xyz switching amp... 
Tom.. 
That is beyond my knowledge and understanding. I only want to control the 230 Volt incoming power. Maybe you can check the service manual and find out.
I believe it first came out in 1975.
I just googled it and ohhh, that thing is a beauty 
both by looks and sound (from what I read). I recently received a Yamaha CR-240 for free, not as impressive as yours, but it was free after all and I got a nice little restoration project at hand.
Ok 12 V and step down converter. Since it's an old amp there should be plenty of room.
I looked into tapping the speaker signal, and AC coupling + a comparator could be something. Needs some tinkering about what values to pick for caps, resistors, to not blow up the comparator and to adjust it for that noise floor.
As I already mentioned the output of the ACS is an analog signal NOT a logic level so a level shifter would not work. You would at least need a voltage divider.
The input levels of the three ESP boards you mention are all different so the divider ratios will be different for each.
An opamp with variable attenuation would be the best solution. You can also add some filtering if needed
The ACS712 is a good solution since it is safe, isolates the speaker signal from the ESP and will not damage the amp or distort the signal like other solutions
The board is a waveshare ESP32-C3, More info in the wiki.
There is also a remote control unit, which will be using this waveshare touchscreen.
From the Arduino documentation
You can use GPIO0
See this for information on how to use the ADC
https://docs.espressif.com/projects/arduino-esp32/en/latest/api/adc.html
So 10k + 10k would do?
That should be fine.
