Cezar Chirila @ allaboutcircuits.com shows how to build a Class-D amplifier which has amazing efficiency.
What is a Class-D audio power amplifier? The answer could be just a sentence long: It is a switching amplifier. But in order to fully understand how one works, I need to teach you all its nooks and crannies.
How to Build a Class-D Power Amp – [Link]
This amplifier has no audio spec’s like its distortion or its frequency response or its damping ratio. Since it has no negative feedback then I think they will be poor.