Overview

The Superlux HD 668B is a semi-open circumaural dynamic headphone targeting budget studio monitoring use, priced at 28 USD. First made available by Taiwanese manufacturer Superlux in 2009–2010, it has remained in continuous production for over 16 years. Superlux was founded in 1987 as an OEM supplier to major Japanese electronics brands before establishing its own consumer headphone line in 2008. The HD 668B ships with 1 m and 3 m detachable cables, a 3.5 mm gold-plated plug with a 6.3 mm snap-lock adapter, and a storage pouch. A successor model, the HD 668B MKII, was announced in late 2024.

Scientific Validity

Third-party frequency response measurements using oratory1990’s GRAS 45CA rig yield a deviation of 1.75 dB standard deviation from the Harman over-ear target [4] — an acceptable result for wired headphones. Audio Science Review [2] and DIY-Audio-Heaven [3] independently confirm that the frequency response tracks the Harman target reasonably below 2.5 kHz, but both document significant treble elevation with peak and trough irregularities above that frequency. The manufacturer specifies a frequency response of 10–30,000 Hz [1] without a tolerance qualifier, and THD below 0.3% at 1 kHz/1 V [1].

Bass THD is independently characterized as elevated at 90 dB SPL across both third-party measurement sources [2][3], with a distortion peak around 4–5 kHz confirmed in both. These qualitative findings indicate that bass-region distortion substantially exceeds the mid-range manufacturer specification at typical listening levels. No single numeric THD percentage for the bass region was stated in text form by either third-party reviewer.

The semi-open acoustic architecture provides minimal passive isolation; no calibrated isolation measurement in decibels was available from any third-party source, and the design type inherently limits attenuation. ASR [2] additionally documented significant channel imbalance in the tested unit, indicating unit-to-unit quality control variability.

With bass THD elevated to a degree that multiple independent measurement sources describe, substantial frequency response irregularity above 2.5 kHz, and negligible passive isolation, the overall measured performance is poor.

Technology Level

The HD 668B is Superlux’s own in-house design, which represents the product’s sole positive technical attribute. The core technology — a 50 mm neodymium moving-coil driver in a passive semi-open housing — was thoroughly commoditised well before the product’s 2009–2010 launch; semi-open acoustic construction has been in widespread use since the 1970s.

The “double dome” diaphragm designation lacks technical disclosure or independent verification. The “patented sturdy construction” claim is unverifiable — no patent number is cited in any publicly accessible documentation [1]. The product is entirely passive and analogue with no digital, software, or signal processing integration. No competitive moat exists: the combination of commodity components and standard passive acoustic design can be replicated by any manufacturer without meaningful delay. The 2024 MKII successor introduces ergonomic changes only, with published measurements confirming the same tonal signature, demonstrating no evolution in acoustic performance across product generations. No proprietary technology with independently demonstrated audible benefit has been identified.

Cost-Performance

The CP comparison uses functional category and measured performance values, without treating driver type or enclosure configuration as independent advantages.

The HD 668B is priced at 28 USD [5]. An exhaustive search for wired passive over-ear headphones with confirmed third-party measurements found no product priced below 28 USD with equivalent-or-better measured performance. All sub-28 USD candidates with third-party measurement data were disqualified for inferior frequency response deviation from the Harman target, or substantially elevated THD.

The cheapest clearly non-inferior comparator identified in the same AutoEq/oratory1990 ranking is the Beyerdynamic DT 990 Pro (250 Ohm), a wired passive circumaural studio headphone. AutoEq lists it above the HD 668B with a preference rating of 91 versus 89 and a lower Harman-target deviation of 1.73 dB standard deviation versus 1.75 dB [4]. A current retailer product page lists the DT 990 Pro at 144 USD [6], so the CP calculation is MIN(1.0, 144 ÷ 28) = 1.0.

CP = 1.0 (no cheaper equivalent-or-better product exists)

Reliability & Support

Thomann provides a 3-year warranty on the HD 668B [5], above the typical two-year standard. Replacement earpads — official Superlux EPK681-V compatible variants and third-party alternatives — remain commercially available in 2026 despite the product’s introduction over 16 years prior, reflecting sustained accessory supply.

Multiple user reports document a recurring structural failure: solder connections between the recessed 3.5 mm earcup jack and the internal driver cable break under mechanical stress due to cramped internal wiring, producing intermittent or permanent channel dropout. The detachable cable design mitigates wear at the cable end but does not address this internal connection point. Support is distributor-based: AVLEX handles US repairs in-house; Thomann provides EU retailer warranty coverage. No confirmed global manufacturer service network is documented. No firmware applies to this passive product.

Rationality of Design Philosophy

Superlux’s “Value in Sound” philosophy explicitly prioritises price accessibility. Cost allocation is directed toward functional acoustic components rather than aesthetics or brand premium — a rational allocation approach for the stated positioning [1].

The product employs a fully passive analogue design with no signal processing capability. Measured frequency response irregularities and resonance artefacts above 2.5 kHz are reproducible across multiple independent measurement systems [2][3][4] and have been documented consistently. The 2024 MKII successor introduces ergonomic modifications with the tonal signature preserved in published measurements, indicating that the development direction does not target acoustic performance improvement. Marketing claims of “professional studio quality” and suitability as a “reference monitor” are not supported by available third-party measurement evidence [2][3].

Advice

The HD 668B’s 28 USD price represents genuine measured value: independent third-party data confirms it is the most affordable wired passive headphone achieving its level of frequency response accuracy from the Harman target. This advantage is conditional on use with equalisation.

Out-of-box, bass THD is elevated at normal listening levels, treble above 2.5 kHz diverges substantially from neutral, and passive isolation is negligible. Users with access to parametric EQ can substantially improve tonal balance; this outcome is independently documented by multiple reviewers. Handle the earcup cable connection with care — solder joints at the internal 3.5 mm earcup jack are a documented failure point, and mechanical stress on the earcup socket should be avoided. Budget for earpad replacement; stock vinyl earpads wear under regular use and affordable replacements are widely available.

Users who require flatter out-of-box frequency response, meaningful passive isolation, or higher confidence in long-term mechanical reliability should evaluate alternatives with verified measurements rather than relying on the HD 668B’s price advantage alone.

References

[1] Superlux - HD 668B Official Product Page - https://en.superlux.com.tw/data-194677 - Accessed 2026-06-22

[2] Audio Science Review (Amirm) - Superlux HD668B Review (headphone) - https://www.audiosciencereview.com/forum/index.php?threads/superlux-hd668b-review-headphone.19178/ - Accessed 2026-06-22

[3] DIY-Audio-Heaven - HD668B Measurements - https://diyaudioheaven.wordpress.com/measurements/brand-superlux/hd668b/ - Accessed 2026-06-22; Brüel & Kjær 4153 HATS measurement rig

[4] jaakkopasanen/AutoEq - RANKING.md (oratory1990 measurements) - https://raw.githubusercontent.com/jaakkopasanen/AutoEq/master/results/RANKING.md - Accessed 2026-06-22; GRAS 45CA measurement rig, Harman over-ear target

[5] Thomann - Superlux HD-668 B - https://www.thomannmusic.com/superlux_hd668_b.htm - Accessed 2026-06-22

[6] Thomann - beyerdynamic DT-990 Pro 250 Ohm - https://www.thomannmusic.com/beyerdynamic_dt990pro.htm - Accessed 2026-06-23; listed at 144 USD

(2026.6.28)