Of course Hifiman is going to recommend their own amplifiers for it: it's called business! I find it interesting, though, what your rationale is behind:
....but usually planars need the right voltage/amperage ratio to be driven correctly. Usually tubes or J-FET diamond buffers can do that properly, but I doubt that 2 paralleled NJM4556 can do it perfectly, especially for the HE6 planars, despite the fact that O2 has enough voltage and current to drive them.
If it can supply enough voltage and current, and provide more than decent distortion figures and slew rates, what's the problem with it? The prototype might've needed 5W to go loud, but at 5W, the production one could go up to ~120 dB.
Exactly.... a lot of people seem to be confused about the relationship between voltage, current, and power.
When you see something like "the impedance is 50 ohms and it needs 2 watts to reach 110 dB", what you need to do is to calculate the voltage and current it would take to produce 2 watts
INTO THAT LOAD, and, if the amp can deliver
BOTH ENOUGH VOLTAGE AND ENOUGH CURRENT to reach that level, then it should be fine.
Remember that most amps have both a voltage limit and a current limit. For example, let's assume I run both a "speaker" amp and a headphone amp using a single 4556 op amp off of a +/- 15V power supply. The 15V power rails limit both to about 8V RMS. However, if the speaker amp is rated to work into 4 ohms, then it can deliver about 2 Amps (2000 mA), while a single 4556 op amp is rated for about 70 mA. If you do the calculations, you will find that, into a pair of 150 ohm headphones, both will deliver the
SAME amount of power - because the power will be limited by their output voltage (a pair of 150 ohm headphones will only draw about 50 mA at 8 VRMS; which both the speaker amp and the op amp can deliver easily). However, into an 8 ohm load, the speaker amp can deliver 8 watts, while the 4556 op amp will deliver less than 40 mW (because its 70 mA current limit limits it to delivering that much power into 8 Ohms).
The upshot of all this is that.....
- With regular headphones (low to medium impedance, high efficiency), most headphone amps can deliver both enough voltage and enough current to get them to run plenty loudly and sound very good. (They'll get loud and sound good.)
- With high impedance headphones (150 ohms, 300 ohms, 600 ohms), most headphone amps can deliver plenty of current, but many amps that run on lower voltage rails can't deliver enough voltage. (They'll sound great but may not get very loud, and you may be able to clip the amp at relatively low output levels, although there's a good chance you'll peg the volume control and it may just not get very loud).
- With low efficiency planars (usually low to medium impedance), most amplifiers would deliver plenty of voltage, but you often need a "speaker amp" or a specially designed headphone amp to deliver enough CURRENT to drive them satisfactorily. (And a headphone amp that can't deliver enough current will sound fine at very low levels, but will clip very easily.) Headphone amps that use op amps like the 4556 will NOT deliver enough current to run low efficiency planars.
- High efficiency planars have pretty much the same requirements as regular headphones.
You also need to consider that the ratings you see on op amps are usually "absolute maximums". An NJM4556 is rated to put out about 70 mA, which means that two paralleled ones could could put out at most 140 mA (if they don't share exactly it could be somewhat less). However, they will probably produce more distortion at current levels near that than they do at lower levels, so they may not sound good at all if you run them near their limits (this will depend on the individual op amp, and on other aspects of the overall design).
