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Low output impedance can also reduce distortion by improving the control that the source has over just where the transducer is in space. This is often expressed as damping factor. For example, a 32 Ω headphone driven by a popular DIY headphone amp with a 32, whereas the same headphone driven with an iPod (5 Ω output impedance) would have a damping factor of just 6.4. If the 120 ohms recommendation is applied, the damping factor would be 0.26.
Of course, output impedance is not the only specification relevant to choosing a headphone amplifier — THD, frequency response, IMD, output power, minimum load impedance, and other measurements are also significant.
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Many headphone amplifiers have an output impedance in the range of 20 – 50 Ohms. The 1996 IEC 61938 standard recommended an output impedance of 120 Ohms. The standard included a note that ”For most types of headphones, the source impedance has verybose in ear headphone.” In 2008 Stereophile published an article that showed that a 120-Ohm output impedance could cause a 5-dB error in frequency response with certain headphones. [1] The article concludes that the effect of noise reducing headphonesoutput impedance on frequency response is ”non-trivial”. Some newer headphone amplifiers have output impedances that are less than one Ohm.
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