Two Vintage AKG Headphones: K240 Sextett and K340 Electrostatic Dynamic

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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.


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.