need help building a 50 W audio amp!

[styoshi]

I am trying to build a 50 W audio amp with automatic gain control. I was wondering if you might have any suggestions for a good AGC schematic. In short, the idea is that if I want to switch audio sources for my amplifier, my power output wouldn't fluctuate with loud or soft sources. My sounds source will be a CD player (~25 mV signal), so source switching would be between loud and soft songs on a CD.

I'm not sure if you have this book ('High Power Audio Amplifier Construction Manual' by G. Randy Slone), but I am using a Darlington array with two pole compensation for my Voltage Amplifier Stage (fig. 5.1h, p. 113) and a complementary feedback circuit for my Output Stage (fig. 6.3c, p. 164). This is closer to a Class B output.

I guess in particular, I'm not sure what kind of global feedback system to implement to maintain a constant power output. I'm thinking of putting in a feedback loop between the output of my voltage amplifier and the base of one of the transistors in my Input differential. Do you have any suggestions? (Also, if possible, I'm trying to stick to BJTs).

Thanks,
Jason