[caption id="attachment_1307" align="alignright" width="300" caption="The Sennheiser HD 650, HD 600, and HD 580 topped the headphone world for almost 20 years. "][/caption] Until about a year ago, my mind had been made up for almost two decades: Sennheiser made the worlds best headphone, and it came in the slowly evolving form of the HD 580, then HD 600, then HD650. Back in the day, say 1991, I felt the best dynamic headphone around was the Sennheiser HD 560. It was a marked improvement over the shrill HD 540 previously, but it was still a bit thin and lacking in umph. In those days, the headphones to beat were the electro-static cans from Stax. The dynamic cans available in those days just wouldn't hold a candle to the speed of the Mylar diaphragms and dedicated electronics of the e-stats. Then Sennheiser introduced the HD 580 and everybody that cared about headphones (there were like twelve of us back then) were stunned. The HD 580 had punch and warmth, most headphones of the day were thinner sounding. There was speed and coherence up top, not the confusion and congestion that characterized the under-engineered "accessories" as they were thought of at the time. Sennheiser had really put some thought into these new cans and it was easy to hear. [caption id="attachment_1309" align="alignleft" width="300" caption="The flexure creasing is easily seen in the diaphragm to the left; connector can be seen at right, the pins and springs barely visible."][/caption] The break-through technology seemed to be their Duofol diaphragm which relies on both the stiffness and the flexibility properties of their patented two-layer plastic diaphragm material. The diaphragm is formed into shape having a central dome and an outer suspension which is strategically creased to enhance it's stiffness and allow very controlled flexing. The diaphragm is firmly attached to a rigid frame to position the extremely light weight aluminum voice coil (which is a ring attached behind and at the outer edge of the central dome) within the gap of the powerful neodymium magnet behind the driver. The basic operating principle of this diaphragm is that some of its creases act like hinges (see Living Hinge and flexure bearing) allowing the rest of the diaphragm surface to move in pure piston-like motion without any flexing. Previous diaphragms of this type had problems with heating at the creases as the driver moved rapidly back and forth with the music. This local heating at the fold would soften the plastic and change the torsional resistance to the bending of the hinge, which would in turn change the characteristics of the driver and introduce distortions. Sennheiser's Duofol material has very well stable viscosity when heated with flexing, and delivers predictable and narrow torsion characteristics at the fold allowing Sennheiser engineers to design a significantly more accurate driver. There are a lot of other things that were just right with the HD 580: the comfort was spectacular; the headband adjusted well; earpads and headband pads were replaceable; and the cable was attached with special connectors at the earpiece. The detachable cable was both a blessing and a curse: A blessing because it allowed enthusiasts to start re-cabling their headphones and discovering the surprisingly big sound quality improvements it delivered --- this spawned a cottage industry of headphone re-cablers that continues today. The replaceable cable was a curse however, because the connectors had a very annoying habit of becoming intermittent over time. [caption id="attachment_1310" align="alignright" width="300" caption="The HD 650 connector is slightly larger then the HD 600 and HD580 for a tighter fit and less strain on springs. "][/caption] The connectors on the cable that went into the earpieces had two small pins each having a an annular indent around a pointed tip. Inside the earpiece, within the diaphragm housing, were two very small springs which made contact with the wires from the voice coil. When the cable was inserted into the earpiece, each contact tip would be pressed between the coils of the springs to make contact. Unfortunately, the springs would relax over time, and soon the gap between spring windings would be slightly larger than the contact tip diameter and it would start loosing contact. Wiggling the connector at the earpiece would get it to work for a little while, but the connector would eventually wind up in its normal position and start to open again and you'd loose audio again. [caption id="attachment_1312" align="alignleft" width="300" caption="The new HD 650 springs are much stiffer and we've not seen any problems."][/caption] Very annoying. Fortunately, the problem was fixed sometime around 2000 with stiffer springs that wouldn't deform over time. Sennheiser was very responsive and repaired defective product free of charge for quite a long time. Currently, if you do have a pair of HD580 or HD 600 with an intermittent connection problem, you can send them in for repairs for a small fee. While the diaphragm has remained a constant, the headphone around it changed over time. First, in 1995 for Sennheiser's 50th anniversary a special limited edition HD 580 Jubilee was produced, which featured thermoplastic carbon fiber body parts, metal grills, and precisely matched drivers. About a year later the HD 600, which was essentially a repainted Jubilee, went into normal production, and is still being produced. In 2003, the headphone was upgraded again, this time with a new paint job, improved internal acoustics, better driver matching, and a much better cable. The sound of all three headphones can be characterized as mildly warm and relaxed, arguably slightly excessively so. In fact, the headphone hobbyist community have come call this character "The Sennheiser Veil." To quote PiccoloNamek from this post: "Veil" is something somebody made up one day when they were dissatisfied with a Sennheiser can, and for some reason, everybody else accepted it as true, including people who had never even heard a Sennheiser headphone before. At least, that's how it seems, because I have never heard a veil on my HD650s or on the HD600s I tried at the Atlanta meet. (Or on the 595s, for that matter.) As for what the "veil" actually is, it is simply their [people who don't like Senn cans] way of describing what they perceive as a lack of treble energy, as if the sound is coming from behind a curtain/outside the studio/shrouded in fog/or whatever terminology they prefer to use." (More Head-Fi threads on the Sennheiser Veil here, here, and here.) [caption id="" align="alignright" width="298" caption="You can see that the HD 650 (blue) has only slightly less energy in the higher octaves than the HD 800 (red)."][/caption]I agree whole heartedly. I find the HD 650 a laid-back sounding headphone, and they do have less treble energy than an AKG K701 or Beyerdynamic DT880 for example, and I'll even admit that the HD 650 may be warmer than neutral in a small way, but they are a terrific sounding headphone. In fact, I would say they remain competitive among the world's best headphones in delivering great listening and a terrific price performance ratio even if you're in row 8 in the audience instead of row 2. I actually prefer this laid back presentation for long listening sessions as it is less fatiguing and I can feel more relaxed as I enjoy my tunes. Both the HD 650 and HD 600 can be operated in balanced mode with a Cardas Cable specifically designed for these cans, and using our Balanced Ultra Desktop Amp. These headphones have been reviewed repeatedly, I suggest the articles here, here, here, here, here, and here, for further reading. I highly recommend this headphone for those looking for a very comfortable and laid-back, long and luxuriant listening sessions with open headph0nes.