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bentl 17th February 2010 08:57 AM

Step-up transformer design

my ESL-project is now in the "transformer"-phase. I have one ESL assembled, and tested with various toroids and EI transformers 230v/6V as discussed in other threads.

I have been playing with the idea of making my own transformers and have looked into toroids; they are not suitable for manual winding. It will take ages to wind a toroid by hand.

Sourcing new parts to built a EI transformer is possible; RS online sells ferrit cores but they are expensive.

So; I have dissasembled a 50VA laminated EI transformer, and sourced several rolls of insulated copper wire in different sizes.

I also have a shipment of suitable bobbins in the mail, so I can try several configurations and easily swap, test and measure.

Also, I have built a jigg for winding copper wire onto the bobbin complete with a digital counter. Picture will be posted later.

The bobbin for this transformer is separating primary and secondary windings - I am not sure if this is what I want?

Has anybody here tried a similar approach, and are any advice? Wire gauge primary/secondary, what insulation, insulation between layers or not?

I will play with different transformer designs for a while, and if not happy I will order a pair of transformers from Sowther.


kavermei 17th February 2010 12:07 PM


I don't think ferrite cores would be suitable. Ferrite has a way too high coercivity, the core losses would be large.

Also, I've never seen separated pri and sec windings side-by-side on audio transformers. They are always wound interleaved. I think, with separate windings, the coupling may be too low, leading to increased leakage inductance and less bass response.

Interleaving has its problems, too: you need better isolation, and the high frequencies may suffer due to capacitive coupling.

I would recommend reading the description of the Williamson output transformer, to get an idea of the interleaving technique.
Another good article:


kavermei 17th February 2010 12:14 PM

To answer some more questions-

Enamel wire has sufficient insulation to the adjacent wires, but insulation between layers is absolutely necessary.

Wire gauges determine the I2R losses (copper losses), so (after determining the number of turns you need,) choose the biggest wire that will fit on the bobbin. Don't forget that the insulation layers take up space too.

analog_sa 17th February 2010 12:39 PM


Originally Posted by bentl (
have looked into toroids; they are not suitable for manual winding. It will take ages to wind a toroid by hand.

Not necessarily. What if you use a toroid with pre-wound mains voltage primary and only add a secondary to be used as a primary? No idea if interleaving is recommended for toroids but at the voltages concerned i woudn't miss it greatly.

bentl 17th February 2010 12:51 PM

Hi, thanks for your replies!

The core I will use for my experimenting is a laminated core, not ferrite.

And Yes - I have thought about rewinding the secondary of a toroid. But I will first have a go at the EI-transformer it should give a faster result.

But if I aim at having 8ohms at the primary it will require 90m of 24 awg if my understanding is correct.

kavermei 17th February 2010 01:27 PM


Originally Posted by bentl (

But if I aim at having 8ohms at the primary it will require 90m of 24 awg if my understanding is correct.


don't confuse impedance with DC resistance. The length and gauge of wire will determine the DC resistance, which you should try to minimize. The 8 ohms you mention is AC impedance, which determines the turns ratio in a normal transformer. For an ESL transformer, the design procedure is a little different:

0. Choose a core size based on the desired audio power at the lowest desired frequency ("guesstimating")
1. Figure out what peak-to-peak drive voltage you want on the ESL and what the maximum peak-to-peak output voltage of your amp is.
2. These two values determine the turns ratio N you need for your transformer.
3. Determine the number of primary turns Np you need, based on amp output voltage, core size, and core material.
4. Determine number of sec. turns: easy, Ns = Np * N
5. Figure out how to interleave the windings (this is the most difficult part, and, together with step 0, will determine the bandwidth of the transformer!!!)
and how thick all the inter-layer insulation will be.
6. Based on 3-5, figure out how thick your wire can be before you run out of bobbin space!
7. Measure & listen
8. Refine
9. Goto 7 :)

geraldfryjr 17th February 2010 01:46 PM

hello,i'm here to invite you to check out my new study on transformers on the"material for esl"thread. i hope that you find the information i just posted useful. jer

woody 17th February 2010 02:09 PM

I know you are talking about transformers but wanted to toss this in. SimiSouth has
some new silicon carbide power jfets that handel up to 1700v! With these a direct
drive electrostatic amp might be realy easy to build. True scary as heck with all that
voltage though.

geraldfryjr 17th February 2010 02:28 PM

thanks for the info i'll be working on that in the very near future. jer

Calvin 17th February 2010 04:56 PM


1700V -of which maybe 1400V are safely useable- is not enough but just for small HF-Panels and Headphone-panels. You need at least something with >2.5-3kV to drive a medium sized hybrid panel. That asks for cascading of transistors -in which case You donīt need those expensive power-JFETs or You use Power MOSFETs or IGBTs with sufficient high breakdown voltages (see IXYSS semi).
The Semisouth JFETs are different anyway, as they are enhancement types with a positive Ugs while typical n-channel JFETs are depletion mode types with negative Ugs values.


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