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homemade 15th September 2005 01:49 AM

Please rec. step-up transformers for ESL
I'm in the process of rebuilding a pair of Electrostatic Research ESL speakers and discovered that one of the step-up transformers is internally shorted. These are a 2 way design with a crossover point of 200hz. The transromers do not appear to have a usable part number. I really do not wish to use old opt's from some discarded tube amp. From the limited info Ive been able to get , the original trans have approx. a 75 to 1 step-up ratio. I'm looking for recomendations to aid my search for a pair of these . I am also willing to purchase a pair from someones past or unfinished project, provided they have been tested(used in system) and are of good quality. I only want to do this once. Help is greatly appreciated.

homemade 18th September 2005 04:46 AM

WOW!!!Nobody?? I'll even take suggestions from some diyers that use 70v commercial systems. I just want to purchase something tried and true. HELP!!!!!!!

moray james 18th September 2005 09:02 AM

Ok cheap, $10.00 70 volt line matching transformers can sound very good and only cost $20.00/pr. so start there run them backwards. They are leaky so expect a rising top end but that's ok with a heavier diaphragm. Probably a help.
People have posted impressive results with european 230 volt to 9volt step down torroid transformers, your second cheapest shot. Used Acoustat interfaces can also be had for very cheap find a pair of those. After all that you will have more experience than most and be ready to modify. Hope this has been of some assistance. Best regards Moray James.

Calvin 18th September 2005 09:49 AM


Toroids work fine indeed. A pair of simple 230V/12V (for halogene lighting, giving 1:40) gave exactly the same results with regard to frequency- and phase- response as a 1:75 Amplimo.
Two 230/9V toroids give a step-up-ratio of 1:50, two 230/6V one of 1:75. Paralleling the trannie-pairs (e.g. as a quad- or hex-package) reduces strayinductance, hence rising the upper frequency limit if needed. Most of these trannies are tested for 4kV voltage, some even higher. I´ve lost quite some special made audio-trannies because of flash-overs and internal shortings but no standard toroid till now!
For most purposes 30W-80W-types will do fine.


homemade 18th September 2005 01:26 PM

Thanks........I feel much better now!!!!!!!!!:)

I_Forgot 18th September 2005 11:40 PM

Re: Toroids

Originally posted by Calvin

Toroids work fine indeed. A pair of simple 230V/12V (for halogene lighting, giving 1:40) gave exactly the same results with regard to frequency- and phase- response as a 1:75 Amplimo.

Are you saying that a power supply transformer works well for the audio step-up?

Has you (anyone?) tried EI core power transformers?


Calvin 19th September 2005 07:51 AM

Some Measurements
2 Attachment(s)
Hi Folks,

some measurements.

Comparison of a ML Prodigy-Panel and my own Panel driven by a 1:75 Amplimo tranny. (Both panels pure and via filter. The lower Fs and more pronounced high-frequency-response on my panel is due to a thinner diaphragm.)


Calvin 19th September 2005 07:56 AM

2 Attachment(s)

my panel with a simple pair of 230/12V halogene lighting trannies
(measured on different positions and distances). Obviously not much difference to the Amplimo (apart from effieciency). 20kHz is easily reached.


Calvin 19th September 2005 07:59 AM

2 Attachment(s)

the same procedure with a quad of Sombetzki´s standard Audio-Trannies (probabely EI-Type). Bandwidth is clearly restricted to below 20Khz. And of my initially 8 trannies only 5 are still working. 3 are already defected by internal flashovers :mad:


Calvin 19th September 2005 08:22 AM

2 Attachment(s)

impedance measurements of my panel with an Amplimo, simple toroids and Somkbetzki-EI-types (1.Amplimo 2. toroids 3. EI-types).

The absolute value of the impedance maximums are indicators for the transformer´s stepup-ratio. The lower the imp-max the higher the stepup-ratio.
The impedance minimum at the upper frequency range is another indicator for the ´quality´ of the tranny (hence little stray inductance).
While both toroids´s minimum lies above 20kHz, the EI-types settle at about 14kHz (even though alrady 4 trannies are connected in series/parallel mode to get the stray-inductance down! It´s even worse using just a pair of EI´s!)).
The same can be seen in the phase response. With the toroids the phase response is more linear and closer to 90° than with the EI´s.
By paralleling the trannie-pairs the upper frequency range can be improved. But that only makes sense with standard toroids because of the high price of Amplimos or the still unsufficient result with EI´s.
Standard toroids often use the same core- and insulation material as good audio trannies, so there is no reason not to try them in first place. ;)


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