I built an inexpensive amplifier because I needed one for my car. With its power supply, this one sells for $3300 but only costs $50.
After a lot of searching, I decided to try the 47 Laboratory 4706 Gaincard. The reviews of it were excellent. After some searching, I came to the conclusion that the rest of the hardware was excellent, but the "heart" of it was an integrated unit costing less than 5€, the LM3875. This particular integrated, if put in correctly, creates a top notch amplifier.
Below is the amp in the case.
The amplifier in its chassis (without the power supply).
12 to 36v DC to DC converter
I also built a DC voltage converter. Below is the schematic of the converter, and below is the converter.
The 1st thing you notice is how much power this amp has. But when you turn the volume all the way up, the amp is pushed to its limits, causing the heatsink to heat up. To make it work properly, I replaced it with an old CPU heatsink, and most of the power supply materials, are from an broken car amp.
Of course later I added a loudspeaker protection circuit that has two main functions: to protect the audio amplifier and the
loudspeakers during start-up sequence when the amplifier output might exhibit DC voltage, and to protect the loudspeakers in the unlikely event of an amplifier power stage failure, where DC component would damage the speaker voice coil.
Compared to other car commercial amplifiers, this one is much better. On the speakers I connected it to, I noticed a stronger bass and higher treble. I have this amp for more than 10 years in my car and works like a charm.