I use an autoradio on my boat and this radio has a subwoofer output. The problem is it needs an active sub-woofer and I had a nice small passive subwoofer (140W) from an old stereo set I did not use anymore.
Amplifiers are not “my thing” and I was busy with work so I needed a simple amp I could build in a few hours. I ordered a LM1875. This is a cheap IC that only needs a handful of parts to make a single rail power supply amp. It can handle 60V but as I hoped it worked fine on 12V (I crank the subwoofer output in the menu to level 4) It took me 4 hours to build on vero-board. The radio has 2 subwoofer outputs (L and R). I combined both channels to 1 input using a 1k resistor for each channel.
It started motorboating so after some measurements and trials I added a 1000uF on the 12V input and that solved the problem. I use the back plane of the cabinet as heatsink. It gets around 30 degrees playing at a louder volume as needed. The amp draws around 40 to 60 mA at 13.8V. In peaks probably more but I read this from the display of my lab-psu. It is amazing you can get so much sound from a small TO220 size IC. It can produce 40W but you need at least 48V for that. It probably only does a few Watt at 13.8V.