HD688 (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 HD688 (Prototype)


The Superlux HD688 has a similar appearance as the HD662-EVO.

The cable on the HD688 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 HD688 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 and HD-662-EVO 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 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 HD662-EVO. drivers
The dome and voice-coil are a lot bigger (25mm ø vs 18mm ø). Also this driver has a duofoil membrane. The dome is thicker (and more rigid) than the outer part of the membrane. The magnet assembly also differs considerably. The centerhole in the magnet is bigger and isn’t round. 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. One hole is open and a few others are sealed with foam. It looks like Superlux did a decent tuning job here.

The efficiency is somewhat lower than that of the HD662-EVO so it plays slightly softer on the same volume setting, that is when comparing both after modifications directly. It plays loud enough from most portable devices such as mobile phones and small players.

The attenuation from outside noises is decent. The HD662 is fully closed. Still some outside noises leak through via the pads so there is no total isolation.
The clamping force of this prototype was uncomfortably high but can easily be lowered though as can be seen on 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.

The sound is ‘impressive’ on first listen. There isn’t much wrong with it at all in stock form already. At least .. with velour pads fitted.

Where the HD662-EVO (in stock condition) was bassy and the mids a bit too ‘forward’ sounding the HD688 is somewhat less elevated in the bass and has much more linear (read realistic) mids.

The subbass is there so great for watching movies. The bass is tight and just slightly elevated (depends on the used pads),. The mids are better integrated into the lows, even though the HD688 still has a little dip around 200Hz the mids and bass do sound much better ‘integrated’ where the bass in the stock HD662-EVO has a somewhat ‘dis-attached’ bass. Mids are also highly detailed with a lot more ‘realism’ than the HD662-EVO. The sound is very spacious and ‘open’. No ‘boxed-in’ or ‘cuppy’ sound at all.

The somewhat 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 HD662-EVO. With the HD662-EVO you can hear treble was not top notch quality, the HD688 certainly improves on that part.
The new Superlux driver certainly is a substantial improvement.

Below the frequency response of the HD688 with pleather pads. Left and Right channel.



Below the difference between pleather pads and the supplied velour pads.


Bass and subbass levels are about 3dB lower and more linear with velour pads. From 300Hz to 7kHz the response is much more linear. Flat within a few dB. The treble peak is also drastically reduced.

Those looking for bigger bass and feisty treble may be well off using the pleather pads. Those looking for a far more linear (with still slightly raised) bass and excellent mids and treble quality may well prefer the velour pads.

Personally I find the frequency response (read tonal balance) of the stock HD688 fitted with velour pads VERY close to my personal taste. Tastefully and not exaggerated bass, excellent and very accurate (read realistic) voices and instruments and clear but not elevated treble with a very high quality.

Distortion plot of the left channel is shown below.

Distortion in the bass area (up to 200Hz) is not very objectionable in general. From 300Hz to 6kHz the distortion levels need to be low though as a higher distortion there usually doesn’t do much good to the sound quality. The HD662-EVO (on the right) has higher distortion between 800Hz and 2kHz and a very high peak at 4kHz reaching 2%. These are not present in the HD688 resulting in much cleaner (< 0.1% in that band) and clearer (not as in clarity but as in resolution) voices and instruments. The peak between 6kHz and 10kHz is a bit misleading as the treble levels themselves are also higher. -50dB for the HD688 and -45dB for the HD662-EVO also shows a lower distortion level.

The CSD plots of the stock HD688 fitted with pleather pads is shown below, Left and Right channel are superimposed.


The CSD is a little ‘messy’ around the treble peak. Very short lived ringing visible at 3.5kHz and around 7kHz.

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.


Of course this headphone has gone through extensive measurements and tests to see if it can be improved a bit more.
The modifications for it are relatively easy to do and don’t require knowledge about modifying it nor about electronics. In order to get to the full potential of this headphone one is required to open it up. This isn’t difficult to do though.

After these modifications the sound signature has improved a bit more.

The improvement in sound quality is audible.

from: Bass is audibly elevated but does not bleed into the mids and has a good ‘cohesion’ with the mids. Voices and
instruments sound very open, spacious and realistic. The somewhat elevated treble (pleather pads) gives the headphone a highly detailed and somewhat ‘sharp-ish’ sound. When using velour pads the bass is still somewhat elevated but more tastefully. The mids are extremely neutral. Tight bass and very clear and wide soundstage (not HD800 wide). Excellent clarity.
The treble (highs) are detailed and clear and do not sound rolled-off. There is a slight hint of ‘etchiness’ with bright recordings. Quite enjoyable and there is no listening fatigue.

to: Excellent bass response with a small boost that does not ‘bleed’ 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 recording. The sound quality of the mids and treble is a notch (small step) better than a stock HD688 fitted with

velour pads.

Below the frequency response of the modified HD688, Left, Right


Much more realistic and linear sounding but without the subbass boost, still with excellent subbass reproduction and extension when the recording has it.

The plot below show the differences between the stock HD688 and the modified version.


Also the distortion has improved. Especially between 300Hz and 10kHz. The distortion levels are much lower than that of the HD662-EVO.


Below the CSD of the modified version, Left and Right channel are superimposed.csd-modif-hd688

Looks a lot cleaner overall.

Improvements can also be seen in the spectrum plot below.


Instead of the squarewave plots and impulse response plots this time I will show a step response. Below the step response of the stock HD688 with pleather pads.


The vertical scale is 5dB/div. the horizontal scale is 1ms/div. It is a bit akin an oscilloscope shot of a step response. The ‘target’ is the top line after the initial rise the line should run in a straight horizontal line from there. The rising edge should reach the top line in a short as possible time. The wiggles after the rising edge should be as small as possible in amplitude. The time it takes for the wiggles to ‘die out’ should be as short as possible.

The ‘wiggles’ in this plot have a frequency of around 8kHz. It shows that in the first 200 micro seconds after a pulse is delivered it rings out with an amplitude of around 5dB.
After 1ms the ringing is already almost irrelevant.

Below the step response plot of the modified HD688 with velour pads.


It is easy to see the ringing is much less and shorter lived. Also the step looks more like a real step. This is a really good performance here. It shows the driver is well damped and has great speed and linear frequency response.

A modification and more elaborate evaluation article can be downloaded HERE

Sound quality.
Good sensitivity, can be driven directly from portable equipment.
Decent isolation from outside noises.
Low sound leakage, other people won’t be bothered by your music.
2 cable lengths supplied (no microphone nor remote) 3.5mm + 6.3 mm TRS jack.

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 15m), 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 a closed 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 HD688 is better than the HD662-EVO. The HD662-EVO and HD688 share the same ‘enclosure’, they just differ in the driver and most likely price as well.
I think Superlux has really taken a considerable step forward again in sound quality.

What I (and other hi-fi loving people) would like to see: A different, more luxurious, build quality with more adjustability and a non-microphonic and suppler cable. Somewhat thicker pads and more room for the ears will also be a welcome change.

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