The AKG K701 has a nominal impedance of 62 Ohm, while Sennheiser HD650 at 300 Ohm. One would assume the HD 650 is much harder to drive. However, experientially this is not the case. It turns out the competitor K701 is about the same, if not harder, to drive than the infamous HD650. How is this possible?
If we assume K701 has a lower efficiency in converting electrical energy to sound, then the problem is solved. To prove the efficiency model, I did the following measurement today following my lab class:
Using 1000Hz sine wave as input, achieving 85dB as measured by SPL meter,
K701 requires: 0.9v, 0.0018mA, 1.62uW (micro watt, 1e-6W)
HD650 requires: 0.48v, 0.0017mA, 0.816uW
Using 100Hz sine wave as input, achieving 85dB as measured by SPL meter,
K701 requires: 1.82v, 0.128mA, 232.96uW
HD650 requires: 0.72v, 0.012mA, 8.64uW
What can we conclude from the data?
The output power is the same (85dB), if we simplify things by looking only at input power, for the 1000Hz case, K701 requires 1.985 times the amount power of HD650; for the 100Hz case, it requires a stunning 26.963 times the amount of power to drive HD650! This is outrageous, which clearly gives out reason why K701 is so bass-shy.
If we do the comparison within the same headphone, HD650 requires 10.588 times power to deliver the same loudness at 100Hz than at 1000Hz. The K701, though, requires 143.802 times! This is totally crazy!
If we look at the voltage for each case, K701 requires 1.875 and 2.528 times the voltage of HD650 respectively for each case, which is roughly twice. For current, K701 requires 1.059 and 10.67 times the current of HD650 respectively for each case.
Conclusion: As can be seen from the calculation results, K701 requires twice the power to drive at 1000Hz, which is to be expected. But as the frequency tested lowered to 100Hz, it asked for substantially more power compared to HD650. The experiment also concluded for the same loudness, K701 asks for twice the voltage than HD650. This isn’t the case in amplifiers. But if we keep believing in K701 asking for twice the voltage, then as frequency goes lower, K701 keeps asking for more current, from about the same as HD650 to 10 times of HD650. This coincides with the fact that K701 “don’t have nearly enough bass” from sources incapable of delivering enough current. Even if the source is perfect, K701 still wouldn’t have as much bass as HD650. If we use the efficiency model, then the model is frequency dependent. As frequency goes lower, K701’s efficiency goes lower too.
Final Thoughts: K701 is freaking hard to drive, much harder than 300 Ohm HD650, especially its bass.
Macrothought 
Hi, I see that is quite an old article to reply, I hope you still monitor it.
My concern is about the math. Theoretically if the headphone impedance is 62 Ohm for 0.9V the current should be about 14 mA, not uA! Please can you help me?
Thanks, Massimo
Hello Massimo,
Thank you for reading my article – that was several years ago. I still have both headphones in possession, albeit using HD 650 a lot more. Regarding your question, I do not have the physics knowledge to answer you well, but I believe it is related to some kind of damping. http://en.wikipedia.org/wiki/Damping
Basically 62 Ohm is what you measure without the coils moving. When they start moving through the magnet, I believe the effective Ohm goes up, thus reducing the current. The effective Ohm depends on the frequency. I hope that makes it less confusing!
This is an old reply but hey, people might stumble in here and want to know. Your coil moving answer is close but not quite.
Coils of wire have a resistance when there is no potential applied to them simply by virtue of the length of wire in the coil. A motor winding, for example, might have a resistance of 2 ohms. If this was all there was to the circuit then it would be essentially a dead short at line votages. Boom!
However, when potential is applied to the coil and current passes through it, a magnetic field is produced that opposes the current that caused it according to Lenz’s Law. This is partially what they refer to as the impedance.
You can guess then, that the *amount* of voltage and current applied would effect this value as well. To make things even less clear, impedance can refer to any type of electrical “friction” not just to specifically magnetic effects (I guess all electricity has magnetic effects though)
Anyway, since the instantaneous impedance will vary with what’s being applied to a coil, unlike a resistor that has a constant value, headphone impedance is probably taken at some reference voltage and frequency.