"Matthew" Headphone Amp
After a couple of false starts and a serious error in part selection where I got caught out with the JLCPCB ordering process and put a 1 ohm resistor where I'd specified a 100K ohm resistor - cue LOTS of magic smoke.
I've now placed an order for some of these and I expect to have a couple available at cost (which is about £10 UK plus shipping/packing). However if someone else has the beans money/patience the actual unit cost will drop to perhaps half that but it means ordering a large number.
The whole project is Open Source Hardware and can be seen here marcdraco/Matthew: High-performance headphone amplifier based on the NE5532DR (github.com)
I have a couple here with the faulty parts (1R 603) which need to be replaced. There are four to do in all but it's not for the feint of heart and although removal is easy, putting the replacement ones in without a re-work station is a bit ... fiddly as they used to say on Blue Peter when I was a nipper.
The schematic is attached - there really is nothing difficult here and it can easily be built (not as compact) on stripboard. It runs from a single supply and is essentially a single-supply current buffer which delivers the same voltage to your headphones but can deliver more current than on-board amps without significant distortion. I haven't got some high-end cans to try it with but it's loud enough to hurt on the cans I have.
3D view for the layout.
And for scale, here is the prototype (it has the red solder mask) in a tin intended for playing cards.
Take everything I say with a pinch of salt, I might be wrong!
(Cybertruck avatar is a riff on my inability to deliver my designs in reasonable time so far.)