Why Is the Sony MDR-ZX110NC 220 Ohms With Power On but 45 Ohms Off?
Back to my Sony MDR-ZX110NC ownership story
Short answer: Sony specifies the MDR-ZX110NC at 220 ohms at 1 kHz with power on and 45 ohms at 1 kHz with power off.
Why are there two numbers?
The headphones contain active noise-canceling electronics powered by the AAA battery. With that circuitry operating, the source sees a different electrical load than it does when the headphones are operating passively.
Does 220 ohms mean you need a headphone amplifier?
Do not interpret the powered-mode impedance number in isolation. Sony designed the MDR-ZX110NC as a portable wired headphone and also publishes different sensitivity figures for power-on and power-off operation. The practical question is whether your source provides a normal compatible headphone output, not whether one specification resembles a studio headphone’s nominal impedance.
What happens when the battery dies?
The manual says the headphones automatically switch to passive mode. That is the same condition under which Sony lists the lower 45-ohm impedance figure.
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The exact model in my Amazon history is the Sony MDR-ZX110NC.