Koss ESP/950 electrostatic stereophones Page 2

This energizer/amplifier doesn't look like much, but it's full of circuitry. I was able to remove the back but not the top, but that was enough to reveal two small circuit boards crammed with parts, the lower board making up the power supply, the upper the amplifier. According to Koss engineering, the latter produces a whopping 63dB of gain (the specs claim 60dB). This is enough, Koss claims, to provide a maximum 126dB (!) spl at a listener's ears. (Why anyone needs this spl capability is beyond me; coming anywhere near it is an invitation to permanent hearing damage.) This amplifier has an op-amp input and all-FET output. The power supply is a high-frequency switching design.

The E/90 can also be powered by batteries; Koss provides a battery pack which must be loaded with six C cells. The front-panel LED is also designed to indicate battery condition: flashing red for batteries running down, solid red for dead. I experimented briefly with batteries and found that a set of alkalines operated for three hours before beginning to flash red. The sound, incidentally, was essentially unchanged with the battery supply. There might have been the slightest loss of openness, but I didn't spend a lot of time pondering it. I did all of my critical listening with the line transformer/supply, as I suspect most users will. The ESP/950 package also includes a handsome carrying case which will hold all of the headphone pieces plus a portable CD player, the latter clearly the functional reason for the minijack front-panel input on the energizer amplifier. So if there's a desert-island stranding in your future, you won't be without music. But the pink bunny with the bass drum had better pay you regular visits.

I encountered a number of functional problems with the ESP/950. First—and perhaps more ergonomic than functional—the design and mounting of the RCA jacks on the rear panel of the E/90 energizer/amplifier make it very difficult to insert audiophile- grade plugs. The jacks themselves are mounted to an internal circuit board and protrude through holes in the rear panel which are not much larger in diameter than the plug itself, making it impossible to fully insert plugs with a wider cross-section than your typical Radio Shack connector. I used a 1m length of Music Metre Silver for most of my listening tests. It worked, as did AudioQuest, Cardas, and Monster's top connectors, but with a less secure fit than I would have liked. The do-it-yourself audiophile could remedy this situation by enlarging the holes in the plastic rear panel (the latter removes easily). I have no idea how this might affect your warranty, but a spare rear panel from Koss—which would return the E/90 to an as-new condition— couldn't cost more than a few bucks. But you shouldn't have to do this; the use of high-quality plugs should be assumed in a product such as this.

But the plug problem was only a minor irritation. There were two other problems with my first sample of the ESP/950: occasional very low-level noises, apparently originating after the volume control in the energizer/amplifier, and mild electrical shocks to my outer ear from the right headset. The latter was infrequent and not alarming—just a small pinprick on the top of my ear—but it was clearly not normal behavior. I asked for a replacement.

The second sample displayed neither of these problems, but threw in a new one of its own. Periodically, usually coinciding with a head movement, both earpieces would emit a peculiar crackling noise which I can only describe as a short, abrupt rainshower. It was not frequent enough to interfere with my enjoyment of the 'phones, and was usually over before anyone even got damp, but it continued intermittently throughout the evaluation period.

Finally, just as I was wrapping up the review, a new intermittent problem reared its head: a right-channel hum clearly coming from the E/90 energizer/amplifier following the volume control (which had no effect on the level of the hum). This hum also modulated the audio signal in that channel, which took on a warbly, underwater quality. I was able to correct the problem by shutting off the E/90 for an hour or two, after which it would work fine for a couple of hours, when the problem would recur. Koss Fed-Ex'd me a replacement E/90. As I write this, I have had it running over a weekend for 60 hours continuously with no recurrence of the problem. I will continue listening to it and report at a later date if the problem returns, but as of now the defect seems to have been specific to the first E/90.

In a product of lesser sonic accomplishment, these would be matters of significant concern. Koss does, however, advertise a No Questions Asked Limited Lifetime Warranty. In words right out of the ESP/950 manual, "if anything ever happens to any part of your ESP/950 System, Koss will repair or replace that part, no questions asked." (my emphasis)

The Koss ESP/950s produced first-rate sound. Were it not for the natural limitations of headphone listening discussed above, they could easily spoil you for the sound of almost any loudspeakers. I think that, perhaps subconsciously, I've avoided extensive serious headphone listening over the years for just that reason.

The Kosses are certainly serious headphones. They impressed me immediately with an often startling clarity with no hint of exaggeration. They excelled at the top end, producing gobs of resolution without screaming DETAIL!! at every opportunity. The entire treble region was silky, open, and transparent to a degree which any loudspeaker designer would envy. If they were slightly softer and more forgiving at the very top than their primary competition (which I'll get to further on), they were so in a manner which never left me feeling that anything was missing. Nojima's flying fingernails on the Mephisto Waltz from Nojima Plays Liszt (Reference Recordings RR-25CD) were clearly evident, as was cellist Nathaniel Rosen's incredibly squeaky chair in Orientale (North Star NS 0027). But neither was a distraction from what are otherwise first-rate recordings—as heard over the Kosses. The natural liveness of Orientale, in fact, came through clearly, combined with the subtly natural ambience of the recording site—New York City's Church of the Holy Trinity. The rather close miking used here threatens to make this recording rather dry, but it did not sound that way through the ESP/950.

Koss Corporation
4129 N. Port Washington Ave.
Milwaukee, WI 53212
(414) 964-5000

ndimoff's picture

Another great news is that the electrostatic transducers of the Koss'es do not age. Bought my pair in April 1993 and they still sound as new. In comparison, the panels of my Martin Logan Aerius electrostatic speakers need to be replaced every 5-7 years.