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Old 14th October 2001, 08:04 PM   #11
hifiZen is offline hifiZen  Canada
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Denis:

sorry, but any class AB amp using any type of output device will not be capable of generating "only 2nd harmonic distortion" over it's entire dynamic range. This is quite impossible. Hint: the measurement frequency plays a critical role in distortion measurement... it's easy to hide all the distortion spectra under the noise floor of your test setup when making a measurement at 100 Hz. Try doing this at 20kHz or 50kHz where your distortion products will be much larger.
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Old 15th October 2001, 08:45 AM   #12
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I've got some (thrown-away) Semikron IGBT-modules intended for use in frequency converters (see for instance http://www.vacon.com). The module consists of three IGBT's rated at 800 V and 200 A (they have models for up to 3300 V and 500 A). They use a couple of modules when making frequencı converters from 0.5 kW up to several MegaWatt. I know that these modules are intended for switching, but it would be fun to try to make a linear class-A amp with them. They are so much overrated that it would be no problem with stabilisation.
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Old 15th October 2001, 09:52 AM   #13
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Default 2nd harmonic

I said " for almost all of the dynamic range". Of course, 3rd and higher order harmonics do appear when the clipping approaches. No sign of something higher than 2nd harminic is visible below -3dB in respect to full output level. Of course, I took all precautions against noise and mains interference! The measurements are similar at any frequency up to 60 kHz. At the output voltage of 2.83 vrms, what corresponds to 1 watt/8Ohms, there is certainly pure 2nd harmonic and THD<0.1%.
By the way, my PP pentode amplifiers using EL509 valves behave the same way. Indeed, they operate in pure class A for 8 or even 4 Ohms, but transit to class AB operation at certain level for lower load resistances. Only those people, who visited my laboratory, now beleive!
The phenomenon of odd harmonisc cancellation in pentodes was first observed in 1950s. The same approach is applicable to bipolar transistors too.
Some details of my solid-state design:
The output stage utilises from 2 to 6 pairs of Motorola TO-3 transistors running at 250-450 mA of total idle current. There are also several paralleled pairs of drivind transistors which run also at considerably large idle current. The front end consists of just three transistors, and there is no differential pair at the input. This topology allowed me to apply moderate global NFB of just 12-14 dB.
If you have more questions, please write.
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Old 15th October 2001, 10:00 AM   #14
djk is offline djk
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You guys just don't get it.Would you like to buy a bridge I own in Brooklyn? Better yet buy my switching transistors. The original MRO wholesale price from Motorola was $44 each.I have about 600 of them.Yep, $25,0000 worth.They are rated at 150V 50A continuous 100A peak 250W and have an Ft of 30Mhz with a typical gain of 100 at 10A.THEY ARE WORTHLESS FOR AUDIO.At 50V they can only handle 15W because of secondary breakdown.And that is at 25*C, you have to derate that over temperature.An IGBT is this kind of a transistor with an FET hung on the base.If anybody is stupid enough to want these they can have them for $1 each.I may hand them out to the kids that come to trick-or-treat this year.
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Old 15th October 2001, 10:51 AM   #15
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Default MOTOROLA

To make a good solid-state amp one has to have the best LINEAR transistors. The best ones are Motorola (OnSemi) MJ15024 and MJ15025! These TO-3 devices handle 250W at 25C, and they are extremely robust against second breakdown. It is possible to build a 400W/8 Ohms amp on just 6 pairs of them, and this amp will withstand continuous operation into 2 Ohms, and the real world short-output current of 40A during 10ms. These transistors are the real workhorses of professional audio manufacturers, and their price is usually $3/each! What is remarkable, they are linear to such an extent that it is possible to make an amplifier without global NFB and not obviously it must be a pure class A one. Other types as MJ21193 and 21194 look even better on their datasheet, but I have no experience with them.
No MOSFET or IGBT is comparable with these brilliant bipolars.
Indeed, I see an advantage of using large MOSFETs in high-power amps where the output devices of more than 250V rated Vceo are needed.
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Old 15th October 2001, 11:16 AM   #16
tvi is offline tvi  Australia
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Theres a good description of IGBTS <a href="http://www.elec.gla.ac.uk/groups/dev_mod/papers/igbt/igbt.html"><b>here</b></a>

The idea of a parasitic thyristor and possible latch-up doesn't inspire confidence.

With so few commercial audio products using IGBTs it makes me wonder about their sound/relability.

If you are looking for a novel output device what about <a href="http://www.ne.jp/asahi/evo/amp/SIT/SITaf.htm">Static Induction Transistors (SIT)</a>
and another <a href="http://www.mobara.ne.jp/~yamada/sit/index.html">here</a>.
Both pages are in Japanes.


Regards
James
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Old 15th October 2001, 11:27 AM   #17
djk is offline djk
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I buy MJ15024/15025 in 100 lot.The MJ21193/21194 have TWICE the safe area at 100V.Three pair will give 400W at 8 ohms and 600W at 4 ohms and with a tiered power supply 1000W at 2 ohms is possible (look at the AB International model 1100A).
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Old 15th October 2001, 11:55 AM   #18
AuroraB is offline AuroraB  Norway
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Default Motorola transistors and beta droop

This should maybe be a thread on it's own , because the subject is quite interesting,----but....

One of the few references I have found in the audio community that deals with the phenomenon of "beta droop", i.e. a reduction of beta as function of load current, is Douglas Self in his E&WW articles about distortion mechanisms. Beta droop definately seems to have an impact on distortion figures, as can well be imagined..

He also has a small survey of relevant parts, where in his test the MJ 15024/15025 comes in third, with MJ21193/21194 second and the often used japanese 2SA1302/2SC3281 as the very best of the ones tested.
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Old 15th October 2001, 01:49 PM   #19
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Default Motorola

MJ21193 and 21194 really look excellent. In fact, from the viewpoint of second breakdown, they have much more than twofold advantage at Vc=150-200 volts, that allows one to operate them even at +-110Vcc. Indeed both 15024/25 and 21193/94 in proper circuits operate at their thermal limitations rather than second breakdown. So, they must be used in equal quantities to ensure reliable service.
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Old 15th October 2001, 01:52 PM   #20
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Default Static induction transistros

I wonder if they are still in production.
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