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Old 23rd April 2005, 07:36 AM   #1
tade is offline tade  United States
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Default esl transformer.

I emplore you of the internet!
i am a pooor college student interested in ESLs. i need someone more competent than i to recommend a high voltage transformer to me! i would like for it to be cheap and readily available.
I expect for my panels to be four feet square and with 1/4 inch spacing.
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Old 23rd April 2005, 06:43 PM   #2
Brian Beck is offline Brian Beck  United States
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Default Trannies from old tube amp

There are two types of high voltage transformers associated with ESLs, the high voltage bias supply for the diaphragm, and the step-up transformer that interfaces your amp and the ESL's stators. The cheap trick for both of these is to use transformers salvaged from an old tube amp (push-pull type). For the bias supply, use the power transformer’s B+ winding. You would make a voltage multiplier (stacked diodes and caps) to multiply the high voltage to even high voltages for the ESL bias, which can be anywhere from about 1000 to over 5000 volts.

For the step-up transformer, you use the tube amp’s output transformer “backwards”. You drive the 4 or 8 ohm secondary (now a primary) and connect the former plate taps to the stators. The bias supply goes between the center tap and the diaphragm. Usually you want a step up ratio of better than 50:1, so try to find a high primary Z tube amp. A smaller amp using, say, EL84s might have 8K primaries. If you drive the 4 ohm tap, you’d get a step up of about 45, barely enough, but maybe good enough for getting started.

There are a thousand factors to consider to really do all this right: leakage inductance, winding capacitances, voltage ratings, ratios versus panel designs, etc. I’d suggest you buy books on ESLs by Roger Sanders and Ronald Wagner. You’ll see references in this forum to those two books, which are especially good for beginners.

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Old 23rd April 2005, 09:20 PM   #3
SY is offline SY  United States
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esl transformer.
Since you're in the US, you should keep your eyes open for old Dynaco SCA35 or ST35 amps. The output transformers work very well in this application used backwards as Brian described. The bias supply needs almost zero current, so a tube amp transformer is overkill (though it will work fine). A high-voltage supply from an old copying machine can easily be pressed into service.
"You tell me whar a man gits his corn pone, en I'll tell you what his 'pinions is."
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Old 24th April 2005, 01:18 AM   #4
I_Forgot is offline I_Forgot  United States
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Default Re: esl transformer.

Originally posted by tade
I emplore you of the internet!
i am a pooor college student interested in ESLs. i need someone more competent than i to recommend a high voltage transformer to me! i would like for it to be cheap and readily available.

There are no cheap, readily available audio transformers. You must either pay a fairly hefty price, or be very good at scrounging them from an old tube amp (this is getting harder and harder to do these days).

You can buy new audio transformers from any of several sites that advertise on the web. Get output transformers with about a 10k Ohm primary winding and 4 or 8 Ohm secondary winding. The primary must be center tapped or you need to get four transformers. 4 transformers will usually make for more sensitive speakers, so if you can get four, do so. 15-20W transformers will be just fine, even if you drive the speakers with a 200W amp.

The high voltage bias supply should be made using the smallest possible transformer if it is to be line powered. It needs to supply very nearly zero current, so to stay safe, you want a really small transformer. Better yet would be a dc-dc converter that converts a low voltage DC input from a wall-wart up to the required bias voltage (3-5 kVDC). There is a plan for such a supply here:

I expect for my panels to be four feet square and with 1/4 inch spacing.
If you mean stator to stator spacing, that should be OK. If you are talking about stator to diaphragm spacing, it is too large and you will not get much sound from your speakers, especially if you use only one audio transformer per speaker.

If your panels will be flat, you will need to break them into smaller pieces. If you try to build a driver that is 4' x 4', the weight of the stators, suspended only from their edges, will behave like a drum with a very low resonance frequency. If you bump the speaker frame with your hand the stators will wobble back and forth for quite a while. This may never be excited by playing music, but it does not give a feeling of confidence about the whole assembly to have parts of it wobbling like that. Also, as it wobbles, it flexes and the diaphragm may wrinkle and make undesirable noises in the process.

I would break a 4' x 4' driver into 4 drivers, 1' x 4', and put some sturdy mechanical support (part of the frame to which they are mounted) between them.

The alternative to breaking the drivers into smaller pieces is to curve the stators. A curved stator is much stiffer than a flat one. This is the reason why Martin Logan curves their stators. Curving the stators creates other problems with tensioning the diaphragm that may be quite difficult to overcome.

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Old 24th April 2005, 01:23 AM   #5
wrl is offline wrl  United States
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Check out www.justrealmusic.com

They aren't really "cheap" but they are definitely less expensive than some other suppliers. They are also very helpful if you have questions about wiring or ESL's in general. I'd recommend calling them.

Also you might want to consider using 1/8 inch spacing rather than 1/4 inch. That way you could use a smaller step up ratio (cheaper transformers) and not sacrifice as much output.

No joke though be careful when working with these.

Anything worth doing is worth doing right... and redoing to make it better... and again to fix it back the way it was.
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Old 23rd February 2018, 08:48 PM   #6
orchidtheaf is offline orchidtheaf  United States
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Default Transformer Confusion - First Time Builder

Hi all, firstly, thank you...I have learned so much on these forums.

I am a hesitant first time builder of ESLs. I am several hours into a full-range segmented copper rod build for my first attempt, with (mostly) all parts ordered/delivered. I still can't seem to gain the confidence I'm seeking regarding transformers. I have ordered 20 Antek AN- 0206, 10 per channel.
My amp is a Crown 1002, 215W at 8 Ohm, 350W at 4 ohm, 550W at 2 Ohm.

If 25VA at 50hz = 12.5VA at 25Hz (low freq cutoff for full range esls?) Right? Don't I need to consider power handling (sharing the load amongst transformers)?

My d/s spacing will be about 1/8" + a very little for adhesive

Should I?:

1. Be satisfied with current components and go with 1:100 step up ratio: (120V* 10 (# of transformers in series) / 12V (low voltage windings in series)

2. Go with higher ratio (1:160, or 1:200) with 10 transformers per channel: (120V * 8)/ 6V = 160 ; or (120V * 10) / 6V = 200

3. buy 4 more transformers to make 1:120 step up ratio: 120V* 12 (# of transformers in series) / 12V (low voltage windings )

I really wish I understood what risks one takes with higher step-up ratios.

My goal (of course) is to not limit sound decibel levels, and maintain clarity of sound throughout possible frequency ranges (already aware some sacrifice is made below 40hz with full range esls).

Or maybe I'm overthinking all of this, and none of those details will matter so much?

Thank you very much.
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Old 24th February 2018, 03:26 AM   #7
golfnut is offline golfnut  New Zealand
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Hi Tade, Orchidtheaf
Step up transformers for ESLs are a bit of a trick, and the problems become more difficult as you push to lower frequencies (more turns on the transformer), bigger panels, and higher step-up ratios.

There are four key specifications for ESL transformers
Step-up ratio: A typical 100 W amp produces peak voltages of the order of 40V, perhaps a bit higher. An ESL requires peak audio voltages (stator-to-stator) ranging from 1.6 kV to 8 kV, depending on the membrane-stator spacing (2.5 to 3mm is good for a full-range ESL). This means the transformer step up ratio should be in the range 40 to 200. 40 won’t cause any trouble, but achieving 200 without compromises on bandwidth will require everything to be right, and with some big ESLs it is impossible.

Primary voltage rating: This figure is frequency dependent. For example if you use a 120:12V 60 Hz power transformer backwards, then the voltage rating at 60 Hz is 12V rms only. With a 100W amp the transformer is likely to saturate (loose its input impedance) causing the amplifier current to rocket. If you limit the frequency to 120 Hz and above (twice 60Hz), then the voltage rating increases to 24 Vrms (twice 12 V). The voltage rating increases in direct proportion to the frequency.

Leakage inductance: Any coil of wire has an inductance, and so too do the primary and secondary windings of a transformer. Some of that inductance couples the two windings (mutual inductance) and some does not – the fraction that does not couple the two windings is called leakage inductance (its more complicated than this, but this explanation will do for the moment). The leakage inductance appears as an impedance in series with the ESL.

Winding capacitance:
Any two conductors placed close together have a capacitance between them, and there are little capacitances between turns on the transformer, between layers of turns, between the primary and secondary windings and between the windings and the core. The cumulative effect of all these capacitance is a single large capacitance that appears in parallel with the ESL. In combination with the leakage inductance this causes the transformer to have a second-order low pass frequency response – modified by the ESL.

There are two lesser things to add into the mix:

Primary winding resistance: To which must be added the resistance of the speaker leads. This resistance helps limit the current through the transformer if it saturates (stop the smoke getting out of the amp) and (ii) it helps damp the transformer resonance. Most people like the total resistance to be about 1 ohm.

The amplifier output inductance: typically 1 to 10 uH, to which must be added the inductance of the speaker leads (~0.7 uH/m). This adds to the leakage inductance of the transformer.

And then there is the type of ESL

Non-segmented ESLs behave as a single large capacitor, which adds to the transformer capacitance – may be up to 2 nF.

Segmented ESL: The load capacitance presented by a resistive segmented ESL is about 1/20 of the panel itself – makes it much easier.

Problem 1: Consider a large panel with capacitance = 1 nF, a step up transformer 1:100, and a 1 ohm primary winding resistance. This looks like an RC filter with the series resistance = 1 ohm x 100 x 100 = 10 kohms in series with the 1 nF panel. The cutoff frequency is 1/(2piRC)=16 kHz. A 1:200 transformer would cut off at 4 kHz. So you cannot run a large unsegmented ESL at high-step-up ratios with a useful series resistance.

Problem 2: Consider the same ESL with an amp with 3 uH OP inductance and 3 m of speaker leads – total primary inductance = 5 uH. The equivalent series inductance seen by the ESL 5 uH x 100 X 100 = 50 mH. This forms a LC filter with a second order filter with curoff frequency 1/(2pisqrt(LC)) =22 kHz. Once again, a higher 1:200 step up ratio would result in cutoff at 5.5 kHz. Even 1:100 leaves very little room to run a transformer. So large unsegmented ESLs require (i) amplifiers with a low output inductance, (ii) short low inductance speaker leads, (iii) transformers with a low step-up ratio and a very low leakage inductance (high wattage toroids are good). It is possible to run a moderate sized panel for a hybrid with high step-up ratios but not a full range ESL. Most commercial ESL transformers are made to drive unsegmented panels.

Problem 3: Ideally the 1 ohm resistor should damp the transformer resonance to give a Q near 1. This requires the total leakage inductance (amplifier+ speaker leads+ transformer), referred to the primary, to be about 8 uH. For high step up ratios, this favours transformers with a high leakage inductance and low capacitance – the opposite of what is available commercially.

So what to do?

First, if you want a large full-range ESL, consider segmentation (see other posts on DIY). Without it you will struggle to get a system with a good frequency response and without the tendency to kill amps.

Secondly, don’t buy commercial ESL transformers for a segmented ESL. Transformers suited to segmented ESLs can be constructed from small (15VA) power transformers run with the high voltage windings in series and the low voltage winding in series and parallel as required to get the voltage rating. For ESLs the voltage rating is more important than the power rating. Do not use transformers with the dual high voltage windings – they have a very high capacitance.

Thirdly, don’t be tempted to aim for high step up ratios unless you know how to model and calculate the frequency response of the entire system. Low ratios (100 ish, perhaps a bit higher) are far more forgiving.

Hope this is helpful – its longer than I expected by still not as helpful as I would like.
“Nothing is more practical than a good theory.”
– Ludwig Boltzmann
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Old 24th February 2018, 06:01 PM   #8
CharlieM is offline CharlieM  United States
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I'm bookmarking that post!
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Old 25th February 2018, 03:11 AM   #9
orchidtheaf is offline orchidtheaf  United States
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thank you for the thoughtful and detailed response, though I would be lying if I said I understood it all (or even half of it). I am a first time builder, with little to no electronics background. Everything I know of the subject has been learned in the last 6 months of research, reading Roger Sanders' book twice, and conversing with other builders. I am still a novice with a capital "N."

I guess I forgot to mention that I am building segmented ESLs, probably could have saved you some explanation. I will however study all that you have detailed, many times I'm sure, so that I may learn more about all of this. I cannot expect to know everything going into my first build...I really just hope to make something that sounds darn good, without frying expensive electrical parts. I had intentions to make a hybrid for my first build, but budget played a role in keeping things simple (no need to make a TL subwoofer, or secondary amplifiers, or digital crossovers). I am saving all of that for my next build IF, I can successfully accomplish my goals with this first build.

Regarding your comments about specifics of transformers; as aforementioned, I already bought 20 of the Antek AN- 0206 with the intention of making a 1:100 step up ratio. I also bought 4 power resisters to make 1ohm resistance which will be wired to the speaker leads (as you mention).

I used the ESL spreadsheet to determine resistor values for my segments.
My (adjustable) DC bias supply will offer ~4000V to the diaphram.

All in all, I do think you answered my question.

Again, thank you for taking the time.
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