Two Vintage AKG Headphones: K240 Sextett and K340 Electrostatic Dynamic

I can't tell you what a treat it is to measure and listen to the unusual headphones I get from enthusiasts. This time, rythmdevils from Head-Fi has sent in a couple of vintage AKG cans.

He's also a photographer, and has kindly allowed me to use his sweet pix of these two cans for the article. Very nice! Thanks!

The AKG K240 "Sextett"
Rythmdevil's Notes - These are the EP (early production) versions which are said to have the most bass, and least treble. They have the stock cable and the stock pads, and I haven't even cleaned them or anything. The only mod is a re-screen job because the stock screen hits my ears. It leaves the passive radiators a bit exposed but is much more comfy.

I think these headphones have glorious mids, some of the best of any headphone, and very realistic tone and imaging. Instruments sound real and pure. Their faults, IMO are a grainy, possibly recessed treble, and bass that isn't very tight. Both these faults improve with more power.

Period advertisement for the AKG K240 Sextett detailing some of its unique features.

Tyll's Notes - Man, these are really good sounding. It's no wonder, I suppose, there's evidently quite a bit of old school engineering going on in these cans.

The K240 "Sextett" gets its sexy moniker from the six passive radiators surrounding the main dynamic driver element. From what I can tell from web info, the surrounding six passive radiators are variously tuned in order to control the natural resonant bass hump of circumaural drivers. In this really cool post on AKG's forum, an AKG forum moderator (and employee, I assume) describes the history of the K240, and has this, in part, to say about the Sextett:

Like in the recent version K240 Monitor a 32mm driver was used but it was surrounded by 6 passive radiators. Each radiator consisted of a membrane attached to a perforated disc covered with carefully selected acoustical friction material. Conventional circum-aural headphones often have a resonance peak in the upper bass region followed by a more or less significant bass roll off. The result: an unnaturally sounding boomy bass. The resonant frequency of the passive radiators in connection with the relatively high friction of the damping material behind these membranes effectively influences this irregularity in the frequency range.

In the region of the bass boost the membranes become transparent to sound waves and thus reduce the air pressure in the coupling cavity between capsule and ears. In that way the bass boost is leveled out. Below this boost-frequency the membranes block the sound waves and the roll-off is compensated. This acoustic principle was invented by Dr. Goerike, co-founder of AKG.

With that as an introduction, I can tell you I found the Sextett to have a remarkably neutral response from upper-bass though the midrange. Unlike rythmdevils, I didn't hear the bass as lacking in tightness as much as it lacked extension. I also felt the dynamic punch of the K240 was quite good. I will agree with him though that the treble is a bit grainy. Cymbal hits, for example, lacked individual characteristic and came out a bit "shhh"-like. I also hear the highs as both fast and somehow subdued at the same time.

Nonetheless, these cans were very nice to hear and did seem like their sound was ahead of their time. Most cans of this era had a rather boring "n" shaped frequency response; the neutrality of the K240 seems pretty remarkable to me for a headphone of this vintage.

Measurements

Click on graphs image to download .pdf for closer inspection.

Raw frequency response plots show some difficulty getting a good seal. Compensated frequency response plot shows fairly flat response with slightly warm tilt between 100Hz and 2kHz. Frequency response above 2kHz is a bit uneven but about right in level. Bass extension drops off below 100Hz at a fairly good clip. You'll note however, that the linearity down to 100Hz is very good, and likely due to the passive radiator influence.

30Hz square wave looks about as expected from FR. One channel is dipping below zero; this will often represent "loose" bass, but I think here it's just not sealing well.

300Hz square wave has pretty good shape and speed, but the dip after leading edge is odd, and may be related to the "both quick and subdued" sound I heard.

THD+noise plot is quite good and shows good power handling and little low end distortion despite likely poor seal; usually a sign of tight bass.

Impulse response shows headphone are wired in reverse and are out of proper polarity (inverted). Impulse response also shows the cans are not settling down very quickly, which may contribute to graininess heard.

The K240 is NOT for use with your iPhone. With approaching 1 Vrms needed to drive these 600 Ohm headphones to 90dBSPL you'll need a proper amp to deliver enough voltage for a solid listening level. Isolation is modest at -6dB broadband.

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COMMENTS
inarc's picture

Thanks for the reviews. I'm personally very interested in AKG's current and former high-end headphones.

Armaegis's picture

I've gone through a pretty healthy share of headphones, even trading away those I liked for the sake of curiosity, but through it all my Sextett was one of my first acquisitions and I don't see myself parting with it any time soon. Paired with a Bottlehead Crack OTL tube amp, it's just happy juice for my ears.

On a semi-related note, my next project is to transplant a SFI 120 ohm ortho driver into a Sextett shell. Unfortunately this requires fitting a 40mm driver into a 35mm enclosure... so a dremel may be involved.

SAS's picture

The odd behavior at 1300 Hz could be a resonant mode of the driver or enclosure. You probably noticed a spike at the same place in the distortion measurement. The Ultrasone PRO 2900 headphones have some similar behavior just beyond 2 kHz. Such a narrow valley shouldn't be obvious when listening to most material, because most material just won't hit that narrow band. For the AKG, it lies around the third or fourth harmonic of the vocal range.

It would be interesting to do an FFT or CSD plot of the impulse response data. Send me the file if you want it done.

rhythmdevils's picture

Thanks for measuring these Tyll! Why do you think the Sextetts had trouble sealing? They are big circumaural pads, I'd think they would seal just as well as any other circum headphone.

Tyll Hertsens's picture
Not really sure. The dummy's ears are a bit stiffer than regular ears, so that can be a difference if the cup isn't deep.
Kool Bubba Ice's picture

The 240 are a true marvel.. No headphone connects me to the music like the Sexetts.. These with my DAC1 & HR2 was utter bliss with a type of emotional response I never encountered before. & I auditioned & owned some mighty fine headphones.. K1000/SA5000/K340/701/HE6/DT48/DT880/DT990/HD650/HD600

amateriat's picture

My very first headphones (1976-77) were in fact, AKG K240s, because the space I was living in was a tad awkward for fitting anything but the tiniest loudspeakers. Wonderful cans...in fact, when I wore out those 'phones (took a handful of years, but they got used most days of the week), I tried out a pair of the then-new K340s, HATED them, and exchanged them pronto for another set of K240s. (Never got the sound of the 340s - my ears picked up a weird sonic signature, along with what I felt was rather anemic bass response vis-a-vis the 240s. Definitely wasn't worth the extra chunk of change for me.

Even after briefly auditioning K701s not long ago, I still think the K240s are the best thing AKG made, or makes.

- Barrett

CptKlotz's picture

This is great stuff. Thanks to Tyll and rhythmdevils!

I can't help but be amazed at the performance of the Sextett that was designed and built almost 40 years ago. Its performance seems to rival modern designs which is mind-blowing.

The K 340 seems to be a bit of a mixed bag and the K 240 might actually be the better headphone from an "objective" point of view.

I really like my vintage AKGs...

My first serious headphone was a K 290 Surround (always used for stereo, though). It's essentially a K 280 with more bass due to different tuning of the drivers' bass holes). I passed it on to my father when I got an actual K 280 which I prefer.

I have a K240 Monitor that I have "moDFied" by plugging the capsules' bass holes as it is done on the DF (I'll probably order "real" DF capsules to see if I can see a difference apart from that). I think it was good when it was a "Monitor" (a little mid-bassy, though) and now that it is a "bastard DF", it's really great. Very transparent and neutral sound. People used to typical HiFi equipment would probably call it "bassless" :-)

When I get my hands on DF drivers, I could probably rig up some cables to run an impedance test... I can't do acoustic measurements, though.

Seeing a Monitor a DF and possibly a moDFied Monitor measured would be very interesting indeed.

The K 240's little brother, the K 141 Monitor isn't bad either but not as comfortable which is why I don't use it that often. It has less treble (and less bass compared to the K 240 Monitor) than the K 240 which is probably a matter of HRTF and/or taste.

I really like the K 280. It has a little more bass than the "DF" but it's still what I'd call neutral. The highs seem to be a little rolled off compared to the K 240 and the K 280 actually sounds a bit more like my monitor speakers in the highs. Hard to say which phone is closer to the "truth". Overall, it's a fairly accurate and non-fatiguing phone that can put out insane levels if properly amped.

Seeing some measurements of this unusual design would be pretty interesting.

The K 270, which I also have, is basically a closed K 280 that uses differently tuned bass ports for each driver. I dug up the old HeadRoom review on the Internet Archive and I think it was treated pretty unfairly back in the day. I think it's similar to the K 280 and has a pretty honest sound. The K 280 has nicer mids though (probably because it is half-open).

The old measurements of the K 270 actually show pretty large differences between the channels which points to some kind of manufacturing defect in the cans.

I have also noticed that this headphone really needs a good seal or the mids will sound hollow and honky. For me, the K271 velours pads with some additional padding underneath work best.

I'd really like to see some new measurements of a K 270 because I really think it's quite good.

I've actually bought a K 702 some weeks ago and while I think it's very, very good, I think the "DF" and the K 280 still hold their own pretty well in comparison.

I'd like to get my hands on a K 500 and a K 501 to compare to the K 702 but the prices on eBay are just crazy :-)

I'd send in some of my phones for measuring, but I live in Germany and I wouldn't really like to send them around the world.

Keep up the great work, though!

Kind regards,
Stephan

baten's picture

I have AKG K240 late production sextett's, probably the exact same as Tyll used in this review. I have to agree with what's been said above. There's no headphone where you can get a.. connection with the music like with the sextett's. At head-fi they're described as "grado'ed" AKG's. At +- $100 these are a steal. For me, this is an end-game set-up. Decent DAC, decent amp and sextetts. The next upgrade would be in the $1000.

n32d's picture

Thanx for nice measurements for k240 monitor. Anyway, it seems that sensitivity measurements of K240 Monitor are too good. Result is about 6dB better than in actual product specification (SPL = 88dB/mW.)

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