cspirou
100+ Head-Fier
- Joined
- Nov 18, 2014
- Posts
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Lately I have been thinking about what makes certain headphones work with certain amplifiers. There is a well known rule of thumb that the impedance of a headphone should be at least 8x the output impedance of an amplifier to have sufficient damping. So high impedance headphones like the HD-650 or the Tesla T1 should work with just about any amp as long as they have sufficient power.
However it appears that a major exception to this rule are planar magnetic headphones like those from Audeze and HifiMAN. Just look at these impedance curves for some of these headphones.
They are all completely FLAT! This means that the impedance is purely resistive. Dynamic headphones have a frequency dependent impedance which typically peaks around the 60Hz-100Hz. Having a low output impedance is important for dynamic headphones because the different impedances will affect the voltage output at different frequencies, leading to distortion. Planar magnetics affect the output voltage uniformly due to the fixed impedance, which means that the result should just be a shift in voltage by a factor x. This appears to be a major reason why planar magnetics are so clear to begin with.
Based on this reasoning it seems that you can completely ignore the 8:1 impedance ratio when looking for a headphone amp for planar magnetics and just focus on other factors like power output. In fact you might even prefer a 1:1 ratio since power output maximizes when the output impedance and headphone impedance are equal, something people tended to do with tube amps due to the lower power output. Probably also means that these are ideal for professional uses since you can plug these into a studio console or a laptop without having to worry about differences in sound coloration.
If I had the equipment I would measure a set of planar magnetic headphones with a variable output impedance to see the differences in frequency response and distortion. However maybe some of the more technical minded readers out there with a bit of spare time would be willing to do this experiment. Or just plug in a pair of Audeze headphones in different amps and see if the result is worse compared to Grados in the same amps.
However it appears that a major exception to this rule are planar magnetic headphones like those from Audeze and HifiMAN. Just look at these impedance curves for some of these headphones.
They are all completely FLAT! This means that the impedance is purely resistive. Dynamic headphones have a frequency dependent impedance which typically peaks around the 60Hz-100Hz. Having a low output impedance is important for dynamic headphones because the different impedances will affect the voltage output at different frequencies, leading to distortion. Planar magnetics affect the output voltage uniformly due to the fixed impedance, which means that the result should just be a shift in voltage by a factor x. This appears to be a major reason why planar magnetics are so clear to begin with.
Based on this reasoning it seems that you can completely ignore the 8:1 impedance ratio when looking for a headphone amp for planar magnetics and just focus on other factors like power output. In fact you might even prefer a 1:1 ratio since power output maximizes when the output impedance and headphone impedance are equal, something people tended to do with tube amps due to the lower power output. Probably also means that these are ideal for professional uses since you can plug these into a studio console or a laptop without having to worry about differences in sound coloration.
If I had the equipment I would measure a set of planar magnetic headphones with a variable output impedance to see the differences in frequency response and distortion. However maybe some of the more technical minded readers out there with a bit of spare time would be willing to do this experiment. Or just plug in a pair of Audeze headphones in different amps and see if the result is worse compared to Grados in the same amps.