This is what I do with my spare time, not that I have much of it at the moment
This is an interface I deviced with one of my friends. We use it to get digital sound in an out off our PC's. It features: 1xOpt/Elec output, 2xOpt/Elec input, 1xOpt->Elec converter, 1xElec->Opt converter, 1xAnalog input, 1x Anlaog output
A small 2x30 Watts RMS amplifier, build with MOSFET transistors in the output stage
A somewhat powerful 2x130 Watts RMS amplifier, build with 8 bipolar transistors pr. channel in the output stage