Proposition: ESLs and Class D are a perfect match
Here is an idea that mixes class D with ESLs.
One of the main difficulties associated with ESLs is the step-up transformer: the magnetizing inductance and core saturation limit the low frequency response, and the leakage inductance causes problems at the high end of the spectrum, especially considering the highly capacitive nature of the load.
Direct drive is the obvious solution, but is difficult to achieve and probably requires tubes, with problems of their own.
I propose the following arrangement:
A conductive membrane is suspended between two metallic grids; the grids are driven by high frequency, high voltage sinusoids having +90° and -90° phases; the center membrane is driven by a phase modulable sinusoid of similar voltage and 0° quiescent phase. Under those conditions, the net force exerted on the membrane will be zero: the force at 2*Fo on one side will be exactly compensated by an opposite force on the other side.
Now, if the phase shifts from 0°, the average absolute potential difference between the membrane and each of the grids will become unbalanced, resulting in a net force on the membrane.
Because the electrostatic force is independent of the sign of the potential difference, the membrane acts as a "phase comparator".
In summary, all we have to do in order to generate an audio signal is to phase-modulate the membrane drive wrt. the reference grids signals.
We need a center-tap transformer with its driver for the grids, and another transformer and driver, plus a phase modulator for the signal channel.
The big advantage is the fact that the transformers only need to operate at a high frequency, making them small, light and non-critical; their amplitude linearity is irrelevant, as only the phase information is critical.
The output drivers can operate in class D/E, the speaker capacitance being part of the output filter.
I see some potential difficulties: with kilovolt signals in the 100~200KHz range, great care would be needed to avoid corona and arcing; the drivers would need to supply a huge reactive power to the system.
For the signal channel, a compromise would have to be found between the Q of the output network and the bandwidth needed to reproduce the PM sidebands.
I think these are practical, solvable issues, what's your opinion about it?
Note that this outline is mainly conceptual, made to ease the comprehension of the principles, but practical implementations could significantly differ.
Enhancements are also possible: for instance, the linearity could be improved by means of a feed-forward analogue signal for correction purpose. This way, the missing parts of the Bessel function due to band-limiting could be compensated for.
I think that would create alot of RF interferance? It does sound like it could work though.
the force on the membrane will be null if both grids are in phase and not 180° out of phase.
A symmetrical ESL is no push-pull device!
In fact is it a push-push or pull-pull-device!
You would need a really incredible powerful amp not only to supply for the reactive power; the real power will be huge too, since the impedance of the ESL will be quite low at ~100-200kHz
You're talking about replacing passive electronics with active circuits. The questions to ask are: can the phase modulator (actually a frequency modulator here) outperform an audio transformer and, can it do it for lower cost with same/higher reliability?
The frequency modulator has to cover the entire audio band and be linear while doing it. How is amplitude controlled?
This design would require the diaphragm to be conductive. The problem of distortion would be the same as any other ESL operating with a conductive diaphragm, though there might be some way to predistort the signals to compensate somewhat.
You also still have the high frequency beaming and low frequency roll-off issues associated with any flat panel driver, though I suppose the modulator could be used to equalize the response within the same limits an op-amp equalizer does now.
Many problems to solve.... could be a fun project.
Although phase- and frequency- modulators can be interchanged if the modulating signal is predistorted accordingly, this one is definitely a PHASE - modulator since it can only modulate phase between + and - 90 degrees which won't give a very large frequency shift.
A phase modulator of this kind can be built using a ramp-generator and a comparator.
This is the reason why the subject is titled ESLs and class D... .
The modulator and the resulting signal are similar to class D
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