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Planars & Exotics ESL's, planars, and alternative technologies

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Old 3rd August 2005, 07:05 AM   #11
bret is offline bret  South Africa
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Hi John,

I am not sure what you are getting at in terms of turns ratios. Remember that you have to use the audio transformer "backwards" and so the higher the "primary" impedance the better.

1650F impedance ratio 7600:4 turns ratio ~44:1
1650K impedance ratio 3400:4 turns ratio ~29:1

i.e. the 1650F has the higher turns ratio, which is why I suggested it. How much power rating do I need? Traditional numbers don't really seem to apply.

I can understand that you would want a high power amplifier for two reasons:
1) Bigger output voltages require lower turns ratio transformers for the same stator voltages (or conversely, higher stator voltages for a given transformer)
2) The load is mostly reactive and hence the amp has to dissipate a lot of power internally (for a class AB)

But what rating transformer is actually required? Mark Rehorst suggests 15W to 20W for 30Hz output on his web page, which is what I based the choice on. Presumably larger is better, but how large?

From my understanding of the physics, ESLs should actually be quite efficient in terms of electrical to acoustical power conversion. (They may have low sensitivity, however).

Audio transformers are somewhat unusual in that they give ratings in Watts rather than VA (which is normally the important value). ESLs may have low W but significantly higher VA requirements due to their low power factors.

Any input I can get on this subject would help tremendously.

Bret
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Old 3rd August 2005, 08:49 AM   #12
MJ Dijkstra is offline MJ Dijkstra  Netherlands
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Hi,

The power which a step-up transformer can handle depends on the frequency. You can have a rated 50 Watt transformer at 50 Hz, but at 20 Hz it can handle only around 10 Watt. Putting more power in it results in (core) saturation; distortion!
Is this 10 watt at 20 Hz enough? It ususally is because most music has little bass content below 50 Hz. If your ESL has low efficiency and your playing organ music on high levels, than you have a problem.
My advice is to make the spacing rather small for high efficiency.
Specially in case of limited step-up ratio which the old tube-amp transformers usually have.
Best regards
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Old 3rd August 2005, 12:18 PM   #13
jmateus is offline jmateus  United States
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Default Audio transformers

Hi Bret

You are absolutely right, somehow I got confused about the two
transformers in calculating the turns ratio.
Np/Ns = Square root of Zp/Zs gives exactly the numbers you mentioned
43.5 for the F and 29 for the K, I must have swapped the numbers
somehow. This way the F sufix is the suitable one for the ESL as you
stated provided the impedance numbers provided by the manufacturer
are correct, even though the turns ratio is not the best suitable for
this purpose, it must be a little higher according to what I've read
since a few years ago. But anyway it's acceptable as mine proved to be.

I've read yesterday an interesting elaboration on the reason why
certain amplifiers - class AB - are not suitable for driving ESL speakers
this was the first time I've read something convincing about the
subject. In another posting on this forum I asked why the amplifiers
especially class AB couldn't drive the ESL without being destroyed
and I never got a convincing reason. Now, by accident, I came across
something that makes some sense to me. And I quote an answer
that was given by Hugh Dean of Aksa amplifiers to the question of
a member of his forum.
"The PP SS amplifier, both class AB and A, has one mortal enemy.
Amp Kriptonite is called cross conduction (CC). CC puts one rail
in direct communication with the other - we can forget about the
speaker here - via the output transistors. A horrific current flows,
aided and abetted by the largish capacitance at the rails, and the output devices lay down their lives in the electronic equivalent of a massive auto burnout. So, what causes CC? Oscillation; uncontrolled switching of the output devices precipitated by phase shifts across the global feedback network. Negative feedback
MUST remain NFB, and when it strays into the PFB region, the amp oscillates in short order."

I really must confess that class A amplifier do not suffer from this
prolem, my JLH is an example of this, it drives my ESL beautifully
without any strain whatsoever, by what concers class AB this
explanation is more than sufficient to me.
If you want to read more about the subject check this site,
http://www.audiocircle.com/circles/v...624&highlight=
electrostatic
Sorry about this long dissertation but I figured that you would
be interested to know about this.
Cheers
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Old 3rd August 2005, 12:33 PM   #14
jmateus is offline jmateus  United States
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Default ESL and transformers

MJ Dijkstra

You've got a point here, the power of a transformer is directly dependent on the frequency. But the same reasoning applies to a less power trafo so I think that the more power a trafo has the better it is.
Of course we should always consider a small spacing in all circumstances and that's where the tube-amp trafos come into place
with reasonable results albeith its less than ideal turns ratio.

Conclusion? A higher primary impedance and high power rating will
do the purpose, right?
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Old 3rd August 2005, 03:34 PM   #15
I_Forgot is offline I_Forgot  United States
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Location: Phoenix, Az.
Default Re: Audio transformers

Quote:
Originally posted by jmateus

I've read yesterday an interesting elaboration on the reason why
certain amplifiers - class AB - are not suitable for driving ESL speakers
this was the first time I've read something convincing about the
subject. In another posting on this forum I asked why the amplifiers
especially class AB couldn't drive the ESL without being destroyed
and I never got a convincing reason. Now, by accident, I came across
something that makes some sense to me. And I quote an answer
that was given by Hugh Dean of Aksa amplifiers to the question of
a member of his forum.

That's very interesting. So the class AB amps I've had driving my ESLs all these years are actually blown? I never would have suspected- they sound great! Hmmmmmm.

ANY class of amp can safely drive ESLs, but it must be designed to remain stable with capacitive loads. Some commercial amps are and some are not. I have found Carver's cheap solid state amps to be unstable with ESLs, for example. I have also found cheap Soundcraftsmen amps to be perfectly stable with ESLs (and ribbons, and every other type of speaker I have ever connected). A cheap, 80s vintage Technics amp I have is also completely stable with my ESLs. None of these are class A amps.

Don't let talk of amplifier instability stop you from making or buying ESLs. There are plenty of options ranging from cheap to expensive to drive ESLs.

I_F
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