Quote:
Originally Posted by

**KamijoIsMyHero** On the topic of planars, is the amount power just something people have blown out of proportions? or current for that matter, I don't get why planars would favor current over voltage.

I heard, HE-500 needs at least 1W but I am a little suspicious as to what it is about this technology that needs that large of an amount of power. Seeing as an amp can consider them purely resistive, according to innerfidelity graphs, wouldn't it need just an ample amount of power?

I agree with both what TMRaven and jcx said.

In regards to quality

Ideal amps should behave as either perfect voltage or current sources (depending on design goals.) They should have very high input impedance and very low output impedance. AFAIK, most headphones (perhaps not all) are designed with a perfect voltage source in mind. A perfect voltage source type amp should deliver voltage in perfect agreement with the source regardless of the load. The current delivered by the amp however would be a function of both the delivered voltage and the impedance of the headphone. In real life, AFAIK a perfect voltage source does not exist but can be approached. Add to that non-linearities and noise and you get quite a bit of different amp designs with different design goals and trade-offs. Innerfidelity also has some amp characterization charts available, and some discussion in this article: http://www.innerfidelity.com/content/headphone-amp-measurements-donealmost-aaaaarghh (I believe the "mW @ 1% THD" numbers need some work, but the "Volts @ 1% THD" seem to be fairly accurate.)

In regards to power

Going through the numbers usually helps. No guarantees here but hope this helps:

Consider the HD600 (http://www.innerfidelity.com/images/SennheiserHD600.pdf.) According to the characterization sheet, this headphone presents a 300 to 550 ohms load.

To get the sensitivity perform the following operation using the "Power Needed for 90d BSPL" number (variable *x*):

*10*log10(10^(90/10)/x) *= 97.696 dB SPL per mW.

These are rms numbers. As jcx said, some people would like the amp to be able to drive a headphone to peak 115 dB SPL given some high quality music material has relatively high dynamic range. To get the amount of power to do this perform the following operation using the sensitivity number above (variable *x*):

*10^((115-x)/10)* = 53.753 mW

Since *P = V^2/R*, this means that for 300 ohms, the amp needs to supply *V = sqrt(P*R)* = 4 V.

Since *P = I^2*R*, this means that for 300 ohms, the amp needs to supply *I = sqrt(P/R)* = 13 mA.

Now consider the HE500 (http://www.innerfidelity.com/images/HiFiMANHE500.pdf)

Using the above formulas we get:

86.904 dB SPL per mW (~ 10 dB less sensitivity than the HD600)

**645.10 mW (to get 115 dB SPL) - Which I guess is close to 1W.**

And therefore the amp needs to supply 5.5 V and 117.2 mA to get to cover 115 dB SPL peak power.

Note however that all these numbers are sort of representative as both impedance and sensitivity are frequency dependent on most headphones. Some people may also not need 115 dB SPL of peak power as they may listen at much lower volumes. Relaxing the 115 dB SPL requirement to 110 dB SPL yields 204 mW / 3V / 65.9mA requirements for the amp in the HE-500 case and

17 mW / 2.25V / 7mA in the HD-600 case.

With some amount of luck I hopefully didn't mess up the numbers

Edited by ultrabike - 4/10/13 at 11:41pm