Arny Krueger wrote:

"Eeyore" wrote
Arny Krueger wrote:

We all know that speakers can be capacitive-reactive and
can jack the load current to unexpectedly high levels.

Entirely calaculable though.


A common power amp will have +/- 80
volt DC rails and may need to deliver up to 15 amps peak
to the load.

15 Amps ? I'd design for 40 !

OK.

In your opinion Graham, what would the quiescent dissipation of a
traditionally-designed output stage like this be?

Phil's already covered that and I'm in accord with what he says.

Graham

Arny Krueger wrote:

"Eeyore" wrote
Arny Krueger wrote:
"Eeyore" wrote

I'm concerned about the transconductance 'modulation'
with signal level since this introduces non-linearity.

Again, covered in
http://www.dself.dsl.pipex.com/ampins/dipa/dipa.htm

Which is what I'm designing out.

Although I'd hoped this thread might be more about any
audible benefits of Class A rather than a discussion of
design principles ( I'm quite au-fait with those ! ).

Like I said before, I've never done any DBTs involving Class-A amps. This is
partially because I've never seen a true Class A amp that was capable of
interesting power levels, in real life.

Fair enough.

I've heard enough positive comment on them to pursue my line of thinking on the
matter already btw.

Graham

Arny Krueger wrote:

"Eeyore" wrote
Phil Allison wrote:
"Eeyore"

In fact I've a sort of grudge to bear in fact.

** So THAT is what " Eeyore " really is -

a bear with some sort of grudge ??

Long story. Only a small grudge really. I must learn to
lie in job interviews too.

** Oh - that job Doug got with Soundcraft ?

Uhuh.


He did the power amp stage of their "PowerStation".

Joke isn't it ?

What's wrong with it?

Do tell!

It's fairly 'routine'. That's the joke.

Graham

"Eeyore"
Arny Krueger wrote:

"Phil Allison"

Yes, there are two problems with class A - both related
to the fact that with class A, the output stage always
has far more current flowing in it. (1) Power transistors tend to be
less
linear at high
currents - the beta falls off. Running an output stage
class A approximately doubles the current that the
output stage has to handle. So, you move the operating
point way out on the output devices. (2) Less SOA from the perspective
of
the load, because
the output devices are pulling so hard against each
other.


** What a load of complete DRIVEL !!!!

Arny - leave commenting on power amp design to people
who know something about it cos they spend their lives
dealing with it at component level.
Cos YOU do not have a bloody clue.

Phil, its real handy for that you seem to be so mentally incapable of
framing a proper technical reply. Everybody is going to dismiss your
ranting.

I'd personally like to see you claim that power transistors get more
linear
when run at very high currents and that there is no such thing as beta
fall-off at high currents.

Oh there is but since a classic Class A output has such high standing
dissipation you use more devices in parallel and you're actually working
them at
lower currents than in A/B.



** Correct.

Arny has made the same ASININE error in FOUR posts.

A "class A amplifer" is one DESIGNED to work correctly in that mode.

His wacky claim about peak device currents being higher in class A is FALSE.




......... Phil

"Arny Krueger"


Agreed again. Building a truely Class A power amp that
can deliver significant amounts of power output is
really a pretty awesome thing. The OPT stage quiescent
current has to be equal to the *peak* current that is
delivered to the load.

** Absolutely FALSE.

The peak load current is *exactly* double the idle
current for an amp operating in class A.

Agreed.


** Self contradiction - right here.



The idle current flow in one device of a pair increases
to double while the other drops to just under zero at
peak level.

Agreed.

That double value current peak flows entirely via the
load to the common point.

Agreed.

Very basic stuff.

So what's your point?


** You are WRONG !!



You just broke down what I said down into steps.


** I JUST COMPLETELY CONTRADICTED YOU

- YOU ****WIT !!.




We all know that speakers can be capacitive-reactive and
can jack the load current to unexpectedly high levels. A
common power amp will have +/- 80 volt DC rails and may
need to deliver up to 15 amps peak to the load.

** Such a power stage has a rated output of over 500
watts and so HAS to use at least 8 large output devices.

Irrelevant.


** Shame how it is MASSIVELY relevant.



So, now the OPT is dissipating 2400 watts per channel
just sitting there, not counting internal losses.

** What a load of complete DRIVEL !!!!

What's your number, Phil? Remember, we are talking about a class-A output
stage that delivers 80 volts peak and 15 amps peak.


** Therefore it runs at 7.5 amps idle.

It also dissipates 1200 watts spread over at least 20 large devices.


Arny - leave commenting on power amp design to people who know something
about it cos they spend their lives dealing with it at component level.

Cos YOU do not have a bloody clue.





........... Phil

"Eeyore" wrote in message
...
:
:
: Don Pearce wrote:
:
: Eeyore wrote:
: Don Pearce wrote:
: I'm concerned about the transconductance 'modulation' with signal level
: since this introduces non-linearity.
:
: Graham
:
: That would be greatest in the transition region between class A and
: class B.
:
: Exactly spot-on Don and readily visible when looking at a distortion analyser's
output.
: This is why Trevor's ideas baffle me.
:
: My intention is to entirely eliminate this with a kind of 'hybrid' output
stage.
:
:
: Graham
:
what? you're going to use tubes ?

;-)
R.

"Eeyore" wrote in
message
Arny Krueger wrote:


I'd personally like to see someone claim that power
transistors get more linear when run at very high
currents and that there is no such thing as beta
fall-off at high currents.

Oh there is, but since a classic Class A output has such
high standing dissipation you use more devices in
parallel and you're actually working them at lower
currents than in A/B.

I think I saw the effects of beta fall-off in my testing. I was working
with 8 devices up and 8 devices down. VCC was about 80 volts. The devices
were complements from the MJE 1502x series.

Admittedly, one go add enough devices so that beta fall-off wouldn't be an
issue.

"Eeyore" wrote in
message
Arny Krueger wrote:

"Phil Allison" wrote

** Such a power stage has a rated output of over 500
watts and so HAS to use at least 8 large output devices.

Irrelevant.

Reason being that my amp running off VCC = +/- 80 volts (no-load) is more
like a conservative 250 wpc @8 ohms amp, not 500 watts. I get about 300 wpc
at clipping presuming perfect power supply regulation which of course is
impractical.

The power supply was based on a toroidal power transformer from a commerical
power amp. My test amp used 16 large (MJE 1502x) output devices, which is
obviously *twice* Phil's 8.

Very relevant indeed since each device is working at a
fraction of the total load current.

Agreed, in the general case.

"Eeyore" wrote in
message
Arny Krueger wrote:

"Eeyore" wrote
Arny Krueger wrote:

We all know that speakers can be capacitive-reactive
and can jack the load current to unexpectedly high
levels.

Entirely calaculable though.


A common power amp will have +/- 80
volt DC rails and may need to deliver up to 15 amps
peak to the load.

15 Amps ? I'd design for 40 !

OK.

In your opinion Graham, what would the quiescent
dissipation of a traditionally-designed output stage
like this be?

Phil's already covered that and I'm in accord with what
he says.

I see no numbers, just a lot of fussing and fuming.

"Eeyore" wrote in
message
Arny Krueger wrote:

"Eeyore" wrote
Don Pearce wrote:
Eeyore wrote:

I'm concerned about the transconductance 'modulation'
with signal level since this introduces non-linearity.

Graham

That would be greatest in the transition region between
class A and class B.

Exactly spot-on Don and readily visible when looking at
a distortion analyser's output.

Agreed.

This is why Trevor's ideas baffle me.

The point that Trevor alludes to and Doug Self describes
in detail, is the fact that there are numerous other
sources of nonlinear distortion that can be bigger
problems in poorly designed amps.

If they're poorly designed !


Don't be proud Graham, go spend some time with Self's
article:

http://www.dself.dsl.pipex.com/ampins/dipa/dipa.htm

I've been there before Arny. There's nought Self can
teach me there.


My intention is to entirely eliminate this with a kind
of 'hybrid' output stage.

How? Other than class-A you'd need to come up with some
kind of Gm-halving circuit.

It is a 'form' of Class A that indeed meets the classic
definition but without the very high idle current. It
involves quite a radical rethink of the output stage.

In the past the usual approach to this has been to use a bias circuit that
keeps the output devices from ever turning fully off.