Sennheiser HD 550: how much amplifier do you need?

The Sennheiser HD 550 is 150Ω with a sensitivity of 98.5 dB/mW, which is 106.7 dB/V. To reach 100 dB SPL — genuinely loud, louder than most people listen — it needs 1.4 mW, or 0.46 V across the drivers.

That puts it in the easy category. A phone, a laptop headphone jack or the cheapest USB dongle will drive it to painful levels. Of the 280 sources in the database, 280 have comfortable 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.

Sennheiser publishes this as 106.7 dB / 1 V RMS (1 V RMS (Sennheiser gives no frequency for this row; the THD row separately cites 1 kHz)) — a figure per volt, not per milliwatt. At 150Ω the two differ by 8.2 dB, so the 98.5 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 8 dB easier to drive than it is — which is roughly the difference between a dongle being enough and not being enough.

Source: support.sennheiser-hearing.com, us.sennheiser-hearing.com, eu.sennheiser-hearing.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 150Ω. “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 106.7 dB/V these are sensitive enough to reveal a source's own noise floor. The noisiest source in the database, the Schiit Vali 2+, would put its hiss at roughly 15 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 150Ω these want a source output impedance below 18.75Ω to satisfy the one-eighth rule. 17 of the 280 sources here miss it, the worst being the Bottlehead Crack (OTL kit) at 120.0Ω. Above that threshold the source starts shaping the frequency response. Check yours here.

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