Hobbyist: Is this a reasonable design for an audio amplifier? (very sketchy design)
All of this powered by a, say, 150W power supply with +/- 15v rails.


With 15V rails, a BJT output stage should output a peak voltage of about 13V. In a 8 ohms resistive load this is 21W peak, or 10W "RMS". Note "Watts RMS" really means "active power in a resistive load"... 

So your amp needs to run on higher voltage rails, which means it can't use opamps to create all the gain, since the output voltage of opamps is limited to +/- 15V. So you have to use discrete, which means opening all the can of worms at the same time, from loop stability to thermal management.

So... If you're a beginner, I definitely wouldn't recommend designing a power amp, because you'd have to get everything right. It could be expensive and risky, and if it smokes there is a good chance one of the output transistors will fail short circuit, which means the amp will output one of the rail voltages continuously and burn the loudspeaker. Also the PCB layout is difficult to get right. Starting with a kit is a much better idea, you'll get all the info on how to choose the heat sink, hopefully a good pcb, etc.

If you want to build something, the line-level, low power part with the opamps is much less risky, and mistakes are less costly. Note a misbehaving preamp can still destroy your speakers if it oscillates, but that's uncommon.

Once I was probing inside a headphone amp ... that stuff isn't scary, right? The scope probe somehow shorted something, one output transistor blew, shorted, and burned the voice coil of the $200 Sennheisers I had forgotten to disconnect. OUCH! That really sucked. Always try the new amp on second hand garbage speakers...

If you want a DIY power amp for full range speakers, you can't go wrong with a LM3886 based kit. Search "LM3886 done right". It has full protection and is easy to use, and LM3886 sounds very good considering the price and simplicity. In fact a properly implemented LM3886 will probably sound better than most BJT amplifier kits, and it doesn't smoke if the output is shorted, which is always a plus...

For a subwoofer, a Chinese class-D module with IRS2092 would be a good option, at an unbeatable price/power ratio.

After you build that stuff, read these excellent books:

"Art of Electronics" by Horowitz &amp; Hill

"Audio amplifier design handbook" by Douglas Self

"Designing audio power amplifiers" by Bob Cordell

The last one in particular covers every tricky detail that really make the difference between smoke and no smoke. I really recommend it, it's practical, well written and "user-friendly", it will answer all the questions you asked, and also the ones you didn't ask. I mean, designing a power amp is not simple, but definitely not rocket science either... having a book that explains all the mistakes you can make and how to avoid them sure saves a lot of time and blown transistors...

There's also something you have to do...

You say "I want a 150W amp", this is something people often say, but do you know how many watts you actually need?

You can measure this with a scope. Probe the output of your current amplifier, using your usual speakers, while playing a bit louder than usual on music you like.

Last time I did that, I measured voltage equivalent to 1W average, and 10-20 watts peak. So my 50W amp actually never clips. It's an interesting experiment to do. Subwoofers usually need a bit more power though, which is why I recommended a Class-D amp.

EDIT:

About 15W amps. You will save a few dollars on the heat sink.

If you use one pair of output transistors (one NPN, one PNP or the MOSFET equivalent) and they're the beefy kind like TO-247 packages then you can do 50-60W RMS. More power requires more power transistors in parallel which opens another can of worms: current sharing, they have to be matched, etc, plus a big heat sink, it gets expensive and annoying real fast, so you only do it if you need it.

A 50-60W RMS amp is never going to clip anyway at normal listening levels with reasonably efficient speakers. So do you need to pay extra for 100W? Only if you determine, by measuring the voltage at the output of your amp, that you need the extra watts! Could be the case for inefficient speakers, a sub, or if you like it REALLY loud.

However a 15W amps will clip, and if it uses discrete components its schematic will be basically the same as the 60W amp, only the heat sink will be smaller. So the 15W amp has no advantage, unless you do it Class A and have super efficient speakers.