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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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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
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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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Old 7th September 2016, 04:56 PM   #35
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Default IGBT vs MOSFET

For what its worth, I am going to drop a reply in this thread. Originally I was working in another thread I created a few days ago regarding IGBT replacement in an existing amplifier manufactured by a brand that is known for excellent products. Ofc as with everything ells, one can talk about brand attachment and such, but I would not say that is the case here. I just happened to like the spec.

My goal is to “replicate” this amp for my own amusement and for the fun of doing so and turn it into an old school style with through hole components and narrow tolerances.

I notice that this thread is very old, spanning over 15 years.

Mister Nelson Pass made his reply back in 2001, that is 15 years ago. Surely that must matter, for if not, then IGBT’s and MOSFET has seen no evolution since they came out.

Let’s look at an article from 1999

The Insulated Gate Bipolar Transistor (IGBT).
University of Glasgow - Schools - School of Engineering

Fig.1 shows the structure of a typical n-channel IGBT. All discussion here will be concerned with the n-channel type but p-channel IGBT's can be considered in just the same way.

Click the image to open in full size.

Operation Blocking Operation

The on/off state of the device is controlled, as in a MOSFET, by the gate voltage VG. If the voltage applied to the gate contact, with respect to the emitter, is less than the threshold voltage Vth then no MOSFET inversion layer is created and the device is turned off. When this is the case, any applied forward voltage will fall across the reversed biased junction J2. The only current to flow will be a small leakage current.

The forward breakdown voltage is therefore determined by the breakdown voltage of this junction. This is an important factor, particularly for power devices where large voltages and currents are being dealt with. The breakdown voltage of the one-sided junction is dependent on the doping of the lower-doped side of the junction, i.e. the n- side. This is because the lower doping results in a wider depletion region and thus a lower maximum electric field in the depletion region. It is for this reason that the n- drift region is doped much lighter than the p-type body region. The device that is being modelled is designed to have a breakdown voltage of 600V.

The n+ buffer layer is often present to prevent the depletion region of junction J2 from extending right to the p bipolar collector. The inclusion of this layer however drastically reduces the reverse blocking capability of the device as this is dependent on the breakdown voltage of junction J3, which is reverse biased under reverse voltage conditions. The benefit of this buffer layer is that it allows the thickness of the drift region to be reduced, thus reducing on-state losses.

On-state Operation

Click the image to open in full size.

The turning on of the device is achieved by increasing the gate voltage VG so that it is greater than the threshold voltage Vth. This results in an inversion layer forming under the gate which provides a channel linking the source to the drift region of the device. Electrons are then injected from the source into the drift region while at the same time junction J3, which is forward biased, injects holes into the n- doped drift region (Fig.2).

This injection causes conductivity modulation of the drift region where both the electron and hole densities are several orders of magnitude higher than the original n- doping. It is this conductivity modulation which gives the IGBT its low on-state voltage because of the reduced resistance of the drift region. Some of the injected holes will recombine in the drift region, while others will cross the region via drift and diffusion and will reach the junction with the p-type region where they will be collected. The operation of the IGBT can therefore be considered like a wide-base pnp transistor whose base drive current is supplied by the MOSFET current through the channel. A simple equivalent circuit is therefore as shown in Fig.3(a)

Click the image to open in full size.

Fig.3(b) shows a more complete equivalent circuit which includes the parasitic npn transistor formed by the n+-type MOSFET source, the p-type body region and the n--type drift region. Also shown is the lateral resistance of the p-type region. If the current flowing through this resistance is high enough it will produce a voltage drop that will forward bias the junction with the n+ region turning on the parasitic transistor which forms part of a parasitic thyristor. Once this happens there is a high injection of electrons from the n+ region into the p region and all gate control is lost. This is known as latch up and usually leads to device destruction.

- end of article -

Lets move forward.


IOR article:

IGBT or MOSFET: Choose Wisely by Carl Blake and Chris Bull, International Rectifier

IGBT or MOSFET: Choose Wisely by Carl Blake and Chris Bull, International Rectifier
With the proliferation of choices between MOSFETs and IGBTs, it is becoming increasingly difficult for today’s designer to select the bes t device for their application. Here are a few basic guidelines that will help this decision - making process.

Device Evolution: Bipolar Transistors, MOSFETs and IGBTs
The bipolar transistor was the only “real” power transistor until the MOSFET came along in the 1970’s. The bipolar transistor requires a high base current to turn on, has relatively slow turn - off characteristics (known as current tail), and is liable for thermal runaway due to a negative temperature co - efficient. In addition, the lowest attainable on - state voltage or conduction loss is governed by the collector - emitter saturation voltage V CE(SAT).

The MOSFET, however, is a device that is voltage - and not current - controlled. MOSFETs have a positive temperature co-efficient, stopping thermal runaway. The on-state-resistance has no theoretical limit, hence on-state losses can be far lower. The MOSFET also has a body drain diode, which is particularly useful in dealing with limited free wheeling currents.
All these advantages and the comparative elimination of the current tail soon meant that the MOSFET became the device of choice for power switch designs.

Then in the 1980s the IGBT came along. The IGBT is a cross between the bipolar and MOSFET transistors (see figure 1). The IGBT has the output switching and conduction characteristics of a bipolar transistor but is voltage - controlled like a MOSFET. In general, this means it has the advantages of high-current handling capability of a bipolar with the ease of control of a MOSFET. However, the IGBT still has the disadvantages of a comparatively large current tail and no body drain diode. Early versions of the IGBT are also prone to latch up, but nowadays, this is pretty well eliminated. Another potential problem with some IGBT types is the negative temperature co-efficient, which could lead to thermal runaway and makes
the paralleling of devices hard to effectively achieve. This problem is now being addressed in the latest generations of IGBTs that are based on “non-punch through” (NPT) technology. This technology has the same basic IGBT structure (see Figure 1) but is based on bulk-diffused silicon, rather than the epitaxial material that both IGBTs and MOSFETs have historically used.

MOSFETs and IGBTs: Similar But Different.
When comparing Figures one and two, the MOSFET and IGBT structures look very similar. The basic difference is the add ition of a p substrate beneath the n substrate. The IGBT technology is certainly the device of choice for breakdown voltages above 1000V, while the MOSFET is certainly the device of choice for device breakdown voltages below 250V.
Between 250 to 1000V, there are many technical papers available from manufacturers of these devices, some preferring MOSFETs, some IGBTs. However, choosing between IGBTs and MOSFETs is very application-specific and cost, size, speed and thermal requirements should all be considered.

IGBTs have been the preferred device under these conditions:
• Low duty cycle
• Low frequency (<20kHz)
• Narrow or small line or load variations
• High-voltage applications (>1000V)
• Operation at high junction temperature is allowed (>100°C)
• >5kW output power

MOSFETs are preferred in:
• High frequency applications (>200kHz)
• Wide line or load variations
• Long duty cycles
• Low-voltage applications (<250V)
• < 500W output power

The hard-switched measurements clearly show the lower losses of the MOSFET in their applications. Chart 1 shows that the losses of the IGBT are approximately equal to the losses of an IRFP460 if the switching speed is reduced to 50 kHz. This could allow a smaller IGBT to replace the larger MOSFET in some applications. This was the condition in 1997.
However, the newer lower charge MOSFETs now available lower the losses at high frequency and therefore re-asserted the dominance of MOSFETs in applications using hard switching above 50kHz.

Chart 2 shows the losses in an application using Zero Voltage Switching at 50kHz, 500W, the IGBT losses of 9.5W are higher than the MOSFET losses of 7W at room temperature. When the temperature is raised up to operating conditions however, the conduction losses of the MOSFET rise more quickly than the switching losses of the IGBT. The losses at elevated temperature increase 60% for the MOSFET while the total losses for the IGBT increase only 20%. At 300 watts this makes the power almost equal, while at 500 watts the advantage goes to the IGBT.

Chart 3 shows the losses at 134kHz, 500W, elevated temperature, the IGBT losses of 25.2W are slightly worse than the MOSFET, with total losses of 23.9W. At room temperature in this same application the losses were 17.8 and 15.1 watts respectively. The switching losses are higher at higher frequency which eliminates the advantage of the IGBT at high temperature
, when switching at the lower frequency. This illustrates the subject of this paper, namely there is no iron clad rule which can be used to determine which device will offer the best performance in a specific type of circuit. Depending upon the exact power level, devices being considered, the latest technology available for each type of transistor, the results will change slightly.

Some of the conclusion: Finally, there seems to be an industry wide perception that MOSFETs are a mature product, which will not offer significant performance improvements in applications and IGBTs are a new technology, which will replace MOSFETs in all applications above 300 volts. No such generalizations are ever true and the huge improvements in MOSFET performance over the last two years certainly confirms that MOSFETs are a very dynamic product, and continues a trend of rapid growth. In fact, new low-charge MOSFETs such as the International Rectifier IRFP460A and IRFP22N50A significantly move performance benchmarks just when the IGBTs appear to offer an alternative in hard-switched applications.

Infineon Showcases 650V TRENCHSTOP™ 5 – Performance of est-in-Class IGBT Gains High Customer Demand
Infineon Showcases 650V TRENCHSTOP 5 &ndash; Performance of Best-in-Class IGBT Gains High Customer Demand - Infineon Technologies




IGBT generation 3,4,5 and 6 have surfaced since late 90's.

In the end what we are looking for is an amplifier that perform well as sound magnificent and the difficult part is what sounds magnificent? That will almost always be a subjective answer. Does a CD sound better than an vinyl record? perhaps, perhaps the CD (digital) is better when it comes to bass notes which vinyl cannot reproduce with the same dynamic range.

Is a tube amplifier worse than a JFET, VFEt or MOSFET amplifier... actually, its different. Efficiensy is only one aspect of things. Some swear by class A amplifiers while some swear by class D.

Some listen to music while some analyse the music. Some don't care about aesthetic aspects while some will pay $10 000 for something that looks good.

My conclusion is that in order to select what I enjoy, I must test both IGBT and MOSFETS, even a Bipolar Transistor.
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