"The Bog Standard" - A good enough amplifier for the rest of us

I calculated something like 488 mW, the voltage drop across the VBE multiplier helps a bit. That is below the rating up to 52.4 degrees Celsius, but taking no margin for signal-related temperature fluctuations. I also didn't take into account that the current drops somewhat at high ambient temperatures, and I guessed the base-emitter voltage.

It's not nearly as bad as what I have seen in a commercial amplifier design from Philips, but it would certainly be good for reliability to reduce the current a bit or improve cooling.
That's the wattage I got too and it lines up with SPICE simulation. However, it's a bit too close for comfort, so I whacked in some BD139/140 instead. Those babies should handle 1W without a heat sink, I believe.
 
Why not protect the input differential pair with antiparallel diodes between the bases? That also protects against excessive input signals (preventing base-emitter avalanching that could reduce current gain and increase 1/f current noise).

By the way, a unipolar aluminium electrolytic capacitor also works as a diode, it starts conducting when the reverse voltage is more than a volt or two
 
We're getting close. Hopefully I can finish up the PCB this weekend so we can get a first impression of what the first prototype sounds like.
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Why not protect the input differential pair with antiparallel diodes between the bases? That also protects against excessive input signals (preventing base-emitter avalanching that could reduce current gain and increase 1/f current noise).

By the way, a unipolar aluminium electrolytic capacitor also works as a diode, it starts conducting when the reverse voltage is more than a volt or two
Yup. That worked even better, especially when I fed way too much amplitude on the input. It adds a tiny amount of THD in the simulation but that will likely drown in the other distortion when this meets the real world. I’m assuming it’s due to tiny leakage across the diodes.
 
Time for some ground routing. Yes, the board is a bit crammed. I'd like to be able to fit it in a compact chassis. Anyway, back to the grounding. It's a best effort star ground. Understanding that power ground is a high power signal carrier, I've tried to avoid having any low-power tracks in parallel. The only thing I don't like are the long traces to the filter caps, but I can't think of a better way. The whole idea of local filter caps on the amplifier board is to have a nearby energy reserve, so I want them as close to the output devices as possible. Any comments? Just because it's Bog Standard doesn't mean we shouldn't pay attention to the PCB design.
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Also give some thought as to how two channels would be connected together. You don’t want ground return currents in the reservoir caps going through any place where a minuscule voltage drop can show up as an INPUT signal. On a single board with a single channel that may not be too hard, but when putting a second board in unexpected things can happen.
 
So you're suggesting I lose the local caps? I have built two amplifiers with caps like that and they work beautifully. But you seem to know what you're talking about and it would definitely make the design simpler without that ground bar in the middle.