Experiences of Class A solid-state ?
 

"Eeyore" wrote in
message
Arny Krueger wrote:

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

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.

It's that kind of thing.

What do you know about previous examples of this.

Used in some Japanese amps from maybe the late 70s and early 80s.

I've just generally heard that they hadn't been brilliantly
succesful.

I think the usual phrase used to describe this is "sliding bias".

It seems to me that sliding bias can make power amps less reliable, by
turning minor faults into situations where lots of power is dissipated in
the output stage. The trick might be to back out the sliding bias feature
when things start going awry, like a shorted load or some such.

Graham

Experiences of Class A solid-state ?
 

"Eeyore" wrote in
message
Arny Krueger wrote:

"Eeyore" wrote
in
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.

IOW, despite the use of a "big name" designer, the wine
came out tasting pretty ordinaire.

Is it class G?

The one I saw was just A/B.


I notice that Self makes some heavy claims about a
proprietary implemention of class G.

I haven't heard that. I'd be curious to know how it can
be made proprietary !


http://www.dself.dsl.pipex.com/ampins/classg/g.htm


"Class-G is now out and about. So what has changed? This would be easier to
answer if there had been more published on the technology; as it is I have
to rely pretty much on my own research. I found that Schottky power diodes
have eliminated the commutation-diode glitching. Having peeled this skin
from the distortion onion, linearity was still signficantly poorer than a
Blameless Class-B design. This proved to be due to Early effect in the
output stage devices, stemming from the sudden changes in collector voltage
they experience in Class-G operation. I had my own ideas about how to deal
with this, and you can read all in Electronics World for Dec 2001 and Jan,
Feb 2002. There is a fully worked-out design, and even a PCB available to
ease building it."

The "Early Effect" amounts to being an increase in beta with increased VCE.
The happens in a class-G amp when the power supply voltage is switched to a
higher voltage.

Experiences of Class A solid-state ?
 

Arny Krueger wrote:

"Eeyore" wrote
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.

Do tell.

I made my last try at making small amounts of nonlinear distortion audible
about 5 years ago, and the 0.1% number was the fruit of that effort. My
results aren't that dissimilar from those found in J. Robert Stuart
(Meridian Audio), "Digital Audio for the Future", Audio, 3/98 pp 30-37.
Stuart isn't exactly conservative on this topic. I know of nobody credible
who has claimed lower thresholds for audibilty.

In my case my I was comparing the H-H TPA 50 amplifier ( a fairly classic A/B
design of its era and quite widely used in pro-audio - including the BBC )
with ~ 0.1% THD on the spec sheet with a friend's recently assembled kit
ampliifer using Crimson Elektrik modules. These used a rather more 'state of
the art' design with the then new high speed Japanese output devices ( the H-H
used Motorola darlingtons ).

Lets just say the difference wasn't subtle !

The H-H sounded gritty by comparison to the much smoother Crimson.

Graham

Experiences of Class A solid-state ?
 

Arny Krueger wrote:

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

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.

It's that kind of thing.

What do you know about previous examples of this.

Used in some Japanese amps from maybe the late 70s and early 80s.

I've just generally heard that they hadn't been brilliantly
succesful.

I think the usual phrase used to describe this is "sliding bias".

Yes.


It seems to me that sliding bias can make power amps less reliable, by
turning minor faults into situations where lots of power is dissipated in
the output stage. The trick might be to back out the sliding bias feature
when things start going awry, like a shorted load or some such.

I couldn't comment without seeing a schematic really. I can't imagine why
that should be so.

You haven't come across any then ?

Graham

Experiences of Class A solid-state ?
 

Trevor Wilson wrote:

**True. What you said was bull****. Now, pay attention to my words. Focus
on: "Otherwise identical". Also focus on: "Poorly designed". If you, like I
have, taken a well designed Class A/B amp and cranked the bias current up to
100 Watts Class A, you will have noted no sonic differences between the
Class A/B operation and the Class A operation. OTOH, I have taken crappily
designed Class A/B amps and cranked the bias up to high levels of Class A
and found significant sonic improvements from doing so.

No, that bears no connection with what I said. I said that if you take a
class B amplifier and crank the gain up into the A/B range, that the sound
quality may be subjectively worse even though the THD is reduced. I said
nothing at ALL about full class A operation.

IOW: The design is more important than the Class of operation. Class A will
help a bad design and, at best, do nothing to a good design.

Absolutely.
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."

Experiences of Class A solid-state ?
 

Arny Krueger wrote:

"Eeyore" wrote in
Arny Krueger wrote:

I notice that Self makes some heavy claims about a
proprietary implemention of class G.

I haven't heard that. I'd be curious to know how it can
be made proprietary !

http://www.dself.dsl.pipex.com/ampins/classg/g.htm

"Class-G is now out and about. So what has changed? This would be easier to
answer if there had been more published on the technology; as it is I have
to rely pretty much on my own research. I found that Schottky power diodes
have eliminated the commutation-diode glitching. Having peeled this skin
from the distortion onion, linearity was still signficantly poorer than a
Blameless Class-B design. This proved to be due to Early effect in the
output stage devices, stemming from the sudden changes in collector voltage
they experience in Class-G operation. I had my own ideas about how to deal
with this, and you can read all in Electronics World for Dec 2001 and Jan,
Feb 2002. There is a fully worked-out design, and even a PCB available to
ease building it."

The "Early Effect" amounts to being an increase in beta with increased VCE.
The happens in a class-G amp when the power supply voltage is switched to a
higher voltage.

I notice that his ideas on the matter can only apparently be read in the copies
of EW mentioned.

His does pontificate a heck of alot !

Graham

Experiences of Class A solid-state ?
 

Eeyore wrote:

My current ruse is to avoid *any* global NFB. Not because it's important but for
pure marketing bull**** factor.

If this is the case, look at some of the Nelson Pass designs. No overall
network feedback, output stages are inexpensive FETs biased well into class A,
and in spite of all the buzzwords, they sound damn good. I haven't run
distortion spectra on any but I'd bet you could make a good guess by looking
at the output device curves. (This being the case, I would suspect that
the particular MOSFETS you chose would make a substantial difference in
the sound.... negative feedback would do a lot to reduce those differences).
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."

Experiences of Class A solid-state ?
 

tony sayer wrote:

Anyone ever made a good commercially viable class D amp?...

Everyone and his brother has. A huge number of portable devices, from
iPods to laptops, are using class D stages because of the low power
consumption. You're also starting to see them inside a lot of powered
speakers. Tripath and TI are some of the popular manufacturers.
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."

Experiences of Class A solid-state ?
 

Scott Dorsey wrote:

Eeyore wrote:

My current ruse is to avoid *any* global NFB. Not because it's important but for
pure marketing bull**** factor.

If this is the case, look at some of the Nelson Pass designs.

I have actually looked at a couple of the explanatory documents on his site but I'm
following a very different path.


No overall
network feedback, output stages are inexpensive FETs biased well into class A,
and in spite of all the buzzwords, they sound damn good. I haven't run
distortion spectra on any but I'd bet you could make a good guess by looking
at the output device curves. (This being the case, I would suspect that
the particular MOSFETS you chose would make a substantial difference in
the sound.... negative feedback would do a lot to reduce those differences).

The output stage I have in mind should totally eliminate these classic problems. I'd
love to say how but I think it may be patentable.

Graham

Experiences of Class A solid-state ?
 

Scott Dorsey wrote:

tony sayer wrote:

Anyone ever made a good commercially viable class D amp?...

Everyone and his brother has. A huge number of portable devices, from
iPods to laptops, are using class D stages because of the low power
consumption. You're also starting to see them inside a lot of powered
speakers. Tripath and TI are some of the popular manufacturers.

I keep hearing good things about Hypex btw.

Graham