> I vote for outside the fb loop. Otherwise, when the output is loaded down, the feedback will be reduced, the amplifier gain will increase and attempt to source more current into the load... and the limiting function of the resistor will be defeated.
The clipping level will be the same, whether inside the loop, outside the loop, or in the power rails.
The real question is: do headphones need damping? The jury is out. My limited tests show that some "300 ohm" phones have a 600 ohm lump in mid-bass, which clearly sounds flatter when driven low-Z than hi-Z. But lo-Z can be 30 ohms. I have not yet found any 32 ohm phone with any impedance kinks below 10KHz, so over the main audio range these phones just do not care what the source impedance is. Some have rising impedance over 10KHz, and I can't trust my ears up there. But I do know that the difference one phone to another in the top octave is greater than the uncertainty of that top-octave rise of impedance.
Anyway: at these levels, if the users insist on clipping, the phones are going to fry.
But so will ears. 250 mW in some phones is over 120 dB SPL. If clipping is obvious, average level is around 110dB SPL, and hearing loss WILL result pretty quick.
I figure 250mW is useful only so you NEVER clip, can trust what the headphones are saying. Also because if you must listen at high levels, clipping increases the damage to the phones and the ears.
My own use for over-100mW is mostly a few seconds at a time. I'm in the same room as the band, playing over 100dB SPL, I need to hear the monitor feed apart from the band, many headphones have only a few dB isolation, so I need 110dB SPL. But only over a few loud notes, to check for clipped recording; once I know I'm clean I back-off to much lower gain and level, and just trust the gear.
I built a live-recording headphone amp with 20dB gain and 5V RMS 100 ohm output. It was ample in many situations, but not quite enough with some phones on some gigs. (It was also too-darn-hot Class-A.) I bothered to design and build another with 34dB gain and 6V RMS 55 ohm output. (And a more efficient though still Class-A output.) Since most of my phones are older 60-300 ohm jobs, this generally does the job. If I worked a lot with some 32 ohm phones, I might need lower output resistance.
FWIW, both my amps have non-zero output impedance because I am not convinced that phones need damping. (Good phones: I have some junk phones that buzz slightly less with low-Z drive, but they are still junk.) In recent thinking, 100 ohms was probably a bit too high, and 55 ohms a wee bit high, but I think 20-30 ohms is good for most phones.
Also: when driving multiple phones, individual 27 ohm resistors semi-equalize the power into various impedances, and isolate shorted wires so the other phones stay live.
