AKG K240 MKII: how much amplifier do you need?

The AKG K240 MKII is 55Ω with a sensitivity of 91.4 dB/mW, which is 104.0 dB/V. To reach 100 dB SPL — genuinely loud, louder than most people listen — it needs 7.2 mW, or 0.63 V across the drivers.

That is a moderate load. Most USB dongles handle it without complaint; the weakest phone outputs will run out of volume before they run out of quality. 276 of 280 sources here have enough headroom. The cheapest portable source that does the job is the Apple USB-C dongle (US) at $9.

Sensitivity here is in dB per milliwatt. Manufacturers often quote dB per volt instead and leave the unit off, which is the single most common reason online calculators give answers that are 10–15 dB wrong. The converter explains the difference.

AKG publishes this as 104 dB SPL/V (1 V at 1 kHz (AKG heads the column 'Sensitivity (dB SPL/V @ 1 kHz)')) — a figure per volt, not per milliwatt. At 55Ω the two differ by 12.6 dB, so the 91.4 dB/mW above is that published number converted, not a different measurement.

This is the conversion that online calculators skip. Quote the per-volt number as if it were per-milliwatt and this headphone looks 13 dB easier to drive than it is — which is roughly the difference between a dongle being enough and not being enough.

Source: akg.com. How this data was built.

Most people listen between 75 and 85 dB. The 100 dB row is the one worth planning around: it gives you headroom for dynamic peaks without designing your system for a volume that would damage your hearing in under fifteen minutes. The safe listening calculator puts numbers on that.

Every source in the database, judged at 100 dB SPL into 55Ω. “Headroom” is how much power is left over above what the headphone needs — 10 dB means ten times the required power, which is what you want for dynamic peaks.

Hiss. At 104.0 dB/V these are sensitive enough to reveal a source's own noise floor. The noisiest source in the database, the Schiit Mjolnir 3, would put its hiss at roughly 12 dB SPL in these headphones — clearly audible in quiet passages. The hiss calculator lets you check your own amplifier's noise figure.

Output impedance. At 55Ω these want a source output impedance below 6.88Ω to satisfy the one-eighth rule. Of the 192 sources that publish one, 34 miss it, the worst being the Bottlehead Crack (OTL kit) at 120.0Ω; the other 88 publish no output impedance at all, so they cannot be checked either way. Above that threshold the source starts shaping the frequency response. Check yours here.

Worked out in advance against its own stable: vs K361 · vs K371 · vs K52 · vs K612 Pro · vs K872 · vs K92. Each of those pages counts the sources that drive one and not the other.

Or open the comparison tool to put any two models side by side, or browse all 578. Or see every AKG headphone in the database, and how much of what this site says about them came from AKG’s own documents.