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Headphone amps for planar magnetics
#1
Hi Guys

Planar magnetic headphones offer very high SPLs at pretty low distortion. The load is almost purely resistive, therefore it is easy to drive. The voltage and power required is also quite low.

Remember: The sensitivity ratingĀ  for headphones is at 1mW input, and this often produces 100dB of sound.

The leader in the planar magnetic headphone camp is Audeze. Their models have become rather diffuse inasmuch as every model is "the best" but they are styled for specific applications and music types. The element to be driven is a resistive track laid down on thin film. The resistance is usually 20R to 110R. At 1mW, the drive required is 140mV up to 330mV. at 7mA to 3mA, respectively. These are RMS values. You can see that driving the magnetic planar headphones is not very difficult.

If you want to push the drivers to their claimed 130dB limit, a bit more drive is needed, at 4V5 to 10V5 respectively for 20R and 110R. These voltage are why you see a recommendation to use a 5W amp, or a 10W amp. It is not that the headphones need the power; rather, that they need the voltage if driven single-ended. Differential drive cuts the voltage swing in half as far as each driver is concerned, except now you need two drivers per cup with symmetric signals.

Whether single-ended drive is used or differential, standard opamp circuits with or without buffers added will work reasonably well. The ultimate performance is attained using discrete fully complementary circuits. Wherever possible, we use inverting gain stages to eliminate common-mode distortion, but we have to be careful to preserve overall phase coherence from input to output. Because this is all low-voltage +/-24V solid-state circuitry, distortions of all types can be pushed to -140dB.
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