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Old 10th February 2013, 01:33 AM   #1
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Join Date: Oct 2009
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Default Making an electrostatic headphone driver.

Dear all,

I am currently building a driver for a Stax Lambda Pro before I start work on my own electrostatic headphone. the first version will be built to low cost, a later version may be more Super HiFi. The headphones come from ebay are in beutiful condition but I assume they are working fine, I dont know yet and getting something working (and cheaply) tested is the first objective.

This is the first installment. It would have the title "Voltage doublers, how many stages?"

I am basing the voltage doubler / "ladder", on the stax circuit. ""high-quality-stax-headphone-adapter""

Now I just happen to have lots (8 unused) torroidal 230 V -> 2X22 V transformers from a bargin ebay buy, but they do look rather old and the sticky tape sealing then is loosing its sticky but they are not so bad as to need electrical tape. I thought I might play with these to drive the headphones in the short term. I suspect I will apresiate the knowledge gained.

So I thought to save my unused transformers I would build more stages on the voltage doubler and run the system off 44 V AC, well the doubler is less and less a doubler with distance from the supply. Tomorrow I will set up a second transformer effectively making a over built 1:1 transformer so will have 240V, and in series making 88V driving it though I suspect if I was buying transformers for the job I might want 100V like the original circuit as my estimates suggest the numbers would be right on the money for voltages. Some times it good to simulate in reality, much as I should learn how to do this on the computer.

Like the Circuit I attached, I used 0.1 uF capacitors (650 and 1000V) and 1000V diodes, but I guess each stage only sees the magnitude of the AC signal sent to the ladder so lower ratings woudl have been fine? I found little benefit after the first 6 diodes. The 7th doubler brings me to about 276V and the 8th makes no benefit at all.

I assume the reduction in effectiveness of the voltage doublers is due to the capacitors smoothing the AC more and more over time, does this mean the frequency of the voltage doubler / "ladder" would be very limited in maximum frequency of operation making frequencies like 1 Mhz out of practicality? Or possibly due to leakage in my breadboard (at such low voltages I hope not, but it was super cheap)

I see Quad uses 8 diodes in its power supply and a 10th of the size of the capacitors, used by Stax, so knowing Quad amplifiers and how they optimise for performance/cost I assume all 8 provide a benefit, so I doubt my experience with 6 stages being the limit to voltage doublers being practical / cost effective. I assume the same frequancy of supply (50Hz) with much lower capacitance's and closer to ideal diodes would improve the stages efficiency in driving an ideal unleaking speaker.

All thoughts apresiated as I am nearly completely self taught in electronics and have a reasonable skill in maths.
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Old 10th February 2013, 01:49 AM   #2
DUG is offline DUG  Canada
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Are the diminishing returns on the V multiplier a result of the meter (loading) you are using to measure the voltage?

Why multiply from 44 or 88?

See jpg

Even if you start and end with 230VAC it will be less multiplication than with 44 or 88.
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Old 10th February 2013, 01:54 AM   #3
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Thanks great suggestion
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Old 10th February 2013, 02:14 AM   #4
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I was using a Multimeter, thankyou, I should have thought of this. I also like your suggestion of using 240V as a good voltage to multiply. Is thier any reason to use much higher transformer outputs, I would imagine beyond 250V the costs just increase?

I have an old osciliscope (again from ebay) that I will have played with not enough and will report back. I read in many places that voltage doublers dont deliver currrent so I was prepared, just did not engage the brain.
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Old 10th February 2013, 11:30 AM   #5
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Ok so my osciliscope gives aproximately the same answer DC wave with 320 V to about 270 V peaking so I guess its got the same order of magnitude of input resistance. the 7th is noticbly smoothing so maybe a bad connection rather than load resistance.

I guess I need to make an extremly large voltage devider with a lot of resistance for meeasurement or is thier a better solution?
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Old 10th February 2013, 12:57 PM   #6
DUG is offline DUG  Canada
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Quote:
Originally Posted by owenhamburg View Post
I guess I need to make an extremly large voltage devider with a lot of resistance for meeasurement or is thier a better solution?
yes...but leave it in place.

Disconnecting it will change the load and the output voltage will change.

Or change to the following cct.

How much V do you need?
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Old 10th February 2013, 01:32 PM   #7
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Thanks Doug,

With a transformer driving a trasnsformer in reverse I can get all the voltage I need. It seems I have over 2000 V at the end of my ladder. Just two diodes and I reach the desired voltage. I have 4 X 2.2 Mega Ohm that I guess a simple rcrcrcr network might help with smoothing if it prooves an issue.
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Old 10th February 2013, 01:46 PM   #8
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So now thats resolved I guess its time to work with the lethal step up transformer parts.

As I stated before I have a few 30 VA 230 VAC to 2 X 22V AC transformers in my bits box.

I am guessing I can just use the two, 22 Volt secondaries in parallel and connect these to the amplifier and use the mains output (230 V) to drive the stators?

Time to chop up some old molex plugs (from broken atx power supplies) until the Stax sockets come in the post for the next stage. This is a little scary as the voltages + power I guess could be lethal on the end of a 50 W power amp. Im going to use the Quad 306 power amp as it has some protection against overload and driving a short circuit that I know works.

I do now wish I had bought some designed for function insulation tape, something to buy on Monday.

I have some candles about the house, can I use any wax as an insulator or is only some wax (bee's wax for example) as a high voltage insulator?
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Old 10th February 2013, 04:42 PM   #9
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It may work,But you may find you will need a higher step-up ratio.
You can add a custom primary to do this.
Head phones typically need about 1:20 to 1:50 transformation ratio.
Also you may need to use 2 or 4 cores to get a good low frequency response without distortions.

Keep in mind what frequency the transformer was designed for (50hz or 60Hz) and to get to half the frequency input 25Hz/30Hz with a 22v winding then 11v is the most you can apply to it or you will get core saturation distortions.
This will only get you 110v maximum with your transformer.

If you were to add a custom primary to give you a 1:40 ratio then your maximum output would still be 110V at 25Hz/30Hz but with only a 2.75Vrms drive signal and you would need 4 cores to get 440Vrms, the range needed to fully drive a headphone.
Although 100V to 220v may be enough for to listen to and much of this will be determined by the D/S and your bias voltage.

The Micro driver I made was the size of a credit card,

A Micro ESL Made From Old Credit Card's

It had a D/S of about .030" and used bias of 500V.
The transformer I used was from a EL34 P-P tube amp.
I didn't measure it but from the requirements of the tubes I am guessing that the ratio was about 1:25 to 1:35 or so.
It got to a comfortably loud level and was a pleasure to listen to.
Even the bass sounded great with no distortions.
Although that transformer was deisgned to go that low as well.

I have gotten similar results with my Mini Desktop ESL's at low levels and listen to them fullrange with no issues providing I don't exceed the voltage/frequency saturation requirements of the 60Hz designed transformers.

Yes, These voltage can be lethal coming from an amp that can deliver the current!!
So do be very careful!!!!!!!!!!!!!
I have been nailed by them before and they pack a Walop.!!!!
More so than a bias supply that has a current limiting resistor on it!!

jer

P.S. A good conformal coating to use is some Plain ole' Clear Acrylic Enamel spray paint.
I have tested this stuff to be good up to 1500V per mil and even as high as 2100v in some cases but definitely good for at least 1000v per mil (.001").
I used this on my HV supply and have had no issues yet at 13.6KV.
I also use Clear silicone rubber as well, it is messy to use and hard to get off should you need to repair something, but it is one of the best thing out there as well.

Last edited by geraldfryjr; 10th February 2013 at 05:06 PM.
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Old 10th February 2013, 08:55 PM   #10
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Default Some success but not HiFi yet.

Thankyou Geraldfryjr.

Your quiet right it does work, and yes I do think I need a higher step up transformer. 1:10 is quiet low volume.

When driving at distortion generating levels on the Quad 306 its an OK volume for listening in a quite house, so I assume a second transformer per channel may give me enough step up volume, definitely further experiments to come.

At first I rigged up the plug in the wrong way. A mirror image of what I want as the articles I read did not make it clear if they where viewing from front or back. The sound was fine on one channel but mains hmm was on the other channel. I doubt I could have broken anything as its not going to make the flash point no mater how you wire it I assume. After wiring it the correct way (see attachment I found on the web) the sound is stereo and I think louder and better.

The Bias is set to 620 V (a little over the targeted 580 V) but I think I will add a few filter stages to reduce the modulation on the Bias and they might bring down the voltage a small amount. At the moment I don't have any filtering on the bias supply.

Since I don't yet have a Stax socket to (yet as three are due in the post), I cut a small terminal block into 5 parts and attached them to the headphone plugs pins. In fact this construction is only terminal blocks to connect the components, and dropped solder on any multi core wire to avoid loose strands.

The treble is already good, and I do have a big smile on my face, only slightly effected by the bass distortion. Expect an update soon with the filtered bias supply.
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