back to Koss
back to measurements

post separation

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.


post separation
Koss KSC75


The Koss KSC-75 is the cheaper (half the price) brother of the KSC-35. It has a different (cheaper) look but a much better earclip. The earclip is a bit thicker but is made of silicon rubber with a metal wire in it that, unlike the KSC35, can be bent in order to adjust the fit and clamping force on the ear. The frequency plot of a stock KSC75 earphone is shown below. Just like the KSC35 very smooth from 30Hz to 3kHz but a bit elevated in the bass area and dropping off above 13kHz. The sharp dip at 7 kHz the KSC35 shows isn’t there. This might have something to do with the titanium flashed coating or the membrane. Some channel imbalance above 5kHz. left, right

KSC75 stock

The CSD of the KSC75 (shown below), however, is not as good as that of the KSC35 and shows more ‘ringing’ between 2kHz and 5kHz. The KSC35 is simply somewhat ‘cleaner‘ in the treble area while the frequency range of the KSC75 is somewhat better as a trade-off.

ksc75 waterfall
This headphone is also often ‘coin modded‘. The difference between a stock and coin modded KSC75 is shown below. The coin mod is discussed above in the KSC35 section.

KSC75 L QM (green)

Of course it would be interesting to see the differences between the KSC35, KSC75 and PortaPro in one plot. What is obvious is that there are more similarities than differences from 20Hz to 2kHz. These drivers all behave differently above that point and the green plot of the KSC75 shows what the titanium coating brings. Both the KSC35 and PortaPro (as well as SportaPro) have untreated membranes.

PortaPro (bl) 75 (green) 35 (pur)
When some smoothing is applied (most plots on other websites are smoothed) the sharp dips and peaks are ‘removed’ and the similarities between the PortaPro and KSC35 are more obvious. 1/6 octave smoothing is applied. The sharp peaks and dips in the plot above do, however, add some value and generally the bigger the peaks and dips the ‘coarser’ sounding the highs will be.

PortaPro (bl) 75 (green) 35 (pur)1-6 smoothed
Both the KSC35, PortaPro as well as the cheaper KSC75 are excellent earphones and I even dare to state that the KSC75 is excellent value and very enjoyable. a HUGE step up from any ‘stock’ earbud supplied with portable players. Highly recommended for on the go. When jogging or riding a bicycle both have some problems with ‘wind’ going past them which drowns the sound when playing softly. The KSC75 can be easily adjusted to the ears and do not get in the way with glasses resting on the ear. The lack of pressure on the ear guarantees you can wear these for LONG periods. It cannot play loud very well though and starts to distort audibly at higher levels.

Grille removal effect and Kramer mod measured.

Some owners remove the grille all together and replace it with  metal mesh (so you cannot touch the fragile membrane).
The question is if replacing the original grille with an open mesh is a smart thing to do. One would think Koss engineers would have done so instead of creating a grille with a certain cut-out pattern in it.

First up the driver with the grille removed. Quarter modded foam in front of it versus stock KSC75 with quarter modded foam pad.  The part below 1kHz is lowered because less pressure can be exerted on the driver against the rig. If more pressure would be used the foam will be touching the driver. More pressure results in relatively less lows (the part below 1kHz)

KSC75 stock QM versus no grille QMAs you can see the lows are a lot less present and we see a peak around 4kHz where there used to be a slight dip. There is more treble extension though. A +10dB peak around 4kHz isn’t desirable. The lack of lows is (partly ?) caused by the fact that the driver is now closer to the ear.  A very open metal grille may prevent that loss in bass in which case the drop in lows  may in fact not be there. The peak around 4kHz will stay. This would basically mean the KSC75 would sound considerably brighter. This mod may have better results with the KSC35 / PortaPro as those drivers (no titanium coating) have much less treble energy.  Haven’t tested that.

Most people will have heard of the Kramer mod. This involves removing the plastic grille in front of the driver and drilling more holes in them.


On the left a ‘Kramer modded grille, on the right original grille, measurements below.
It should be noted that differences below 1kHz may also come from slightly different pressure exerted on the earpieces. As can be seen in the PortaPro section the differences seem to mainly address the extension in the highs which is also observed in the plot below.

stock KSC75 QMvs kramer mod QM
The improvement is more or less inbetween the stock and no-grille configuration. You get a little more clarity and slightly less lows (could also be because of slightly more compressed pads)  and more highs extension.  Kramer does recommend to use thicker pads in which case the driver is somewhat further from ear resulting in relatively more lows. This is shown below.  Stock KSC75 versus Kramer modded KSC75 with slightly thicker foam pads.

KSC stock vs kramer mod thicker padThe result is slightly warmer signature with a tad less energy around 3kHz and slightly more extended in the highs (air around instruments).

The Kramer mod (with original foam pads) may be a very good idea for KSC35 and PortaPro owners as you get more clarity in return. For the KSC75, which by itself has more ‘presence’ than the KSC35/PortaPro may best be left alone or have the Kramer mod done and some thicker (more comfortable ?) pads used.


The KSC75 has a varying impedance. This results in a different frequency response when driven with a low (0.5Ω) output resistance amplifier and high (120Ω) resistance amplifier. The 2 plots have been level matched (120Ω = 9.1 dB raised) to show the differences in tonal balance.120 Ohm 9.1 dB
The bass levels are raised when the KSC75 is connected to a higher output resistance amplifier. When connected to a 10Ω output resistance amplifier the ‘boost’ will be well below 1dB so probably inaudible.

newer version

Recently (sept 2018) I got 2 recently manufactured KSC75’s in and measured (and listened) to them. Well, one of them is the KSC75 and the other is teh Massdrop KSC7X.
It looks like the newer versions behave slightly different from the older versions. Before one states.. the new ones aren’t burnt in yet I have to disagree. One of the newer KSC75’s had been used for quite some time where KSC7X was new.
In the plots below only the used ‘new’ one is compared to an older KSC75 (about 10 years old).
The newer and older version look exactly the same inside and outside.
Below an older KSC75 and a current one (2018) both have been used a lot of hours.

KSC75 new-oldBass extension seems to be the biggest change. At -3dB (opposite 1kHz) the old one is 35Hz the newer version 55Hz.

Below the corresponding CSD’s CSD comparo
These do not differ that much in behavior above 500Hz. Both have substantial ringing at around 4kHz.

spectrum KSC75 oldWhen looking at the spectrum plots (which dig a little deeper in dB and longer in time) wecan see that the older KSC75 (above) is somewhat better damped around 4khz than the newer version (below)SPECTR KSC75 new

When comparing the step response there is a difference in bass extension between these 2 (also seen in the FR plots). The older KSC75 has better bass extension and seems better damped in the higher frequencies than the newer (but used) KSC75. step comparo

This leads me to believe that maybe the materials or titanium flash coating has changed a bit over the years with the newer materials being stiffer or due to many years of aging the older membrane is a bit less stiff.
In my personal opinion I found that my impression of the older KSC75 is that it did not change over the years. The newer one sounds slightly less in the bass department.

distortion measurements

dist at 90dB
Above the distortion measurements of the older KSC75.
Below the distortion measurements of the newer KSC75.dist KSC75 new 90dB
Basically they are very similar. Higher (clipping alike) distortion in the lower frequencies. There are some differences though. The newer version seems to have lower higher harmonic distortion levels above 150Hz but has higher 2nd harmonic distortion below 200Hz.

A well known problem of the KSC75 is that it starts to distort quickly at a bit higher than ‘normal levels’. The 90dB at 1kHz is clearly a much higher level than ‘normal’ so below the distortion plot (newer KSC75) but measured at 80dB SPL.
dist at 80dB SPL
The distortion below 200Hz has gotten a LOT smaller and this is what the distortion is under normal usage. The ‘rise’ of the higher harmonics is not ‘real’ and caused by the increase of system and ambient noises.
The 2nd harmonic distortion is probably lower than 0.1% in the middle frequencies. This is the measurement floor of the used microphone/amp.

Comparison between various Koss models

Below a (smoothed) plot to better show tonal balance differences between various models of this driver. KSC75 old, KSC75 new, used, KSC7X new, KSC35 (old), PortaPro (old) and PortaPro (2016)

comparo various Koss
The most recent models seem to have less bass extension. The PortaPro have more bass extension but behave differently in the treble area.


Pads are a problem with these headphones. They start to decay as soon as one starts using them. After a few years they lost all springyness and fall apart.
On e-bay one can find cheap replacements and even find ‘better’ pleather options for these headphones. Below some plots made with the same driver (new KSC75) but fitted with various pads.4 different pads KSC7x
Needless to say the tonal balance differs substantially with different pads. The pleather pads (green and blue) make the KSC75 gain 15dB in efficiency but make these headphones sound really dark. Usable for telephone answerring duties but not hi-fi as all details and treble is gone.
The red trace is made with the stock pads, the purple trace with cheap Chinese ‘replacement’ pads.
I recommend to buy the original pads.


The KSC75 is a good sounding ear-clip headphone that sounds quite balanced. It has its issues.
Not being able to play very loud. Bass extension is not that great. Ringing at 4kHz.
It’s price and overall sound quality (for that price) and comfort are pro’s
The newer versions (> 2018) have measurably less bass and don’t sound as good.

post separation

back to Koss
back to measurements