KOSS ESP6 REFURBISHED VINTAGE ELECTROSTATICS
Apr 22, 2007 at 11:54 PM Thread Starter Post #1 of 94

edstrelow

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The Koss ESP6 is the first electrostatic headphone made by Koss, back in the Jurassic period. Its age, size and weight make it a veritable dinosaur of headphones. However, to anticipate what follows, it shows up surprisingly well against more recent and more highly regarded products and it is more than a vintage phones curiosity piece.

ESP6.jpg


The ESP6 was introduced in 1968 and phased out in 1973 according to the Koss website.

http://www.koss.com/koss/kossweb.nsf...m&60s^Products

Prior to this, Stax had made only one electrostatic
model, the SR-1.

http://www.stax.co.jp/Export/History.html

I had bought an ESP6a more than 20 years ago. In fact it was the first headphone I ever bought. I later sold it and moved on to the Koss ESP9 and several Staxen.

The ESP6 and 6a are, as best I can tell, identical, although I am sure there are some minor circuit design changes. They are probably the heaviest headphones ever sold, almost 2 lbs in weight. I am not sure if even the Jecklin Float

http://www.head-fi.org/forums/showthread.php?t=236434
is heavier.

The weight of the ESP6 comes principally from Koss' decision to put the transformers, needed to allow these to be run from a regular power amp, in the earcups themselves. Evidently Koss felt that users would prefer this to a separate transformer box, employed by Stax and later by Koss to in the ESP9. The effect is somewhat spoiled because these phones still come with a small adapter box, which evidently is mainly used to protect the phones from the full power of a regular power amplifier.

While these phones have a large regular stereo headphone plug, they tend not to run well from a conventional jack. Koss suggests removing the protective resistors found in most such set-ups and putting a smaller value resistor (2-5 Ohms) in each channel.

These phones are also self-biased, i.e. the additional charge applied to the diaphragm is generated from a portion of the musical signal, versus being powered separately from the mains. I am not sure who first came up with this feature, certainly Stax also has used this feature. The ESP manual implies that Koss pioneered this. I can't tell from the Stax site, whether or not their earlier models were self-biased or mains powered.


The current pair was sold on Ebay a few weeks ago. When I got them, I didn't have great expectations for them, because I was aware of the tendency for these early Koss phones to deteriorate. Evidently, the foam used as damping has a tendency to rot, short circuiting the electronics in the earphone cups. As well, Koss had used liquid-filled earpads, which generally leaked after a few years, often into the phones. Thus you faced the additional problem that the electrostatic transducers could be ruined by the liquid. This may also be why the foam itself rotted so badly.
Anyway, for $37.00 I thought they were worth a try.

I know nothing about the history of this particular set except that it came looking almost like new, except for the deflated earpads. I was able to purchase e new set of conventional, i.e. no liquid filled earpads, from Koss for $5.00

A test strip came with the set showing that they were made in 1969.

Chart.jpg


The test results look amazingly flat, however this was run with a coupler rather than a dummy head, so it would not totally reflect what your ear is likely to hear. On the other hand it is not bad either and I think it is remarkable that there was a time when headphones came with individual test results attached!

Their sound out of the shipping parcel was not good. You could hear something, but it was very distorted with the volume surging.

On the good side, I noted that the sound, crappy as it be, was balanced in both ears and that the diaphragms seemed to be intact. You could tell this because pressing each cup produced a static burst (some call it the electrostatic fart). This is caused by the air pressure of the movement pushing the diaphragm to the outer stator, thus creating an audible static discharge. This is a usually harmless characteristic of most electrostatic designs. I heard this as a sign of intact diaphragms, because if they were torn or punctured, they will leak and won't move to the outer stator.

THE CLEAN-UP

I remembered that a few years back, either on this site or HEADWIZE, someone had sent in a post indicating that old Koss phones could be brought back to good running order if they were cleaned up, especially the circuit boards, because of the above- mentioned problem with rotten foam. He believed that the foam actually short circuited the circuits. At any rate he got his Koss working, a set of ESP 9's if I recall correctly.

Other persons on this site have set to work to rehab various ESP9's, but I have not yet heard of a 6 being brought back to its original operating characteristics.

Upon opening the cups, I found the expected gooey mess. I took the circuit boards out of each cup, and cleaned them with contact cleaner. I also removed every crimped contact and cleaned them with Progold contact enhancer. I scrapped off the crud from any contact surfaces .

The photos show the component side of each board. Note the transformers built in each cup.


cup2.jpg


Cup1.jpg


I have done three cleanings so far. With the last I am starting to get something like HiFi sound at last. In particular, the mid and upper frequencies have come back with considerable definition.

BIAS/VOLUME ISSUES
On first hearing, the volume drove up and down badly. This is almost certainly due to problems with the self-biasing operation. While the bias charge is supposed to be currentless (Koss' term) in fact it leaks out. With a dirty set like the present, evidently there is so much leakage that the sound goes up as the set is charged, and then the bias charge drops off almost immediately. However with each cleaning, this has become less of an issue and the phones are pretty stable now for most rock/pop music and even a lot of classical, which is more difficult because of the wide dynamics in much classical music.

I would say these phones are pretty good now, and I may experiment with means of making the bias more stable. Anyone know if larger capacitors would help?

THE SOUND

I let everything warm up for an hour before making comparisons. It's been more than 20 years since I last had a pair of these phones, so it's hard to recall exactly how they sounded. Still, my general impression is that the ESP6 sound now pretty much like what I recall from the ESP6a. I put on Roberta Flack's "Killing Me Softly" which I used to use to demonstrate the 6a's and it sounded like the same glorious intro, with the ringing bell sounds and smooth background vocals. I spent some time going back and forth between the ESP6 and a Stax 404 and Stax Sigma/404 (a Sigma rebuilt with the newer 404 cable and drivers.

http://www.head-fi.org/forums/showth...15&highlight=s igma%2F404

I used the previously-mentioned Roberta Flack, a Mahler symphony and Russian and Christmas Choral music. The ESP6 was generally as much up to the task as the Stax, except in the Christmas music, a somewhat disappointing Telarc recording of the Mormon Tabernacle choir, in which several tracks sounded murkier with the ESP6 than the Stax.

Each of the phones had a somewhat similar frequency extent, i.e. much the same range from top to bottom. The Sigma/404 has more upper mid bass and better "air" around the instruments/voices and a beautiful ambience and openness. The 404 less air, more bass punch than either of the other 2 phones. For the last few months I have been ambivalent as to which of the 2 Stax was best. When the Sigma/404 arrived I felt it was the best phones I had and that the 404 was somewhat harsh sounding. A recent tweak, the use of Silclear contact enhancer on all contacts, really helped the harshness of the 404 as well as an older Nova Lambda, and the 404 may have moved ahead of the Sigma/404 in performance because of its overall better definition. The Sigma/404 has a more diffuse sounding as a result of its unusual front mounted drivers, but basically they are both great phones, each being better with different source materials.

The ESP6 was somewhat muddy in the bass, with less bass punch, less clearly defined instruments/voices and generally lacking the airiness of the two Stax. However what it does have something that neither of the Stax have, more smoothness to the sound. In other words, the Stax have a slight raspiness compared to the Koss.

COMPARISON WITH STAX 404, SIGMA 404

It is hard to make direct comparison with phones run from a different amplifying system. While the Stax and Koss phones are hooked up to the same source, the amplification differences favour the Stax phones.

The system I am using here consists of a CEC TL5100Z (used as a cd transport) with a Musical Fidelity A324K DAC plus Monarchy jitter reducer. The DAC feeds a Stax 717 amp, which runs the two Stax phones. An output from the Stax runs to a Pioneer 50 W/CH receiver. The Koss ESP6 is connected to the Pioneer through the Koss adapter box which is plugged into one of the speaker outlets of the Pioneer.

The amplification is not really comparable here. For a start the Stax 717, even bought direct from Japan cost about 10 times the Pioneer receiver. The Stax is class A, the Pioneer a conventional amp. As well, the 717 is fed directly from the DAC, the Pioneer has one addition connecter stage. The physical set-up made it hard to run the connections direct to the Pioneer.

Several contributors have recently been raving about another old phone, the Stax SRX3 run from amps through an amplifier and transfomer adapter box, compared to various Stax headphone amplifiers. Clearly a lot will depend on the quality of the amp.

I tried to check the quality of the Pioneer by running a dynamic phone from the headphone jack, the Sennheisser CX300. It sounded too grainy by comparison with electrostatics so that this comparison made no sense.

COMFORT ISSUES

Weighing in at almost 2 lbs, these are not phones you are likely to want to boogy with. They also fit very tightly, although you can bend back the metal bands to a better comfort level. They are quite isolating, and sometimes that is good. I can listen to these in bed with my wife asleep, without disturbing her. In fact listening on a bed with a soft pillow is the ideal way to use these phones, the weight is fully supported and the listening experience is very comfortable. You can not do this with the Stax because they are open back and any close surfaces, such as the pillow will cause a backwave which will muddy up the sound.

CONCLUSION

The ESP6 remains an interesting product almost 40 years after it was made. It has much of the clarity one expects from an electrostatic with good treble and bass extension. It lacks the "airiness" of the comparison phones and has a more congested sound in complex passages. However, it has smoothness that the other Stax lack and they show some harshness by comparison.

If I had to run out of the house on fire and could only grab one set, I would still grab the Sigma/404, because of its sound quality and rarity.

However, if you can get a working pair of ESP6 under $100.00, you might very well find use for them. They are fine with rock/pop but really need some work on their bias problem to be considered a good all-around set of headphones.

Below is a schematic and board diagram for anyone else working with these.

SCHEMATIC.jpg

BOARDS.jpg


I am adding the 2 photos below to show the adapter box and its schematic.

TR3Pic.jpg


TR3Schematic.jpg
 
Apr 23, 2007 at 1:34 AM Post #2 of 94

facelvega

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a fine contribution to the forum, not just for today but one of those nuggets that the search function will turn up again and again when we need it. Many thanks!
 
Apr 23, 2007 at 3:40 AM Post #3 of 94

wualta

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Great photos, nice comprehensive look at the history. Kudos.

Interesting that Koss introduced their first ES 'phone the same year Stax introduced their second, the SR-3, the one that furnished OEM parts to many US Koss competitors (Magnavox, Radio Shack, Marantz, and, according to Spritzer, Superex).
 
Apr 23, 2007 at 7:59 AM Post #4 of 94

spritzer

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Great post and thanks for the internal pictures and schematic. Now I can finally turn mine into ESP-7's.
icon10.gif
Just to clarify, that there was actually a Stax SR-2 and if was released in the mid 60's but in limited numbers. The SR-3 was released in early '68.

Quote:

Originally Posted by wualta /img/forum/go_quote.gif
Great photos, nice comprehensive look at the history. Kudos.

Interesting that Koss introduced their first ES 'phone the same year Stax introduced their second, the SR-3, the one that furnished OEM parts to many US Koss competitors (Magnavox, Radio Shack, Marantz, and, according to Spritzer, Superex).



I don't know if Suprex used OEM Stax drivers or if the just stole the design. Next time I open them up, I'll look for any similar materials or construction unique to Stax. There is a mint SR-3 on the way to me now so I can compare the drivers and also see what is the difference between the SR-3 models.
 
Apr 23, 2007 at 1:50 PM Post #5 of 94

spritzer

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Thanks to the pictures and schematic edstrelow posted I have a working ESP6-ESP7 hybrid. The ESP7 was the same phone as the 6's but with all of the electronics external so I guess that makes mine a ESP7. I just finished this 10 minutes ago but first impressions are good. The bias is slightly higher then the Stax normal standard and so is the turn ratio of the transformers. The bass is slightly fuller but the highs are a bit shrill or etched. Now for a quick clickable picture:



They use a cable from a Magnvox 1A9217 and the headband from my ESP9 because the stock doesn't fit my head.
 
Apr 23, 2007 at 6:13 PM Post #6 of 94

edstrelow

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Quote:

Originally Posted by spritzer /img/forum/go_quote.gif
Thanks to the pictures and schematic edstrelow posted I have a working ESP6-ESP7 hybrid. The ESP7 was the same phone as the 6's but with all of the electronics external so I guess that makes mine a ESP7. I just finished this 10 minutes ago but first impressions are good. The bias is slightly higher then the Stax normal standard and so is the turn ratio of the transformers. The bass is slightly fuller but the highs are a bit shrill or etched. Now for a quick clickable picture:



They use a cable from a Magnvox 1A9217 and the headband from my ESP9 because the stock doesn't fit my head.



Cool!

Even for me the headband was a bit tight, and my head is somewhat narrow. Bending the metal portions of the band was sufficient to make them comfortable.

The ESP7 photos in the Koss historyshow what looks more like a ESP9 cup which is smaller.

http://www.koss.com/koss/kossweb.nsf/kmuseum?openform&60s^Products

I assume that you now have an empty cup behind the transducers and that you can try different kinds of damping. Do these sound any good without the backs of the cups? I.e. as open back phones, or partially baffled?

What is the bias on these phones? The ESP6 owners manual was a bit imprecise but implied that they should run about 500 volts, which is more like Stax pro bias. Obviously they run at lower bias, and with their somewhat dodgy self-bias operation considerably below this. The manual refers to signficant reductions in volume as the bias drops, but only minor changes as the bias goes higher.

My impressions of mine are that they sound best after the bias has been boosted by a loud signal, even if the phones are turned down afterwards. They seem show more detail and "air."

I wondered if mine might run better with higher value capacitors in the bias circuit?

I am curious about the Stax SR2 because I can't find it listed on their history page. http://www.stax.co.jp/Export/History.html Evidently Stax wasn't too proud of it.
 
Apr 23, 2007 at 7:13 PM Post #8 of 94

spritzer

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Quote:

Originally Posted by edstrelow /img/forum/go_quote.gif
Cool!

Even for me the headband was a bit tight, and my head is somewhat narrow. Bending the metal portions of the band was sufficient to make them comfortable.

The ESP7 photos in the Koss historyshow what looks more like a ESP9 cup which is smaller.

http://www.koss.com/koss/kossweb.nsf/kmuseum?openform&60s^Products

I assume that you now have an empty cup behind the transducers and that you can try different kinds of damping. Do these sound any good without the backs of the cups? I.e. as open back phones, or partially baffled?

What is the bias on these phones? The ESP6 owners manual was a bit imprecise but implied that they should run about 500 volts, which is more like Stax pro bias. Obviously they run at lower bias, and with their somewhat dodgy self-bias operation considerably below this. The manual refers to signficant reductions in volume as the bias drops, but only minor changes as the bias goes higher.

My impressions of mine are that they sound best after the bias has been boosted by a loud signal, even if the phones are turned down afterwards. They seem show more detail and "air."

I wondered if mine might run better with higher value capacitors in the bias circuit?

I am curious about the Stax SR2 because I can't find it listed on their history page. http://www.stax.co.jp/Export/History.html Evidently Stax wasn't too proud of it.



The ESP7 used smaller cups much like the ESP9, no need for the extra space.

The cups were at first empty but I left the pelt pads in place at the back of the drivers. There was a slight echo and some colorations so I put some medium density foam in there and they sound better that way. They sound very different driven from a Stax transformer, slightly darker and more closed in, with a fuller midrange and much sweeter highs. The Koss unit is a bit more upfront, with better bass and the slight etch on the highs.

Higher value caps might help but it didn't work with the Beyer ET-1000. In the stock bias supply Beyer specs 1uf caps and they can still only maintain the bias for a few minutes. A larger charge could help with those quiet passages. It's a cheap tweak.

I've tried them without the cups only when I was testing the drivers. These older phones are not to keen on the open backwave with the Sr-Lambda as the best exception.

I'll try and test the bias when I put the pcb's in a case. That was meant to be today but somebody has taken my 16mm stepped drill so I drill any holes for the Stax sockets.
mad.gif
I was listening to them on the SRD-6 just prior to the Koss modules and I had to jump for the volume control so the bias might be higher. The ratio on the transformers is higher then what Stax used so those warning lights were a necessity. I've seen to many burned diaphragms lately in phones that have been used with transformers. Flick the wrong switch and the phones are dead.

I have seen two sets of SR-2's for sale over the years but stupid me I didn't buy them. There isn't much known about them but I'm certain they were a step up from the SR-1. Here are some pictures of them.
 
Apr 28, 2007 at 8:22 PM Post #9 of 94

edstrelow

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I started having a problem with the Pioneer receiver that I was using to run the ESP 6. The 6's are hooked up to one of the two sets of speaker terminal. The regular speakers to the other. However even when the ESP terminals were switched off at the receiver and adapter box, the protection circuit of the amp was cutting in on loud passages when the speakers were on.

I finally guessed that it might be related to the fact that the phones use a common ground, even though the adapter box has separate grounds. The phones themselves use a conventional headphone jack with L, R and ground.

Just pulling the right ground connection from the receiver to the adapter box cleared up this problem, without making any obvious difference to the performance of the ESP6 when it was switched in. The adapter box apparently works fine with the left ground connected.
 
Apr 28, 2007 at 10:27 PM Post #10 of 94

sandyr

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Newcomer here so hope I'm not chewing over old fat.

Been looking for info for some time on the old Koss phones since I picked up an ESP 6A headset minus interface box for one pound (US$2.00) at a fleamarket. After repairing the leads which had failed at the entry point to the left cup the sound seemed promising when run from the loudspeaker posts but with the well documented volume variation with programme content. I was therefore delighted to come upon this superb contribution which gave me considerable insight into the workings of these headsets but I am left with the question of what was in the black box and whether a similar schematic exists for the esp 6A or in what regard this differs from the externally biased ESP 6.

I would be most grateful for any observations.
 
Apr 29, 2007 at 11:33 AM Post #11 of 94

spritzer

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Quote:

Originally Posted by edstrelow /img/forum/go_quote.gif
I started having a problem with the Pioneer receiver that I was using to run the ESP 6. The 6's are hooked up to one of the two sets of speaker terminal. The regular speakers to the other. However even when the ESP terminals were switched off at the receiver and adapter box, the protection circuit of the amp was cutting in on loud passages when the speakers were on.

I finally guessed that it might be related to the fact that the phones use a common ground, even though the adapter box has separate grounds. The phones themselves use a conventional headphone jack with L, R and ground.

Just pulling the right ground connection from the receiver to the adapter box cleared up this problem, without making any obvious difference to the performance of the ESP6 when it was switched in. The adapter box apparently works fine with the left ground connected.



This was a big problem for Koss back in the day. Everything is connected together to ground and many amps don't like it. The grounds for both cups are connected together in the TRS plug so lifting one return should have impact on performance.

Quote:

Originally Posted by sandyr /img/forum/go_quote.gif
Newcomer here so hope I'm not chewing over old fat.

Been looking for info for some time on the old Koss phones since I picked up an ESP 6A headset minus interface box for one pound (US$2.00) at a fleamarket. After repairing the leads which had failed at the entry point to the left cup the sound seemed promising when run from the loudspeaker posts but with the well documented volume variation with programme content. I was therefore delighted to come upon this superb contribution which gave me considerable insight into the workings of these headsets but I am left with the question of what was in the black box and whether a similar schematic exists for the esp 6A or in what regard this differs from the externally biased ESP 6.

I would be most grateful for any observations.



There are probably only some resistors inside the adapter box for the 6's. Both the ESP6 and ESP6A are identical apart from some of the grounding was reworked with the 6A. Mine run fine without the external box connected (it wasn't included with my set either) directly from the speaker outs.
 
Apr 29, 2007 at 11:43 AM Post #12 of 94

sandyr

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Quote:

Originally Posted by spritzer /img/forum/go_quote.gif
This was a big problem for Koss back in the day. Everything is connected together to ground and many amps don't like it. The grounds for both cups are connected together in the TRS plug so lifting one return should have impact on performance.



There are probably only some resistors inside the adapter box for the 6's. Both the ESP6 and ESP6A are identical apart from some of the grounding was reworked with the 6A. Mine run fine without the external box connected (it wasn't included with my set either) directly from the speaker outs.



Interesting that Spritzer's ESP6As run OK direct from the speaker posts while mine seem to lose their charge with the modulation. This suggests that following the cleaning procedures advocated by Edstrelow ought tp be the next step to follow.

Many thanks
 
Apr 29, 2007 at 11:56 AM Post #13 of 94

spritzer

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Quote:

Originally Posted by sandyr /img/forum/go_quote.gif
Interesting that Spritzer's ESP6As run OK direct from the speaker posts while mine seem to lose their charge with the modulation. This suggests that following the cleaning procedures advocated by Edstrelow ought tp be the next step to follow.

Many thanks



Mine were pretty beat up when I got them and didn't work at all. The removed the electronics and all wiring and converted them to the Stax standard plug. Then I cleaned the boards as well as I could, lifted all bad traces, resoldered all connections and simplified the circuit a bit(removed extra connections, extra wires etc.).
 
Apr 29, 2007 at 4:05 PM Post #14 of 94

sandyr

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Quote:

Originally Posted by spritzer /img/forum/go_quote.gif
Mine were pretty beat up when I got them and didn't work at all. The removed the electronics and all wiring and converted them to the Stax standard plug. Then I cleaned the boards as well as I could, lifted all bad traces, resoldered all connections and simplified the circuit a bit(removed extra connections, extra wires etc.).


Mine seem to be in reasonable order but I haven't examined very closely but that will be the next job.

Meant to ask on the last note - are you using transistor or valve (tube) output stage on your amp which might contribute to the problem?
 
Apr 29, 2007 at 4:25 PM Post #15 of 94

spritzer

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Quote:

Originally Posted by sandyr /img/forum/go_quote.gif
Mine seem to be in reasonable order but I haven't examined very closely but that will be the next job.

Meant to ask on the last note - are you using transistor or valve (tube) output stage on your amp which might contribute to the problem?



I'm using an old 1974 Kenwood solid state integrated to drive them. It doesn't have any problem with the common ground.

I found out when I recased the internal PCB's that some wires were broken inside the headphone plug so that would have caused them not to work and some of the bare copper traces were corroded all the way through. The foam inside the earcups rots away with time and turns into this oily mess that corrodes all metallic parts it touches
 

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