MOSFET output stage

[Phil Allison]

"Gareth Magennis"

"Phil Allison">

Not if you clip them like you might a valve amp for example.

** Only fucking half-wits listen to clipped audio from a home hi-fi.

Would a Mosfet amp clip more "nicely" than a BJT amp?

** No.

Any more fuckwit MOSFET myths you want to give an airing here ?

Imbecile.



...... Phil

 

[Eeyore]

Eeyore a écrit :

Uhhh.

Please explain how you obtain even distortion products from a
symmetrical (suppose paired mosfets) push-pull.

If you're such an expert, built one and see for yourself. And use LATERAL mosfets
designed for audio.

TWIT.

Graham

 

[Eeyore]

"Eeyore" wrote

Well in that case the OP's question is thus answered, as far as I can tell.
It sure is noisy in here.

It is indeed, with a whole bunch of nutcases spouting nonsense.

It's a fact that 'open loop' with no NFB, a mosfet output stage will indeed
distort more than a bipolar one, but the distortion characteristics are
different. Put some gain on the front and close the loop and the mosfet wins
every time (admittedly requiring more overall NFB but it can take it).

That 0.0008% THD amp I mentioned, you could not see ANY vestiges of crossover
distortion on an Audio Precision analyser output, and what you could see was
mainly 2nd harmonic.

Oh and its THD floor is 0.0007%, so doubtless the amp was rather better than the
display said.

Graham

 

[Eeyore]

Not if you clip them like you might a valve amp for example. Would a Mosfet
amp clip more "nicely" than a BJT amp?

Not really.

I'm thinking driving bass speakers.
Is this what some of these "audiophools" or pehaps PA guys are getting at by
saying they sound better?

I know of no 'PA' amps currently using mosfets. Everyone wants cheap these days
and bipolars are cheaper.

Graham

 

[Eeyore]

Er, perhaps. Mosfets don't have the same thermal characteristics as BJT's,
which are more prone to thermal runaway. So perhaps when you are runing
Mosfets hard, there is some kind of compression thing going on which sounds
nicer than a BJT amp exploding after clipping a lot.

Competently designed BJT amps don't 'explode'. It's not difficult, but the
Chinese haven't quite mastered it yet.

Graham

 

[Phil Allison]

"Gareth Magennis"

"Phil Allison"


Er, perhaps.

** Wot a glutton for punishment we have here - folks.

Mosfets don't have the same thermal characteristics as BJT's, which are
more prone to thermal runaway. So perhaps when you are runing Mosfets
hard, there is some kind of compression thing going on which sounds nicer
than a BJT amp exploding after clipping a lot.

** Another 100% fuckwit MOSFET myth.............

Go away - you IDIOT !!!



....... Phil

 

[Phil Allison]

"Eeysore"

I know of no 'PA' amps currently using mosfets.

** Huh ????

All those hundreds of thousands of MOSFET audio power amps made since the
mid 1980s have not justs disappeared you know - power amps made by H-H &
Harrison Electronics plus C-Audio in the UK , Perreaux and ZPE of NZ,
Australian Monitor and ARX (still in full production) and Jands here in
Aussie - plus many other less well known brands.

The majority are STILL in use, maybe looking just a bit the worse for wear.

Even in the UK - Chevin Research ( based in Yorkshire) A-series amps are
all lateral mosfet designs.

http://www.chevin-research.com/products_a_series.php



...... Phil

 

[Phil Allison]

"Gareth Magennis"

** Bugger off - wanker.

If a Mosfet amp compresses the bottom end slightly over a BJT,

..... blah, blah , blah .....

 

[Phil Allison]

"Gareth Magennis"

No, this is good fun, and you might eventually tell me why my supposition
is untrue.

** Cos nothing makes it true.


YOU FUCKING Z- GRADE IMBECILE !!!




..... Phil

 

[Audio Expert]

Who do MOSFET sound better than bipolar, as an audio amp output
driver?

MoSfEtS zound batter becuaze thOze work wiz EleKtRONS, LikE TubeS.
Traanzisters WOrk wiz HOLES, AND Thoze HoLES YoU wILL HeAr in Ze Muzick.

 

[Vladimir Vassilevsky]

Who do MOSFET sound better than bipolar, as an audio amp output
driver?

In most cases, this is fallacy, and the result is just the opposite.
Reason: FETs have lower transconductance compared to BJTs. It is
impossible to build a half bridge stage with an ideal transfer curve.

However there are few special cases when a FET output stage has an
advantage:

1) With FETs, it is simpler to control bias current, because of the
negative dependency from the temperature. That simplifies the life.

2) If the global warming is not an issue, then the class A stage made of
drain follower loaded by the current source can be very linear indeed.

3) FETs are free from BJT high injection effects and the charge
accumulation in the base.

4) FETs do not require high base currents of BJTs; that simplifies the
driver stage.


Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com

 

[Eeyore]

"Eeyore" wrote


The point is that when people prefer one amp over another it may not be easy
to tell technically what it is they prefer.

Flatter frequency reponse perhaps ?

If a Mosfet amp compresses the
bottom end slightly over a BJT, for example, this might in the long term be
a nicer sounding amp. Maybe absolutely nothing to do with crossover
distorion, linearity, feedback blah blah blah.

How would this 'compression' occur ?

Graham

 

[Eeyore]

Who do? That's voodoo!

Only to be expected from Arny "Any amp with less than 0.1% THD at full
power sounds the same as all other amps."

Not quite as inane as some comments so far.

Graham

 

[Eeyore]

Not in any relevant way for audio power amps.

The first two are highly relevevant in ANY circuit using NFB. Basic
stability criteria.

For Christ's sake has everyone except Phil's and my brain turned to jelly
overnight ?

Graham

 

[Eeyore]

"Eeysore"


** Huh ????

All those hundreds of thousands of MOSFET audio power amps made since the
mid 1980s have not justs disappeared you know - power amps made by H-H &
Harrison Electronics plus C-Audio in the UK , Perreaux and ZPE of NZ,
Australian Monitor and ARX (still in full production) and Jands here in
Aussie - plus many other less well known brands.

Not to mention Studiomaster's Mosfet series of old. Initiated by none other than
myself.

The majority are STILL in use, maybe looking just a bit the worse for wear.

Even in the UK - Chevin Research ( based in Yorkshire) A-series amps are
all lateral mosfet designs.

http://www.chevin-research.com/products_a_series.php

Goodness are they still going ?

By current, I did mean in current manufacture. So there are a few. But all the
big boys are bipolar only now.

Graham

 

[Eeyore]

"Gareth Magennis" wrote


Straw man. It is entirely practical to build power amps that are never
clipped in actual use. If you don't like how your power amp clips? Get one
with enough output so that it never clips.


Power amps aren't just devices, they are circuits. Circuit design can
easily trump device characteristics.

In fact the sharpness of the clipping of a power amp relates to things like
how much negative feedback it has, all things considered. If you have a
circuit with lots of negative feedback, it is very likely to clip very
sharply and ideally. If you have a circuit with less negative feedback, the
clipping will be softer, but will occupy a larger proportion of the transfer
characteristic.

Precisely so.

Graham

 

[Eeyore]

This whole "amp sounds better" stuff was fully debunked 30 years ago. Good
power amps sound the same and they sound like a piece of wire with gain.

There are tons of power amps that can't be distinguished from a piece of
wire with gain, while driving well-designed speakers.

There are quite a few amps that meet the same criteria while driving even
the weirdest speaker load.

A really good power amp will destroy a poorly-designed speaker before it
starts sounding bad, and really good power amps aren't all that unique.

The speaker is a large part of it. Highly reactive ( i.e. lots of inductance or
capacitance ) speakers can prematurely trigger device protection. We discovered
that EV's SX500 was particularly bad in this respect in PA for example. Now that
is certainly audible.

Graham

 

[Eeyore]

In most cases, this is fallacy, and the result is just the opposite.

Another one off with the faeries.

Graham

 

[Eeyore]

4) FETs do not require high base currents of BJTs; that simplifies the
driver stage.

So what charges and discharges the damn gate capacitance you dozy bugger ?
The classic Class A voltage gain driver stage probably needs as much current
as if you were driving darlington output devices with a simple design..

In my ultra-low THD design I had a bipolar complementary Class A emitter
follower driving the mosfet gates ! It also removes the capacitive loading
from the voltage gain stage, increasing phase margin and loop HF response.

Graham

 

[Eeyore]

Where did you put the dominant pole cap? - still around the voltage
amplifier I'm guessing. It is vital that the dominant pole cap is the
only one that shows up in the phase response up to unity O/L gain, so
buffering of the fet gate caps is pretty much a given.

It wasn't dominant pole compensated. But you got the location right after a
fashion.

The big problem you need to overcome with mosfets is that they have an
essentially log response, resulting in (or from, depending how you look
at it) huge - maybe 10/1 Gm changes over the operating range of
currents.

Not on laterals.

Graham