Grado SR80 on a Chord Mojo 2: what $650 actually buys when 2.1 V was already enough
The SR80 needs 0.776 V. Mojo 2 publishes 5.2 V at 300 Ω and 4.24 V at 30 Ω — 126.5 or 124.7 dB depending on which point applies. Neither number is the reason to buy it: the 0.06 Ω output impedance gives a damping ratio of 633 where the rule asks for 8.
A $175 open-back headphone that needs 15.85 mW, plugged into a $650 portable DAC. The level question was settled two hundred dollars ago; the interesting number is somewhere else.
The SR80 is 38 Ω at a clearly labelled 98 dB/mW — 112.20 dB/V — and needs 0.776 V for a 110 dB peak. Chord publishes 5.2 V RMS at 300 Ω for the Mojo 2, which puts the SR80 at 126.5 dB SPL, +16.5 dB. That is more headroom than any listening situation can use, and a plain DragonFly-class dongle at 2.1 V already delivered +8.6 dB.
The pearl: Chord publishes two operating points, and together they show that "maximum output" is not one number. The spec line reads 600 mW into 30 Ω (4.2 V RMS) and 90 mW into 300 Ω (5.2 V RMS). The higher voltage appears at the higher impedance — which looks backwards until you see what limits each case: into 30 Ω the amplifier runs out of current, into 300 Ω it runs out of volts. For a 38 Ω load the 30 Ω figure is the relevant one, so the honest reading is 4.24 V and 124.7 dB, +14.7 dB. Every single-number "max output" spec on any other product page hides which of these two you are being sold.
What the money buys, in one figure: 0.06 Ω. Against the SR80's 38 Ω that is a damping ratio of 633, where the 1/8 rule asks for 8 — a factor of 79 in hand. It is worth putting that next to the Audient EVO 8's published <50 Ω, where the same 38-ohm-class headphone lands at a ratio of 0.8 and the divider eats several decibels. Output impedance is the specification that separates these two devices; the volts do not.
The rest of the chain. The iPhone 16 Pro connects over USB-C, and the Mojo 2 runs on its own battery, so the phone's 4.5 W port carries data only and the level above does not depend on the handset. The SR80's 3.5 mm plug goes straight in. One caveat on the headphone side: Grado's store page lists this model as SR80 with a 20 Hz–20 kHz response, which does not match the SR80x datasheet — the numbers here come from the page we cite, so check your own box before reusing them.