I'm asking for the HE-560 which has 90.0 dB SPL/mW. The thing is I know my amp outputs 4W max but don't know at what impedance. I thought planars, being purely resistive wouldn't change the power being delivered based on the impedance. I guess I'll have to test the headphones when I get them. Or I could find someone to get me the actual power of my amp at ~50 ohm.
Yeah, the Beresford Capella doesn't list the specs in detail...but I'm 99% sure it has more than enough power for the HE-560. For some reason, the power requirements of HiFiMAN headphones seem to work differently than normal headphones. I calculated that it needs 1,000 mW at 50 ohms to reach 120 dB...but
this link says it needs 1,500 mW. Go figure. It's not likely that the amp can't handle that, and you may not need to reach 120 dB for dynamic peaks anyway. You'll need ten times
less power to handle 110 dB, so you should be fine.
You should be correct, but it's probably a rounding error. Or, it's quite possible the SPL for the HE-560 is not exactly a straight exponential relationship. At any rate, remember that we're dealing with exponents when using dB. At 1000mW, the next 10dB increase will need 10,000mW total (130dB)! At 120dB, an increase of only ~1.765dB will need another 500mW resulting in 1500mW total.
Power output on amplifiers at different impedances is simply ohms law. The power available is proportional to the impedance of the headphone the higher the impedance the lower the power.
Actually, no. The power required by the headphone load is a function of its sensitivity and the sound pressure level desired. It's impedance determines the makeup needed of the vector quantities, voltage and current - that's Ohm's Law.
The power output of an amplifier depends on its design as to whether it can supply a certain power at a certain impedance. Ohm's Law still dictates, but it has to be applied to the internal circuitry as to whether it can supply certain quantities of voltage vs. current for load impedance.
The 10 dB difference in sensitivity is the key. Low sensitivity needs an amp capable of high voltage output. Ray Samuels talks about this in the description of The Dark Star: http://www.raysamuelsaudio.com/products/dark-star
It's also discussed here with respect to the HE-6: http://www.head-fi.org/t/529873/amps-that-can-drive-the-hifiman-he-6-planar-headphones/2565#post_10326971
No. Sensitivity has no bearing whatsoever on what voltage vs. current is required. That's dictated by the impedance of the load.
Let's say for the HE-560 at 120dB it needs 1500mW -
If its impedance is 50 ohms, then Current = sqrt(Power/Resistance), or sqrt(0.03), Current = 173ma. Voltage = 8.66V (which is what the HiFiMAN link says)
If its impedance is 32 ohms, then Current = sqrt(.047), Current = 217ma. Voltage = 6.93V
If its impedance is 300 ohms, then Current = sqrt(.005), Current = 70.7ma. Voltage = 21.2V!
All of those impedances above are at 1500mW power, while the current ranges from 217ma down to 70.7ma. Meanwhile, the voltage ranges from 6.93V to 21.2V!
A particular amp may not be able to provide the higher currents needed at low impedance or the high voltage needed at higher impedance - to meet that 1500mW.
