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Side 3]
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Pitfalls
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Although it doesn't always appear in the specifications of headphone amps, output resistance has a
nontrivial effect on frequency response, and hence on tonal balance. Many headphone amps have output resistances in the range of 20–50 ohms, the principal justification being that this helps even out the substantial differences in sensitivity between different headphones. Output from low-impedance/high-sensitivity models is attenuated more than that of high-impedance/low-sensitivity types, making it less likely that users will inadvertently expose themselves to potentially damaging sound-pressure levels. Insertion of a resistor in series with the amplifier output also helps the designer ensure short-circuit protection and unconditional feedback stability. The IEC 61938 standard goes even further in recommending an output resistance of 120 ohms, noting that "For most types of headphones, the source impedance has very little effect on the performance."
Whoever wrote that must live in a fantasy world. Many of the headphones currently available in the hi-fi market have a medium nominal impedance of 30–60 ohms and typically display a quite wide variation of impedance with frequency. Fig.7 shows overlaid graphs of modulus of impedance vs frequency for the four aforementioned headphones, and fig.8 a simulation of the effect of using them with a source impedance of 120 ohms. The model with the largest impedance variation, the AKG K530 (orange trace), would suffer a total response error of almost 5dB.
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