Experiences of Class A solid-state ?
 

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.

Graham

Experiences of Class A solid-state ?
 

"Phil Allison" wrote in message

"Arny Krueger"


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.

Experiences of Class A solid-state ?
 

Arny Krueger wrote:

As my distortion measuring gear progressed to residuals in below 0.01% there
was always a mixed bag of measurable differences. But they were arguably
quite small, given that its often darn hard to hear distortion below 0.1%
or so.

I had a 'revalation' that blew away that idea about 30 years ago.

Graham

Experiences of Class A solid-state ?
 

Arny Krueger wrote:

"Eeyore" wrote

I've browsed it occasionally I don't consider him to be
especially any guru though.

I've read Self's little article about power amp distortion mechanisms
several times, and have been struck by the benefits that I might have
received from it, had it been available about 20 years ago, when I was
trying to design a SOTA power amp.

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

Do tell.

See Phil's post.

Graham

Experiences of Class A solid-state ?
 

"Phil Allison" wrote in message

"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.

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 just broke down what I said down into steps.

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.

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.

Experiences of Class A solid-state ?
 

"Eeyore" wrote in
message
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?

Experiences of Class A solid-state ?
 

"Eeyore" wrote in
message
Arny Krueger wrote:

"Eeyore" wrote
in

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.

Experiences of Class A solid-state ?
 

"Eeyore" wrote in
message
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!

Experiences of Class A solid-state ?
 

Arny Krueger wrote:

"Phil Allison" wrote in message
"Arny Krueger"

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.

Graham

Experiences of Class A solid-state ?
 

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.

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

Graham