Is the FiiO K11 loud enough for the Sennheiser HD 600?
Same 300 ohms, same family, 6 dB apart — and the K11 is exactly the source where those 6 dB flip the verdict. HD 650: 115.5 dB, comfortable. HD 600: 109.5 dB, target reached with nothing to spare. The catch is that the deficit is in volts, and the K11's headline 520 mW says nothing about volts.
The HD 600 reaches the target and stops there; the HD 650 clears it by 5.5 dB. One DAC, one impedance, two verdicts.
Both headphones are 300 Ω. Sennheiser publishes the HD 600 at 97 dB/V and the HD 650 at 103 dB/V — six decibels, which is four times the power. For a 110 dB peak the HD 600 needs 4.467 V, 66.51 mW, 14.89 mA; the HD 650 needs 2.239 V and 16.71 mW.
The FiiO K11 publishes 4.24 V with a 1.2 Ω output impedance. After the divider the HD 600 sees 4.223 V and reaches 109.5 dB — half a decibel under target, while the HD 650 reaches 115.5 dB. Damping is a non-issue for both: 300 / 1.2 = 250, well past the ratio of 8.
| Source | HD 600 | HD 650 |
|---|---|---|
| FiiO K11 (4.24 V) | 109.5 dB (−0.5) | 115.5 dB (+5.5) |
| FiiO KA13 dongle (1.22 V) | 98.7 dB (−11.3) | 104.7 dB (−5.3) |
| SSL 2 MkII (3.46 V) | 107.8 dB (−2.2) | 113.8 dB (+3.8) |
The pearl: the K11's headline number is measured at the wrong impedance to answer this question. FiiO publishes 520 mW — at 32 Ω. The HD 600 asks for 66.5 mW, which looks like an eighth of that and therefore trivially easy. It is not, because at 300 Ω the K11 cannot deliver 520 mW at all: 4.24 V into 300 Ω is 60 mW, and that is the ceiling. Power figures do not travel across impedances; volts do. Read the voltage, not the milliwatts — the HD 600 wants 4.467 V and the box has 4.24, and that 5 % shortfall in volts is the entire verdict.
So the practical reading: on a K11 the HD 650 has room for a dynamic recording; the HD 600 will be sitting at maximum volume on quiet material, and every 6 dB the mastering engineer left below full scale comes straight out of your listening level.