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The Heil Air Motion Trans former (AMT), the technology that’s at the heart of the Ergo AMT headphones, has been around for decades: It was invented by Oskar Heil and first used in speakers built by ESS in the early ‘70s. (For more in formation, see “You Say You Want a Revolution,” Ivan Berger’s article on unorthodox speaker technology, in the November 1997 issue.) Years ago, I taped an interview with Heil at his home for broadcast on my radio program, Audiophile Audition. He was a charming gentle man, and the single omnidirectional speaker that stood in the middle of his living room had many unusual and superior qualities, including its frequency response, which sounded uniform from anywhere in the room. I never broadcast that inter view, however, be cause when I listened to the tape, I couldn’t figure out what Heil was talking about and concluded that most listeners probably wouldn’t be able to, either!
The Ergo AMT, from Swiss manufacturer Precide (which also builds speakers that incorporate AMT drivers), is the world’s first high-end headphone to use these ingenious transducers. (Two otherwise similar Ergo ‘phones have conventional drivers.)
The application of AMT technology in a headphone is a natural, as some of the obstacles that prevent full-range speaker reproduction with an AMT driver are circumvented in a headphone. These obstacles are not that different from the problems of achieving full-range audio output from electrostatic speakers.
So what, in simple terms, is the basic idea behind the Ergo AMT ‘phones? (The brief owner’s pamphlet doesn’t even attempt to explain it. But I will.) It begins, like some electrostatic designs, with a rectangular diaphragm to which conductive strips of aluminum foil are bonded. The diaphragm is then folded into many narrow pleats, like a miniature version of a pleated window covering. As the alternating audio signal travels through the conductive strips, they function like voice coils and are attracted to and repelled by magnets on either side, causing the pleats to open and close rapidly. Air is alternately squeezed out and sucked in, like a bellows, in stead of being pushed and pulled in the pistonlike manner of a conventional speaker cone. It is said that an AMT driver moves air five times more efficiently, thereby producing greater clarity and sonic detail.
When I unpacked the Ergos, I was surprised to discover that they don’t operate from a ‘phone jack but, rather, from an amplifier’s speaker outputs. The Ergos include a switch- box, enabling connection to an amp’s speaker output terminals. The amp signal is needed to power the Ergo AMT’s large 4-ohm drivers in what really are mini-speakers for the ears. With my biamped surround system, I didn’t have easy access to a full-range speaker-level signal, but I did have an 80-watt amp with level - controls. Unfortunately, it caused a low-level, 120-Hz buzz in the extremely sensitive Ergos, even after padding down the amp’s output with resistors. Next I tried a 100-watt Marantz integrated amp that powers my surround channels; that solved the problem.
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For most of my listening, I alternated between the Marantz amp and a Mesa Tigris integrated tube amp.
Because I couldn’t use the stereo head phone jack on the front panel of the Tigris, I connected the Ergo switchbox to the 4-ohm speaker terminals on the Tigris’s rear panel.
The Ergo AMTs are much closer to ear- canal ‘phones, such as Etym ER-4S, than they are to conventional “crush-the- ear” types. Like the dynamic and electrostatic headphones from Jecklin and the dynamic AKG Kl000s, they keep their drivers entirely away from the pinnae (outer ears). This has several advantages. Nothing presses against the head, so many people find that this headphone type provides maxi mum comfort. Further, because the ridges and valleys of the pinnae are not com pressed against the head, their intrinsic ability to supply directional cues is fully retained. Additionally, open drivers produce some leakage, with signals from each ear going around the head to the opposite ear; this is like the mixing of left and right signals, or interchannel crosstalk, that occurs when you listen to speakers. The effect is a liability with loudspeakers, because it narrows the soundstage. But it’s an asset with the Ergos, as it alleviates one of the most annoying aspects of headphone listening: the sense that half the band is jammed in your left ear while the other half is in the right.
With binaural source material, having the drivers away from your head is particularly advantageous. Because the natural shape of your pinnae is retained, the Ergos supply a more accurate localization than headphones that press the ears against the head. With stereo recordings, the leakage of one channel’s signal into the opposite ear inhibits the illusion that sounds are occur ring inside your head rather than outside it.
Weighing 20 ounces (580 grams), the Ergos feel heavier and heftier than most other headphones. They also have the widest headband of any ‘phones, being nearly the same width—3½ inches—as the rectangular driver housings. But the headband features a thick foam pad to soften the weight of the ‘phones on your head. Two narrow foam pieces at the back of each driver housing prevent the Ergo AMTs from falling off your head as you bend forward. No sliding adjustment of the drivers on the headband is necessary; they are so large that they will line up with most people’s ear canals just fine. Without the usual adjustments, the overall design is sturdier and less subject to resonances.
When I compared the Ergo AMTs to non-AMT Ergo and Jecklin ‘phones, they shared a common sonic characteristic: very natural distancing from the sounds in the source material and a more palpable sense of the acoustic environment in which the music was performed. It is the exact antithesis of the effect of those high-end headphones (especially electrostatic de signs) that seem to put an acoustic magnifying glass on the music. Initially, it may seem as though something is missing, but with more relaxed and extended listening, the amazing depth of detail and transparency comes through; few ‘phones would be less fatiguing for extended listening sessions.
The Ergos’ dynamics are excellent, and transient response is fast. They also share with high-end loudspeakers a hypersensitivity to the slightest fault in ancillary components. If you put your ear close to any loudspeaker with the volume down, you’ll normally still hear some buzz or hum, no matter how wonderful the S/N specifications of the amp and your other equipment. ‘When the sensitive Ergos are connected to the same amp’s speaker terminals, you’ll hear that buzz intensified. (This plagued two of the three circuit modes in the Tigris amp—the tube-tweaker’s special. The Tigris has three selectable modes of operation: all-triode, all-pentode, or mixed pentode/triode. But there was little opportunity to explore why there is so much fuss about triode sound when using the Ergo ‘phones, because both the triode mode and the hybrid position generated a buzz. Only the all-pentode mode—and a moderate amount of feedback, which is also adjustable!—offered relief and a silent back ground. This allowed the Ergos to perform at their best.)
I listened to standard stereo and binaural CDs using the Ergo AMT ‘phones; in addition, I used my reference headphones (the AKG K1000s) and Grado SR80s. One of my longtime favorite test discs is Test records 1, 2 & 3 Selection, a gold CD on the Opus 3 label (CD 99500). Track 2 is a guitar quartet playing a transcription of a Telemann concerto, and track 3 is a five-piece traditional jazz band performing “Buddy Bolden Blues.”
When I listened through the Ergos, the four guitarists seemed farther away and the string overtones were not as prominent as with the Grados. But the Ergo ‘phones conveyed a better sense of the church in which the single-mike recording was made, more like the ambience rendered by my reference ‘phones. In the jazz selection, I noted an even greater sense of hall ambience with the Ergos. Switching from tube amplification to solid-state power brought the guitarists closer to the listener on the Ergos while making their string noises almost annoying on the Grados. With my AKG K1000 ‘phones powered by a dedicated AKG Class- A amp (a combination that costs more than twice as much as the $1,000 Ergos), the similarities in the AKG’s and Ergo’s off-the-ear drivers showed up in improved naturalness and a reduced tendency for performers in stereo recordings to be split between one ear and the other. The jazz track again had a good feeling of acoustic space on the Ergos, but some of the subtle accompanying sounds—e.g., the musicians moving around and instrument mechanical noises—were missing when compared to the AKGs. Switching to solid-state power only marginally improved this.
I also used a stereo CD of the Gilbert Kaplan Edition of Mahler’s Second Symphony with the London Symphony Orchestra (Conifer 75605 51277). The rich, massed string tones at the beginning of the work were reproduced with a good sense of hall ambience when the Ergos were powered by the Mesa Engineering Tigris amp, but I heard more string overtones when I used the Marantz amp; the Marantz also improved the fill between my ears. With this setup, the Ergos acoustically placed me in about the tenth row (orchestra) of the con cert hall; the AKGs set me up in the first row, and the Grados put me on the podium! The dynamics were awesome with the Ergos, but the AKG ‘phones surpassed them in high-end extension and air.
When I auditioned the Ergos with binaural CDs, the out-of-head effects were better than with the on-ear Grados and as impressive as with my reference AKG ‘phones. The Ergos, though, made the sound almost too mellow.
When I first tried out the Ergo AMT, I thought it would be ideal for binaural listening. I was right. The first binaural CD I used was Vox Humana, with Stefan Palm (Projekt Freies Kunsthaus LC8 139), a pipe organ recording captured live in a cathedral in Aachen, Germany, with the Aachen dummy head binaural microphone. The second binaural disc was Gershwin to Sousa (Grado Signature Recordings, no catalog number), a recording of the brass quintet Solid Brass made by Joe Grado with his own spherical semi-binaural mike system.
(Editor’s Note: These two binaural CDs are available from The Binaural Source, Box 1727, Ross, Cal. 94957; 800/934-0442.—A.L.)
On the Vox Humana disc, the toccata from Leon Boellmann’s Suite Gothique is a stirring pipe organ demo track, and with both tube and solid-state amps, the Ergo AMT head phones rendered a most believable sense of the cathedral’s huge interior space. With the solid-state amp, the sound seemed more palpable and acquired a slightly stronger high end. The Ergo AMTs’ low bass reproduction was al most as good as with the Grado SR8Os. (In my opinion, the Grados have the edge in bass response over all other ‘phones.) How ever, the Grados and the reference AKGs revealed more of the subtle overtones of the pipes and their occasional chuffing sounds.
Listening to the Solid Brass CD with the Ergos, I felt that the sense of recording space was overpowering; I got the impression that the quintet had been miked at too great a distance. But with the AKG and the Grado headphones, the balance was better. The Grados yielded the best brass “blat” on attacks, reproduced xylophone transients with great clarity on two Gershwin preludes, and did a fine job with other percussion instruments.
I also listened to solo violin tracks using all three ‘phones and both amplifiers. The Grados were too shrill on most, while the Ergos sounded just right when powered by the solid-state amp.
The switchbox is more than just a switcher between loudspeakers and the Ergo AMT headphones: It also contains a simple equalization circuit that the designers feel is needed to make the frequency response more pleasing, rolling off some of the high end for a mellower sound. When I first learned of this, I was concerned; extensive listening did not alter that reaction. So I opened the little black box that says “Made in Switzerland.” Inside I found each channel lead was bridged by a large capacitor, a small capacitator, two 5-watt resistors, and a small coil.
After seeing this, I bypassed the switch- box by feeding the signals from the amps directly to the large, six-pin plug on the headphone cable. (Using alligator clips, I soon identified the correct four pins.) Voila! The missing high end! Distinguishing the Ergo AMT ‘phones from my reference AKG Ki000s now became an extremely difficult task; they sounded al most identical!
Even when I used the tube amps, the uber-mellowness I had noticed earlier with the Ergo AMTs had vanished. And though the highs were still less extended than with the solid-state amp, I felt the sun had come out! I think Precide should add a switch on the box to enable you to bypass the equalization. An alternative approach would be to provide a separate six-pin sock et like the one on the box, with pigtails to hook up directly to a small amp. On the other hand, with certain amps you may not want to disable the equalization in the switchbox after all.
I would like to try the Ergos without the box but with a good Class-A amp of about 50 watts output, which might be a synergistic combination of great potential. So, why didn’t I power the Ergo AMTs with the AKG Class-A headphone amp? Because the 4-ohm load might have destroyed the amp, according to AKG’s Vienna-based service center.