MOS-FETS exclusively, input to output, why Nelson? - diyAudio
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Old 28th November 2007, 07:39 AM   #1
Jazzo is offline Jazzo  United States
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Cool MOS-FETS exclusively, input to output, why Nelson?

I think I have an idea of why you design with MOS-FETs for outputs in your later designs. You've explained this in a paper, if I remember.

However, amplifier designers have many different views on devices. Curl and McCormack, plus many others, believe in the J-FET input, MOS-FET driver and bipolar output topology. Others, like David Belles, believe that bipolars are better devices for input and driver stages, with MOS-FETs as outputs. Levinson uses bipolars in every stage.

So, you seem to be alone in your choice of using MOS-FETs for every stage. I'm sure you have good reasons. Still, I would think different devices might excel better in one amplification stage than another, but you obviously made a decision to not use J-FETs or bipolars in any stage.

Share with us how you came to believe this to be the best topology.
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Old 28th November 2007, 08:16 AM   #2
h_a is offline h_a  Europe
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Well, I'm sorry to destroy your illusions, but Nelson is not the strict believer in Mosfets as you seem to think.

Quote:
So, you seem to be alone in your choice of using MOS-FETs for every stage.
For a quite long time now this is no longer true. It is well known here that Nelson Pass uses Jfet inputs in all his amps and he uses bipolars for cascodes. Only the output stage still consists of Mosfets and I guess his reply to why he's doing so, is something along 'because it allows simple designs'.

All the best, Hannes
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Old 28th November 2007, 02:48 PM   #3
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This is one of those questions that either takes a really, really long answer or a really short one. The short answer, as has already been noted, is that Nelson doesn't always use MOSFETs in every circuit position; it certainly isn't the case at present.
The longer answer...hmmm...don't seem to have time for the longer answer, so I will attempt the impossible and try for a mid-length answer.
If you were to look at Nelson's earlier designs, you'd find that a lot of them were all bipolar. Then you'd see JFETs here and there. Then you'd see MOSFETs. Most recently he used power JFETs in one of the First Watt pieces. It's not so much a question of whether a device fits into the FET or bipolar category, it's a question of what characteristics the device has--gain, voltage limitations, current limitations, heat dissipation, input impedance, capacitance, and on and on throughout the entire list of specifications. When you sit down to design a circuit, you choose from a menu of available parts. Some people like steak, some like seafood, and some are vegetarians. Those inclinations tend to show up in what people choose from the menu.
When I was younger, I'd listen to one designer hold forth on why his design was better...and there'd always be this portion where he'd say that bipolars were clearly better than FETs because of this and that. And for a week afterwards, I'd go around thinking that bipolars were better. Then I'd hear another guy telling how FETs were better. And, sure enough, for a week I'd go around repeating what he said. And then a week for tubes, and finally back to bipolars...'round and 'round in a circle, with no end in sight.
So who's right?
The answer is that all gain devices are pretty pathetic. You pick a set of compromises that you think you can work with and you do the best you can.

Grey
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Old 28th November 2007, 02:56 PM   #4
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keep it up Grey...I think you're getting really good at the mid-length answers
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Old 28th November 2007, 04:34 PM   #5
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Quote:
Share with us how you came to believe this to be the best topology.
Like Grey said in his short answer, it is about compromises.

If you are able, get an aleph amplifier, pair it with some nice sealed box speakers (my preference) and have a listen. You will be hard pressed to find any compromises!

JJ
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Old 28th November 2007, 07:08 PM   #6
Jazzo is offline Jazzo  United States
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Quote:
Originally posted by h_a
Well, I'm sorry to destroy your illusions, but Nelson is not the strict believer in Mosfets as you seem to think.



For a quite long time now this is no longer true. It is well known here that Nelson Pass uses Jfet inputs in all his amps and he uses bipolars for cascodes. Only the output stage still consists of Mosfets and I guess his reply to why he's doing so, is something along 'because it allows simple designs'.

All the best, Hannes

You guys are right. He uses other devices. I failed to say I was referring to his present X series, "regular" commercial product. I'll wait for the Nelson himself to add some salient comments.
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Old 28th November 2007, 07:09 PM   #7
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Here in the technical vastness of the future we have a myriad
of gain devices to choose from, all offering different disadvantages.

The decision to use Mosfets on the input of the Alephs is an
interesting one - they were chosen because I wanted to have
a very simple 2 stage circuit. Mosfets are noisier and generally
have less linearity that JFETs, but they offer a higher
transconductance and higher current/dissipation.

Currently we use bipolar devices for cascodes on JFET front ends.
They don't measure better but they sound better and their
bandwidth corroesponds to our needs. Recently we re-evaluated
bipolars against our current Mosfets in front end level shifters.
This time we get the same measurements, but our panel doesn't
like the sound.



Live in the Future! It's just starting now....
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Old 28th November 2007, 07:38 PM   #8
EUVL is offline EUVL  Europe
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> They don't measure better but they sound better and their
bandwidth corroesponds to our needs.

Would you be so kind to explain the importance of bandwidth of the cascoding device ?

Thanks,
Patrick
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Old 28th November 2007, 07:50 PM   #9
Zen Mod is offline Zen Mod  Serbia
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Quote:
Originally posted by Nelson Pass
.......
Live in the Future! It's just starting now....



that sounds so promising for waaaaaaay over-spoiled kids ........
__________________
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clean thread; Cook Book;PSM LS Cook Book;Baby DiyA ;Mighty ZM's Bloggg;Papatreasure;Papa... by Mighty ZM
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Old 29th November 2007, 02:20 AM   #10
Jazzo is offline Jazzo  United States
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Quote:
Originally posted by Nelson Pass
.... Mosfets are noisier and generally
have less linearity that JFETs, but they offer a higher
transconductance and higher current/dissipation.

Currently we use bipolar devices for cascodes on JFET front ends.
They don't measure better but they sound better and their
bandwidth corroesponds to our needs. Recently we re-evaluated
bipolars against our current Mosfets in front end level shifters.
This time we get the same measurements, but our panel doesn't
like the sound.



Live in the Future! It's just starting now....

Mr. Pass, javascript:smilie('')

You stated that you currently use bipolar devices for cascodes on JFET front ends. What amps are you talking about? The X and XA series? Or are you talking about First Watt stuff?

You also state that you recently re-evaluated bipolars against MOSFETs as "front end level shifters," and the MOSFETs sound better. What are front end level shifters?

So, what is the compliment of devices on the X and XA series?

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