Two Vintage AKG Headphones: K240 Sextett and K340 Electrostatic Dynamic Page 2

These mint-condition AKG K340s are new old-stock, fresh from the box with about 15 hours of play time on them.

The AKG K340 Electrostatic-Dynamic
Rythmdevil's Notes - I bought these NOS (new old-stock) from Germany they only have about 10-15 hours of use. They fit a bit loose on my noggin, so I added some felt spacers to force the cups to swivel to their max (they don't swivel enough, an obvious flaw to an otherwise brilliantly engineered headphone), and I also put foam spacers in the earpads to make them thicker on the bottom. I didn't notice any changes to the sound because of this, it just seemed to help them keep a better seal. They have to original earpads. I tried some newer ones, and they seemed to offer a smaller soundstage because they aren't stretched as big. No other mods have been done.

I think they are a great combination of warmth and speed that is quite unique to the hybrid design. Soundstage is great, imaging is great, and the treble is clean, smooth, and natural. Their faults IMO are a slightly bright top-end that doesn't seam terribly integrated with the rest, and a gap somewhere in the FR, or just something strange or missing in the upper mid-range area between the vocals and cymbals. I know the stat tweeter only produces the higher frequencies so this really cant be due to the crossover, but I can't help feeling that the dynamic driver starts to roll-off and then the FR picks up again with a slight peak. This may be amp dependent though; I haven't heard them on enough amps.

Tyll's Notes - The K340 is a very unusual design that uses both a dynamic driver and an electret (permanently charged electrostatic driver). I have measured a couple of modified versions of these cans, and you can find additional technical information about the AKG K340 on this page.

I had troubles with my treble before with the K340, and I had them again. I listened a bit more carefully this time as rythmdevil spent some time describing his experience with the treble. Again I found the highs too strong with the K340, but not as objectionable as last time. It might have just been the day, or maybe that they're virtually brand new, but I thought these sounded markedly better than the K340s I've previously heard.

The highs didn't seem quite as strident as the other K340s, but I did hear something which may be similar to rythmdevil's comment above, "Their faults IMO are a slightly bright top-end that doesn't seam terribly integrated with the rest..." I heard some things in the mid-to-high treble that was quite good; I thought the cymbals were quite well defined and natural in character. But the treble didn't seem to integrate well with the spectrum of sound lower in frequency. To me, it seemed the highs were closer in space than the rest of the instruments.

Measurements

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

Raw frequency response measurements show modest variation with position indicating reasonably good seal. Compensated FR shows somewhat typical of the era "n" shape response from 10Hz to 1kHz, with relatively poor bass extension. There's a very deep and narrow notch in the frequency response at about 1300Hz. I've not seen a notch like this on any other headphone I've measured except for the other two K340 headphones I've measured; I suspect it's some sort of destructive interference from the two drivers interacting. On the other hand, the electret is said in an AKG ad to have the crossover set at 4000Hz with a 6dB/octave slope, which makes it sound like there shouldn't be a problem at 1300Hz. It's a mystery. Frequency response is quite uneven above 1.3kHz, and highs above about 6kHz are maybe 3-5dB higher in level than I would like.

30Hz square wave shows poor bass extension but proper phase.

300Hz square wave is VERY strange, as is the impulse response. I really think we're seeing a time delay between the two driver elements with the electret's high frequency energy coming in before the main drivers lower frequency energy. I'll talk to this math geek I know who might make some proper sense of it.

THD+noise plot shows a headphone that's having a bit of trouble playing loud, and I think I heard them getting grungier sounding as I turned the volume up. The isolation is fairly good at -12dBSPL broadband (100Hz-10kHz). Impedance is 350 Ohms with a big spike at 10kHz --- probably an interaction with the electret transformer/crossover network.

At 670mVrms to reach 90dBSPL the K340 is a little more efficient than the K240, but you still won't be using it off your iPod.

Summary
I think it's really cool to look at these vintage AKG phones and see too what great lengths they went as AKG moved forward in headphone design. It seems to me they were quite successful in some ways, but at the expense of falling short in others. I had a quick listen to my Quincy Jones Q701s and thought they trounced the K340, and bested the K240 in clarity and articulation. But the early production Sextett had a more relaxed and warmer sound that I would prefer in the long run for listening pleasure.

Good luck finding one though.

Resources
Index to rythmdevil's cool headphone photos.
Wikiphonia page for the K340 and K240 series including the Sextett.
AKG Page to download some Sextett ads.
Head-fi member reviews here, and threads here, here, and here.

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