Electrostatic Phones: membrane issues?

[davstev]

Hi Folks,

Not too long ago, I bought a near-mint, still boxed pair of early 70s, Koss ESP-9 electrostatic headphones. Obviously old, but in good cosmetic shape & seems barely used.

After several weeks I found the time to hook them up. I wired the E-9 energizer to the back of my Kenwood 700M (170W per channel), power amp. (Pretty sure I hooked them up right!)

The result was disappointing to say the least. The signal is barely audible (even when cranked waaaay up) and very distorted at that, and all treble. Just wrong.

My audio tech, who is quite knowledgable, says that there's a chance it's the membrane coating, which may have deteriorated with time. Says too, if not the membrane, then the polarizer could be the issue.

He'll get to the root of the problem, but in advance of me handing them over to him, wondering if anyone has any issues with electrostatic 'phones and membrane coating issues, and would like to comment, and/or knows of anyone who could remedy this problem.

Thanks
David

 

[bimasta]

In "the old day", the coating was graphite powder rubbed on/into the mylar. It didn't last. I had early Martin Logan 'statics (more recent than your Koss) and their output diminished, such that I had to replace them 10-15 years after they were made. ML told me they were graphite, and the new ones had a chemical coating that would last "forever". This was in the mid-90s.

DIYers still make 'statics using graphite powder. You could make replacements for your Koss using graphite. Matching factory specs would be less important than L and R ears matching.

Or as you say, the electronics may be the fault. At their age, components deteriorate. If it hadn't been used in years, decades, and you just plugged it into 110-20V, that alone would have destroyed electrolytic caps. Fixing the electronics would be easier: it's a small unit. I'd try that first.

 

[davstev]

That's interesting! And of course slightly daunting. I have access to graphite powder quite readily, actually. Is it a matter of re-coating the existing mylar in the phones? How successful is it in reviving the phones to sound as good as original (or close)?

Yes, will pursue the electrolytic possibility first, it sounds more promising, or at least, simpler!

 

[bimasta]

I wish I could help you more Dave, but what I said above is just knowledge picked up along the way, not hands-on exprerience — that was limited to putting the ready-made new panels into my MLs: easy.

But continuing to theorize, I'd definitely check the electronics first. And you ask ask a great question: "Is it a matter of re-coating the existing mylar in the phones?"

I didn't think of that! I was thinking of making new ones; but getting the tension right, and then inserting them in place of the old would be a nightmare. It would so easy if you can use the old diaphragms, and just add new graphite, to restore conductivity.

However, rubbing graphite into them is forceful, could tear them, they'd need to rest on something solid.

Try to find out if they can be taken out, intact — I'd think Koss might have made them that way, and offered new replacement diaphragms, like a new stylus on a cartridge. You won't find those replacements nowadays, but if you can remove yours, add graphite safely, and put them back, that might work.

 

[Pio1980]

Also, check bias power supply.

 

[twiiii]

Are you using a power cord with the energizer? Its by far the best way to use the phones. You really shouldn't use much more than 50 watts.

 

[bimasta]

You really shouldn't use much more than 50 watts.

I didn't know that. I had similar Koss 'phones, 9B. Also old. I didn't like them — not as bad as davstev's (they weren't as old when I had them) just lifeless. Maybe I over-drove them? Or they were just on the way out...

 

[davstev]

Are you using a power cord with the energizer? Its by far the best way to use the phones. .

It's plugged into the wall, and, it is hooked up to the speakers, too. Is it an option to use it without plugging in to AC power? Seems that it requires that to charge the plates, no?

 

[Pio1980]

It's plugged into the wall, and, it is hooked up to the speakers, too. Is it an option to use it without plugging in to AC power? Seems that it requires that to charge the plates, no?

Yes, afaik the polarizing bias voltage is provided by an internal power supply.
You don't need the power supply to just play the speakers alone

 

[murugan19]

In "the old day", the coating was graphite powder rubbed on/into the mylar. It didn't last. I had early Martin Logan 'statics (more recent than your Koss) and their output diminished, such that I had to replace them 10-15 years after they were made. ML told me they were graphite, and the new ones had a chemical coating that would last "forever". This was in the mid-90s.

DIYers still make 'statics using graphite powder. You could make replacements for your Koss using graphite. Matching factory specs would be less important than L and R ears matching.

Or as you say, the electronics may be the fault. At their age, components deteriorate. If it hadn't been used in years, decades, and you just plugged it into 110-20V, that alone would have destroyed electrolytic caps. Fixing the electronics would be easier: it's a small unit. I'd try that first.

Hi bimasta,
You are quite right that it is graphite but it is not "powder" - it is "Colloidal Graphite" - colloidal graphite in isopropanol (without the polymer binder) - trade name Aquadag. And it needs to be rubbed into the membrane with an alcohol pad. Continue rubbing (alcohol wetted pad with the colloidal graphite) until the resistance (probes 1" apart) needs to be about 1M ohm. Both L & R membranes have to be the same - so if you have one good membrane, measure the conductance and duplicate it on the other, rubbing a very little colloidal graphite at a time and EVENLY over the entire surface. The membrane is VERY delicate and assuming that the membrane is intact (no holes from prolonged touching the polarizing plates (300-500VDC) from continuous overdriving) then you should be good to go. Also the membrane should be taut - not loose and sagging.
A DELICATE touch and lots of patience are the keywords.
I made my own electrostatic headphones (and then tweeters) from an old article (from the mid 1970s ) of Wireless World (I'm sure if you search for Wireless World Archives, you will find it (Try: https://keith-snook.info/wireless-world-articles/wireless-world-articles.html). The bias voltage came from the plate circuit from my tube amp. One problem that plagued me (and also in the Wireless World article) is that the wires connecting the headphones have to be low capacitance (<90pF). Years later (in my professional life), I realized that Servo Motors also needed low capacitance wiring and the best are those from Olflex (with Helukabel, Omron and Lutz a close third and Belden dead last).
I own a pair of electrostatic headphones - Koss ESP 9B and Stax SRL700 (lambda). Unfortunately, if you like heavy bass, then Electrostatic is not for you True of Quad and Martin Logan speakers too - they use separate cone drivers for that part of the frequency spectrum). The Stax is more "flat" than the Koss and not heavy (by weight) and "clamping over the ears" (by pressure) like the Koss. Also, the Koss has air-filled ear cushions that tend to leak and will go flat against your ears (then the clamping on your ears is really painful). At high altitudes (like when I had a project in Denver) the air-cushions will somewhat regain their "cushion"! I finally resorted to having them replaced with closely matching Sennheiser foam ear cushions. (Sennheiser has a fantastic support department - on a par with Oppo).
(BTW: Each Koss comes with an individual frequency plot of it's range!). Even though the plot is fairly flat (in the usual +- 3dB range), the Stax has a better transient response. The Koss has better bass (due to it's closed back design) than the Stax (open back design). Both my Stax and the Koss are doing fine (except I tend to use the Stax more often).
Note: Stax made "Electret" headphones too - not to be confused with "electrostatic" headphones - they were piezoelectric based.
If I ever win the lottery I will buy the Sennheiser Orpheus HE90! (Not the HE91 or the latest H-1)
Sincerely,
John Graham

 

[Pio1980]

Hi bimasta,
You are quite right that it is graphite but it is not "powder" - it is "Colloidal Graphite" - colloidal graphite in isopropanol (without the polymer binder) - trade name Aquadag. And it needs to be rubbed into the membrane with an alcohol pad. Continue rubbing (alcohol wetted pad with the colloidal graphite) until the resistance (probes 1" apart) needs to be about 1M ohm. Both L & R membranes have to be the same - so if you have one good membrane, measure the conductance and duplicate it on the other, rubbing a very little colloidal graphite at a time and EVENLY over the entire surface. The membrane is VERY delicate and assuming that the membrane is intact (no holes from prolonged touching the polarizing plates (300-500VDC) from continuous overdriving) then you should be good to go. Also the membrane should be taut - not loose and sagging.
A DELICATE touch and lots of patience are the keywords.
I made my own electrostatic headphones (and then tweeters) from an old article (from the mid 1970s ) of Wireless World (I'm sure if you search for Wireless World Archives, you will find it (Try: https://keith-snook.info/wireless-world-articles/wireless-world-articles.html). The bias voltage came from the plate circuit from my tube amp. One problem that plagued me (and also in the Wireless World article) is that the wires connecting the headphones have to be low capacitance (<90pF). Years later (in my professional life), I realized that Servo Motors also needed low capacitance wiring and the best are those from Olflex (with Helukabel, Omron and Lutz a close third and Belden dead last).
I own a pair of electrostatic headphones - Koss ESP 9B and Stax SRL700 (lambda). Unfortunately, if you like heavy bass, then Electrostatic is not for you True of Quad and Martin Logan speakers too - they use separate cone drivers for that part of the frequency spectrum). The Stax is more "flat" than the Koss and not heavy (by weight) and "clamping over the ears" (by pressure) like the Koss. Also, the Koss has air-filled ear cushions that tend to leak and will go flat against your ears (then the clamping on your ears is really painful). At high altitudes (like when I had a project in Denver) the air-cushions will somewhat regain their "cushion"! I finally resorted to having them replaced with closely matching Sennheiser foam ear cushions. (Sennheiser has a fantastic support department - on a par with Oppo).
(BTW: Each Koss comes with an individual frequency plot of it's range!). Even though the plot is fairly flat (in the usual +- 3dB range), the Stax has a better transient response. The Koss has better bass (due to it's closed back design) than the Stax (open back design). Both my Stax and the Koss are doing fine (except I tend to use the Stax more often).
Note: Stax made "Electret" headphones too - not to be confused with "electrostatic" headphones - they were piezoelectric based.
If I ever win the lottery I will buy the Sennheiser Orpheus HE90! (Not the HE91 or the latest H-1)
Sincerely,
John Graham

Sorry to report that valued member Bima Stagg passed a couple years ago, R. I. P. Bima.

AudioKarma Member Bimasta Has Passed

Bimasta and I are friends on social media and there are some vague comments that are suggesting that he passed away. I don’t want to set off alarms, and it does seem like there could be some unintentional shadings of language or conclusions being jumped to. But if it’s true, we’ll miss you...

audiokarma.org

Your information is still a worthwhile contribution to the thread.