The Orpheus project came about as the result of a decision to try to make the best headphones system possible, regardless of cost and complexity. With an unsurpassed reputation for the quality of their everyday headphones, the company felt that it was time that they should come up with a 'statement' on the subject. The project is a limited edition of 300 sets and they have apparently sold some 50 so far.
Most headphones drive units are moving-coil types but even the most sophisticated of these have limitations which ultimately compromise the end result. Laser Interferometer analysis of MC unit behaviour reveal s the inevitable departures from the piston-like movement which would ideally obtain across the frequency spectrum, imperfections caused by mechanical and/or electrical resonances and so on. All MC drivers have a coil of wire running in a magnetic field to translate the electrical signal into it s mechanical equivalent, but all coils have a finite mass and all masses have a resonance. Further problems occur at the junction between coil and diaphragm and between the diaphragm and its termination, and then the diaphragm it self contributes its own set of imperfections.
A more costly but undeniably vastly better approach is the electrostatic driver, in which a membrane charged with a high DC voltage is suspended between two perforated fixed plate s to which the audio signal is applied in anti-phase. The membrane is simultaneously attracted to one plate and repelled by the other, thus moving the air and replicating the electrical waveform. Because it is driven across its entire surface it needn't be at all rigid to behave like a piston (unlike the MC type where the drive from the coil is connected only at one annular region on the dome). It can therefore be extremely light, storing little or no energy, and thus respond with exceptional accuracy to the profile of the electrical signal.
All this is widely known and accepted. What is unusual in the Orpheus is the manner of its execution and the care with which it has been realized. The membrane here is 1μm thick polymer film to which a gold coating approximately one atomic layer thick has been applied (so as to make it conductive). A large elliptical shape was chosen, which effectively eliminates the standing-wave reflections that compromise the more common circular or broadly rectangular - shaped diaphragms. The fixed plates are made of an exceptionally rigid glass ceramic, less than 1mm thick, with a 70 per cent honeycomb of precision-machined perforations, and are gold-plated on the outside face (thus preventing the membrane shorting against them and being damaged by a spark in the event of being overdriven-instead it collides harmlessly with the insulating ceramic). Some idea of the precision needed in assembling this composite construct ion can be gained from the knowledge that the gap in which the membrane operates is a mere 0.8mm or so wide.
The frames into which these drivers are set are made of laminated beechwood, a material used widely in the aircraft industry for propellers on account of its high strength-to-weight ratio and resistance to torsion. These qualities are complemented by its inherently well damped acoustic properties. On the outside the diaphragms are protected by a perforated steel mesh and on the inside they are held off of the ear by large and extremely comfortable circumaural cushions. An adjustable light metal headband completes the unit, this also well padded. Visually the HE90 headphones are elegantly styled, unobtrusive and business-like.
That is more than can be said, in my opinion, for what is the essential other half of the design, the dedicated HEV90 valve amplifier which converts the line-level input signal to the high AC voltage required by an electrostatic driver and also provides the 500V DC polarizing charge for the membrane. Designed and made for Sennheiser, this is a curious, bulky, rather wild looking unit, a mixture of modem high-tech and Bauhaus with its chromed covers and valve protectors, and weird semicircular shape. The only visual link with the headphones themselves is the curved wooden trim.
To many people the use of valves seems rather eccentric in 1992, yet they are well suited to this particular type of application, since they happen to operate at the high voltage and minimal current required and therefore have no need of the output transformer which is normally employed to convert the output to the low voltage, high current environment of loudspeakers. There is, as well, a magic about valves even now, a warm, benign quality which Sennheiser clearly wanted to exploit. Six individually selected and matched low-noise valves are used: two ECC83 for the preamplifier stage and four ECL86 for the bridged Class A output. At switch-on (which is via a key-operated mains switch, not unlike a car ignition switch!) a delay operates to allow the valve heaters to come up gradually; the valves are also under-run and a life of more than 5,000 hours is anticipated. The HEV90 also incorporates a Bitstream digital-to-analogue converter (one-bit, 256-times oversampling) which will accept, and automatically lock on to, all three common digital bit-rates: 32, 44.1 and 48kHz. A minimalist system could therefore comprise simply a CD transport and the Orpheus.
The rear panel has a pair of stereo analogue inputs and two digital inputs, one coaxial and the other optical, with miniature toggle switches to select between them. The only other control is a single rotary knob at the front for volume-a high quality type with a claimed channel separation better than 80dB and interchannel level matching of closer than 0.4dB across its working range. Two pairs of HE90 headphones can be connected at the same time via their unique five-pin connectors, the only limitation here (apart, that is, from the additional cost of the second set) being that the listeners will need to come to some agreement about level, since the volume setting is common to both.
How they performed
I am not in a position to state categorically that these are the best headphones in the world because, have not been able to make A/B comparisons with other contenders, but neither would I refute the suggestion that they are. Certainly, cannot imagine anything better. In several weeks of using them I really cannot find fault with the sound, which is so well articulated and effortless, transparent to a larger degree than, have hitherto experienced and tonally so complete, as to seem literally like a window on the recording session itself. Electrostatic designs usually have these qualities in some measure but have never before been quite so unaware that the sound source was a membrane vibrating in close proximity to my ears. The subtlest details in a recording are revealed with a clarity that you can almost 'touch' and the sound-stage, which given the extreme separation of headphones can never be correct if it was intended in the first place for reproduction via loudspeakers, is utterly stable regardless of level. Loud music poses no strain, gives no sense of compression, the bass is extended and tight, freed of the colorations imposed by any room-a true foundation to the score.
Such clarity is not without its downside, the limitations of less than perfect recordings being exposed quite ruthlessly, oddities in balance severely punished. But surely it is right for the reproducing system to tell the truth in this way and, truth to tell, can think of no closer approach to the original sound. My hope is that Sennheiser will produce a less elaborate version of this set, perhaps using a more modestly styled, solid-state amplifier without a digital input and with a headset based upon the existing parameters but employing less difficult and expensive-to-fabricate components. If they could begin to approach this performance for a tenth of the price they would still set a standard.