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Old 23rd August 2007, 08:55 AM   #11
Calvin is offline Calvin  Germany
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Hi,

clever idea indeed, but it is rather restricted to hybrid panels, because itŽll work as a constant voltage design. A CV-electrostat is a inherent nonlinear design, creating distortions, that can only be kept sufficiently small when the movement of the membrane is very small. To allow for more movement or lower distortion YouŽll have to linearize the system with some means of feedback, creating the problem of how to create the feedback sinal and to get the system stable.
The well proven alternative is the ConstantCharge-design (with small d/s and rather lots of membrane area) with (relatively) low voltage demand, using a tranny with lower transformation factor or a more classical HV-amp running on 1kV to 2kV supply voltage. This is much easier to design, much cheaper and imo the best is, that You use a inherently linear, low-distortion transducer that doesnŽt need feedbacking in first place.

@AK47: In Your example You assumed a constant impedance of 8 Ohms. This will definitely not be the case with a tranny-coupled ESL.
The impedance varies between fractions of an Ohm up to several 100s of Ohms and the impedance curve is shaped similar to a gaussian curve.
Too You assume and calculate with rms-values. Its common and more reasonable to calculate with p-p values when You are dealing with ESLs!
Based on the assumptions of 28Vrms and 90Vrms this would give 8kVpp resp. 25.5kVpp in Your example!

jauu
Calvin
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Old 23rd August 2007, 09:11 AM   #12
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You need some type of synchronous rectifier to turn the voltage into DC - and you need to do it for both sides seperately (out of phase) - then it can be applied to constant-charge electrostats.

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Charles
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Old 23rd August 2007, 09:14 AM   #13
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One might argue that you have switches again on the HV side but these would be easier to implement than a class-d stage and it can be made transformer-driven.

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Charles
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Old 23rd August 2007, 09:47 AM   #14
Elvee is offline Elvee  Belgium
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Quote:
Originally posted by Calvin
Hi,

clever idea indeed, but it is rather restricted to hybrid panels, because itŽll work as a constant voltage design. A CV-electrostat is a inherent nonlinear design, creating distortions, that can only be kept sufficiently small when the movement of the membrane is very small. To allow for more movement or lower distortion YouŽll have to linearize the system with some means of feedback, creating the problem of how to create the feedback sinal and to get the system stable.
The scheme lends itself to feedback application: the movement of the membrane can be accurately monitored by applying opposite DC voltages to the grids and picking up the displacement signal on the membrane.
A practical way of implementing this could be the insertion of a capacitor in the cold connection of the drive transformer, forming a capacitive divider with the membrane-to-grid capacitance. The superimposed HF ripple can easily be removed using cancellation techniques, with no impact on the bandwidth, thus easing feedback stability issues.
LV
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Old 24th August 2007, 05:27 AM   #15
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Interesting! Diaphragm feedback, thats something to think about!
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Old 27th August 2007, 03:29 AM   #16
GK is offline GK  Australia
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Quote:
Originally posted by phase_accurate
He then built a topology of multiple low(er)-voltage class-d amps that add up their voltages. I don't know if he even did them in some multiphase modulation scheme which would further reduce ripple (or make lower switching frequencies and therefore lower switching losses feasible).

Actually, this is real easy to do do, as the output of each class D amp (prior to the filter) can be transformer coupled. Then you just have to series connect the secondary windings, and all your class D amps can share common supply rails.
Reminds me of a pair of 495kW 500kHz RF generators I once had to fix when employed at Flinders University. These things were run in unison for a few mS (powered 16kV plate supply provided by a room full of capacitors) for plasma fusion experiments, providing the RF excitation for the Rotomak.
Each unit had a ~10kW PWM driver, built transformer coupled as described, to provide kV's of drive to the pair of tetrodes used in the final amplifier. PWM was used to ramp up the drive voltage as the plate voltage supplied by the capacitor bank dropped off, providing a constant 990kW of RF drive for a few mS or so.
This equiptment is currently operational in some university In Houston, AFAIK (I had to repair the PWM RF drive drive unit in a hurry for an operational demonstration to the prof who buying the whole rig) That was a fun thing to work on.

Cheers,
Glen
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Old 27th August 2007, 06:20 AM   #17
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Quote:
Actually, this is real easy to do do, as the output of each class D amp (prior to the filter) can be transformer coupled.
If there is something that I wouldn't do then it is exactly that !!

Regards

Charles
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Old 27th August 2007, 06:27 AM   #18
GK is offline GK  Australia
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Quote:
Originally posted by phase_accurate


If there is something that I wouldn't do then it is exactly that !!

Regards

Charles

Why?
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Old 27th August 2007, 11:14 AM   #19
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Hint: Simply bexause you don't gain anything compared to a class-d amp driving the usual step-up transformer !

If you intend to do what you mentioned you'd need audio tranformers that have at least their lower cutoff frequency equal to the woofer/ESL crossover frequency - preferably somewhat lower.

If you are going the class-d route you'd rather use insulated stages where each one is driving the (floating) ground of the subsequent stage. Then you can implement the floating PSUs using small SMPS.

Regards

Charles
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Old 27th August 2007, 12:12 PM   #20
GK is offline GK  Australia
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Quote:
Originally posted by phase_accurate
Hint: Simply bexause you don't gain anything compared to a class-d amp driving the usual step-up transformer !

If you intend to do what you mentioned you'd need audio tranformers that have at least their lower cutoff frequency equal to the woofer/ESL crossover frequency - preferably somewhat lower.

If you are going the class-d route you'd rather use insulated stages where each one is driving the (floating) ground of the subsequent stage. Then you can implement the floating PSUs using small SMPS.

Regards

Charles

I'm sorry, but you've got it completely wrong - I said "prior to the filter". The transformers connect directly to the MOSFET outputs - ie they work at the switching frequency.

The 500kHz 10kW PWM driver I mentioned earlier consisted of 10 synchronised 1kW half bridge modules, each the size of a eurocard. The 10 transformers were toroids, about 5cm in diameter, mounted on the PCB's.

Cheers,
Glen
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