HD687 (Prototype)

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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.

sound descriptions mine

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Superlux HD687 (Prototype)


NOTE: The production version differs from the prototype version described below.
Modifications therefore also no longer apply.

The Superlux HD687 has a similar build as the HD681-EVO, but that’s where similarities end.

The cable on the HD687 is detachable. It comes with a 3 meter and a 1 meter cable which can also be combined to create an even longer cable. Also a clip is included so you can fix the headphone plug to the cord and can’t pull it out any more. Personally a find the removable cable just fine as it is. It doesn’t come off easily and when the wire gets caught it unplugs itself.

Alas, the cable is microphonic. This means that mechanical contact sounds, when the cable rubs against clothes for instance, are transferred to the cups and can be heard during quiet passages and silence.
This headphone also comes with gold-plated 3.5 to 6.3mm TRS jack connector.
A soft carrying pouch is also included.

Another pleasant addition in the box is an extra set of velours pads. Stock the HD687 comes with pleather pads already mounted.

The headband has a thin layer of rubber on the underside with ‘air filled cushions’.
The shiny surface of the headphone parts are
not fingerprint magnets but scratches will be quite visible. Looks-wise it seems to be targeted at younger people or the classier looking Hi-Fi market though.
People with large heads may not be able to correctly place this headphone properly on the head. Just like the HD681-EVO, HD-662-EVO and HD688 the height adjustment range is too limited.
The height can only be varied 15mm. Most other headphones can be adjusted between 25mm and 60mm. The most common adjustment range is around 30 – 40mm.

There are small L/R markings for the non-visually impaired users, Braille markings are present on the outside.
The impedance is around 40
Ω which is excellent for connecting it to portable equipment.
The efficiency is high enough so it plays decently loud from most portable devices such as mobile phones and small players. It has large 50mm drivers which differ substantially from the drivers found in the HD681-EVO. The dome and voice-coil are a lot bigger (25mm ø vs 18mm ø). The magnet assembly also differs considerably. The centerhole in the magnet is bigger and isn’t driversround. Also the damping of the centerhole (which allows the air behind the dome to flow in and out). The damping on the outer side of the voice-coil also differs a lot. When looking at the driver from the membrane side it looks the same as that of the HD688. Looking at the rear side it is obvious they share the same frame but the damping differs. The paper is different and is much more damped (all holes sealed) compared to the HD688 driver. The reason for this is that the HD687 is ‘semi-open’ and the HD688 cup is closed.

The attenuation from outside noises is poor due to the semi-open design. The difference between semi-open and fully open is that in a fully open system you can clearly see the drivers from the outside and the rear sound is directly sent to the outside where in semi-open the rear sound is coming out though ‘ports’. Outside noises can come in just slightly attenuated. People around you can hear what you are listening to. Just slightly more attenuated compared to a fully open headphone.

The clamping force of this headphone was VERY high. Fortunately this can easily be adjusted by locally bending the rods. A video showing how to do this is found in this instruction video

They are a bit cheap plastic looking/feeling. The steel rods connecting the ear-cups are thinner and can be bent and twisted without things breaking off or able to bend back. cheapish look about it you can bend the headphone in all kinds of positions and it always comes back in the original position (within limits of course).
The headphone seems sturdier than it appears to be based on its looks. I have had reports of headbands breaking off from HD681-EVO’s though which share the same headband and connection to it.

In the end it is always about the comfort and sound. The comfort part is excellent ! NO complaints there from me, but my head is average sized.

Where the HD681-EVO (in stock condition) was extremely bassy and the mids a bit too muddy sounding the HD687 is quite the opposite. The sub-bass is there but not in abundance. The bass is ‘tight’ but sounds slightly ‘pinched/rolled off’ as opposed to the HD681-EVO which sounds overblown, fat and muddy. The mids of the HD687 sound detailed, loose and realistic and have a slight hint of warmth to it. Maybe on the verge of lacking some clarity. The sound is very spacious and ‘open’. No ‘boxed-in’ or ‘cuppy’ sound at all.

The elevated treble gives the headphone overly-detailed and with some music a slight ‘edge’. For longer listening sessions this could result in listening fatigue. Treble quality and also quantity is better than the HD681-EVO. Above 16kHz it rolls off.
The new Superlux driver certainly is a substantial improvement over the ‘older’ drivers that are still used in the current Superlux line-up.

Below the frequency response of the HD687 PROTO TYPE with pleather pads. Left and Right channel.


Channel matching is excellent below 3kHz.

Another important aspect of sound quality is distortion. Below the distortion plot of the HD687 (Right channel)


Distortion in the bass area (below 200Hz) is quite decent (-35dB ~ 1.8%) and mainly 3rd order. This means the driver is over-damped and compresses larger amplitudes. From 300Hz to 6kHz the distortion levels need to be low though as a higher distortion in that range usually doesn’t do much good to the sound quality.
Around 300Hz the distortion (2
nd harmonic) peaks at 0.3%.
The peak between 6kHz and 10kHz is a bit misleading as the treble levels themselves are also higher. -50dB (0.3%) is quite good.

The good news is that Superlux provides both pleather and velour (Superlux calls this velvet) pads with this headphone. This is seldom seen in this price class. Often there are quite some differences in tonal balance between pleather pads and velour pads. Below the differences in frequency response between a stock HD662-EVO with pleather pads and with velvet pads.


Below the CSD (Waterfall plot) of the HD687 with stock pleather pads, left and right are superimposed.

pleather pads on the left                           velvet pads on the right

The CSD is a little ‘messy’ around the treble peak with pleather pads. Very short lived ringing visible at 2.5kHz, 4kHz and around 6kHz.
The CSD of the velvet pads is better with only some short lived ringing around 7.5kHz. The treble peaks are much better damped.
Below the Spectrum plot which is more or less a waterfall plot but looked at from the top where the level is colour coded with the time progressing to the top of the plot. In the CSD the time-scale is 5ms, in the plot below the time scale is 50ms.

pleather pads on the left                           velvet pads on the right

In the pleather pads plot the ringing at 2.5kHz is visible. This is improved with the velvet pads. Below 1kHz the response with the pleather pads is slightly better.

Step response is another way of judging impulse behaviour.
The plot below shows a step response of the HD687 (Right channel) with pleather pads and velvet pads.

The ‘wiggles’ in the pleather plot have a frequency of around 8kHz. It shows that in the first 200 micro seconds, after a pulse is applied, it rings out with an amplitude of around 7dB. In the plot on the right (velvet pads) the amplitude of the first wiggle is smaller and around 5dB. Wiggles in the right plot are damped a bit sooner as well.

The areas that could do with some improvements is the somewhat recessed bass, the clarity could be improved and the treble peak should be addressed to turn this headphone in an even better sounding one.

With some modifications (alas not easy to do) this headphone can be changed into a better sounding HD681-EVO or an excellent sounding headphone that can compete (sonically) with almost Top Of The Line headphones. Comfort wise it still is a budget Superlux headphone.

Below the frequency response of the HD687 with pleather pads. Left and Right channel.


Given the fact that the headphone discussed here is merely a prototype chances are the final version that will be in the store may measure and sound different from this one and may also need other modifications.


The improvement in sound quality of the HD688 PROTO is quite audible.
NOTE that the description of the prototype DIFFERS from the final production version.

Bass is tight but lacking somewhat in ‘body’ and sounds a bit ‘compressed’. Voices and instruments sound very open, spacious and realistic. The overblown treble gives the headphone an overly-detailed and somewhat ‘sharp’
sound. When using velour pads the bass the treble quality is improved.
Tight bass and very clear and wide soundstage (not HD800 wide). Clarity is lacking slightly when compared to other headphones that do have proper clarity.
The treble (highs) are detailed and clear and do not sound rolled-off. There is a slight hint of ‘etchiness’ with bright recordings.

Excellent and fuller, more dynamic, bass response with a small boost. Excellent integration into the mids. The
midrange is exemplary. Clear, neutral and very realistic. Very dynamic and clean sounding. Not a ‘wall of sound’
type of headphone.
There is an excellent clarity with a very ‘lifelike’ treble. No harshness, ‘etchy’ sound anywhere, unless it is in the r
ecording. The sound quality of the mids and treble is better than a stock HD687 fitted with velvet pads.


  • Sound quality when modified !
  • Decent sensitivity, can be driven directly from portable equipment.
  • 2 cable lengths supplied (no microphone nor remote) 3.5mm + 6.3 mm TRS jack.

Less positive:

  • The inside pad diameter is on the small side for some people.
  • Spacing between the driver and ear is too small for some people.
  • Height adjustment is way too little (just 15 mm), people with larger than average head won’t get a good fit.
  • Cable is microphonic (touching the cable is quite audible)
  • Looks and build quality are not on par with the excellent sound quality.

When you are looking for an open and neutral set of headphones and don’t want to spend a fortune and are willing to take the mentioned negatives for granted and they fit you fine then you will probably thoroughly enjoy this new headphone.

In my opinion the sound quality of the stock HD687 is much better than the stock HD681-EVO.
I think Superlux has really taken a considerable step forward again in sound quality.

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