Headphone Impedance and How Much Power You Need

Every headphone amplifier publishes its power into 32 ohms, because that is where the number looks biggest. Plug in a 300-ohm or 600-ohm headphone and the published figure falls by an order of magnitude — and the manufacturers who publish that second figure are telling you something the ones who do not are hiding.

The published figures, side by side

All FiiO figures are published at THD+N below 1%, which is the condition — and a maximum-output condition rather than a listening one. iFi is the only maker here that publishes a 600-ohm figure at all, and it is the one that answers the “will this drive my old studio headphones” question.

Why the number collapses

An amplifier delivers power into a load, and how much depends on whether it runs out of voltage or out of current first. Into a low impedance it needs current; into a high one it needs voltage. Desktop amplifiers run from modest supply rails, so voltage is what they run out of.

The other impedance number: the amplifier’s own

Output impedance is the amplifier’s internal resistance in series with your headphone, and it does something power figures do not describe.

A headphone’s impedance is not flat — it peaks around the driver’s resonance, often substantially. Put a significant output impedance in series with that and the divider between the two changes with frequency, so the response changes with frequency. The audible result is usually a lift where the headphone’s impedance peaks.

The convention is to keep the amplifier’s output impedance below about one eighth of the headphone’s. Almost nobody on this page publishes a headphone output impedance — Topping publishes 20 Ω for the E30 II’s line output, which is a different thing, and WiiM publishes none for the Pro Plus at all. That is a genuine gap in the category, and it is worth knowing that it is a gap rather than assuming a low figure.

Matching, practically

• Low impedance, high sensitivity (most consumer and IEM headphones). Everything here has far more power than you need, and the risk is hiss rather than insufficient volume. A high-output amplifier on a sensitive IEM can be audibly noisy at low volume.
• 250–300 Ω (classic studio headphones). This is where the second column matters. The FiiO K7 publishes 140 mW into 300 Ω single-ended; the K11 publishes 60 mW single-ended and 250 mW balanced. The balanced output is doing real work here.
• 600 Ω.Only iFi publishes a figure, and it is 18 mW. Whether that is enough depends entirely on the headphone’s sensitivity, which the headphone manufacturer publishes and we do not cover.

What balanced actually buys

Both FiiO units publish substantially more power on the 4.4 mm balanced output than the 6.35 mm single-ended one: 2,000 mW against 1,220 mW on the K7, and 1,400 mW against 520 mW on the K11.

The mechanism is voltage. A balanced output drives both sides of the headphone with opposite polarity, doubling the voltage swing across it, and power scales with the square of voltage. That is why the gain is roughly fourfold on the K11 rather than twofold — and it is the most useful thing balanced connection does at this price. Noise-rejection arguments are real in a studio with long cable runs and largely irrelevant on a desk.

It also requires a headphone cable terminated in 4.4 mm, which is an extra purchase and sometimes not available for a given headphone at all.

Which one to buy

FiiO K7 if you own or might own high-impedance headphones. It publishes the most power at both impedances, publishes the 300-ohm figure, and has three output modes including a pure DAC mode so it can also feed a speaker system.

FiiO K11for the same job at less money, provided you use the balanced output for high-impedance headphones — 250 mW balanced against 60 mW single-ended into 300 Ω is the difference between adequate and marginal.

iFi ZEN DAC 3 if 600-ohm headphones are the question, because it is the only unit here whose maker answers it. Note that iFi publishes no output voltage figure at all, and does not name its Burr Brown DAC part number.

WiiM Ultraif the headphone output is a convenience rather than the point. It publishes a TI TPA6120A2 with THD+N of −99 dB into 300 Ω falling to −92 dB into 32 Ω — note the direction: it does better into the higher impedance, which is unusual and worth knowing.

For the roundup version of this comparison, including which of these doubles as a preamplifier, see the DAC and amp combos page. If the headphone output is the secondary job and the box also has to be your preamp, the all-in-one DAC, preamp and headphone amp roundup ranks a wider field on inputs and published output voltage, including two units that publish power at three impedances rather than two.