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ak_47_boy 10th December 2006 04:10 AM

Help With ESL Transformers
How do i tell a transformer ratio from the primary and secondary ohm's?
I want to use audio transformers made for tube amps. If i have a 100 watt amp do i need a 100 watt audio transformer in my esl?
If i had a 10kv bias supply would i need a 250:1 transformer (40volts x 250 = 10000)?

Calvin 10th December 2006 10:07 AM


You canīt tell the transformation-ratio by the coils (DC-)impedances, since the ohmic (DC-) value depends on the length of the wire, the material and the gauge.
You have to measure the voltages for a given frequency (start with 1kHz).

The wattage You need depends on what kind of ESL You want to drive. For a fullrange ESL 100W is ok, for a hybrid-Type working from 100Hz or above, smaller types are useful.

If You really need a powersupply that delivers 10kV, Iīd rethink about the ESL alltogether. Tubeamp-trannies are not designed to withstand such high voltages and would soon fail because of internal flash-overs.

The formula for the voltage -even though formally correct- doesnīt apply here. The output voltage of the amp (40V in this example) is measured as a rms-value, but we need peak-values here.
40Vpk~28Vrms --> 1:250 for 10kV ...or...40Vrms~56Vpk -->1:180 for 10kV.
Still both numbers are much higher than with any tubeamp-tranny.
Additionally such high figures present the tranny-designer with serious problems and the driving amp with ridicoulosly dangerous impedances. Bandwidth will come down considerably, as well as safety....and uhhh... btw.... it doesnīt sound good either ;-)


SY 10th December 2006 11:41 AM

With a small risk to personal safety (zero if you're a careful sort), you can get that ratio at 50/60Hz. Measure mains voltage. Then connect your meter across the lower voltage winding of the candidate transformer. Connect the mains across the higher voltage winding, then note the meter reading. Turn ratio is the ratio of mains voltage to outputvoltage. Simple, eh?

maudio 10th December 2006 12:14 PM

With a tube output transformer you can also calculate stepup using

N = Squareroot (Zprim/Zsec)

Zprim and Zsec are the values provided by the manufacturer, NOT the DC-resistances of the windings!
So if you have an output trannie spec'd at 6kOhm / 4Ohm, it has a stepup of 1:39. You'll find it very difficult to find a tube trannie with a stepup >1:50...60.

I fully agree with Calvin about the problems using 10 kV and high stepups. Better rethink your design, reduce spacing, maybe increase surface, try to keep bias < 5kV and stepup < 1:150.


With a small risk to personal safety....
If you have acces to one, better use a sine wave generator to put around 1 V over the primary, then measure Vsec.

I_Forgot 10th December 2006 02:44 PM

100W transformers are going to be huge, and very expensive, and completely unnecessary. A 30W transformer is sufficient. Audio transformer power ratings are a spec that is dictated by the marketing dept. A transformer rated for 30W at 20 or 30 Hz will be rated for 100W at 250 Hz. When comparing audio transformers the weight is often the best clue to see which can really handle power because most of the weight is the core and core size is the main factor in determining the power handling.

As previous posters have said, the enormous transformation ratios you are looking for are not easy to find. You can make life easier if you use two transformers per speaker.

I have not yet tried it myself, but other posters on this forum claim to have excellent results using toroidal power transformers for the audio. You can get surplus troidal power transformers for $5-20 each. The low voltage winding doesn't matter much- what you're after is the core and the dual 115V windings. Look for low voltage secondaries, but if you can't find anything less than 12-24V it doesn't matter because you can either remove some of the wire or just add your own winding (as long as the core isn't potted).

Here is one that is much largr than needed:
Here's one that a more reasonable size (and price) but will require either unwinding the secondary, or adding your own (just a few turns of wire):
Here is one that looks good, but has only one 117V winding.
Here's yet another:

You can get 4 surplus power transformers for less than the price of one audio specific transformer.


ak_47_boy 10th December 2006 10:33 PM

So i would need 6 transformers per speaker if i were triamping? Or 3 transformer per speaker is they are 12-0-12 style? Would a 120primary 12secondary mean its a 10:1 transformer?

I_Forgot 10th December 2006 11:41 PM


Originally posted by ak_47_boy
So i would need 6 transformers per speaker if i were triamping? Or 3 transformer per speaker is they are 12-0-12 style? Would a 120primary 12secondary mean its a 10:1 transformer?
A driver needs either a transformer with a center tapped winding or two transformers with the windings wired in series, thus creating a center tap.

It is hard to get huge transformation ratios in a single transformer, so using two transformers will enable a higher transformation ratio. Also, when using two transformers, you can use a smaller core than when you use a single transformer.

If you want to triamp the system, you will need at least 3 transformers or as many as six per speaker.

The voltage transformation ratio for audio transformers requires that you take the square root of the winding impedance ratio. For power transformers the voltage transformation ratio is simply volts out / volts in. A 120VAC to 12VAC transformer has a 10:1 voltage transformation ratio when used to drive an ESL. If that transformer has dual 120VAC windings, as many do, you can get 20:1 from it.

Toroidal transformers that are not potted (embedded in plastic) are easy to change. All you have to do is wrap a few turns of wire over the core and you can get just about any transformation ratio you want.

Since you are asking these basic questions I assume you have not built an ESL yet. I would advise against planning a triamped design as a first undertaking. Build a few small test drivers, use one or two transformers to drive them and see how you like them. ESLs are very wide bandwidth drivers. They don't require a lot of complex crossovers and multiple drivers to get them to sound good. Very complicated, expensive projects are the type that get started and never finished. Start simple and cheap and your results will inspire you to build on your successes. Almost all the parts you use to make a small driver/system are reusable in a bigger system, so nothing will be wasted.


ak_47_boy 11th December 2006 12:43 AM

Ahh i understand now. Just one more think i don't understand, does the bias voltage tell you what ratio you should have? example, can you use a 40:1 with 5000V and also use 40:1 with 8000V?

I_Forgot 11th December 2006 04:37 AM

It doesn't matter. Use the largest practical for both. 5kV is about as high a bias as you can use without having corona problems or diaphragm tension problems.


ak_47_boy 12th December 2006 07:09 PM

would a 100:1 stepup be ok with a 5000V bias?

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