The Superlux HD 681 are very similar to the HD 668B in shape and sound. They're a bit more comfortable thanks to the different headband design that's not as tight on the head. They also sound better than a lot of pricier headphones, but they're not very versatile. They're cumbersome, they don't block a lot of noise and leak quite a bit so they're best used in isolation.
The Superlux HD 681 have the same ear cups as the HD 668B but a more comfortable headband. The strap design of the headband adjusts better to different head sizes and shapes compared to the previous model. However, they're less stable than the HD 668 since they're not as tight and do not have a detachable audio cable. They're also quite bulky and cumbersome to carry on your person if you don't have a bag.
The Superlux HD 681 look very similar to the HD 668B. They have practically the same ear cups but a different headband design. This makes the HD 681 look even more like the AKG K240 MKII. The color scheme is pretty bland but this model stands out a bit more compared to the HD668 thanks to the red accents on the ear cups and headband. Unfortunately, their plasticky build quality makes them look cheap and not durable.
The Superlux HD 681 are more comfortable than the HD 668. They're just as lightweight and have a better headband design that doesn't feel as tight on the head and extends a bit further than that of the HD 668. They also have large ear cups that fit well around most listeners' ears. Unfortunately, the padding on the ear cup feels a bit stiff and may cause a bit of uneven pressure. It's not particularly uncomfortable but it is noticeable after wearing them for a while.
The Superlux HD 681 like the HD 668B are cheaply made and plasticky. They feel like a budget version of the AKG K240, which shows in their build quality. They have a plastic design with no metal parts, and all the materials used in their build quality feel slightly low grade. They won't break if they accidentally fall once or twice, but they don't feel like durable headphones.
The HD681 are not as tight on the head as the Superlux HD 668B and are slightly less stable overall. They won't be the best for sports since they have big ear cups that sway during physical activity but they don't move much during casual listening sessions. Unfortunately, this model does not have a detachable cable so the headphones will get yanked off your head if the cable gets hooked by something.
The Superlux HD 681 are a good sounding pair of semi-open over-ear headphones. They have a nearly flawless Bass, an excellent Mid Range and a decent Treble. They also perform quite consistently across multiple humans, but if you wear glasses there's a chance you get slightly less Bass compared to those who don't wear glasses. Additionally, they have an average Soundstage and image decently, but their mediocre distortion could make their Bass and Treble slightly harsh under heavy loads.
Excellent Bass Range performance. Low-frequency extension is at 14Hz, which is great. The entire Bass response is virtually flat and within 1dB of our target.
Very good Mid Range performance. Low-mid and Mid are virtually flat, but under our target by about 2dB. This pushes vocals/leads slightly to the back of the mix and giving more emphasis to Bass and Treble Ranges. High-mid shows a small tilt favoring higher frequencies, but the effect will be very subtle.
Decent Treble Range performance. Low-treble is well-balanced but slightly inconsistent. Treble has a 10dB bump between 7KHz and 12KHz which makes the sound of these headphones noticeably sharp and sibilant.
Good Consistency performance. The Bass Range performance is very consistent across our human subjects, except for our subject who wears glasses as he gets 3dB less Bass than others at 20Hz. The Treble Range consistency is also very good, especially below 10KHz.
Mediocre Harmonic Distortion performance. The amount of harmonic distortion in the Bass Range is high, especially at 100dB SPL. This could make the Bass of these headphones slightly unclear. The peak in distortion at around 9KHz will have a negative effect on the Treble Range by making it noticeably harsh sounding.
The HD 681 like the HD 668B are not designed to block ambient noise. They have semi-open ear cups that create a more immersive Soundstage but also leaks and lets ambient noise seep into your audio. This means the HD 681 won't be the ideal headphones for commuting or to use at the office since you will struggle to hear your music in noisy environments and the level of leakage will be too distracting to the people around you in quieter settings.
Poor Isolation performance. Due to the semi-open back of the HD 681, they don't isolate at all in the Bass and Mid Ranges. In the Treble Range they reduce the outside noise by about 20dB, which is decent.
Poor Leakage performance. The significant portion of Leakage on these semi-open back headphones sits between 500Hz and 20KHz which is a broad range. Additionally, their overall level of Leakage is also relatively loud.
No active features.
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Wired connection negligible latency.