Headphones from power amp questions

I have previously seen discussion WRT driving headphones from power amp outputs.

My interest has been sparked by recent acquisition of a rather cute, 2 stage, single valve SET amp with a power output of approximately 2 watts into 8 ohms and I thought to add a headphone out to this.

Which naturally raises a few questions for me

I found a general description at http://sound.westhost.com/project100.htm however, the table of resistor values given does not cover my requirement.

Question 1 - is there a formula readily usable that might yield appropriate values ?

Question 2 - setting appropriate output impedance, particularly for K701, how to determine ?

Question 3 - what effect does varying the output impedance have on heaphone performance in general terms ?

I would only use the speaker tabs of a power amp for very special and demanding cans. For now I can only think of:

Tony, the rms output is going to be 4 volts (check sum: 4 squared divided by 8 Ohms = 2 watts).

4 volts is not uncommon for a HPA output before the usual (and advisable) series resistor. I would expect there not to be any DC here (check with meter). Now, if that were the only OPT tap it really should need to switch to a 8 Ohm dummy load (or the amp could "motor-boat"), but if there is a 16 Ohm tap a 16 or 15 Ohm (preferable) dummy load resistor would not have to be rated so high - perhaps a 10 watt wirewound would not get so hot. So, once suitably loaded via a switch to select between speaker and dummy load, then take the HP feed series resistor from there. I suspect a 15 Ohm 1 watt resistor would bring good results with Grado's and K701's.

If this is the wrong answer then please break it to me gently.

An aside: a solid state amp should not need a dummy load resistor normally...


Edited by Graham Slee - 07 May 2012 at 3:33pm