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Old 16th February 2013, 06:13 PM   #61
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Location: Jackson,michigan
HV fet's will be around for quite some time.
Even just a few year's ago voltage voltage rating's above 1200V were are rare find.
Now voltages up to 2500V are a common place today and can be had for just a few dollars a pieces.

Remember that the voltages coming out of a step-up transformer just as dangerous as with any ESL type of system.
Typically a direct drive amplifier should employ some DC blocking capacitors as shown in the stax schematic as C33.

Regulation Is not necessary but is a good measure as good filtering will suffice as well.
Ripple voltage is a function of the value of the filter capacitor and the current drawn from the power supply and not its voltage output.

I can understand if one is not comfortable with working with such DC voltages.

In order to design a good cheap transformere that you can get down to 20Hz you need to select a rather large core as the number of primary turns is determined by the cross sectional area,Voltage input to the winding and its lowest frequency.

For instance the 100 watt toriod core that I am working with right now has 26 turns for the 6V winding and has a 1:40 transformation ratio with the two 115v winding that are already on it.
This makes about 500 turns for each HV winding and takes up the whole circumference of the core so there are two layer for both windings.
This increases its self capacitance and this particular one is not Bifilar wound as some are which makes it worse.
So to get to 20Hz at 6V input to the primary this core would require 3X the number of turns or about 78 turns (75 to 80) this also will also triple the number of turns for the HV windings to about 1500 turns each.
Because these turns will be bunched up together with little to no space in between each turn this will greatly increase the self capacitance of the transformer and thus increasing the extra load on the amplifier by have a lower impedance at the higher frequency's.
If you were to use a bigger core it would take much less primary turns to achieve your goal while keeping the HV turns down as well as having more room to not have the turns overlapping or touching each other and allow for a thicker insulation in between layer should you require it.
This helps to keep the transformers self capacitance at a low level and the resonate frequency above the audio band.

In the the first graph that esltransformer posted on his build states that it has 77pf of self capacitance.

Antek Toroidal power transformer for Step-up, Measurements (part 1/2)

These are very good parameter's!!
After seeing the picture of it I can believe this as it is a fairly large C-core with lots of winding space to work with.
Form what I have read on a core this size it can handle about 880 watts at 50/60 hz.
And more exotic materials are very costly as well.

I don't know what is available in your area but over here we have a few company's that will supply cores and alphacore is one of my favorites and have very reasonable prices.
The also have C-cores and exotic materials as well,

Magnetic Cores - In Stock

something like a PN#130 for $31.50 might work well for your application if you don't mind winding it yourself.

Silicon Steel Toroidal Cores - In Stock

This core has volts per turn rating of .613 Volts per turn at 60Hz.
So for 6V at 60Hz this would mean only 10turns on the primary and 20 turns for 30HZ and finally 30 turns for 20Hz.

Multiply that by your ratio of say 40 and you have 800 and 1200 total secondary turns for 30Hz and 20Hz respectively.
Larger core core would be even better for less total turns!!
With 26ga. magnet wire you can get about 700 turns on one layer and with 30ga. you can get maybe 960 turns in one layer.

A C-core is much easier to work with but they do cost more.

There are company's that offer the HV winding already applied the trick is to have them wind it for a higher voltage than 230V and then add your own primary turns.

jer

Last edited by geraldfryjr; 16th February 2013 at 06:34 PM.
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Old 16th February 2013, 06:26 PM   #62
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Join Date: Feb 2008
Location: Jackson,michigan
Yes, I have used stacked cores and it worked great except for the added capacitance takes a toll on the amplifier as shown in the post after this one,

Step-up transformer design

However it works in a pinch and I did enjoy listening to my panel full range at a fair level.
I used to hold the panel next to my ear just like if it was a headphone driver and it just sounded incredible.
Although I did have a much higher D/S spacing so the bass went really low and clean!!!
Except (in my case) for the diaphragm resonance in which I had to filter this range out a bit.
But when I did I could believe what I was hearing!!!

jer

Last edited by geraldfryjr; 16th February 2013 at 06:36 PM.
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Old 17th February 2013, 02:57 AM   #63
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Toroid wise it can be done the way shown. Primary on top with required spacing and isolation barrier. Secondaries on each toroid in series. This way you have rather "small" voltage on each secondary. Self capacitance of the secondaries is of lesser importance due to "sectioning".
Attached Files
File Type: pdf HV_TRANS.pdf (79.6 KB, 22 views)
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Old 17th February 2013, 08:51 AM   #64
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Quote:
Originally Posted by geraldfryjr View Post
HV fet's will be around for quite some time.
Even just a few year's ago voltage voltage rating's above 1200V were are rare find.
Now voltages up to 2500V are a common place today and can be had for just a few dollars a pieces.

Remember that the voltages coming out of a step-up transformer just as dangerous as with any ESL type of system.
Typically a direct drive amplifier should employ some DC blocking capacitors as shown in the stax schematic as C33.

Regulation Is not necessary but is a good measure as good filtering will suffice as well.
Ripple voltage is a function of the value of the filter capacitor and the current drawn from the power supply and not its voltage output.

I can understand if one is not comfortable with working with such DC voltages.

In order to design a good cheap transformere that you can get down to 20Hz you need to select a rather large core as the number of primary turns is determined by the cross sectional area,Voltage input to the winding and its lowest frequency.

For instance the 100 watt toriod core that I am working with right now has 26 turns for the 6V winding and has a 1:40 transformation ratio with the two 115v winding that are already on it.
This makes about 500 turns for each HV winding and takes up the whole circumference of the core so there are two layer for both windings.
This increases its self capacitance and this particular one is not Bifilar wound as some are which makes it worse.
So to get to 20Hz at 6V input to the primary this core would require 3X the number of turns or about 78 turns (75 to 80) this also will also triple the number of turns for the HV windings to about 1500 turns each.
Because these turns will be bunched up together with little to no space in between each turn this will greatly increase the self capacitance of the transformer and thus increasing the extra load on the amplifier by have a lower impedance at the higher frequency's.
If you were to use a bigger core it would take much less primary turns to achieve your goal while keeping the HV turns down as well as having more room to not have the turns overlapping or touching each other and allow for a thicker insulation in between layer should you require it.
This helps to keep the transformers self capacitance at a low level and the resonate frequency above the audio band.

In the the first graph that esltransformer posted on his build states that it has 77pf of self capacitance.

Antek Toroidal power transformer for Step-up, Measurements (part 1/2)

These are very good parameter's!!
After seeing the picture of it I can believe this as it is a fairly large C-core with lots of winding space to work with.
Form what I have read on a core this size it can handle about 880 watts at 50/60 hz.
And more exotic materials are very costly as well.

I don't know what is available in your area but over here we have a few company's that will supply cores and alphacore is one of my favorites and have very reasonable prices.
The also have C-cores and exotic materials as well,

Magnetic Cores - In Stock

something like a PN#130 for $31.50 might work well for your application if you don't mind winding it yourself.

Silicon Steel Toroidal Cores - In Stock

This core has volts per turn rating of .613 Volts per turn at 60Hz.
So for 6V at 60Hz this would mean only 10turns on the primary and 20 turns for 30HZ and finally 30 turns for 20Hz.

Multiply that by your ratio of say 40 and you have 800 and 1200 total secondary turns for 30Hz and 20Hz respectively.
Larger core core would be even better for less total turns!!
With 26ga. magnet wire you can get about 700 turns on one layer and with 30ga. you can get maybe 960 turns in one layer.

A C-core is much easier to work with but they do cost more.

There are company's that offer the HV winding already applied the trick is to have them wind it for a higher voltage than 230V and then add your own primary turns.

jer
Thx. This esl transformer i made for my own home made esl. It's not made for a headphone.
The core that was used is a c-core type SU90B a 600w core, core material is HiB (M0).
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Old 17th February 2013, 09:38 PM   #65
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Dear all,

More interesting posts, thanks

I have been giving direct drive more consideration but not yet, maybe in a month or so.

I have now wired up the 4 transformers but have yet to attach it to headphones, as I will wait until the plugs come in the post due Tueday.

I picked up a cheap STAX SRD-4 which is an designed for electret headphones, so I think it does not include the HV bias of a normal or pro stax headphones. This is not an issue as I have successfully built HV bias supplies on breadboard for the torroidal step up transformer circuit. Good news though is it does have a genuine 5 pin pro socket

The SRD-4 also contains a pair of transformer for stepping up the output of and amplifier, (1:50 from what I have read) and these are common on ebay at cheap prices. I will make a small box, to put under the SRD-4 and drill a socket in the bottom or side to pass the HV in.

This seems a cheap solution to the transformer solution and is a LOT smaller than 8 X 30 VA torroidal transformers. I will let you all know how they compare later.

Developments on the SR-3 and SRD-5:

I have mostly been listening to this system (late at night) while I built the supplies.

The SRD-5 started to hmm a lot, after a few hours listing, (rather than a little as before), I guess the capacitors are showing their age worse than I am, but they are older. So I took out the transformers, noting that Stax had been cheap and no terminal connectors on the transformers which is saving them a little money but not making fixing easy as fragile primary and secondary winding can easily be broken. To solve this I hot glued a 2 and a 3 way PCB terminal block to the top of the transformer and replaced the capacitors which I could now access without the transformer blocking access.

Sadly my stocks did not have 1uF 190 V capacitors so it only got 0.1 uF on the HV bias, this means that it wont blow up but sadly does mean that the main hum is still with me for now, at about the same level when pluged in so I guess the original electrolytic had about 1/10 of their value. ( ifdidnt

I cant help thinking the AKG's have better, cheaper and most importantly more comfortable ear pads than with either of these Stax headphones. I do suspect I might end up building some electrostatic headphones before the year is out designed to use AKG ear pads (easy to get as spares for a reasonable price).
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Old 17th February 2013, 09:41 PM   #66
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Location: Hamburg
Out of interest I tried to measure the capacitors for you guys, so see how bad they are, but discovered my Multimeter is not worth trusting in this area.

Regards

Owen
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Old 17th February 2013, 11:46 PM   #67
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Quote:
Originally Posted by owenhamburg View Post
Dear all,

More interesting posts, thanks

I have been giving direct drive more consideration but not yet, maybe in a month or so.

I have now wired up the 4 transformers but have yet to attach it to headphones, as I will wait until the plugs come in the post due Tueday.

I picked up a cheap STAX SRD-4 which is an designed for electret headphones, so I think it does not include the HV bias of a normal or pro stax headphones. This is not an issue as I have successfully built HV bias supplies on breadboard for the torroidal step up transformer circuit. Good news though is it does have a genuine 5 pin pro socket

The SRD-4 also contains a pair of transformer for stepping up the output of and amplifier, (1:50 from what I have read) and these are common on ebay at cheap prices. I will make a small box, to put under the SRD-4 and drill a socket in the bottom or side to pass the HV in.

This seems a cheap solution to the transformer solution and is a LOT smaller than 8 X 30 VA torroidal transformers. I will let you all know how they compare later.

Developments on the SR-3 and SRD-5:

I have mostly been listening to this system (late at night) while I built the supplies.

The SRD-5 started to hmm a lot, after a few hours listing, (rather than a little as before), I guess the capacitors are showing their age worse than I am, but they are older. So I took out the transformers, noting that Stax had been cheap and no terminal connectors on the transformers which is saving them a little money but not making fixing easy as fragile primary and secondary winding can easily be broken. To solve this I hot glued a 2 and a 3 way PCB terminal block to the top of the transformer and replaced the capacitors which I could now access without the transformer blocking access.

Sadly my stocks did not have 1uF 190 V capacitors so it only got 0.1 uF on the HV bias, this means that it wont blow up but sadly does mean that the main hum is still with me for now, at about the same level when pluged in so I guess the original electrolytic had about 1/10 of their value. ( ifdidnt

I cant help thinking the AKG's have better, cheaper and most importantly more comfortable ear pads than with either of these Stax headphones. I do suspect I might end up building some electrostatic headphones before the year is out designed to use AKG ear pads (easy to get as spares for a reasonable price).
Owen, maybe you could do some measurements about your transformers. Frequency respons?
I think that will help much
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Old 18th February 2013, 12:43 AM   #68
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Quote:
Originally Posted by esltransformer View Post
Owen, maybe you could do some measurements about your transformers. Frequency respons?
I think that will help much
I think so too, how should I do this best?

My scope does not like > 300 V AC, and says things like it will die in it manual.

It looks like about 100 V is the maximum input that can be displayed.

My multimeter only 600 V AC.

I guess a potential divider into a ADC and optical the computer is the easiest?

Regards

Owen
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Old 18th February 2013, 12:51 AM   #69
Bazukaz is offline Bazukaz  Lithuania
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Quote:
Originally Posted by owenhamburg View Post
I think so too, how should I do this best?

My scope does not like > 300 V AC, and says things like it will die in it manual.

It looks like about 100 V is the maximum input that can be displayed.

My multimeter only 600 V AC.

I guess a potential divider into a ADC and optical the computer is the easiest?

Regards

Owen
Basically even integrated sound card should be enough.
You will have to make a resistor divider by yourself to drop voltage to <1V, lets say, 1000:1.
Optical decoupling is nice to have but is not absolutely necessary as long as you manage to avoid ground loops.

Lukas.
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Old 18th February 2013, 01:00 AM   #70
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I describe how to do this in the links I posted in post 23.

Making an electrostatic headphone driver.

I have just recently posted some screen shots of some real tests as well.

Frequency response examples from are the HV output of the transformer as well as input and output voltage comparisons to the transformer.
There will be more stuff on this matter and a more detailed descriptions of how to make such measurements and what they mean.

I use both my scope and the computer to monitor the signals.
The transformer jig is working great and flawlessly so far.
I have been monitoring signals as high as 5Kv p-p into the computer and scope with no arcing or breakdown issues.

This is the very reason I started such a project, as is to show how to do such tests.

jer
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