A very rare electrostatic headphone

Maruni ES-801 Direct Drive - Professional Series Electrostatic Headphones were one of the two headphones I borrowed from an audio sales technician friend in 1979.

Description (sounds like one of the two I borrowed):

• machined aluminium shells with black plastic spacers
• chrome steel and black leather headstrap in the traditional STAX/Herton/Teledyne style

However if these are electrets, like the Maruni's I've seen pictures of, I can't see how they would be transparent.  They have a phone jack and no energizer, therefore electret (not electrostatic) design.  I think these were the ones that were convenient but didn't sound great.  I remember agonizing over the convenience, I wanted something more convenient than my Infinity's, but I also wanted it to sound good, and these Maruni's sounded dull like old Koss Pro3a's, IIRC.

The ones that sounded great, even more transparent than my Infinity's, may have been the Jecklin Floats.  They are the right color (brown).  Though somehow, I didn't remember the transparent phones I borrowed as being Jecklin Floats.  I do vaguely recall talking to my friend about them, he certainly had a pair for himself, but wasn't inclined to sell them, and therefore wasn't inclined to let me borrow them either.  It's possible he finally let me borrow them for a night.  But there could also have been some other electrostatic phone he let me borrow.

More Things I've learned reading Head Case blog on Koss ESP 950's

I've been reading this blog at Head Case full of good information (and also much OT and flaming unfortunately).




1) Koss is generous with warranty.  If you have recabled it is only recommended that you remove cable before sending in, otherwise you'll never see your cable again.  Sending in with no cable is fine. You must include energizer and phones.

2) Lots of people use Stax, KGSS, and many other custom amps.  There's a whole cottage industry of electrostatic headphone amp makers, including some guy in Germany.  Most such amps are actually made for Stax Pro, but work fine with Koss because specs are virtually identical. Some amps have adjustable bias.  People have little good to say about E90 energizer, except it's included with phones. It also can run on batteries, and you can carry a player and everything around in the case Koss conveniently provides for that purpose, but since your D cells only last 3 hours, nobody is much interested.  Koss and/or Stax connectors are available from Amphenol and other sources such as Allied Electronics.  The Koss has more in-head sound than Stax, but when Stax amp is used much of that goes away, and the image is then similar to Stax, more in front and outside of head.  Some people also think the larger ear offset in Stax Omega phones, or the driver angling, has a further beneficial effect on the image.  [since writing this, I've learned out-of-head is a coloration, see later post]

3) Recabling is easy, except for removing the screws on headset which are very tight.

4) There was a long problem with squealing noise.  Also present but to much lesser degree with Stax phones and/or Koss using Stax amps. Nobody seems to have figured it out.  Koss will only fix if you send in both headset and energizer, then sometimes it appeared that they adjusted energizer more than doing anything with phones.  Many people suspected that the reduced bias, to as low as 300V, but that could be measurement error (these internet guys are not engineers).  Many thought it was dust or hair in the driver, but another theory was arcing around the headset terminals.  Koss finally redesigned the terminals (they now point away from each other) in 2008 and it seems less of a problem (or maybe not, and maybe squealing results from too high bias, the Koss 600V spec may be misleading).

5) One of the favorite stats is the Stax Omega MkI.  People hated the MkII, and thought the Koss was similar or better than that.  Koss has more forward midrange, Stax more laid back.  Which has better bass depends on who is talking, some think Stax bass is exaggerated and less defined.  Koss phones are obviously much more cheaply made.  But people say "good enough," especially with lifetime warranty.  Sennheiser made an astronomically priced Orpheus system, which may have been the best ever, but people were mortified about the potential repair cost.

7) They had picture of energizer main board, but on edge mainly to show very simple bias network (basically just resistors and capacitor, with DC taken from HV rail that amp uses, which comes from power board). That's the best I can do until next week.  You can see the rest of board has lots of small transistors and other parts.  Definitely not a purist design and consistent with my PWM guess, as are some findings above.



I think I won't remove foam, especially at the risk of letting dust or hair into the driver.

Koss ESP-950, very nice

Koss ESP-950 Electrostatic Stereophones with E90 energizer.  Very nice electrostatic headphones.  The most comfortable I've ever used, I love the circumaural design which doesn't touch my ear at all.  Very light weight.  Nice system including amplifier with 100K ohm high impedance inputs that can be powered by any preamp.  Very low sonic coloration, probably the lowest I've ever heard.  Extremely transparent from the mid-bass through the midrange; I always effortlessly hear new details I've never heard before even on my very transparent Acoustat speaker system.  And unlike with the Acoustat system, I don't have to keep my head locked into the correct position.  (What's actually worse about the Acoustats is that I am always paranoid that the image is always just a little wrong, and so I constantly need to readjust my chair position or the speakers to make things right.)  The Koss image (such as it is) is effortlessly always the same and perfectly coherent.  Very nice sonic balance from lows to highs, no etched sound (as is widely reported with other electrostatic headphones), no artificial thinness, no electronic sounding high frequencies, nothing unpleasant at all.  They are made in USA (Wisconsin) by a venerable family-run company that pioneered stereo headphones in the 1950's and has been making electrostatic headphones since 1968.  They are guaranteed for life.  I am happy I have bought them for critical monitoring purposes.

The downsides: lacking in deep bass impact and high frequency transparency.  Classical music is generally fine but rock music lacks passion.  The image is largely inside my head, though this is a an problem with nearly all headphones (I haven't tried binaural recordings).  By comparison, the troublesome Acoustat image is full sized and glorious, the whole soundstage is laid out in front of me from far right to far left.

At one time, I had Infinity ES-1 electrostatic headphones which I recall (though this was long ago, and I could be wrong, especially because my standards were much lower, I was using large Advent speakers) sounded both more transparent in the highs and less in-your-head.  They blew away the sound quality I had from any speakers at the time.  However, the Infinity's (actually made by Mechano Electric in Japan) were an on-the-ear design which was far less comfortable (hard plastic rested right on my ear flesh) and they often made a crackling sound.  I was very sad (and still am) to have sold them, but actually never used them much anyway when I had them.

While I still owned and occasionally used the ES-1's, I worked at Audio Dimensions (parent of Audio Directions) for awhile.  The store sold Stax electrostatic phones, and I had an opportunity to try all of the ones we sold in 1978.  The smaller ones with transformers sucked, I thought, sounding no where near as transparent as my Infinity's.  Some even had hard to ignore resonant colorations.  Only the large Lambda phones with a transistor amplifier was competitive, and I even thought it sounded a bit wooly by comparison with the ES-1's (though they had deeper bass, and were far more comfortable, and no crackling, and I certainly wouldn't have turned them down as a gift).   One of the notable sales technicians at the store had a personal pair of electrostatic phones whose very obscure name I can't remember.  I borrowed them for awhile, and thought singularly they were even more transparent than my Infinity's.  I rediscovered the name on The Audio Circuit awhile back, and looked up the company history.  They didn't make headphones for very long unfortunately.  After doing that research, I promptly forgot the name again.

It is commonly believed (and I believe it to be true) that the Koss system is limited mainly by the very small and lightweight E90 amplifier.  Power by a wall wart, weighing about a pound, it is the antithesis of the current generation of top Stax electrostatic headphone amplifiers which can be as heavy as a home theater receiver and consume even more continuous power.  A good number of people have gone out of their way to adapt ESP950 phones for use with Stax amplifiers and are pleased with the result.  According to many reviewers, Koss's ESP950's same rank as Stax's best headphones when used with Stax amplifiers.   (Some like the Koss better than any Stax even with the E90.)  And the Stax large size Omega phones by themselves (without the amplifiers) cost 5 times as much as the complete Koss system.  The Stax amplifiers cost nearly that much also.  Though many find the Koss lacking in high frequency transparency compared with the Stax, the tables may be turned when the Stax amplifiers are used for the Koss, with the Koss having greater high frequency transparency still.

I still lust after the top Stax phones, and though I haven't heard any of the recent top models, I wouldn't be surprised if a Stax Omega 007 with top Stax amplifier would sound better than my Koss.  I'd love to have the Stax Omega 007's (or 009's) with their top of the line balanced class A transistor headphone amplifier.  The minimum price for that package with 007's is a cool $4500.  (The 009's cost that much just for the phones.)  It's hard to plunk down that kind of change for something I don't even know I'm going to use very much.  I got the ESP950 package for $632 with free shipping.  I could get a smaller cheaper Stax setup (say, 303 phones with 323 amplifier) for as low as $1280.  But the smaller Stax phones rest on (not around) the ear, and people complain about an etched high frequency sound.  I'm worried that while the smaller Stax phones might have greater transparency than my Koss, they might have some unpleasant aspects as well.  I wouldn't mind trying them, but I don't want to waste the time of my local by-appointment-only dealer for something I'm not that likely to buy anyway, because I don't know if my limited usage would justify paying so much.

Another largely unappreciated factor may be the pedestrian cable the Koss uses to connect the phones to the energizer, but quite likely the Stax could benefit from better cables as well.  A proper cable would use Teflon insulation with small gauge silver plated solid core or litz wire.  It would not be nice and flexible, but it would maximize audio performance.  I recall my Infinity phones had a fairly stiff cloth wrapped cable.  Under the cloth, there was likely polypropylene coated wire, far better than what appears to be the soft vinyl used by Koss and maybe even most Stax.  That could be a large part of why the ES-1's sounded more transparent than even Stax phones at the time.  The cables connecting energizer to phones could have a substantial fraction of the overall capacitance in the high voltage circuit, and the poor dielectric absorption (if not outright leakage) of vinyl coated wire could easily do a lot of high frequency smearing, far more so than in the far lower impedances of interconnect circuits.

Here's a review comparing several pre-Omega top-of-the-line Stax phones with the ESP950's, using Stax amplifiers in all cases.  He doesn't even bother to review the Koss phones with the E90 amplifier.

Here's a general discussion of the Koss phones, some pictures of the electrostatic elements, and discussion toward making an adapter to run the Koss phones from a Stax amplifier.

Even the very fine Stax amplifiers are not perfect, and a properly engineered electrostatic phone amplifier might do even better still.  I am especially convinced that the very popular Stax tube amplifier is far from an optimal tube design.  It is commonly believed to soften the sound and reduce transparency compared with the top Stax transistor amplifier.  A friend of mine might step up to the challenge.  His last project was a direct drive tube amplifier for Quad ESL-63 type electrostatic speakers.

Here is a very interesting review and measurement of dozens of headphones, in which the Koss ESP950 is among the top few in flat response, if not the outright flattest.  The author doesn't select the ESP950 as his new reference phone because he needs a sealed back unit for his job.  While the Koss has more extended HF response than all but a few, (a tiny bar at the very top which most don't even have) it does show a couple dB of average HF rolloff above 4k which some others don't show (while they may show more HF roughness instead of an average rolloff).  That makes me think a couple dB of boost above 2-4k might help.  The deepest bass also shows a tiny drop, but actually most others (even his reference) shows a larger decline, but preceded by more upper bass boost.  Roller coaster bass boost and hole in the mid treble are par for many other headphones.




I've found one company, APureSound, which will make an ESP950 to Stax Amplifier adapter for $140 (that seems too high), or recable the ESP950 with a Stax connector for $150 (sounds better).  The recable includes cleaning out the dust inside (great for older phones) and removing the foam pad on the ear side (which they say improves transparency).  I believe "recable" also includes different better cable of their choosing.  They also offer custom modifications, that might permit me to choose an even better cable, say with Teflon dielectric.

Here is a truly amazing thread (but unfortunately filled with OT rants and flames) on the Koss ESP950, with lots of people using Stax amps, other amps like KGSS, homebuilt amps, experiencing an infamous "squeal" problem (may be dust, hair, or terminal arcing, Koss changed terminal design in 2008 apparently in response), Stax gets squeal also but less frequently.  In this thread you will see pictures of the inside of the E90 energizer and more.

Here is a great site with homebuilt E.S. headphone amps, and the following response curve (don't link due to web design).

AIX Recordings

After several days of listening the the great AIX recording "Moonlight Sonata", I switched over to the AIX sampler.  I think I could pick out at least 5 more discs worth getting.

One thing is very curious about the two above DVD-Audio discs, however.  When I play them, and bring up the information screen, it shows that I am listening to 2 channel LPCM.  I go to the audio menu and there are no other choices.  Despite the fact that picture shows "96/24/5.1" and stereo isn't mentioned at all and there is no audio selection menu.

Just to be sure I wasn't losing my mind, or that the player itself doesn't automatically select stereo because I have only two speakers selected in the setup, I went back to King Crimson Court of the Crimson King, and saw that it shows 5.1 channels in the information screen when I select (or default) that audio mode, despite the fact I only have stereo outputs connected and have only two channels selected in the setup menu and am therefore listening to a stereo downmix.

On another AIX recording I also get the 5.1 display even when downmixing to stereo.

Either Moonlight Sonata and the Sampler somehow detect that I have an all stereo setup and deign to play accordingly, which would be a new but welcome feature, or I need to update my Oppo firmware to get the 5.1, if I ever want 5.1.

I do want to update my Oppo firmware eventually anyway.  I'm hoping they've fixed the startup bug.  I find that if I run the BDP-95 from a cold start, the music hiccups once or twice in the first 5 minutes, possible concurrent with the fan coming on.

Behringer DCX limited malfunction

I've noticed that from time to time my Behringer DCX 2496 goes into a wierd state where the input and output level lights don't display and the controls don't operate.

It still produces output in the mode, however.  But I had been wondering if the output was compromised either by crossover not operating (all outputs might be full range, for example) or whether the infamous high frequency cutoff of the digital input ASRC starts happening.

This is not related to a powerline cutout, though I leave my Behringer (and the Tact RCS 2.0 which feeds it AES/EBU) turned on all the time.  I have my whole system (except power amp) now running from a Belkin PureAV AVU 1500 power conditioner and sine wave UPS, which kicks in seamlessly (I've watched it do so without any noticable glitch) except that the backup power is balanced mode.  And recently (strangely enough for my neighborhood) there haven't been any power outages anyway.

So when nevertheless I noticed the Behringer go into control lockup state, I decided to leave it that way until I could get around to doing a set of comprehensive measurements.  I listened to it in lockup state for weeks, even auditioning some new Nordost Baldur balanced interconnects for my Oppo BDP-95 to Lavry AD10 connection which allows me to play SACD and DVD-Audio discs from the uncompromised analog outputs of the Oppo into the digital processing that forms the core of my system.  I decided I liked the old version and sale priced Baldur better than the Belkin 1800F balanced wires I was using previously.  That was obviously some serious listening, but I didn't notice any defect in the sound like a HF cutoff, it was sounding better than ever with the Baldur interconnects.  On the other hand, I did worry just a bit that some HF cutoff might be romanticizing the sound somewhat.  I've heard some systems with obvious HF cutoff sound romanticized, though when I deliberately low pass my own system it actually makes it sound flat and harsh.

Finally I got around to doing the measurements.  Everything seemed to be working normally.  Frequency response was essentially flat at 22kHz (actually, it measured about 0.3dB down, which could be mostly or partly due to rolloff in my Fluke 8060 true RMS meter and/or the analog inputs of the Tact which I rarely use) through the midrange outputs.  The supertweeter outputs rose sharply above 16kHz, peaking at 20kHz, and falling down a bit at 22kHz, as intended (I actually shelve high frequencies slightly to protect my supertweeters from spurious high frequencies found on some discs).  Anyway, there was no evidence of either a 10kHz cutoff or malfunctioning of the crossovers.  I also verified the high pass filter on the midrange outputs, but didn't bother verifying the subwoofer outputs.

So now I know the lockup state has nothing to do with ASRC or crossover malfunction.