When I bought these off of Ebay I sent the seller a personalized 'thank you' email for really going above and beyond in fulfilling the purchase with great communication, following my packing specifications, and even including a few replacement parts which came in handy when I partially refurbished these. He replied by stating that after hours of listening he's hard-pressed to find any area where the technology has improved since the 1970s.
The item in question is the (late 70's or early 80's) AKG K240 Sextett LP (late production). The Sextett is apply named for the six passive radiators that surround each headphone driver. To my understanding the radiators are membranes that allow certain frequencies to pass through them and not others, resulting in a fuller frequency response.
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I refurbished mine with new rubber loops for the suspension headband and new velour cushions. I power mine with either a dedicated headphone amp, the Headroom Micro Amp, or the output from my Yamaha R-S201 receiver. With 600 ohm impedance, these need a lot of voltage swing so they're hardly suited for portable use.
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Compared to the newer crop of AKG headphones, the older 600-ohm models have a completely different tone and timbre. They're much more natural sounding, at the expense of having a little more color, albeit, in different parts of the frequency range. The Sextett especially has a warmer and lusher sound compared to something newer like the K/Q701, which has a slightly artificial sounding upper midrange, and in my experience, has come off somewhat screechy, compared to older models. The newer headphones have also had additional tuning in the upper midrange and treble to try and hide any distortion or recording imperfections. You can take Norah Jones' first album, or Soul Sessions by Joss Stone, two female vocal albums with a significant amount of distortion. The newer 701's tend to mask it more while the older Sextett does the poor recording no such favors and really lays the faults bare for all to hear. Some have also compared the Sextett favorably to the Sennheiser HD600, one of the best-selling headphones of all time, while other's have described the Sextett as offering great micro-detail and transient response. All from a driver, the DKK32, that AKG developed in the early to mid-1970s. Critics have complained that compared to newer headphones the Sextett has a slightly grainy treble, while others have complained that they're too bassy compared to other AKG's. For me, the natural tone is worth a little bit of colored bass. And while colored, it still comes across as pretty natural. Especially with smaller arrangements, bossa nova, soul, and funk. I do not personally hear graininess in the treble at all.
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I only just recently began acquiring vintage K240s. I am beginning to find that I appreciate their current tone and timbre more than the newer headphones, which appear to have more detail, and have a much wider soundstage, yet they seem to be designed to mask imperfections and seem to be more concerned with making the material played sound good. When you discover a pair of headphones are playing tricks, even if it sounds good, you begin to wonder, what other tricks are these headphones playing on me? Are they taking me closer to the recording or further away from it? Your mileage may vary, and everybody hears differently and has different preferences for listening. For a while now I have wanted to get closer to the recording, and further away from tricks and color. The AKG K240 Sextett aren't perfect, however they've been a fun step along the way and I feel like I'm probably going to want to keep these much longer than my 701s.