Based on a design originally found in Elektor magazine. It's a classic headphone amplifier circuit that augments an opamp with an emitter follower output buffer stage. The entire design is routed on a single layer so that it can be cheaply made and easily made by home-etching or milling.
As soon as I have some video of the build I will embed or link it here. I'm using the Bantam Tools Desktop PCB Mill to make the PCB. I'm using a K40 Laser Cutter for the enclosure made of plywood, which will be shielded with foil on the inside.
The output stage gets its DC bias from the opamp. Though this design shows an NJM2122D in the picture, that opamp has limited supply of 14V so I made a revision separate for that one. You can use any good, low-noise audio opamp with reasonable outut current drive capabilities, I recommend NE5532 or even TL072 or LM833 is fine. For example, the original circuit used an NE5532.
The three diodes across the output transistor bases bias them into class A so if they get warm bolt a lump of aluminium on each (aka heatsink) though strictly speaking this should not be necessary.
The 3.5mm and 6.35mm jack outputs are in parallel so if you want two people to be able to listen at the same time you can. This will happily drive loads well down to 8 ohm. So it's intended for home studio recording monitor use (precisely what I will be using it for while editing all my Altium Academy videos late at night!)
The switched outputs are brought to a header so if you like you can wire an additional jack to a rear panel which becomes disconnected when the front panel has a plug in it. Therefore this output could go to your speaker amplifier and be auto-disconnected when the 6.35mm headphones are plugged in.
My source of inspiration was the Elektor book "High-End Audio Equipment" which was a collection of articles from the 90s of pre and power amps. This book is no longer in print but there seem to be second hand copies around the web for sale. Check the external links.
The entire design is 100% routed on the bottom layer with no jumpers, so that it can be milled and assembled quickly.