Say you had a 100watt amplifier, 1kw amplifier, and a panel with a 100:1 step up transformer.
The final drive voltage would be 2800v for the 100watt amp and 9000v for the 1kw. The 1kw will obviously be able to play louder but will it actually be using 100mA @ 9000v?
I don't understand where 1kw of power goes. It's not heat or sound.
The final drive voltage would be 2800v for the 100watt amp and 9000v for the 1kw. The 1kw will obviously be able to play louder but will it actually be using 100mA @ 9000v?
I don't understand where 1kw of power goes. It's not heat or sound.
Actually, heat and sound are exactly where it goes.
An ESL is a big capacitor. To get from zero to some voltage requires current. To get from zero to that voltage even faster takes more current. Now, when it's time to go back to zero, the higher the available current, the faster it can happen. But where does that current go? Yes, right back to the amplifier. Being reactive, to a first approximation the speaker absorbs no power, but the amp does. And that's where the heat goes; remember power factor, too.
Now, when we refine the approximation, we can see where the sound energy goes, but it's still a small proportion of the electrical energy that the amplifier needs to source and sink charging currents. It's an interesting exercise to calculate how much current it would take to drive a 1nF capacitor (a typical ESL value) at 20kHz.
An ESL is a big capacitor. To get from zero to some voltage requires current. To get from zero to that voltage even faster takes more current. Now, when it's time to go back to zero, the higher the available current, the faster it can happen. But where does that current go? Yes, right back to the amplifier. Being reactive, to a first approximation the speaker absorbs no power, but the amp does. And that's where the heat goes; remember power factor, too.
Now, when we refine the approximation, we can see where the sound energy goes, but it's still a small proportion of the electrical energy that the amplifier needs to source and sink charging currents. It's an interesting exercise to calculate how much current it would take to drive a 1nF capacitor (a typical ESL value) at 20kHz.
A class D amplifier would be a good alternative for driving a highly reactive load because all the reactive energy is returned back to the PSU. In case of a capacitive load (ESL), the amplifier may be designed so that the load forms part of the output LC filter (although the resonance that results when the step-up transformer is added has to be tuned to avoid trouble).
I agree that a purpose-built class-d would be great for driving ESLs. But I am afraid that we might not be able to do a "classic" class-d amp that is capable of doing the desired voltage swing. I once consulted someone who was developing an amp intended to drive piezo actuators up to 1000 volts. The idea of an ordinary class-d half-bridge was given up quite quickly due to the huge voltage swings involved and the subsequent high snubber losses.
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).
A mutlibit (i.e. multi voltage output stage) delta-sigma topology might be quite cool as well !
The RF transmission problem could however be minimised IMO using conventional methods.
Regards
Charles
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).
A mutlibit (i.e. multi voltage output stage) delta-sigma topology might be quite cool as well !
The RF transmission problem could however be minimised IMO using conventional methods.
Regards
Charles
There is an alternative I've already proposed, based on high frequency transformers and phase modulation:
http://www.diyaudio.com/forums/showthread.php?s=&threadid=93433
http://www.diyaudio.com/forums/showthread.php?s=&threadid=93433
Hi,
clever idea indeed, but it is rather restricted to hybrid panels, because it´ll 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 You´ll 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 doesn´t 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
clever idea indeed, but it is rather restricted to hybrid panels, because it´ll 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 You´ll 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 doesn´t 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
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.Calvin said:
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
phase_accurate said:
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
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
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
phase_accurate said:
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
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.