I've been fiddling with my prototype - enabling the wifi on the ESP32 sends the temperature of the LM7805 through the roof, so it's fair to say that it's not going to cope - even with a better heatsink. Even if it could, the sheer power consumption is drawing too much power from the transformer which no doubt was never designed to provide this much power on the electronics line. Sadly, there's no way to find out what the rating for the transformer is. It's obviously more for the 12V line (since this was previously driving a 35W Class-AB amplifier) but the line for the electronics (which was powered at a surprising 20V, down to 12V via an LDO) isn't designed for what I'm drawing from it.
The solution I can see is a switching regulator instead of a linear one. This will generate considerably less heat (potentially negligible) and put less strain on the transformer. In my hunt for a good switching regulator, I've tried to avoid the low frequency switching regs (~150kHz) and instead have settled on the LMR14020 - a 2A switching regulator at 1MHz. The components are pretty minimal so it shouldn't be hard to integrate... but it's SMT, not through-hole. My plan was to use through hole components for ease, but footprint (I have less space than expected) and component design has forced my hand. Once I've gone down this road, I may as well go all-in. I've swapped the 3.3V regulator from a through hole LM1117T to an SMT LM1117T-MPX and the associated components. I've also swapped the supporting components for the ESP over to SMT. To squeeze the TPA3122 into the smallest space, I've also swapped a few components over to SMT (just a few capacitors) but most of it is the same as before.
Finally, I've taken a brave step. While I have a lot of experience soldering - even the odd bit of SMT, I've never dealt with anything tiny. I've ditched the large DAC module for a much smaller design of my own, leveraging the PCM5102A that I know works with the ESP32. It's a pretty tiny chip - SSOP - so that's going to be fun! Given all the SMT components, I've ordered an SMT template as well which should help putting down the solder paste. That's another first - so wish me luck!
This is my next PCB design: