Making an electrostatic headphone driver.

[owenhamburg]

On further listening to bass heavy music the peaks are I fear too high to think just two mains transformers can do the job.

I dropped into conrad (german version of Tandy/Maplins) to collected some reistors for measuring current and making a voltage ladder, only to find one lady serving on the cash register and a queue of 15 people in frount and waited 5 mins and no one left and 20 people came behind me, so I walked out, saying that the serivice was too poor and left my good at the cash register, at least 10 people followed me. We will have to wait a little longer for resistors to measure current as I will now order online.

[geraldfryjr]

Yes, every time you Double the bias voltage you will get a 6db increase in the sensitivity.

Just be sure you don't exceed the rating's of your headphones.
I don't have any experience with those so I don't know how high you can take them.
But DIY ones are not hard to build either!!

Typically your voltage will be 230v * 1.41 * Nstages= Vbias
The more stages you have there will be some losses but not much unless you have as many that are in my supply.
There I have probably over 1Kv or more of loss with 14 stages (2 diodes per stage).

jer

[esltransformer]

Originally Posted by geraldfryjr

Yes, every time you Double the bias voltage you will get a 6db increase in the sensitivity.

Just be sure you don't exceed the rating's of your headphones.
I don't have any experience with those so I don't know how high you can take them.
But DIY ones are not hard to build either!!

Typically your voltage will be 230v * 1.41 * Nstages= Vbias
The more stages you have there will be some losses but no much unless you have as many that are in my supply.
There I have probably over 1Kv or more of loss with 14 stages (2 diodes per stage).

jer

I makes sense to have a high bias voltage but there is also a limit. After 2000V/mm you don't get much more sensitivity because the air will be a conductor itself

[zeonrider]

I have a Koss ESP 9, does the driver for Stax responsible for the Koss?

Regards zeoN_Rider

[owenhamburg]

Originally Posted by Bazukaz

Core saturation happens when almost all microscopic magnetic zones called "Domains" become oriented in a specific direction by magnetic field. No amount of current can increase magnetization by much. The current draw tends to increase very rapidly and not "collapse" when this point is reached with increasing voltage as long as the source can supply it.

Thank you you are quiet right here, I realised I said this after posting.

Originally Posted by Bazukaz

However, as I already mentioned, even small amounts of DC current from the amp can partially magnetize the core so much less is left for audio signal.
So stay away from custom-tuned audiophile's amplifiers from which all of the DC blocking capacitors are removed from signal path

Good to know, Quad amplifiers are good here, the Quad 306 has a servo and the Quad 303 has an output capacitor.

Originally Posted by Bazukaz

The magnetization curve of the transformer core is non-linear. This effect is much more prominent in low frequencies where core magnetization is high. The low impedance(mainly caused by feedback) of a typical discrete amplifier compensates for this : despite current draw varies, the voltage on the transformer secondary follows the voltage at amplifier's output(remember transformer's formula : U1/U2 = N1/N2).
Therefore I view tube amplifiers a bad choice for driving an ESL; their relatively high output impedance causes increased distortion from step-up transformer, in addition to attenuating high frequencies.

Very good points, and although not a fan of valve/tube amplifiers on cost grounds some are class A and many have class A bias AB amplifiers which in my opinion is a factor in good sound. This is under conditions favorable to Valves / Tubes. I would rather not get distracted by the Valves V Transistors V Mosfet discussion as I accept that all can sound good and all can sound bad so its clearly not a transistor or Valve that's deciding the issue of sound quality but the many other factors in the design such as expected output impedance which you highlight here as important and I suspect you are right in this regard.

Originally Posted by Bazukaz

You don't need a scope to determine polarity of transformers. Primary goes in parallel(this case from amplifier) and secondary in series. After connecting, run some steady state signal through the trafos and measure voltage across secondaries via voltmeter. If you wire them in wrong polarity, voltage is subtracted otherwise summed. Just be careful, you may be having hundreds of volts there.

This is actually how I have been doing things by ear, I just don't like the hassle, maybe I am moaning too much but I would like to not have to disconnect and reconnect as much as I have with 2 transformers per channel.

Originally Posted by Bazukaz

After having been hit by a small ESL panel a long time ago I started to greatly respect step-up transformers and their capabilities

Glad your still with us to tell the story

Originally Posted by geraldfryjr

Yes, every time you Double the bias voltage you will get a 6db increase in the sensitivity.

And its really noticeably changed the sound signature too, maybe this is just that the transformer are doing less so distorting less though. This said I am sad to report even passing the official Bias voltage has not made these headphones sensitive enough for typically synthetic low bass. Bands like the Eurythmics really highlight this clearly particularly their earlier works has very low loud bass synth notes that add to the music and sound noticeably better on my Peerless 830668 10" sealed subwoofers (big heavy to move and very heavily braced) I run from another Quad 306. My Quad ESL 63's and the subwoofers are crossed over second order at line level. The Stax should sound better as they are headphones.

Originally Posted by geraldfryjr

Typically your voltage will be 230v * 1.41 * Nstages= Vbias

Thank you, another error on my part using RMS when I should have been talking peak to peak, and I have no reliable voltage ladder yet to measure. Still the internet order is half finished. (Growls at Conrad's lack of respect for customers, particularly the middle manager who said that its not his job to man the cash register and just continued "managing", this is what caused me to walk out, and others to follow. If I managed IT projects this way I would not have the carrier)

Originally Posted by esltransformer

After 2000V/mm you don't get much more sensitivity because the air will be a conductor itself

So I assume 1000V bias is potentially flash point as I read these headphones have a 0.5 mm gap. I think that three stages is the limit of Bias boost from 230 V.

Interesting when switching from voltage doubler to voltage trippler I forgot to reattach the power. and even without powering up for 12 hours the headphones still played OK, so I think leakage is very low on these Stax headphones.

My Stax headphone socket order from the USA has finally been dispatched after nearly 4 working days sitting in there order queue. I guess I will not be using Alied components again, and get things sent from Farnel to my parents in the UK as Farnel only sell to companies in Germany.

I will have to look at measuring the frequency response of the headphones + driver a little later in this project, I guess a measurement microphone very close to the headphone driver is best way to do this?

I got a 5 ply board now to mount the transformers (MDF is one of the things I have given up on for 2013) on which is a step forward and will set up a 4 transformer system soon. Expect more updates soon.

[owenhamburg]

Originally Posted by zeonrider

I have a Koss ESP 9, does the driver for Stax responsible for the Koss?

Regards zeoN_Rider

Dear zeonrider,

I did a very quick google and could not find the details of Bias voltage and stator spacing, or even any estimates / or driver circuit diagrams of drivers. Maybe you can search these things out or measure your own driver circuits (the best solution) and add it to this thread?

With this information even an amateur like me can tell you if the project will just work or need some modifications, that said unless you want to help with design, best to hold off starting until the design is a little more mature and adjusted. Currently it is still not correct but I am hoping the move to 4 transformers per channel will resolve the last big issue of low bass distortion, then I must build measurement equipment to see if the sound is a good as I think as I know from experience, pride in a build can hide glaring flaws in the product. Currently the sound is acceptable but I suspect we are 50% through and the last 10% will take the most effort.

I suspect that with very small changes to the circuit or components any electrostatic headphones can be driven using the driver I am making with good sound, but some knowledge of the electrostatic drivers parameters will be needed to avoid arcing the headphones (and so making them worse) while adjusting making the driver.

With knowledge of Bias and driver voltage, or step up transformer ratio, we cant guess that the same Stax Lambda Pro driver may or may not be used without modification with the Koss ESP 9.

[Bazukaz]

Originally Posted by owenhamburg

I will have to look at measuring the frequency response of the headphones + driver a little later in this project, I guess a measurement microphone very close to the headphone driver is best way to do this?

The membrane of an ESL headphone is considerably loaded by cavity of the ear.
So over most of the frequency range its working in "compression" mode.
You may get quite different results if you try to measure it in free air compared to what you hear. However if one could make a cavity similar to ear say from rubber and sound absorbing materials and place a microphone inside, maybe its possible to get sensible measurement?
Anyway take measurements with a big grain of salt.

Lukas.

[zeonrider]

Originally Posted by owenhamburg

Dear zeonrider,

I did a very quick google and could not find the details of Bias voltage and stator spacing, or even any estimates / or driver circuit diagrams of drivers.

I do not have the original Energizer!

I tried the headset Stax, but works poorly.

Regards zeoN_Rider

[owenhamburg]

Originally Posted by zeonrider

I do not have the original Energizer!

On dear, then we are in this boat together. What I think we want to know is the and voltage that the Koss headphones should be driven at. From this can be guessed from many things.

Originally Posted by zeonrider

I tried the headset Stax, but works poorly.

Maybe if you can tell us how it sounded wrong it can give us some clues.

1) Was it a Modern (Pro) or Old (standard) Stax driver that you used?

2) Was the sound too loud or too quiet?

3) What else can you say about the wrongness of using a Stax Driver with your Koss head phones?

Regards

Owen

[zeonrider]

Originally Posted by owenhamburg

1) Was it a Modern (Pro) or Old (standard) Stax driver that you used?

2) Was the sound too loud or too quiet?

3) What else can you say about the wrongness of using a Stax Driver with your Koss head phones?

I forgot, 303 or 404.

Sound was quiet and high raised THD.

Regards zeoN_Rider