Two numbers on an amplifier get mixed up all the time. Gain is how many times the amplifier multiplies the voltage coming in. Maximum output is the ceiling: the most voltage and current it can put out before it clips. Power is what you get when that voltage meets your headphone’s impedance, P = V² / Z. Gain decides how quickly you reach the ceiling as you turn the knob. It does not move the ceiling.
So “not loud enough” has two different causes with two different fixes. Either the ceiling is too low for the headphone, and only a stronger source helps. Or the ceiling is fine but the signal arriving at the amplifier is too quiet, and the volume knob runs out before the amplifier does. Buying more watts does not fix the second one.
A headphone does not ask for watts so much as for a voltage, and for the current that voltage pushes through its impedance. These are 15 widely owned models whose makers state their sensitivity with a unit, sorted by the voltage they need for a 100 dB SPL peak — loud, louder than most people listen.
| Headphone | Impedance | Power for 100 dB | Voltage | Current |
|---|---|---|---|---|
| Shure SE215 | 17Ω | 0.20 mW | 58 mV | 3.4 mA |
| Moondrop Aria | 32Ω | 0.19 mW | 79 mV | 2.5 mA |
| Audeze LCD-X 2021 | 20Ω | 0.50 mW | 0.10 V | 5.0 mA |
| Truthear Hexa | 20.5Ω | 0.49 mW | 0.10 V | 4.9 mA |
| Sennheiser IE 200 | 18Ω | 0.69 mW | 0.11 V | 6.2 mA |
| Sony MDR-7506 | 63Ω | 0.25 mW | 0.13 V | 2.0 mA |
| Meze 99 Classics | 32Ω | 0.50 mW | 0.13 V | 4.0 mA |
| Focal Clear MG | 55Ω | 0.40 mW | 0.15 V | 2.7 mA |
| Audio-Technica ATH-M50X | 38Ω | 1.3 mW | 0.22 V | 5.8 mA |
| Beyerdynamic DT 1990 Pro (250Ω) | 250Ω | 0.63 mW | 0.40 V | 1.6 mA |
| AKG K702 | 62Ω | 5.1 mW | 0.56 V | 9.1 mA |
| Sennheiser HD 650 | 300Ω | 1.7 mW | 0.71 V | 2.4 mA |
| Sennheiser HD 800S | 300Ω | 2.1 mW | 0.79 V | 2.6 mA |
| ZMF Atrium | 300Ω | 2.5 mW | 0.87 V | 2.9 mA |
| Sennheiser HD 600 | 300Ω | 6.6 mW | 1.41 V | 4.7 mA |
Those requirements are fixed. Nothing upstream changes them: whatever the rest of the chain does, the HiFiMAN Susvara still needs 1.73 V at its terminals for 100 dB. What changes is whether the source can still deliver that voltage once the signal reaching it has been turned down.
Loudness normalisation in streaming apps, ReplayGain, and the preamp of an equaliser all lower the digital signal before it reaches the DAC. Every 10 dB lost there divides the most a source can put out at full volume by 3.16 in voltage, and by ten in power. The table counts how many of the 567 non-electrostatic headphones in this database each source reaches 100 dB on at full signal, and after 6 and 10 dB of digital attenuation; the first two columns are the source’s power into 32Ω and its voltage into 300Ω.
| Source | Into 32Ω | Volts into 300Ω | Full signal | −6 dB | −10 dB |
|---|---|---|---|---|---|
| Apple USB-C dongle (EU) | 15 mW | 0.77 V | 516 | 404 | 293 |
| Apple USB-C dongle (US) | 30 mW | 1.10 V | 547 | 457 | 386 |
| FiiO KA11 | 200 mW | 2.57 V | 566 | 556 | 540 |
| Moondrop Dawn Pro | 125 mW | 2.00 V | 561 | 548 | 505 |
| FiiO KA13 | 50 mW | 1.22 V | 555 | 514 | 451 |
| Qudelix 5K | 125 mW | 2.00 V | 561 | 548 | 505 |
| FiiO BTR17 | 280 mW | 3.00 V | 566 | 561 | 549 |
| Focusrite Scarlett Solo 4th gen | 31 mW | 2.57 V | 552 | 499 | 413 |
| MOTU M2 | 333 mW | 3.27 V | 565 | 557 | 549 |
| JDS Labs Element III MK2 | 2.3 W | 9.36 V | 567 | 566 | 565 |
| Schiit Magni+ | 2.0 W | 8.83 V | 567 | 566 | 564 |
The Apple USB-C dongle (EU) goes from 516 to 293. Across all 280 sources in the database, 266 reach fewer headphones once 10 dB is gone, and the median source loses 18 models. None of that is a lack of power in the usual sense. The amplifier is the same; it is being asked to do the same job with a third of the input voltage.
The HiFiMAN Susvara is the headphone everyone calls hard to drive. HiFiMAN prints a sensitivity of 83 dB without saying per what; read per milliwatt, it needs 50 mW, or 1.73 V, for a 100 dB peak. The FiiO KA11, a $29 dongle, works out at about 2.53 V into 60Ω from FiiO’s own published power figures — 3.3 dB more than the Susvara needs. Lose 10 dB before it and the same dongle tops out at 0.80 V, well short of 1.73 V, with the volume already at maximum.
The headphone did not get harder to drive. The fix in that case is not a bigger amplifier but either less attenuation before the DAC or more gain after it.
Many amplifiers and dongles have a low and a high gain setting. High gain lets you reach the ceiling with less of the volume range; low gain spends more of the knob on the quiet end, which is where sensitive in-ears live, and keeps the amplifier’s own noise lower. Some makers publish a different maximum output for each setting, and on those the high setting does raise the ceiling. Where a maker publishes a single maximum, the gain switch is about how you get there, not how far you can go.
Related: what digital volume really throws away, when the usable volume range is a sliver, what EQ headroom costs in watts and how a power rating changes with the load.