Amplifier Power Across Impedance

Every amplifier is advertised with power at one load — “1 W into 32Ω” — and that number tells you almost nothing about what it does with your headphones. This works out the rest of the curve, and shows you where the amplifier stops being limited by voltage and starts being limited by current.

Why one number is not enough

An amplifier has two ceilings. It can only swing so many volts, and it can only push so many milliamps. Which one you hit first depends entirely on the headphone you plug in.

Into a high impedance load the current stays small, so the voltage ceiling decides everything — and power falls as impedance rises. Into a low impedance load the voltage needed is tiny but the current is not, so the current ceiling takes over — and power stops rising, or even falls. The impedance where the two ceilings meet is the knee, and it is one of the most useful numbers about an amplifier — and one almost nobody prints on the box.

It is also why one advertised figure tells you so little. A dongle rated 250 mW into 32Ω is holding about 2.8 Vrms, and that voltage into 300Ω is 27 mW — four times what the hardest 300Ω headphone in this database asks for at a 100 dB peak. A dongle rated 30 mW into 32Ω is holding 0.98 V, which is 3 mW into 300Ω, and that one really does run out. Same headline unit, opposite answer — and the load a maker chooses to quote is usually the one that flatters the product.

Once you know the power available, the power calculator tells you how much your headphones need, and the matcher does both at once across every source in the database.

How many volts each maker’s own figure implies

A power rating is a voltage rating in disguise: V = √(P × Z). Run that on the 240 of the 280 sources here whose maker publishes a power figure at a stated load — not on a figure this site derived, only on their own — and you get the number that actually decides what they will drive. Sources with no published figure at any load are not on this list.

The median is 3.3 V. The most voltage-hungry headphone in this database, the HiFiMAN Susvara at 60 Ω, needs 1.73 V for a 100 dB SPL peak. So the median source on this list has 1.9 times the voltage that the hardest headphone in the catalogue asks for, 192 of the 240 clear two volts, and 35 do not reach the 1.73 V the HiFiMAN Susvara wants — a statement about volts alone, which is not the same as driving it. That is the shape of the problem this site exists to describe: the question is almost never whether there are enough volts.

The highest figure belongs to the SPL Phonitor se at 35.4 V, from its own rating into 250 Ω; the lowest to the Sony NW-A306 at 0.75 V from its rating into 16 Ω.

Read the third column before the fourth. A figure of thirty volts derived from a rating into 600 Ω does not mean the amplifier swings thirty volts into 16 Ω — at low impedance the limit is current, and some makers publish numbers from which their own voltage can be seen to sag as the load drops. Where that happens the product page says so. This column is the voltage implied at the load the maker chose to quote, which is the most favourable reading available and still, for 35 products out of 240, not enough for the hardest headphone here.

Which load makers choose to quote

246 of the 280 sources here publish at least one power figure. 104 of them publish exactly one, and 76 of those single figures are at 32Ω. That is the whole spec sheet for more than a third of this catalogue: one number, at the load that makes a headphone amplifier look strongest.

Read the last column. 32Ω is not just the most quoted load — it is overwhelmingly the load chosen by makers who quote only one. 300Ω appears 97 times in total, and as a maker's only figure once. Nobody sells an amplifier on the number that flatters it least.

The same headline, three times the amplifier

Here is what that costs you. These are the products in this database rated between 200 and 300 mW into 32Ω that also publish their own figure at 300Ω — both numbers from the maker, nothing derived:

Near-identical headline figures, and 3.0 times the power at the load where a 300Ω headphone actually lives — 4.7 dB between the ends of that list. Every one of these makers measured both, and the second number does not follow from the first. If you own something high-impedance, the headline figure is not conservative and not optimistic. It is unrelated.

And that is only visible because these seven publish both. For the 104 products that publish one figure, everything on their page past that one load is this site holding the published voltage steady and working the rest out — labelled as derived, load by load, because it is a model of the amplifier rather than a measurement of it.