One additional point would I would avoid polarized caps at the input and the output. If possible I would also skip non-polarized electrolytics.
Not sure about the virtual GND. My first thought: No way. But the more I think about it I think it might work. As it's only used as reference for the OPAs. But for my taste the 2x 4.7kOhm is a bit too high to help stabilize it at the center. At power on it rely on identical cap-values as voltage divider. In real world they differ +/-20% (or +/-10%) so does the GND until the resistive voltage divider equalize it. The result will be a offset at the output during power on.
Wrt the thermal protection indicator. The datasheet works GND-Based and not V- I'm not sure if your circuit will do. Plus the 3.43V is fairly low for the 2N7002. Probably still OK, but if you have an alternative go for it.
Layout:
* The +12V trance close to the OPA551 seems very weak. In general I would increase the power traces a bit.
* Since you're already SMD, why do you use through hole caps?
* If possible place the parts a bit cleaner. For example R2 close to C3; R3 to C4. With the current design that doesn't make a big difference. but it looks cleaner. In general try to clean up the placement (e.g. swap R2 and R3)
* Why the strange layer jumping between C3/C4->R12? You're already at the top layer going to R2.
* Do you really want to have the LEDs spread over the board? No connectors for front panel mounting?
* Depending on the way you plan to solder the board and your solder experience: The pad-size seems rather small.
Thank you. This helps.
I've tried SMDs electryltics a few times, decided to go back to through hole. The SMDs are usually just the THT versions with the leads mounted in a plastic base. I found them trickier to solder too, found to reflow them with hot air the bases melt while the case soaks up the heat.
I could probably go 0805 on the 10uF, maybe, but I have 10uF THTs in stock.
I had the thermal protect mosfet on the GND, but I moved it to V- for some reason. I can move it back to AGND and pick a different resistor to give me closer to 5V. Thanks, this is my first circuit with a mosfet and I know they can be tricky. I did simulate it in Spice with a 2N7002 model, but ... that doesn't always equate to the real world.
This is a prototype for part of a much larger project, so some of the design decisions are a bit weird. In the full project I have +-15V rails to play with and melt headphones, but as I was making the prototype I considered...
1. Test the general amp works as expected.
2. As I need to make it anyway, it could realistically replace my current amp, so I need it to run on a single 12V rail.
On that last point, I will take another pass based on your comments and a general tidy up, given it should be a decent looking device in it's own right, no need to be messy. Having all the LEDs in one corner in a neat row would be cool. I had already laid it out before I picked a case and thus a board shape/size. I can now let things spread out a bit more while keeping the important stuff close.
On soldering, this board is sparse compared to my last one. I use a microscope, paste and hot air, so as long as I can get the teasers into drop the components, it's fine.