The Superlux HD 668B is 56Ω at 98.0 dB (a reference the maker contradicts); the Superlux HD 681 EVO is 32Ω at 98.0 dB/mW. This page is only about what they ask of an amplifier — not how they sound, which no specification on either sheet can tell you.
In amplifier terms these two are the same headphone. The gap between what they need for 100 dB is under half a decibel, which is smaller than the step between two clicks of most volume controls.
| Specification | HD 668B | HD 681 EVO |
|---|---|---|
| Impedance | 56 Ω | 32 Ω |
| Sensitivity | 98.0 dB | 98.0 dB/mW |
| Sensitivity (voltage) | 110.5 dB/V | 112.9 dB/V |
| Power for 100 dB | 1.6 mW | 1.6 mW |
| Voltage for 100 dB | 0.30 V | 0.23 V |
| Current for 100 dB | 5.3 mA | 7.0 mA |
| Highest source impedance that leaves them alone (one-eighth rule) | 7.00 Ω | 4.00 Ω |
| Sources that reach 100 dB out of 280 | 280 | 280 |
Every source in the database that drives one of them drives the other. On power, the choice between these two changes nothing about what amplifier you need.
1 source is low enough in output impedance for the HD 668B under the one-eighth rule but not for the HD 681 EVO (Shanling H5).
1 source stays below the audible-hiss threshold with the HD 668B but crosses it with the HD 681 EVO (Schiit Mjolnir 3).
280 of the 280 sources here reach 100 dB on both. That count is about power only — the output-impedance and hiss lines above are separate questions, and a source can pass this one and fail those.
The gap between these two in decibels does not change with volume — it is a property of the two headphones. What changes is the size of the numbers, and it changes fast: every 10 dB costs ten times the power. The row that matters is the one nearest the level you listen at, and for most people that is nearer the top of this table than the bottom.
The HD 668B is more demanding than 74% of the 567 non-electrostatic models in this database; the HD 681 EVO than 63%. Both figures use the same measure as the rest of the site: whichever runs out first against a 2 V, 100 mW dongle, voltage or power.
| dB SPL | HD 668B power | HD 668B voltage | HD 681 EVO power | HD 681 EVO voltage |
|---|---|---|---|---|
| 80 | 0.02 mW | 30 mV | 0.02 mW | 23 mV |
| 85 | 0.05 mW | 53 mV | 0.05 mW | 40 mV |
| 90 | 0.16 mW | 94 mV | 0.16 mW | 71 mV |
| 95 | 0.50 mW | 0.17 V | 0.50 mW | 0.13 V |
| 100 | 1.6 mW | 0.30 V | 1.6 mW | 0.23 V |
| 105 | 5.0 mW | 0.53 V | 5.0 mW | 0.40 V |
105 dB is included because forum power figures are usually quoted there or higher. It is also a level that damages hearing in minutes — how many minutes is its own calculator.
| Source | Price | Headroom, HD 668B | Headroom, HD 681 EVO | Output impedance | Noise |
|---|---|---|---|---|---|
| JadeAudio JA11 | $20 | 10.3 dB | 12.8 dB | 0.5 Ω | 1.7 µV |
| FiiO KA11 | $29 | 18.6 dB | 21.0 dB | 0.7 Ω | 1.7 µV |
| FiiO BTR11 | $36 | 10.3 dB | 12.8 dB | 0.5 Ω | 8.0 µV |
Of the 280 sources here, 280 reach 100 dB on both. That is the easy axis. 69 of them also publish an output impedance low enough for both under the one-eighth rule and a noise figure that stays below the audible-hiss threshold with both — the rest either fail one of those or never printed the number at all, and 208 of them are in the second group.
The cheapest of the 69 is the JadeAudio JA11, at $20. It leaves 10.3 dB of headroom on the HD 668B and 12.8 dB on the HD 681 EVO, its output impedance is 0.5 Ω against a one-eighth limit of 7.00 Ω and 4.00 Ω, and its noise figure puts the hiss floor 25 dB and 22 dB below the 20 dB SPL this site treats as the point where a quiet room stops hiding it.
Cheapest that satisfies three published numbers is not the same as best, and none of it is about how either headphone sounds. To put either model against a source this page does not mention, or to change the target loudness, use the amplifier matcher.
| 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 |
Thomann's own product datasheet, not the maker's lists 10 Hz – 30 kHz for the HD 668B and 10 – 30,000 Hz for the HD 681 EVO. Both of those figures stand on a shop page rather than a document published by the maker.
Neither range has a tolerance printed beside it, and that is the whole of what this section can tell you. A wider range on one sheet is not deeper bass or more treble on that headphone: without a tolerance, neither number says how loud either model is at the ends of its own range, so the difference between the two is a difference between two pieces of marketing copy. A ±dB figure is what turns a range into a specification, and of the 490 models whose maker prints a range, 58 print it.
None of this is a measurement. What each maker publishes, and what it is worth, is set out in the frequency response audit.
Each model has a page of its own with the full source list, the hiss estimate and the provenance of every figure above: Superlux HD 668B and Superlux HD 681 EVO. Each figure on this page is labelled on those pages with where it came from, including the ones Superlux never published — what that maker publishes, counted.