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Old 17th June 2009, 09:05 AM   #31
milosz is offline milosz  United States
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Join Date: Oct 2006
Default IGBT / Forte 4A

Forte 4A was a Threshold product that successfully used IGBTs. Forte 4a's are very reliable, very stable and sound particularly good. With Soderberg's mods a Forte 4A can challenge just about any 50 watt/ ch amp made today in terms of sound quality. I bought a Soderberg'ed Forte 4a for $650 recently. Show me another amp that sounds substantially better at anywhere near this price..... so, Nelson Pass' comments notwithstanding, it apparently IS possible to build a good-sounding amp with certain IGBT's. Bladelius accomplished this with the Forte 4a.

But, of course, the point is well taken: Using IGBTs requires a lot of extra matching to get good sound, so why go through all the trouble of building a good amp with IGBT's when you can use prefectly fine bipolar or FET devices to build a good amp without all that fuss. You don't gain anything by using IGBT's except some sort of "exoticity" - which maybe was seen by some marketing guys in the 1980's as a plus re: IGBTs.
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Old 29th November 2013, 11:49 PM   #32
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Join Date: Oct 2011
Default IGBT Amplifier

Originally Posted by Samuel Jayaraj View Post
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.
The speaker cone moving in the opposite direction is probably caused by speakers being wired 180 degrees out of phase i.e. + to the - and - to the +.
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Old 30th November 2013, 12:23 AM   #33
milosz is offline milosz  United States
diyAudio Member
Join Date: Oct 2006
Default Forte 4a

By all accounts, a Forte 4a amplifier with the "Soderburg" mods is a 'giant killer' amp. sounding as good as the best, and better than most other amplifiers.

I have one and I LOVE the sound on my Quad ESL-57's. I've recently bought another to use on the midrange panels of my tri-amped Magneplanar MG-3.6's, although I have not put it into service yet.

Forte' was a 'sister brand' of Threshold, meant to offer excellent performance at prices lower than Threshold - as I understand it. The Forte' IGBT amps were designed at the time that Nelson Pass was in charge of engineering, or maybe just after he left, I do not know. I do know he did NOT design these IGBT amps, and in fact does not think that IGBTs are a good choice for audio designs. I think that, in general, his points are valid, but that in the case of the Forte 4a at least, the designer "got it right" - and in spades. (Nelson Pass comments: http://www.diyaudio.com/forums/showt...71#post1153171 )

Forte 4a is 50 watts at 8 ohms, and 100 watts at 4 ohms per channel, class A. I don't think it's excellent sound is owed to the presence of IGBT's, I think it stems from a very good design that just happens to use specific IGBT's.

I've never heard a Forte 4a that had any kind of problems in the bass. The bass from my amps is solid, taut, extended and tuneful; the mids are the closest to the 'liquid' mids of an EL-34 amp that I've ever heard from a solid state amp, and the treble is 'smoother' and less 'grainy' than just about any other solid-state amp I've heard, while still offering loads of detail and extension.

I have not heard a 'stock' Forte 4a amp, so I don't know if these good qualities are due solely to the modifications performed by Mr. Soderburg, or if his mods just provide 'more of a an already good thing.'

Last edited by milosz; 30th November 2013 at 12:42 AM.
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Old 9th July 2015, 04:42 PM   #34
Rsmlabs is offline Rsmlabs  Mexico
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Join Date: Jul 2015
Hi: i have a "stock" Forte 4 amplifier, since new (and stored until some weeks) and i agree with member Milosz about great liquid mids and smoother and grainless treble, but bass is not taut or litle loose compared with other two small power amplifiers like Levinson 29 and Adcom 545II driving a pair or B&w801 series 3. i thinkthat one of Mr. Soderburg mods that improve bass are the caps and fast bridge rectifiers.
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