This page does not review anything Superlux makes. It counts one thing: how much of what this database says about their products came out of their own documents, and how much had to be worked out or left blank.
| Model | Impedance | Sensitivity | Source of the figure |
|---|---|---|---|
| HD 668B | 56 Ω | 98.0 dB | Maker disagrees with itself |
| HD 681 EVO | 32 Ω | 98.0 dB/mW | Published per mW |
Impedance is almost always printed. Sensitivity is the one that goes wrong, because a decibel figure means two very different things depending on whether it is per milliwatt or per volt — about five decibels apart at 300 Ω and eighteen at 16 Ω. The conversion is here.
| Model | As printed | Tolerance | Standard |
|---|---|---|---|
| HD 668B | 10 Hz – 30 kHz | not printed | not printed |
| HD 681 EVO | 10 – 30,000 Hz | not printed | not printed |
Superlux prints a frequency range for both models here whose sheets have been read.
Not one of those ranges comes with a tolerance in decibels. Without a ±dB figure a range says nothing about how loud the headphone is at either end of it, which puts Superlux with the majority: of the 490 models across this database whose maker prints a range, 58 print the tolerance.
Every range stands under the same heading: “Frequency Response”.
2 of these figures stand on a shop listing rather than on anything Superlux published; those model pages name the shop.
No measurement is published here — only what was printed. The whole count, maker by maker, is in the frequency response audit.
The hardest of the 2 to drive is the Superlux HD 668B: 1.6 mW and 0.30 V for a 100 dB SPL peak. Their impedances run from 32 to 56 Ω, which matters because the voltage a headphone wants rises with impedance while the power falls.
280 of the 280 sources here reach 100 dB on all 2, and the cheapest of them is the Apple USB-C dongle (EU) at $9. “Reaches” means the maker’s own published power figure is enough at that load; it is not a judgement of how it sounds.
Their lowest impedance is the Superlux HD 681 EVO at 32 Ω, so a source wants an output impedance at or below 4.0 Ω to satisfy the one-eighth rule on everything they make. 134 of the 169 sources that publish an output impedance manage that.
These counts use figures the source makers published themselves. A product that publishes nothing is not counted as a pass anywhere above, which is why the three numbers differ so much. The collection matcher runs the same arithmetic over whatever you actually own.