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Karl71 10th July 2008 09:56 PM

DIY SS Amp for Just Real Music ESL Panels
I am awaiting delivery of a pair of Just Real Music Model .5 panels. Each one will be mated to a woofer crossed over at 375 Hz. The panels are minimum 2.5 Ohms at 12 Khz and a nominal 6 Ohms.

I am searching for a diy ss amp design that will meet the requirements of my panels. I have been reading the Randy Slone Book "High-Power Audio Amplifier Construction Manual." I really agree with his design philosophy, especially regarding Class B and correcting crossover distortion.

Any thoughts or suggestions?



Bazukaz 12th July 2008 02:22 PM

The requirements for ESL amplifier are not very special , except for stability at capacitive loads and good output transistor protection. I used LM3886 chips and these worked fine and are very difficult to fry out. It was not a SS amp, though.

Calvin 14th July 2008 09:36 AM


the correct answer should have been: "The requirements always depend on the load."
ESLs can be very special, but don´t need necessarily be so. The deciding factors here are the quality of the audio tranny and the panel construction.
The worst case that could happen is the combination of a non-segmented panel driven by a lowloss tranny. The results are very low ohmic values at the upper end of the freq-range (<1Ohm possible even with low transformer factors around 1:50) and very high values of phase shift in the upper mid-freq-range (>80° possible). Such a load can hardly be driven properly by a chip amp (just 3-5W of useable power, but 30-50W of reactive power!). Some measurement systems feature such chipamps and you can easily measure and hear the distortions of those amps from quite low volume levels on....up to the complete exitus of the device!
The other ´extreme´ is the combination of a segmented panel driven by a lossy tranny. The reduced capacity at high freqs leads to a higher ohmic value in this range and it reduces the value of phaseshift to levels which are similar to dynamic speakers (<30°, see Audiostatic).
Most of the commercial producers use the´ lossy-tranny-trick´ because otherwise they´d be faced with the problem that hardly any amp on the market would be suitable. Of course does this technique give up on good sound to a certain degree, because nothing comes free, but it relieves the user from the stress of finding a ´capable´ amplifier.
So to evaluate if a amp is suitable power-wise You should check on the impedance plot of the device and its claimed efficiency.
The claimed 2.5Ohms@12kHz should be no problem to any decent amp.

The second problem with ESLs is that most amps feature global feedback. The capacity the ESL presents modifies the feedback loop and as such influences the ´behaviour´ of the amp immidiately and very effective. The amp even can change its behaviour from amplification to oscillation. Its up to the designer to implement ´reserves´ for capacitive loading and to make the amps behaviour stable. But those techniques usually reduce bandwidth and give worse specs on paper. Especially amps that claim excessive high bandwidths (>>50kHz) are prone to oscillation.

The JRM panels seem all to be of the non-segmented type (high capacitance), but the trannies look more of the lossy type (though apart from the 50:1-version their transformation factors are higher than its useful for such panels), so you might get along with a chip amp, or a modificated chipamp .
From the specs the JRM trannies are rated at 4.5kV maximum, which translates to ~16Vrms input voltage, ~32W@8Ohms). So a high power amp would mean trouble over time (flashovers internal to the trannies, that destroy the part).
Modifications to a chip amp could be: reducing bandwidth, adding compensation networks, parallel devices, larger valued power transformer, increasing power supply capacitance.
In discrete designs working with emitter followers or source followers it sometimes helps to cut the feedback loop at the output node and take the feedback from before the power stage (see Erno Borbely´s Class A MOSFET amps as example of this technique).
Switching amps which are normally well suited to drive such reactive loads fall out of count here because of the low quality of the audio trannies (it leads in conjunction with the output filter of the amp to insufficient bandwidth). And sure they are low quality as you can see that the impedance minimum is at a low 12kHz (that is the niveau you can already reach with cheap and simple EI-power trannies! And those are more robust too voltage wise).
Above this frequency the response drops by 12dB/oct. Subtract even a more couple of dBs if they still use half micron films for the diaphragm.
So You might try with a single chip amp, but be prepared that you might have to modify it.


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