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Class D Switching Power Amplifiers and Power D/A conversion

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Old 24th December 2009, 09:12 PM   #1
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Default Flyback-mode class D amp to direct-drive electrostatic loudspeakers

I'm building a pair of electrostatic loudspeakers following some easy recipes I've found on the net. While they are rather easy to build, the big problem seems to be driving them, as they aren't exactly a nice load and they require a step-up transformer with it's distortion and bandwidth problems.

Since I've been helping friends in building high-voltage power supplies for some time, it seems pretty easy to me to build a "controlled" high voltage power supply to power the ESL's, that's it, a class D amp. I have found no information on the net about this kind of amp so it's either a very crank idea or I'm the first one to seriously try this.

My approach would be:

Triangle wave generator -> comparator -> mosfet -> flyback transformer
Audio input -> summing amplifier (bias) -> to comparator

Switching frequency would be around 200KHz. I believe that this setup could sound OK with no feedback at all, but this has to be seen.

Any input?
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Old 25th December 2009, 07:43 AM   #2
Calvin is offline Calvin  Germany
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Hi,

I donīt think the idea is crank and Youīre definitely not the first to think in that direction. Youīd be probabely the first if You managed to build a working -and of course legally(!) working- item
On the Pro-side there are:
- the capability of switching amps to control even very complex loads like ESLs with ease.
- their inherent high efficiency.
On the Con-side there are:
- finding fast enough switching power transistors or circuits that are capable of at minimum 2kV, preferable 4-5kV, with large ESLs even up to 10kV supply voltage.
- EMV-rules and - compatibility is already not that easy to accomplish with typical classD, let alone highvoltage-classD.
- distortion values will probabely be higher than those of a well executed ESL panel driven by a first class audio-transformer. Why spoil the performance? Sonically every class-D I listened to performed rather uninspiring and dry. A good tube amp seems still be to be the best sounding match, wether in Direct Drive or coupled via audio transformer.

Assumptions You made, which imho donīt hold up in practise:
- ESLs are easy to build. No they arenīt! Itīs easy to build a sound generating device with an electrostatic motor. A decent ESL is something very different. If it were not, then there would be many more manufacturers who would try this superior loudspeaker principle. With ESLs the devil is in the details.
- audio transformers restrict in bandwidth and distortion. That is true, but a well designed audio tranny allows for such good values that most direct drive amlifiers donīt reach better results either. If You look at common directdrive designs there is hardly one that reaches a full power bandwidth of just 20kHz!
- Apart from some enigmatic devices fabricated from unobtainium, there are hardly any output devices on the market that might be usable as power output stage. IXYS specialized a bit on highvoltage type transistors up to 4kV (IGBTs), but I doubt You get those to switch cleanly at 200kHz and 4kV. With the upcoming of the SIC-technology new power-JFETs which look quite promising may make it to the market. But to my knowledge there are only two companies that offer samples at the moment at considerable high costs. Both restricted to 1200V Uds at the time. One offers enhancement, the other company depletion mode N-channel types. Stacked devices switches with up to 6.5kV have already proven their functionality using the latter devices. Especially those dpletion mode JFETs look very promising for a common classA/AB directdrive design with stacked devices. There might be some transmitting tubes, but at considerable cost too.


jauu
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Old 25th December 2009, 08:18 AM   #3
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high voltage transistors?
why not tubes? -good for high frequencies and high voltages, i would look at HV triodes like the GP5, 30kV and 2mA and similar tubes.
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Old 25th December 2009, 11:17 AM   #4
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I believe that in the flyback topology the power transistor will not have to withstand the full output voltage as the load is connected to the secondary of the transformer.

The idea of using high voltage tubes looks really good. I will look for class D tube designs.

About whether it's easy to build a ESL loudspeaker or not, I've still to decide if I'm doing an audio project or a science projects. Probably it will start like a science project and turn into an audio project after the first working results.

My initial idea was to use a TV flyback output transformer, but I'm not sure if it will last long driving an ESL.
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Old 25th December 2009, 12:52 PM   #5
Eva is offline Eva  Spain
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For half or full bridge class D I think there are useable MOSFET and diodes up to 1500V. A 1500V full bridge could drive the panels directly, or drive a step-up transformer with a low turns ratio, which should perform much better in terms of frequency response, distortion and parasitic capacitance than conventional audio transformers. But getting a switching stage right at 200V is already tricky, at 450V (the highest I have done) it's hard and frightening every time you get a set of exploded output transistors, and at 1500V add "risk of death" to the equation...

Flyback is a different matter, I would not recommend it for audio, transfer characteristic is not linear, output voltage has a "1/(1-duty_cycle)" term in it that makes it highly non-linear.

Anyway, audio power transformers with high step up ratios are terrible, most of the difficult load associated with electrostatic speakers actually comes from the capacitive and resonant nature of the transformer. Class D amplifiers tend to behave much better. Nowadays many designs have load independent frequency response (the one in which I'm working changes less than 0.1 dB from no load to full load) and low THD.

Most tube amplifiers with output transformer will easily show several +/-3dB peaks and dips across the audio band depending on load, thanks to their impressive unity damping factor
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Last edited by Eva; 25th December 2009 at 01:08 PM.
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Old 25th December 2009, 12:54 PM   #6
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i guess that a ignition coil from a car would be able to deliver more power than one from a TV-set, but i also think both would be underrated in terms of power.
I will follow this thread, it would be an interesting amplifier, especially if you would combine class-D with a tube, new and old technology integrated in one amplifer
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Old 25th December 2009, 02:04 PM   #7
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There's a patent #68444777 where a classD amp is made by using transformer. But I don't know whether this idea really works or not, considering flux imbalance.
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Old 25th December 2009, 02:57 PM   #8
Eva is offline Eva  Spain
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A phase shifted full bridge driving a step-up or step-down transformer, and then an output filter, can make a class D amplifier with reasonably good open loop linearity. Doing it that way, the transformer is not subject to audio frequencies. Peavey (and probably others) patented it a long time ago. But the problem is rectification: Polarity of output winding has to be flipped on every cycle too, which requires active devices on the secondary side to achieve "four quadrant" I/V operation, not a good thing at high output voltages.

Same happens with flyback.

An audio amplifier must be able to both source and sink current regardless of the polarity of output voltage, particularly with a capacitive load like ESL. A flyback can only work in one quadrant. Half bridges and full bridges can be easily operated in 4 quadrant mode (conventional class D).
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Last edited by Eva; 25th December 2009 at 03:05 PM.
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Old 25th December 2009, 04:05 PM   #9
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So it seems that the biggest problem is rectification, as i'm thinking about using a triode (Something like a PD500, I have some of these lying around) to sink current, this would make the amplifier extremely inefficient and will not be an elegant solution but it will allow some experimentation with the class-D part working on a single quadrant and doing the hard "voltage lifting" part.

The tube seems a nice part to experiment as it can withstand enormous voltages, the only concern being x-ray emission.

Any suggestions for rectification part?
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Old 26th December 2009, 01:22 AM   #10
Eva is offline Eva  Spain
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You could do a half bridge with triodes or pentodes as switches, tube or solid state diodes and solid state grid driving, operating at several kV

I wonder if tubes can switch fast enough with proper grid drive... Less than 250ns total switching delay would be good, 500ns would probably be the maximum acceptable for 20khz class D.
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