ESL headphones troubleshooting
I had this posted on the headphones section but didn't receive any responses for five days, perhaps I should have posted it here instead.
I recently made ESL headphones, just to test my materials - I am in a (long) process of building big electrostatic panel speakers (160cm x 25cm), and I wanted to get a taste of the ESL sound.
I had a piece of 1mm aluminum, and I fashioned round stators from it. 5cm in diameter. I made the spacers from a cover of a plastic bucket - also 1mm in thickness. I don't know what the material is. Spacers are about 5mm in width, so the area for the diaphgram to move is 4cm in diameter. The diaphgram is cheap wrapping plastic (of the Elmu variety for those in Finland) - it's thickness is stated as 10µm on the box, material is LD-polyetylene.
I glued all the parts together with superthin soldering strips lodged in the right places for the electric contacts. Then I applied a bit of graphite powder on the diaphgram with cotton pieces. I measured the surface resistance to be in the order of 22 to 42 kilo-ohms per centimeter.
After the glue had dried, I soldered all the wires, put the drivers in their case (full sized headphones previously closed, now drilled open, with relatively tight ear cushions). For driving the ESLs I have two Hammond 125D push pull transformers and a high voltage source. Signal comes from my 1W Darling amp with approximately 250v signal swing, 5k / 4R OT.
I also put a lot of cotton on the backs of the drivers.
I thought I was good to go, and started experimenting. On the first try I got some sound, but it was very very weak even with the source and Darling amps volumes at max. Barely able to hear anything. At this point the bias was 405v and transformers at 4R / 27k. So I tried progressively bigger transformer ratios, and got a tiny bit more sound. Then I made a new voltage source, with 742v bias and transformers at 1.5R / 27k.
With this I got an ok listening volume, maybe 3/4 of what I usually use (I don't listen super loud), but even now I have Darling volume at 100% and source volume at 100%. What is more puzzling is that I have absolutely no bass. None at all. When comparing with my AKG601 headphones and with EQ (my source is a computer) I found I have no action at all pretty much below 700Hz. The response starts dropping around 1 - 2 kHz.
The sound is quite good on the highs - excellent imaging and presence. I find ESLs to be quite promising. However my first experiment is clearly not done right.
So, no volume or bass. Where did I go wrong?
Is the diaphgram area too small, with only 4cm diameter?
Did I use too much graphite - I think I did, but could this contribute to these problems?
Is the LD-polyetylene completely unsuited for ESLs? Could I have used too much tension?
I'm pretty much at a loss here for explanations. Any help would be appreciated.
Welcome to the club! I know your frustration. Believe me, I've been there. :)
May I point you to my DIY electrostatic headphone thread? http://www.diyaudio.com/forums/headp...headphone.html
From what I've read and understood, you might have a problem with your graphite coating. I didn't like it. I did try it and found that it was very difficult to get a good coating using it. If you just want to give it a try, liquid soap could be an easy alternative. However, the coating won't last very long. You can also try to mix your graphite powder with PVA glue and water like what I did. The link to the thread is here http://www.diyaudio.com/forums/headp...ml#post1837517
Calvin's formula works too - http://www.diyaudio.com/forums/headp...ml#post1844005
If you need further help, don't hesitate to yell. There are many experts here.
I think that the darling amp you use is not powerful enough to drive the headphones to a good listening level. If you can try driving the headphones with a more powerful amp, you will notice better and louder sound.
Your stators are very small. That could be the reason why you didn't get good bass. With the spacer on, your diaphragm vibrating area is only 40 mm. That is too small.
You might want to try to use 0.5 mm spacer instead of 1 mm. You will get a lot louder sound by doing so.
The food wrap plastic isn't all that bad. Trust me; I know. :)
I think the diaphragm may be too conductive. You could try putting a 10M...100M series resistor between the HV bias supply and the diaphragm; this will help to maintain constant charge on the diaphragm and would help bass reproduction.
If nothing else works and you just want to try something temporary to determine if the diaphragm coating is the problem, try this:
Remove the diaphragm, thoroughly clean the spacers and surrounding areas and replace the diaphragm. Mask off the edges of the new diaphragm and apply a thin coating of any hand dishwashing soap containing sodium laurel sulfphate. I assure you that this coating works perfectly because I've tried it but I can't verify how long it would last. I use Licron Crystal ESD coating on all of my diaphragms. Good luck with it.
You're right, Charlie, graphite is a total mess and will do that! Here's some tips to try to find leakage paths:
- darken the room, give your eyes time to adjust, and check for corona
- listen for sizzle sounds
- put a parallel circuit of a capacitor and a neon indicator bulb in series with the diaphragm. The capacitor will charge up to the threshold voltage of the neon bulb -- when the bulb triggers the capacitor charge is delivered to the diaphragm. Thus: the less frequently the neon bulb flashes, the less leakage current you have. If it never flashes, well... probably a bad connection.
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