K501

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Published: Sep-8-2017, updated: Sep-19-2017

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.
The 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.

AKG K501

The AKG K501 was the successor of the K500 with different color-scheme.
The elastic bands that pull down the headband do wear after time and need to be replaced sooner or later.
It has the familiar AKG look and is simple and elegant. The gold color ring on the grey cups gives it a fancy look. The pads are cloth instead of pleather or velour and very comfortable for longer listening sessions.
The clamping force is low and because the earpads are large in diameter the headphone may shift around on the head. The weight is very low as well.
All in all a very comfortable headphone to wear.
The cable is also low in microphonics but non-replaceable.

It is an open headphone design and sounds from the outside enter almost unattenuated. People around you can hear what you are playing.
The efficiency is not very high so really only well suited to be driven from home equipment with an amplifier that can supply enough voltage.
This fact gave it the honor of being ‘hard to drive’. It isn’t, you just need enough voltage from your amp and need to turn up the volume knob a bit higher.
On not well suited amps and portable devices it will sound ‘raw’ and compressed at louder volumes simply because those amps run out of voltage swing.

It sounds quite similar to the K500 but has a bit more clarity.
Bass and mids sound identical to the K500 and with the same shortcoming… the lack of body and bass extension. A very ‘open’ sound.
This headphone is not suited to be driven very loud and starts to sound harsh/sharp and distorted when you do.

specifications:

Type: Over-ear, Open
Usage: Home, Studio
Isolation: poor, as it is an open design
Driver type: dynamic
Pads: replaceable, cloth
Inner pad dimensions: diameter: 60mm,  24mm
Collapsible: no
Headphone connector: fixed
Cable entry: left sided.
Cable: 2.5m with 3.5mm TRS plug and screw-on 6.3mm adapter.
Driver size: ø 42mm
Nom. power rating: 200mW
Max. voltage: 4.9V
Max. current: 41mA
Max. S.P.L.: 116dB
Impedance: 120Ω
Efficiency: 94dB @1mW
Sensitivity: 103dB @ 1V
Weight: 235g. (without cable)
Colour: grey with black accents
Clamping force: low
Accessories: screw-on 6.3mm adapter

sound

This headphone is all about the mids. There is absolutely no bass to speak of.
Bass is rolled-off considerably. For that reason the bass sounds ‘tight’ but ‘thin’.
Bass integrates nicely into the mids. Therefore bass sounds realistic albeit way too thin, lacking ‘body’. The mids have a clarity that it is a bit too much. It has an overly bright-ish edge to it which can make female voices a bit too present. The K500 did better in this aspect.
The treble is also accentuated and can have a bit of sibilance and sharpness to it.
Even though the treble doesn’t sound like it is rolled off it has a bit of raspy sound, as in not very refined, to it and lacks ‘air’.

measurements:

Below the frequency response of the K501 (Left, Right)

The channel matching is excellent. period.  The tonal balance is bass shy/anemic. The bass starts to fall off quickly below 100Hz. At 30Hz (where the lowest bass signals are) it is already -10dB meaning that signal is half as loud as it should be.
The integration between bass and mids and the mids themselves have a very good tonal balance. From 100Hz to 1kHz it is very ‘flat’. The K500 was flat from 100Hz to 3kHz which was even more impressive.
The +5dB peak around 1.5kHz gives the mids a too forward and ‘shouty’ character on a lot of music.
Stereo image  is somewhat wider than some other headphones but not nearly as wide as say the HD800 (yes, I know… not a fair comparison).
The treble is mediocre quality. A bit sharp and sibilant at times (depends on the music). The treble quality itself does not sound rolled off, on the contrary there is a bot more of it than ideal. It lacks the finer nuances and ‘air’. The peak at 8kHz is the most offensive one here. As can be seen later on the driver behaves very poorly at that frequency.

The K501 sounds a lot like the K500 so below a direct comparison between these two.

As can be seen these headphones have more in common than they differ.
The most obvious difference is between 1kHz and 7kHz. The 3 peaks are seen in most AKG headphones.

Below the distortion measurements of the K501 (Left channel).

Distortion levels are rather high. What stands out is that the 3rd harmonic distortion in the bass is higher than the 2nd harmonic distortion. This points to ‘clipping’ alike behavior. Below the same data but shown in percentages.

The distortion levels are quite high, most certainly in the bass where 2nd and 3rd harmonics reach 2% .  At around 8kHz there is a sharp rise. The 3rd harmonic even peaks at 2% which also helps explain the sharp treble along with the rather big amplitude spike there (+15dB !)

output resistance / damping-factor

As this is a dynamic headphone the frequency response might be amplifier output resistance dependent when certain higher output resistance amplifiers are used.
To test this the headphone is measured via a low impedance amplifier (0.2Ω) and a high impedance amplifier (120Ω). On a higher output resistance amplifier the output level will be considerably lower. To compensate for this the amplifier is cranked up to the same level (at 1kHz) as the low impedance amplifier. This way the plots are overlay-ed and it is easy to see how the tonal balance changes. In reality when a 120Ω resistance is used it will play about 6dB softer.

The effect is very small. You get about 1dB more bass but not more bass extension. The difference is subtle.

Below the CSD (Waterfall plot) of the K501. (Left and Right channel are superimposed)

This plot shows the problem areas of this headphone quite clearly. Some ringing around 2kHz. A big one at around 7kHz-8kHz and one higher up at around 14kHz and a smaller one at around 20kHz. The 7-8kHz one is quite audible and degrades the sound.

The spectrum plots also tell the same story of course. Below the Right channel is shown.
There is some ringing visible at around 800Hz, 1.8kHz, 3kHz (short lived), 6.5kHz and 8kHz as well as around 14kHz. This shows the driver probably has some cone break-up issues.

The step response (Right channel)

It shows the elevated treble part and ringing lasts quite long. The downwards sloping line indicates the poor bass response/extension.

Square-wave and impulse response (left channel):The absolute phase of this headphone is turned 180 degrees compared to all other headphones I measured. This absolute phase difference is not audible so not really and issue but strange none the less.
This only shows up when looking at square-wave and impulse plots.
For this reason I ‘doctored’ the 100us impulse plot a bit by flipping the measured trace around the horizontal axis so the plot looks like the other ones on this website.

The 40Hz square-wave response shows the considerable lows roll-off. The 440Hz square-wave shows a constant ringing at higher frequencies.
The impulse response shows a rather slow rising edge and some overshoot. This is indicative for a poorer treble response. The ringing is also quite visible and lasts much longer than desirable.

some experiments/tweaks

The K501 accepts the same pads as the K601, K701, K702/K712. These pads aren’t really cheap anymore though.
Also to take the treble experiments with toilet paper can be done.
And last but not least there are 3 ‘ports’ around the driver which feed signal back from the rear in a specific frequency range.

3 different pads were used with this headphone. Below the results with the stock K501 pads, K601 pads and K702/K712 pads.

The first thing that stands out is the K702 pads having substantially more signal below 1kHz. This means it goes about 4dB louder with the same volume control setting when these pads are used.
Below the same plot except the ‘volume’ of the K702 pads has been lowered so all the differences are easier to see,  stock K501 pads, K601 pads and K702/K712 pads.

Below the same plot but smoothed to 1/3octave which shows tonal balance differences more clearly/easy.

What is obvious is that the K501 and K601 pads aren’t very different in tonal balance.
The extremely small difference in bass extension is inaudible. The treble peaks with the K601 pads are slightly lower and shifted in frequency a bit.
The K501 pads are cloth and slightly less thick (probably due to aging)  and made of cloth. These are quite soft (also aging ?) and very comfortable.
The K601 pads are grey velours and much stiffer as well as thicker so slightly more room for the ears in depth. The K702 pads are even stiffer and black velours.
Comfort goes to the original pads.
The K702/K712 pads sound (and measure) quite different. The Tonal balance is a bit warmer and less ‘forward’. One could say somewhat more ‘laid-back’. Clarity isn’t ‘overdone’ and maybe even slightly lacking. It also isn’t sharp sounding any more but  ‘smoother’. Where the K501 pads give the illusion that it isn’t rolled-off in the upper treble the K702 pads just slightly are.
Clarity is what the K501 is all about though. When you feel it has too much of it and too much treble…. fit the K702 pads and loose out on some comfort.

There is another way to lower the clarity and amount of treble though. This can be achieved by sealing 1, 2 or 3 ports around the driver. This can easily be done with some tape. Very reversible as well, no damage is done. (picture will follow)
Below the effect between all 3 ports ‘open’ (= stock) and all ports closed (tape over the paper covered slits).

What’s interesting is that below 2kHz everything is flatter and the 2kHz ‘bump’ is gone.
Also the response between 2kHz and 3kHz is changed. Sealing the port thus lowers the clarity somewhat and increases the efficiency by about 2.5dB. Also the treble is relatively lowered now by the same 2.5dB.
Of course, one could also seal just 1 of the 3 ports or 2 of the 3 ports to subtly vary the effect. Stock and all 3 ports closed are also shown.

As can be seen there is no influence on the bass. The roll-off is the same and seems a driver issue. Also everything above 3kHz remains the same. BUT because the frequency part below 2kHz differs relatively the overall effect is that when sealing the ports the amount of clarity and treble levels are slightly reduced.
Not nearly as much as the K702 pads do though.

The amount of treble can also be ‘adjusted’ by the use of toilet-paper.
Simply remove the pads (twist them a few mm counterclockwise and they can be removed. The pads are ‘locked’ this way.
In this case I had some 2-ply toilet paper and cut a disc about the same diameter as the inside of the pads is. Pulled the 2 layers apart so you end up with 2 plies.
The same can be done with 3-ply toilet paper. The cheap one ply is not really suited.
Below the difference between stock K501, 1 ply of toilet paper between the driver and pad and 2 plies of toilet paper.

Below the same data but smoothed 1/3 octave so tonal balance differences are easier spotted.

It is easy to see the toilet paper only attenuates the upper treble above 7kHz.
The upper plot shows a serious drop in the peaks (approx 7dB) when using 1 ply.
The smoothed plot suggests just a 2dB drop in treble level which is audibly more correct.
The second ply removes another 2dB.
Downside is the treble extension is also much less and wasn’t great to begin with.
It would have been better when above 10kHz the level would have been unaltered.
So toilet paper can remove the ‘treble sting’ but 1 ply is already enough. This at the cost of a slight loss of treble (not loss of clarity, this remains the same).

Another solution is the Kameleon, below the tonal balance differences with and without the Kameleon. This filter can only partially compensate the lows. The filter also can only be used with the portable amp, not with the G1217 inline which would most likely clip the bass when high input levels are present.
Another downside is the reduced maximum SPL which already isn’t that great to begin with.

summary

The K501 is an older headphone and no longer available. It remains a sought after headphone. Mostly because of the comfort, the mids, clarity and stereo image.
The headphone seriously lacks a bottom end. When you listen to it exclusively for a longer period the lack of bass isn’t that obvious. When you know there should be deep bass in a song, and it isn’t there, you do miss it. The treble can have a nasty edge to it.
Comfort is great.
To drive it with some authority you really need an amplifier that can supply enough voltage.

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