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Old 22nd August 2003, 05:54 PM   #1
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Default Power Mosfet Choices

I received the following email question, which I think makes for worthwhile discussion:

(Quote) I have posted several questions on diyaudio Aleph-X thread concerning the more esoteric issues in selecting a mosfet for this amp. It had occurred to me that Nelson may be uncomfortable giving an outright answer on the optimum rail voltage and current bias in an open forum so I have decided to ask this privately. When I look at the transconductance figures of the IRF family when compared to their input capacitance, the IRFP240 looks like a relatively poor choice. As per Nelsons suggestions, I read the Zen v5 article and payed particular attention to the fig. 5-7 distortion graphs. The IRFP240 is a 200V part and so it is logical to me that it would provide much lower distortion figures as you increase the voltage rails but would there be commensurate effect with the other IRFP mosfets? To me, the IRFP044N, IRFP150N and IRFP150V have huge a huge transconductance to Ciss ratio compared to the IRFP240 and were designed for a lower voltage application so I would like Nelson (or any of his highly competent engineers) to just give it to me straight. What combination of rails and bias make the best sounding Aleph-X for each of the following parts?

IRFP044N
IRFP150V
IRFP150N
IRFP044
IRFP150 (end quote)

For the record, I experience no discomfort. It is true that I tend to pick IRF244's, partly because I own a zillion of them, having cornered the market on TO-3's. Of course the reason I did that was that I like the 244's sound in general.

Each of these Mosfets brings something different to the table, and each results in a different sound. Some people like the lower transconductance models, such as the early Mosfet like Hitachi's, and some like it higher. A lot higher.

It is my experience that picking the optimal part (or parallel group of parts) is an exercise in experiment and experience, and it is my opinion that for a given amplifier design there is an optimal characteristic for the transconductance.

I recommend the 044 for Zens because they benefit from maximal transconductance, which you get with the lower voltage types. For a Zen amp, high transconductance is a good thing, however it is traded off against Gate capacitance, which is not.

IRFP150's are nice parts. They look to me like two IRFP140's in parallel (the same applies to 250/240 comparison). The advantage in that case to the 140 is that two of them have greater dissipation capability, being in two packages.

According to my book, the 150 has about 1.5 times the Ciss of the 140, but also about 1.5 times more transconductance (this at 10 amps).

Just about any decent Mosfet will work in any of the Zens, X's, and XA's. Besides reliability, it's a question of sonic character.

As the aggregate transconductance gets higher in simple amps, the bottom end becomes more pronounced and you do get better control, but it seems that you are trading off against sparkle on the top, and vice versa.

You will also experience differences in the sound between Mosfets of the same type - remember that the published curves are typical examples. If you really want to nail the thing down, you have to substitute parts in and out and evaluate each.

So we pick a middle ground (or not), and try to enjoy what we've got.

Is this a good argument for bi/tri/quad amping? Yes, I think it is, as you can tailor the amps for best performance in a given spectral area.

It is also a fine example of something a DIYer can do that a manufacturer will never do? I think so.
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Old 22nd August 2003, 07:06 PM   #2
Bricolo is offline Bricolo  France
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Nelson, is there any (methematical, physical...) reason for the relation between transconductance and bass/highs?
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Old 22nd August 2003, 07:07 PM   #3
MBK is offline MBK  Singapore
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Mr Pass - It would be a fine DIY'er that could combine the experience of decades of manufacturing with in depth knowledge of theory and practice of topology vs. device choice... So there are the things a manufacturer can do which the DIY'er most often can't.

That being said I very much like the idea of tailoring the amp and the devices - if the DIY'er can sample all the offerings and knos what h's doing - to the bandwidth and characteristics actually needed per band in a bi or tri amped system. Example of how I *wouldn't* do it: a recent poster wanted to build 6 Aleph 2's for a triamped system, the low end of which is sub-100 Hz - I find it much more elegant to build the right amp for the job.

So, one could trade power handling needs vs. tonal characteristics vs. phase linearity in certain bands, and types of distortion and where they falls frequency wise.

In my case for a biamped system (low 25-310 Hz, mid/hi 310-20k by fullrange driver), I am just beginning to use this line of thought:

Low end by easy chip amps. They save power and I can even build one chip per driver to keep I vs. V limitations consistent. Potential high order distortion or grain of SS chip amps will likely fall out of band. Drawback, phase not linear by data sheet in the bass.

Mid/High: I just built Mini-Aleph prototypes for that using IRFP140 at fairly high bias. I only need moderate power here, but good tonality.

Ultimately I'd like to add a tweeter >2kHz, then the Mini-A would have a natural home there - lowest power, best smoothness, and its drawback, the harmonics it likely produces, fall outside the critical Fletcher Munson range. An X or AX would then be perfect for the 310-2kHz midrange.

To perfect the idea of getting rid of harmonics after the fact in triamping, one would like to make the last LP passive after the power amp rather than part of the line level XO.
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Old 23rd August 2003, 04:33 AM   #4
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I find chip amps not as DIY as I like, but then that is just personal taste - after all I don't make my own transistors, for example.

The choice of Zen, Aleph, X, or X Aleph kind of falls into certain aesthetic categories, but you are best off actually trying as many combinations as you can. You might be surprised sometime.
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Old 23rd August 2003, 07:02 AM   #5
MBK is offline MBK  Singapore
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Actually I initially chose the chip amp approach because I had no electronics knowledge whatsoever and needed a working system, where I could be confident that performance - or lack thereof - wasn't due to my poor understanding or building practices. I might have tried the Zen for starters, but wasn't aware of it at the time. The chip amps done by data sheet serve me as a reference of "commercial engineering standard" - I want to do better than that of course!

I really enjoy the discrete approach now, much more fun than the chips! Since the system already runs the pressure is off, and I now understand a lot more. So now I try the more sophisticated discrete approaches. Since I already have the PSU's I can easily pop in different circuits, and make them permanent in case of success. For a specific topology, say my Mini-A, I can easily substitute the MOSFET's because I don't have a matching issue. Or I can double up the Mini A into an XA. A lot of fun!

You suggest that in practice the various designs might work best where I don't expect it... interesting... I will try more permutations then.
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Old 23rd August 2003, 07:55 AM   #6
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Default To The Father of The Child....

Hello Nelson,
Can you give us an appraisal of the sound qualities/characteristics of your range of Aleph amplifiers, and your preferences according to application as you hear them ?.
I have a friend with a factory built Aleph 2 (which sounds delicious) and I have heard a few factory built Aleph 0's in the past, so I have some idea of the sound of these two models.

Thanks, Eric.
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Old 23rd August 2003, 10:24 PM   #7
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I have often said that the Aleph 2 was generally my favorite in
terms of aesthetics, power, and tonal balance. Aleph 3's are
cute as bugs, but don't have the power or control and lean a
little toward the mid/hi. The Aleph 1.2 had the best bottom end,
but was heading off toward a little less detail on the top than
the smaller models.

The Aleph 0 was a 3 gain stage piece. It measured better in
every respect, and could drive any impedance, in fact it had
almost exactly the same distortion vs power curve into 2 ohms
as it did into 8 ohms, which made it quite unique. It did,
however, sound more like a lot of the other high quality solid
state amps, a little less romantic by comparison.
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Old 24th August 2003, 04:30 AM   #8
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Default To The Proud Father.....

Thankyou Nelson,
That describes things very nicely.
I agree that the Aleph 2 is extraordnarily user-friendly sounding, enjoyable and fun long term - well done.

Regards, Eric.
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Old 19th November 2010, 08:19 PM   #9
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I would like to build the Aleph 2. I want to use the IRFP 250 for the outputs because I have many of them. Is it possible to use that version without other changes to the circuit?
RG
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Old 20th November 2010, 12:13 AM   #10
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The IRF250 is a lot like two IRF240's in one package. I
suggest using somewhat fewer of them in parallel to get
the best performance.

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