Superlux HD 668B: how much amplifier do you need?

The Superlux HD 668B is 56Ω. Superlux prints a sensitivity of 98.0 dB against a reference its own figures contradict, so this page neither converts it nor pretends the unit is settled. Read as per milliwatt it needs 1.6 mW to reach 100 dB SPL, or 0.30 V across the drivers; the provenance note below sets out the contradiction.

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, 274 have at least 10 dB of headroom and another 6 have enough. 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.

Superlux publishes 98 dB SPL (1 W), and that reference cannot be right: converting it gives a figure no headphone of this kind could have. Some makers print a watt where they mean a milliwatt. Rather than silently pick whichever reading looks plausible, this page reports the contradiction and leaves the figure above as it was.

Source: images.thomann.de, en.superlux.com.tw. 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 56Ω. “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 110.5 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 19 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 56Ω these want a source output impedance below 7.00Ω 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 HD 681 EVO. 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 Superlux headphone in the database, and how much of what this site says about them came from Superlux’s own documents.