Qudelix 5K with Sennheiser HD 650: enough, with nothing to spare
109.0 dB against a 110 dB target — the verdict that sits between comfortable and too quiet, and the whole distinction is 0.24 V. The load asks for 7.5 mA, which any phone can source. What a 300 ohm headphone runs a battery source out of is volts, not current.
Enough, with nothing spare: 109.0 dB against a 110 dB target.
Level. The HD 650 is 300 Ω and 103 dB/V. Qudelix publishes 2 V. Peak level 109.0 dB, headroom −1.0 dB — the verdict our calculator calls «enough without headroom», which sits between comfortable and too quiet. What separates the two is 0.24 V, or 12 %.
What the load actually asks for. 2.24 V, 7.5 mA, 16.7 mW. Seven and a half milliamps is nothing; a phone sources it without noticing. Compare the Truthear HEXA at 20.5 Ω on the same target: 15.4 mA, twice the current, from a source that has to supply a third of the voltage.
The pearl: 300 ohms is not a hard load. It is an easy load that needs a tall voltage. The forum shorthand «high impedance means you need a powerful amplifier» has the mechanism backwards. Impedance divides current, so the higher it goes the less current the same level takes; power falls with it. What a 300 Ω headphone demands is volts — and volts are the first thing a battery-powered dongle runs out of, because they cost supply rails rather than output devices. That is why a 32 Ω in-ear and a 300 Ω Sennheiser fail on opposite sides of the same small source, and why «is it powerful enough» is the wrong question to ask about either.
What we do not know. Qudelix publishes no output impedance, so the damping line here reads «unknown» and not a ratio. We would rather print «unknown» than print zero: a zero would render a perfect damping ratio out of a number nobody has measured. Neither device in this pair has a spec discrepancy on file with us — the uncertainty on this page is an absence, not a contradiction.