I've been reading this great thread about headphone waterfall plots.
The author has measured Koss ESP950 and likes them a lot. They have very flat and smooth frequency response compared to most, and very good lack of stored energy in the upper mids and highs (just some tiny wriggles, similar to but possibly even better than most Stax electrostatic headphones, which are typically much more expensive).
There is considerable stored energy in the lower mids and upper bass. This is possibly attributable to the less-than-open enclosure. The author strongly disagrees with the proposition that the Koss is bass weak, in fact he thinks it has a slighly dark sound, partly from the orientation of the stored energy, and partly from rolloff in the extreme highs. However, there is rolloff in the very deepest bass.
He believes the ESP950 sound much better through a midrange Stax transistor amplifier, the 323 (which sells for about as much as the entire ESP950 package, with other stax amplifiers selling for far more). He feels it opens up the sound compared with the Koss E90 amplifier, however waterfall plots show essentially no difference in the mids and highs. There is one measured difference, a significant extension of the deepest bass.
One thing nice to know is that apparently the Koss E90 doesn't do any special equalization. Other than the deeper bass response shown with the Stax 323, the frequency response is the same.
Elsewhere, I've been reading many many blogs devoted to making electrostatic headphone amplifiers (usually for Stax, but often for the Koss as well). Often these amplifiers are very elaborate, and even the parts cost might exceed the price of all but the most expensive Stax amplifiers. There is an enormous and unexpected number of websites and articles devoted to such projects.
Above is picture the first tube amplifier from about 10 years ago by Kevin Gilmore, who went on to design others. He complained that the Stax SRM-T1S tube amplfiers use 6FQ7, which isn't really up to the job. He uses 654A's.
Above is pictured the prototype of the famous Blue Hawaii amp also designed by Kevin Gilmore. It's a hybrid transistor/tube design with EL34 outputs with current source loading, and power supply in separate chassis, and the whole package looking mind boggling.
Above is the commercial version of the Blue Hawaii and another electrostatic amp. Kevin Gilmore also designed the KGSS (the KG is Kevin Gilmore again) can be ordered from HeadAmp! Well, actually the KGSS does not appear on the order page, but the Blue Hawaii does, as does a fancier but ultimately not as serious looking all tube amp, the Aristaeus. You don't actually order these because they are cheaper than currently available Stax units, the Blue Hawaii, for example, appears to run $4995.00, and they want 1/4 down payment up front, that's about double the price of the top Stax tube unit, but it looks worth the extra cost in sophisticated parts and design (read the Blue Hawaii link above to hear Kevin Gilmore describe it). HeadAmp says they make their own products in the USA and can customize just about anything. I suspect they'd be able to build a KGSS if you really wanted it. It appears that unlike Gilmore's prototype, the commercial Blue Hawaii has two pairs of outputs for two heads. Four EL34 tubes in all.
One much simpler change involves merely changing the AC wall wart. The Koss E90 amplifier is powered by a small 9VDC 1amp wall wart. This provides rather high impedance power. Unloaded, it actually puts out 12VDC, though the constant draw of the Koss brings it down. Some people report significant improvement using lab grade power supplies instead, with DC voltages as high as 12V. I'm wondering if lower impedance power might help with the bass response. Most E950 tweakers, however, think a whole new amplifier is needed. Removing the back felt is said not to be a good idea, that's the only defense against dust getting on the diaphram, a potential cause of the infamous E950 squealing that may be the number one reason units are returned to Koss.
I got my Koss a nice Stax HPS-2 stand (I was worried about spilling a drink on the coffee table and mucking them up) and Stax CPC-1 dust cover, very nice.
The author has measured Koss ESP950 and likes them a lot. They have very flat and smooth frequency response compared to most, and very good lack of stored energy in the upper mids and highs (just some tiny wriggles, similar to but possibly even better than most Stax electrostatic headphones, which are typically much more expensive).
There is considerable stored energy in the lower mids and upper bass. This is possibly attributable to the less-than-open enclosure. The author strongly disagrees with the proposition that the Koss is bass weak, in fact he thinks it has a slighly dark sound, partly from the orientation of the stored energy, and partly from rolloff in the extreme highs. However, there is rolloff in the very deepest bass.
He believes the ESP950 sound much better through a midrange Stax transistor amplifier, the 323 (which sells for about as much as the entire ESP950 package, with other stax amplifiers selling for far more). He feels it opens up the sound compared with the Koss E90 amplifier, however waterfall plots show essentially no difference in the mids and highs. There is one measured difference, a significant extension of the deepest bass.
I've studied some response curves, and decided Koss would sound a tad better with a mild boost starting around 2.5k, and possibly more up higher. Tonight I tried 2dB and 3dB boosts at 3k, which is the lowest my Tact digital pre will do an easy treble boost from. These boosts are surprisingly subtle, but both seem to make the sound lighter and have a more open quality, more like my old Infinity ES-1 headphones. I'm sticking with the 2dB boost for now on the least harm principle, though 3dB might actually be better. And guess what, bass instruments sound less muddy also.
One thing nice to know is that apparently the Koss E90 doesn't do any special equalization. Other than the deeper bass response shown with the Stax 323, the frequency response is the same.
Elsewhere, I've been reading many many blogs devoted to making electrostatic headphone amplifiers (usually for Stax, but often for the Koss as well). Often these amplifiers are very elaborate, and even the parts cost might exceed the price of all but the most expensive Stax amplifiers. There is an enormous and unexpected number of websites and articles devoted to such projects.
Above is picture the first tube amplifier from about 10 years ago by Kevin Gilmore, who went on to design others. He complained that the Stax SRM-T1S tube amplfiers use 6FQ7, which isn't really up to the job. He uses 654A's.
Above is pictured the prototype of the famous Blue Hawaii amp also designed by Kevin Gilmore. It's a hybrid transistor/tube design with EL34 outputs with current source loading, and power supply in separate chassis, and the whole package looking mind boggling.
Above is the commercial version of the Blue Hawaii and another electrostatic amp. Kevin Gilmore also designed the KGSS (the KG is Kevin Gilmore again) can be ordered from HeadAmp! Well, actually the KGSS does not appear on the order page, but the Blue Hawaii does, as does a fancier but ultimately not as serious looking all tube amp, the Aristaeus. You don't actually order these because they are cheaper than currently available Stax units, the Blue Hawaii, for example, appears to run $4995.00, and they want 1/4 down payment up front, that's about double the price of the top Stax tube unit, but it looks worth the extra cost in sophisticated parts and design (read the Blue Hawaii link above to hear Kevin Gilmore describe it). HeadAmp says they make their own products in the USA and can customize just about anything. I suspect they'd be able to build a KGSS if you really wanted it. It appears that unlike Gilmore's prototype, the commercial Blue Hawaii has two pairs of outputs for two heads. Four EL34 tubes in all.
One much simpler change involves merely changing the AC wall wart. The Koss E90 amplifier is powered by a small 9VDC 1amp wall wart. This provides rather high impedance power. Unloaded, it actually puts out 12VDC, though the constant draw of the Koss brings it down. Some people report significant improvement using lab grade power supplies instead, with DC voltages as high as 12V. I'm wondering if lower impedance power might help with the bass response. Most E950 tweakers, however, think a whole new amplifier is needed. Removing the back felt is said not to be a good idea, that's the only defense against dust getting on the diaphram, a potential cause of the infamous E950 squealing that may be the number one reason units are returned to Koss.
I got my Koss a nice Stax HPS-2 stand (I was worried about spilling a drink on the coffee table and mucking them up) and Stax CPC-1 dust cover, very nice.
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