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Published: Sept-7-2013, updated: Dec-2-2015

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NO SMOOTHING is applied to the shown plots. Most measurement sites have some smoothing applied which ‘irons flat’ sharp peaks and ‘wiggles’. I do not use smoothing because some info about sound quality is lost when plots are smoothed.

Aside from a small correction of the microphone itself also some correction in the lowest frequencies is applied to the plots to compensate for the perceived loss of bass when using headphones. This is described HERE in more detail.
A ‘horizontal‘ frequency response curve on the shown frequency response plots on this website thus indicates a perceived ‘flat’ tonal signature.

ALL measurements are made with a good SEAL on a flatbed measurement rig.
shape of your head, bone structure, pad size, pad ‘softness,  (compliance), hair or no hair and or wearing glasses may (drastically) change the frequency response of some headphones, so… your personal experience may differ substantially from these plots.

Frequency response (tonal balance) is the most sound-determining aspect of headphones. A horizontal line shows audible neutral response in the plots on this website. Deviations in different severities at different frequency bands have an effect on the sound character.
The bigger the deviation the stronger the effect.

Below an aid to help determining the sound character of headphones with relation to the frequency response.


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Superlux HD681-EVO


The Superlux HD681-EVO is twice as expensive as the ‘original’ HD681 shown above this part. It has better better looks but the older version ‘sits’ better on my head. The pads are slightly thicker and slightly ‘softer’ feeling but both are ‘thick’ pleather and can get hot/sweaty pretty quick. Sonically they differ a LOT. Where the HD681 did not sound ‘dark’ this one does. Where the old version had piercing highs (stock) this one doesn’t. Where the original one had good bass but not overpowering ‘warmth’ this one sounds a bit ‘overly warm’ I would even dare to say muffled with big bass. Those that only want to hear bass and no crisp mids and clear highs might like the stock EVO.  My preferences lie elsewhere (in a flatter and more realistic presentation).

Stock Frequency range:

hd681 EVO stock (L+R)Bigg bass, overly warm mids and no piercing highs. Sharp ‘dip’ around 4kHz. The dip being narrow and a dip instead of a peak makes it inaudible.

Below the watrefall plot (CSD)

waterfall evo stockThe CSD shows some slight ringing around 2.5kHz and 5kHz. No idea whether that is a coincidence or not seeing as they are ‘harmonics’.

A modification article is in the future and being worked on. For those looking for a quick ‘fix’ by changing pads to Fostex T50RP ones and removing the felt that is glued in front of the driver could end up with the headphone plot below:

hd681-EVOwith T50RP pads

Hello mids, hello silky and present highs, hello bass and not overly warm sounding but full and fun. The pads are considerably less thick and have about the same ‘feel’ as the original pads. Alas they aren’t velour ones but pleather (vinyl)

The article about modifications of the HD681-EVO can be found HERE. An electronic filter and modifications are discussed there.
After modifications you can end up with the result below. All it takes is just a few components and a set of Shure SRH-940 pads. The roadmap how to get there is discussed in THIS ARTICLE.

end result 940 pads l+r

With some pads bought from e-bay the enormous bass levels are reduced in a big way and makes a very good sounding headphone.

There is also a Kameleon filter module for this headphone when used stock.
Difference between stock and via Kameleon is shown below.


This headphone can also be fitted with cheap Chinese aftermarket pads intended for the K240 with results as shown below. Left Right

l-r evo chinese pads

Sounds excellent this way, unless you like really big bass in which case the stock HD681-EVO may be more interesting.

Below the distortion plot of this headphone fitted with the Chinese ‘Wang Yifei’ pads. Just like the original HD681 lots of 2nd and 3rd harmonic distortion in the lows and a peak around 5kHz.

DIST HD681EVO chinese pads

Below the 40Hz squarewave plots of the (modified) HD681. It shows a less bass response but still not bad given this is the version with the Chinese pads. With original pads it would look quite different. You can see the ringing already…

40Hz SQR HD681-EVO

Below the 440Hz squarewave plot. It shows sharp rising and falling edges with quite some overshoot. Also there is substantial ringing visible and it is at a lower and more audible frequency than the HD681.

440Hz SQR HD681-EVO

And finally the 100μs needle pulse. Overshoots the target line but shows quite some resonating effects afterwards at around 5kHz. Some of this is also visible in the CSD. Note that this is the modified version.

100us HD681-EVO

The HD681-EVO, once modified, sound quite well. Without mods it is a very bassy headphone with a dark signature.
Not ‘sharp’ in the treble like a stock HD681 and has better pads as well.It is twice as expensive as the original though.

One must take into account that longevity of the little cable ‘dongle’ on the left ear cup is not so great. The cable has a tendence to break rather sooner or later. (see below)broken wire SL
This is something to consider when buying this headphone.

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