What do you think about IGBT amps and your experiance with them...
The Crescendo Amplifier (Millenium Edition) article in a recent Elektor magazine compares the sound of the Crescendo Mosfet amplifier with that of their earlier published IGBT amp (Compact AF Amplifier - using Current Feedback instead of the usual Voltage Feedback). The report says that the sound of the MOSFET circuit is more laid back but less detailed than the IGBT circuit. This could be due to topology differences but....
I have heard an IGBT amplifier that goes by the name of Forte'. If I am not mistaken, this is a sister-concern of the famous Threshold brand. The amp sounded very detailed and authoritative. But on prolonged listening, the bass seemed to be "out-of-phase" with the rest of the sound spectrum; it seemed that the speaker cone was moving in the opposite direction than required. However, in the same set-up, the Ultra-fast amplifier design published in Electronics World, far out-performed the IGBT amplifier in every aspect of sound reproduction. Smaller amplifiers, both Bipolar and Mosfet output types, did not suffer from the afore-mentioned bass problem, but failed to exhibit the same authority and speed as the Ultra-fast or IGBT amplifiers.
This is only an observation that I could make as well as verify with other expert listeners but could not arrive at any conclusions, for lack of enough data regarding circuit details etc.,
I do have at hand a pair of Toshiba IGBTs and the PCBs for the Compact AF Amplifier (Elektor design) but have not yet assembled these and tested them.
John Linsley Hood who was asked by a Japanese manufacturer to test IGBTs for audio purposes, reported his findings in Electronics World, along with the circuit used in this comparative study of Bipolars, Mosfets and IGBTs. You could probably have a look at this informative article.
Before I left Threshold, I delivered a white paper to
the "new management" entitled "IGBT's: Threat or Menace?"
and it was my opinion that the only advantage to them was
The amplifiers to which you refer were designed by
As you may be aware, IGBT's are essentially two devices
on a chip; a MOSFET whose output Drain goes into the Base
of a high power Bipolar device whose Collector attaches
back to the Source of the FET. In an N channel IGBT the
MOSFET is N channel and its output is boosted by a PNP
For high end linear applications there are a couple of
problems. First, there is no way of biasing the MOSFET
independently of the Bipolar. Normally we like to run
previous stages at significantly higher bias than the
needs of the following stage, which gives better linearity
and faster response. Since all the current from the Drain
of the MOSFET goes into the Base of the Bipolar and there
is no external access, this cannot be adjusted.
Secondly, matching is essential when using them in
parallel, but the match has to occur not only between
the MOSFET portion, but also the Bipolar device, and this
is very difficult to do, resulting in current hogging and
The parts were a pain in the butt, both Threshold
and Counterpoint seemed to have abandoned them, and
the manufacturer discontinued offering complementary
parts for audio use.
IGBT's remain popular for switching applications, but
I haven't seen them in an audio amplifier since.
IGBTs and CFB
About 10 years ago, a design was published as an Analog Devices application note, by a guy called Mark Alexander. The amp was a current feedback design. This design was taken on by the Danish magazine " High Fidelity", and very slightly altered by Poul Ladegaard et.al. This amp used IGBTs in the output, and was loudly praised for its qualities, "authority " being one of them.
I'm quite sure NP has seen this app.note. It would be quite interesting to hear NPs comments on this, also on the subject of current feedback. Also since I have two fully populated PCBs of this amp in my drawer marked " eternal projects". ( BTW- anyone else but me that has such a drawer ?? )
[Edited by AuroraB on 10-12-2001 at 05:24 AM]
A drawer? I got a whole house marked this way. Indeed I think my ex hung a sign so marked on my butt before she desided to quit :)
Thanks for replays!
Elektor Electronics has released a few projects with Toshiba IGBTs. At least a mono power amp and a sub-woofer amp at about 250 W with four output IGBTs. I think the amps are very good, and in the article was no complain about the IGBTs. The transistors used were GT-20D-101 and GT-20D-201.
IGBTs and MOSFETs vs. BIPOLARS
What will you say about all-bipolar amplifier wihch generates only 2nd harmonic of distortion for almost all it's dynamic range? And this amp even runs in class AB.
I think the best amplification devices after the vacuum tubes are the bipolar transistors.
"IGBT's remain popular for switching applications, but
I haven't seen them in an audio amplifier since." Hex-fets are poor choices for high voltage swiching due to the lossses from the Rds.The IGBT does not have this problem.On the other hand it has the same problem that all high voltage BJT switching transistors have.It is not suitable for linear operation due to very poor secondary breakdown characteristics.Apex makes some 10KW class D modules with IGBTs.You would be lucky to get 100W with the same parts in a class AB amplifier.
My only expirience with the IGBT´s is in ELEKTOR magazine´s amp. you can see that in my web site :http://download.tripod.de:81/Promitheus/950077.pdf
The original design was with small hexfets I think irf540 and irf9540. After that they used the same topology scaled up with the Toshiba IGBT´s GT20D101 and GT20D201. I wasn´t so satisfied with the sound and I dind´t like the idea after that they used the IGBT´s in place of the mosfets since IGBT´s are like two in one devices (bipolar driving a mosfet). When I changed the output devices back to hexfets like the original smaller version but with bigger devices, it sounded perfect. I used in the new version irfp250 and irfp9250. I prefer them a lot over igbt´s.
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