The AM 80MKII amplifier module is designed for the use in an amplified subwoofer. I bought it from Lautsprechershop in Germany. I mounted this plate amp into an old JBL 125 subwoofer, it worked great after some tuning of the level and phase settings.

However, the sensitivity of the auto-turn-on circuit is too low for my taste. Probably ok for a home-cinema and its bangs and booms, but for low-level music with near-field monitors, I could not get it to turn itself on soon enough.

I asked the schematics from / and I was surprised to get it almost immediately. I will not publish it entirely to protect the copyright.

I identified the inverting amp U3 for which the original gain (100) can be tuned. In my case, and for music levels I listen to, replacing R38 by a 3,3 kΩ resistor did the job. The U3 circuit has now a gain of about 820. Before the change, the amp needed 450 mV peak-to-peak to turn itself on. After the change, something in the range of 80-100 mV is sufficient.

R38 (3,3K) after replacement

To avoid completely dismounting the PCB (many screws and 2 pots), you could as well solder a resistor in parallel, under the PCB. On the image below, 42mm from the border, R38 is soldered between the two black dots. Solder an additional resistor of about 3K9 or 4K7 in parallel. If you find it too sensitive, 10K is worth another try. Avoid a combined value less than 3K3.

the 2 black dots at the “0” of the ruler are the soldering points of R38