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Old 25th October 2001, 02:51 PM   #11
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Location: Netherlands
A very good reference, I've read it several times but still I don't understand it fully.
My experience with the low (and complex) impedance of an ESL turns out most transformers are the pain in the ***. Many transformers are rather poor, with a too big secondary capacitance and leak inductance.
An example: 1:100 ratio, Cs=1nF will give 0.8ohm at 20kHz. These numbers are are not uncommon!
If you hook up an ESL (1nF), this number will halve!
To check the influence of the transformer, do an impedance measurement without an ESL hooked on. This will make a lot clear.

I don't like series resistors in the primary winding, though sometimes it's the last way to go. The best way it to try to get a better transformer.

I ahve done some spice simulations with an ESL and transformer, using microcap. I generated a spice listing, mayby you can use it...:
CESL 4 0 {1.2N*100*100}
CMYLAR 5 4 {C(CESL)*2.5}
CP 1 0 20P
CS 4 0 {1N*100*100}
LAIR 6 0 450M
LP 0 2 0.2
LS 3 4 5U
RAIR 5 6 44
RLOSS 0 2 220
RP 1 2 0.2
RSEC 2 3 0.1
VGEN 1 0 AC 1 PULSE (0 1 0 1e-006 1e-006 0.000499 0.001)

.AC DEC 171 1 1e+006
.PLOT AC (V(VGEN)/I(VGEN)) 0.3,300
.PLOT AC VDB(5) -80,20
.PLOT AC VP(5) -180,180

success with it...
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Old 29th October 2001, 06:24 AM   #12
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Thank you very much for the info. I agree completely that a series resistor is pretty crappy, even if the frequency dependant attenuation wasn't a factor, you're still losing power for no other use but heating the room. Any method of helping our impedance that doesnt resort to power resistors would be ideal.

Also, thanks for the netlist, that did help a lot. And on that topic I've got some questions. First of all, given a physical transformer, what sort of calculations must be prepared to represent it in the model you've proposed? Are the values for R,L,C primary/secondary just the raw measurements of the transformer? I assume Rloss is just an efficiency estimate. Is this model of a transformer really accurate? If it is, damn, why dont they use that in most spice models . . .

Next, i see you have plot defined as V/I of the voltage source. Is this a plot of the impedance that the amp would see in the real world?

And finally, Im a bit confused about your esl model, as shown. What are Cesl and Cmylar, and why are they in parallel? What are R(air) and L(air), and what do they relate to in the real world, and why are they in series with Cmylar? And lastly, you're plots refer to the phase and voltage at point 5, between Cmyl and Rair. Whats the physical significance of that point? Why is it important? I realize these questions are going to seem pretty elementary, I've got a lot of holes in my electronics experience. Thanks for any help you can offer, and for the help you've already given me.

- Jonathan
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Old 29th October 2001, 08:52 AM   #13
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Hi Jonathan,

The model of the transformer I got from an ESL building friend of mine, I'm not an expert on this topic, but I'll try to answer your questions.
RLC prim/sec are just the values of the corresponding variables of a transformer. Rloss is, I think, the loss of the core. This is not a very accurate model, is doeas not include the core for example, but can be used for most cases. The simulation I, among others, did correspond very well with the actual measurements.
I think other models are around.....

V/I of the generator is indeed the impedance of the total schematic, so this is what the amp will see.

The ESL model I used is from "Loudspeaker and Headphone handbook", chapter 2. This describes in a lot of detail an ESL.
Cesl is just the capacitance between the two plates,
Cmylar has to do with the stretch-strength of the mylar, the force which pushes the mylar in it's middle position,
Rair and Lair is a model of the accoustical resistance of the air, so the actual sound is the power from these. This last point is my personal interpretation of the theory, but it looks ok, I think. The values of Rair and Lair are just chosen to give a nice plot, I don't really know what to put there.

By the way, all secondary values are transformed to primary values, so divided by 100*100, for a 1:100 turn ratio.

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Old 3rd November 2001, 01:52 AM   #14
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My friend Dan, as mentioned above, has built a working set of ESLs and using this god-aweful huge 'inpedance analyser' we have in the lab, I generated the below impedance data. This is the impedance across the entire load circuit (across what would attach to the amplifier speaker terminals). Shown vs. frequency are the magnitude and phase plots of the transformer by itself, and also the transformer attached to the esl (but without the bias supply activated).

The project now, as stated before, is to try to even out the impedance the amplifier is driving. The peak around 500 hz isn't too troublesome, nor is the corresponding phase shift there (34.5 deg.). What bothers us is the low ends of the peak; 0.4 ohms at its worst. Also, the phase shift extremes approach 85 degrees at either end. This, I've been told, could do bad things to amplifier stability, sending it into ultrasonic oscillation.

When constructing a crossover circuit for regular magnetic drivers, there's a correcting device called a Zobel Network, that fits between the speakers and the crossover. Basically, its a filter of some sort (or so I gather). I've seen plots of before / after input impedances, and it seems to be doing what I would like to do here. I've mentioned it before, but here it is again: this site talks about this zobel network . The problem, of course, is that this is not a simple mostly-inductive load like most magnetic drivers.

What would anyone suggest here, to even out the impedance curve, keep the impedance away from dangerously low values, moderate the phase shift extremes, and do all this without loosing too much power to resistive heating? As mentioned above, sticking a resistor in series with the transformer is not the best option. Thanks for any help.
- Jonathan

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Old 8th November 2001, 08:19 AM   #15
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what I did not mention before, a well known way to improve the impedence and performence of an ESL is to place series resistors between the secondary of the transformer and the ESL, so one on each side. This will lower the Q of Lsec and Cesl, which improves impedence, phase (I prefer to talk about groupdelay), and amplitude around the top end of the audio spectrum. Due to Csec you hav some limits, and the resistors must be choosen carfully, but with this the dip can be made almost the same as without an ESL attached.

I have with a Cesl of 500pF the best result with two resistors of 10k each.
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Old 15th November 2001, 01:16 PM   #16
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Exclamation Try a 10 OHM Power Resistor in Parallel with the transformer

I am using the same kinds of transformers on my ESL panels and they have been running for almost a year on a Crown DC-300A power amp. I spoke with Barry at the ESL info exchange, and he put me in contact with a friend of his who suggested that for a passive crossover you should put a ten ohm power resistor in parallel with the transformer's primary. This will produce a smoother curve when using passive high-pass crossovers (and also gives the amp a resistive load to drive). This suggestion works, just remember to use a 50Watt power resistor mounted on a heatsink.

My ESLs use a 47uF solen cap in series with the primary, and the 10 ohm resistor in parallel(after the capacitors). This rolls mine off at around 500 hz. Then I use a conventional woofer.

See my website for more details:
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