Tube amp specifically for driving ESLs - Page 17 - diyAudio
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 13th December 2007, 05:09 AM #161 diyAudio Member   Join Date: May 2005 Location: Bavarian Forest In an other thread someone told me that Berning probably uses such a complex topology because a simple modulator-amplifier-rectifier topology would only have 25% efficiency because a capacitive coupling is necessary to get the signal on ground. But since this is not the case for an ESL, would such a simple topology work here? Additional info: Berning cascades low voltage rectifiers, so high voltage at the output doesn´t seem to be a problem.
 18th December 2007, 06:54 AM #162 diyAudio Member   Join Date: Nov 2007 On the point of panef efficiency: I've once did a calc, which gave me really high efficiency of a panel.. Here it is: Panel size: 0.5*0.5m, spacing 6+6mm. At 20kHz that gives 43kOhm. Assuming 2kV driving signal, it's 50mA, ~100Wt. Now, let's calculate the pressure: P=Isig*Eht/(2*d*c*pi*r) d is spacing, c - speed of sound, r- distance to measurement point. Assuming 12kV EHT - high, yet possible, especially with such a spacing, value. That's 24Pa = 27dbPa. 1 dbPa = 94 dbspl, 27+94=121. At 100Wt! So sensitivity is 101 db? A somewhat too high value.. That's an just excellent sensitivity. I'll do some more calcs and post em soon.
 18th December 2007, 02:01 PM #163 diyAudio Member   Join Date: Nov 2005 Location: Southern Germany Hi nevod, first you mentioned 100 wt at 2Kv, later on you calculate the soundpressure at 12kv, which is certainly not 100 wt, but some kilowatts !!!! Capaciti
 18th December 2007, 02:56 PM #164 diyAudio Member   Join Date: Nov 2007 Capaciti 12kV is not the driving signal, it's EHT. The driving signal is still considered as 2kV.
 18th December 2007, 05:52 PM #165 diyAudio Member   Join Date: Nov 2007 Some more thoughts: A switching amp is extremely efficient when driving ESL - it recuperates energy, transferring it from PS caps into panel and back. A low-voltage amp has been built by a guy, which showed recuperation. There are two issues with switching amps: quality and voltage capabilities. Quality is a hard question, guys here at Class D say that D-class is good, but, really, how good it is? That's whai I do not know. Voltage capabilities: you'd need a switching device both fast and able to withstand HV. Trafo is of no help, as it reduces efficiency and makes recuperation much less desired. There are no such transistors, tubes aren't switching, then what? In some future, possibly, GFETs will be able to do so. But then, are Class D really good? In the pretty much hi-end sense good, I mean, comparable to a class A tube amp, directly driving an ESL? Is it possible without boosting switching frequency into 100 MHz range, as that range has it's own problems with switching, which won't be solved by any type of active element?
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Join Date: Apr 2004
Location: Northern California
Quote:
 Originally posted by Nevod ...edit.... There are no such transistors, tubes aren't switching, then what? In some future, possibly, GFETs will be able to do so. ..edit...
Powerex makes IGBT (insulated gate bipolar transistor) up to 6500 V There are others.

 19th December 2007, 03:52 AM #167 diyAudio Member   Join Date: Nov 2007 They, afaik, don't switch anywhere as fast enough.
 19th December 2007, 04:20 AM #168 diyAudio Member   Join Date: Apr 2004 Location: Northern California Yea, I know they were meant as an example. Those are also much faster at lower current densities. Smaller (i.e. less than 200 Amp) IGBTs exist, they are also faster. MOS-FETS of up to 1500V switching at up to 5 MHz are available from SANYO. Tubes rightly or wrongly deliver a certain sonic signature when used as linear amplifiers. Some like the signature others do not. But that special effect doesn't make tubes a superior or automatic solution to other topologies such as class D. While tubes could make a pretty good linear ESL driver by taking advantage of their nature. I would spend more effort in looking for a solid state switch if I was pursuing a high voltage class D design.
 19th December 2007, 10:58 AM #169 diyAudio Member   Join Date: Nov 2007 Yes, that's quite logical to choose tubes for linear amp and SS for switching. I'm trying to address a different point: Can a D-amp be on par with A-amp, regarding only quality? For long time, D-amps weren't good for anything but subwoofers, lacking highs. At once, ESL is much better than dynamic head for highs reproduction, so, coupling ESL with highs-lacking amp is certainly a good idea. I haven't been reading Class D forum for long, but from what i understood, currently the best sounding D-amps are 'analog' designs, self-oscillating, having an analog input, and analog feedback into input from the end of LP filter. That is, as i understand, essentially a 1-bit ADC with output directrly connected to a 1-bit high-power DAC Theory seems to give quite good DD and noise for even a relatively low frequency unit with simple filtering, but that seems not to be very true, actual well-performing amps have high order filtering and high sampling frequency, which results in RF-type problems. Besides, an analog input isn't very appealing if you have a digital source, but fully digital amps doesn't seem to perform well, they are said to be inferior to 'analog' ones. And we don't know for sure how well a 'analog' D-amp performs against a good A-amp at highs..
 19th December 2007, 04:33 PM #170 diyAudio Member   Join Date: Apr 2004 Location: Northern California There is no reason that an all digital amp could not be made or sound as good as the best analog designs, but with todays technologies it is damn hard. It's not really true that the current class D designs are really one bit DACS. Think about it; A one bit DAC would need to switch at 2^16 * 44.1 KHz or 2,890,137,600 Hz (2,890 Mhz) in order to exactly cover every possible output state. All current class D amps truncate information. Probably all or nearly all analog amps truncate as well, a 100 watt amp needs to output 400 microvolts for the least significant bit, this is below the amps noise floor. And that's at 100 watts, at a more normal listening level of say 5 watts we now need to control down to 20 uV. I think most class D designs use a triangle wave fed to a comparator to generate a variable output pulse width. This is really an analog design disguised with a switching output. Making a triangle wave linear to 1/(2^16) is no picnic either.

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