I made this thing. Looks a bit like a dog's breakfast but works pretty well. Very similar to a Rod Elliot kit (Project 113) It sort of grew back in that direction after lots of sukkeling with other ideas with cascode output stages. A pair of 15-0-15V transformers connect to the blue terminal blocks (or you could use a single one.) The grounds are linked at the headphone plug of course. If you want real "monoblocks" you could fit a 4pin XLR to the cans and keep things separate. Output impedance is low but there is provision for adding an output resistor on the PCB if that floats your boat. This one had NE5534s but most any single opamp will work. There is provision for "inner loop" compensation (I'm sure there's an educated name for that) The outputs (2SB649/2SD669) idle at 100mA so things are in Class A up to silly levels. If you look carefully at the bad pic you'll see the output transistors are mounted in pairs on top of each other. The top one is the bias reference. Gets freakin' hot like it is in the pic. If one adds a big plate to the heatsink it works fine. There are 2 LEDs at each opamp that allow nulling the offset voltage due to opamp input bias current to a fraction of a millivolt. Input impedance is about 100kOhm. Coupling cap is 330nF so bass cut is at 5Hz or so. The output bias current is set by 2 JFets matched for the purpose, and the opamp output stage is pulled into class A by a third JFet.
Think it's about time I draw it out and put it in the DIY section.
Lots of tweaking and adjustment possible. But all of that guided by one's own oscilloscope and at one's own risk. I have made these with 100MHz GBWP video opamps and somehow got it stable into 2nF, I think, but I'm not at all convinced that it is a sensible thing to do. The TPA chip and an output resistor is the way to go if one wants insane slew-rate on paper etc.
I'm willing to make complete (populated & tested) PCBs like this so you can just add transformers wires pots and plugs of your choice. And a box of course with a suitably big heatsink plate. PCB is 145mm x 96mm. Will suck my thumb about a price if there is interest. It is not particularly cheap but should be a lot less than importing.
