What a complex subject impedance matching can be. Given the reasonably low frequencies we are at in audio we don't suffer from partial wave wave reflections and standing waves in the cables since wavelengths are so long. But, we still should strive for a goal impedance that satisfies the needs of the source and load devices.
The resistor in parallel with the outputs of the amps is for allowing us to use an amp comfortable with a speaker load into a much higher impedance load (headphones). Since most amps are very happy with an 8 ohm load, something around 8 ohms would let the amp see its desired load and prevent any oscillations due to the high load impedance (cans). For efficient headphones like the HD800 5 to 10 watt resistors should be safe.
As far as throwing away power with the parallel and load resistors, this is a fact of life. However, the power waste is very minimal with efficient headphones. for example, I am using the same volume setting that I use with my 100 db efficient speakers, That means that at 85db I am using between 1/16 and 1/32 watt total average steady state power out of my amp. Instant impactful peaks might be using a watt. That's one of the beauties of using a speaker amp: even a 5 watt SET has lots of headroom into efficient headphones.
Now we have a reality that the speaker amp will likely have more gain than would be practical for use with high efficiency headphones such as the HD800. To be in a useful volume range with the preamp and to prevent blowing the headphones we can load the + line going to the headphones with a series resistor. I started with the standard load (though many headphone jacks don't totally follow this load) of 120 ohms (I believed this is the value, no need to be exact). For the HD800, being so efficient, a 1 to 2 watt resistor should be safe.
Be aware that bridged amps and dual mono amps may destruct themselves hooking up a common ground set of headphones with a resistor network. Common ground would be a wire set that terminates in a headphone plug. The plus and minus headphone grounds are connected at this plug. There may be other amp designs, other than bridged and mono, that would destruct also. A good check would be to check the resistance between the 2 negative speaker connectors of your amp without anything plugged into the amp. If they are shorted together (very low resistance) then you are safe with the common ground load of the headphones. If not, you better have a balanced cable set where each channel of the headphone has a separate ground.
When using common ground headphone cables with an amplifier that has common ground outputs I run ground to the headphone connector ground from only one channel of the resistor network. This brings ground together in the jack anyway, but doing it this way prevents any parallel resistors in the network (i.e., resistors going from + to -) from introducing quite a bit more crosstalk between left and right channels.. .
Hooking up the described setup could blow your amp and/or headphones if a wrong connection is made.
Hooking up the described setup could blow your amp and/or headphones if the amplifier does not like its ground terminals to be connected. Check with your amplifier manufacturer to be safe.
If you are not sure of what you are doing do not do it. I'm not suggesting that anyone hook up their speaker amps to drive the headphones. I am relating what is working for me.
I also am not suggesting that folks should not buy headphone amps. I don't need to be an enemy of those makers. In my case, I already had perfectionist amps and preamps, some costing $40k each. In that situation it only made sense to try to utilize them. And it is no criticism of a $2k ore $5k headpone amp that a $20k or $30k or $60k amp and preamp set might outperform it.
Edited by Operakid - 4/16/13 at 3:24am