Originally Posted by Pappucho
No problem, I think another thing we need to consider is the output voltage of the line out of what you will be using. The DX50's line out for example is rated at 1.5v's. An amplifier with a gain of 7 will output a maximum of 10.5Vs (7 x 1.5vs) when using the DX50 line out. This means that across a 50ohm load (HE-6) you need 210mAs (10.5V/50ohms) and across a 38ohm load (HE-500s) you will need 276mAs (10.5V/38ohms) to get the full 10.5Vs across the load. These values represent what you will get at the output of the amplifier when the volume knob is set at the highest setting. NO ONE listens to their amplifier that high. Most will keep it around the 12:00 position which is about half that voltage out (around 5-6Vs) so you will need roughly half that current.
The ideal amplifier should have enough voltage swing at the desired gain and enough current to adequately drive headphones from a particular source.
Volume pots have a log taper, so 12 O'clock is not going to give you half the voltage. We don't perceive loudness nor pitch linearly.
Since amplifiers amplify voltage and must be able to supply enough current under load to achieve this, let's consider headphone sensitivity specs at 1V rather than 1mW.
HE-6: 96.5 dB/1V
HE-500: 103.2 dB/1V
What's dicey is how much headroom to allow for.
Let's say you listen at 95 dB as a baseline, which is probably loud to many:
HE-6: 0.841 V or 16,83 mA or 14.16 mW
HE-500: 0.398 V or 10.24 mA or 3.98 mW
Let's say you'd like the peaks to reach 120 dB, the threshold of pain:
HE-6: 14.96 V or 299.25 mA or 4.48 W
HE-500: 6.92 V or 182.06 mA or 1.26 W
Just to show how we work in a logarithmic fashion, at 115 dB:
HE-6: 8.41 V or 168.28 mA or 1.416 W
HE-500: 3.89 V or 102.38 mA or 0.398 W
So if you need to hit peaks of 120 dB then the HE-6 is a bit much for many amps. How many of us want to hit 120 dB peaks? The HE-500 fares better than much of the fanfare at HeadFi calls for.
If 115 db is enough, then the HE-6 is still a challenge for many amps and the HE-500 makes the grade more often than some think. So how often do we need an exotic amp that can deliver 6W or more? Perhaps not as often as some would make us believe.
Special thanks to Rob Robinette for his Excel spreadsheet, a nice time saver.