Experiences of Class A solid-state ?
 

Trevor Wilson wrote:
"Scott Dorsey" wrote in message
...
Bret Ludwig wrote:
High bias AB amps operate in class A through most of the power range
they spend all their time in, giving the advantage of Calss A operation
where it is needed, and rather than running out of power when this is
exceeded they simply transition into Class B.

This is true. The problem, however, is that although the distortion may
be much lower in class AB than in class B, the distortion spectrum is
often
less pleasant, at least to my ear. I often find a given output stage
sounds
better biased into class B than up higher into class AB.

**Bull****. Only poorly deisgned Class A/B amps sound significantly worse
than an otherwise identical Class A amp. Pay careful attention to my words:
"Otherwise identical".

That's not what I said. That is not at ALL what I said.

For low power studio-grade applications, though, there's no reason not to
just go whole-hog and run everything Class A. Electricity is cheap.

**There are plenty of reasons NOT to go pure Class A and very few to do so.

Other than size and heat?
--scott

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

Experiences of Class A solid-state ?
 

Trevor Wilson wrote:

"Eeyore" wrote in message
Trevor Wilson wrote:
"Eeyore" wrote in message
Trevor Wilson wrote:
"Eeyore" wrote in message

Have you ever spent any time listening to a true Class A solid state
amplifiers ?

**We all have. Pretty much every amplifier operates Class A to a few
tens of milliwatts.

That's not what I meant by true Class A though.

**'True Class A' is a very rubbery definition. A 'True Class A' amplifier
rated at (say) 50 Watts @ 8 Ohms, will only be 25 Watts Class A, when
driving 4 Ohms, 12.5 Watts @ 2 Ohms, and so on. Given the fact that a
typical 8 Ohms speaker can easily exhibit impedance minima down to 3
Ohms, you'll see the problem.

My understanding of the textbook definition is that the device(s) are
always conducting for the entire cycle i.e. never switch off at all output
power levels.

**And, unless the precise load is specified, then the definition of Class A
is meaningless.

It can be specified as a minimum load impedance.


Plenty loud enough for quiet listening. Some high bias Class A/B
designs operate to a couple of Watts in Class A. Even with 90dB/W/M
speakers, this is pretty loud.

Sure enough but what you need to be aware of is that working in the
crossover
region the output device transconductance is varying widely and putting
non-linearity into the transfer characteristic

**Well, you've managed to narrow your choice of devices down,
significantly,
since the amplification factor of BJTs is expressed as hFE. Current
gain.
The variation in current gain of modern BJTs is spectacularly low, over
very
wide Collector currents. You need to examine the curves on this page:

http://www.futurlec.com/Transistors/2SC5200.shtml

Pay close attention to the hFE/Ic curves. You'll note that the device is
linear from 10ma all the way through to 3 Amps.

Current gain doesn't much bother me. It'll be coming from a very low
impedance
stage.

**Current gain SHOULD bother you, since that is what we're discussing.

It's not what I'm discussing. I've had excellent results with 'super beta'
output stages. Commercial ( pro ) amplifiers simply can't used matched devices
for practical production and servicing reasons so you design the issue out.


The
linearity of the devices is exemplary, over a very wide current range and
down to quite low currents.

For my own part, I am very familiar with an
amplifier which has user switchable Class A operating points. Since
the
design eschews the use of Global NFB, it is far more sensitive to bais
current changes than most high Global NFB designs. Once the Class A
operating point goes beyond 10 Watts Class A, there is bugger all
difference.

In this case the crossover point has 'moved up the scale' and has less
overall effect as a percentage of signal level. It doesn't totally go
away though.

**More bull****. If one goes to the trouble to match output devices,
then,
combined with the excellent linearity I previously mentioned, crossover
distortion does not exist. It will be swamped by other forms of
distortion and noise.

You can do all this without matched devices.

**Using matched devices eliminates all possiblity of problems.

I don't want to used matched devices. In any case the gm still varies hugely at
small currents.


I simply can't conceive that the
change in gm won't ever have an effect.

**It probably would. However, I am suggesting that modern BJTs have such
excellent current linearity, that no problems will occur when using in Class
A/B.

That's not my experience. I should point out that I am targeting *ultra-low*
distortion figures.


Except, when using the thing on a US power receptacle. At 100
Watts Class A, the power consumed from the mains supply is too high
for regular US outlets, without losing mains Voltage.

Really ?

**Yes, really. US 117VAC receptacles are rated for 1,500 Watts. In
reality,
I have measured significant Voltage drops with as little as 1,000 Watt
loads. IMO, it is for this reasona that manufacturers such as Krell
stopped building Class A amplifiers for domestic consumption.

At 100 Watts Class A, in
the US, the sound becomes slightly 'compressed'.

I find some difficulty believing that.

**After you try it, get back to me. US 117VAC power receptacles and
associated wiring is pretty ordinary, compared to almost anywhere else on
the planet.

How would ac power voltage sag cause a compressed sound ?

**Think about it. Think about it in the context that few power amplifiers
use a regulated power supply.

It's a crappy design that's supply rail sensitive !


Pretty much anywhere else
on the planet is fine.


Any comments on how it sounded compared to any other types ?

**IMO, Class A is used by so-so designers who can't figure out how to
design an amplifier properly. Once bias current is set above the
'knee' of the
device (around 100mA for BJTs and 1 Amp for MOSFETs) any more
Class A bias is superfluous.

There are still some advantages in not turning the devices hard off
though.

**No. However, you can name those alleged advantages, if you wish.

Elimination of switching effects.

**Those effects can be elimintated through the use of matched linear
devices, constant operating temperatures and modest amounts of Class A bias
current. No full Class A operation is required.

I'm talking about *switching effects* - carrier storage and stuff.


Think of it this way: A badly designed Class A/B amplifier will
probably
sound better when operating in Class A. A well designed Class A/B
amplifier will probably sound slightly worse when operating in Class
A.

I can't understand the rationale for your 2nd idea there.

**Douglas Self explains it very nicely.

Call me a sceptic then !

**Read Self's work.

I've browsed it occasionally I don't consider him to be especially any guru
though. In fact I've a sort of grudge to bear in fact.

Graham

Experiences of Class A solid-state ?
 

Mike Rivers wrote:
Amplifiers are amplifiers. Some sound better than others. There's
nothing magic about a Class A amplifier, but it sure sounds good on the
marketing literature. Most people have the good sense to not use a
Class C amplifier for high quality audio.

You'll find most of the fancy "Class D" amplifiers do in fact have an
ordinary class C output stage before the integrator.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

Experiences of Class A solid-state ?
 

"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 ??






........ Phil

Experiences of Class A solid-state ?
 

"Scott Dorsey" wrote in message
...
Trevor Wilson wrote:
"Scott Dorsey" wrote in message
...
Bret Ludwig wrote:
High bias AB amps operate in class A through most of the power range
they spend all their time in, giving the advantage of Calss A operation
where it is needed, and rather than running out of power when this is
exceeded they simply transition into Class B.

This is true. The problem, however, is that although the distortion may
be much lower in class AB than in class B, the distortion spectrum is
often
less pleasant, at least to my ear. I often find a given output stage
sounds
better biased into class B than up higher into class AB.

**Bull****. Only poorly deisgned Class A/B amps sound significantly worse
than an otherwise identical Class A amp. Pay careful attention to my
words:
"Otherwise identical".

That's not what I said. That is not at ALL what I said.

For low power studio-grade applications, though, there's no reason not
to
just go whole-hog and run everything Class A. Electricity is cheap.

**There are plenty of reasons NOT to go pure Class A and very few to do
so.

Other than size and heat?

**Cost, reliability and the fact that high bias Class A/B is lower in
distortion.


--
Trevor Wilson
www.rageaudio.com.au



--
Posted via a free Usenet account from http://www.teranews.com

Experiences of Class A solid-state ?
 

"Scott Dorsey" wrote in message
...
Trevor Wilson wrote:
"Scott Dorsey" wrote in message
...
Bret Ludwig wrote:
High bias AB amps operate in class A through most of the power range
they spend all their time in, giving the advantage of Calss A operation
where it is needed, and rather than running out of power when this is
exceeded they simply transition into Class B.

This is true. The problem, however, is that although the distortion may
be much lower in class AB than in class B, the distortion spectrum is
often
less pleasant, at least to my ear. I often find a given output stage
sounds
better biased into class B than up higher into class AB.

**Bull****. Only poorly deisgned Class A/B amps sound significantly worse
than an otherwise identical Class A amp. Pay careful attention to my
words:
"Otherwise identical".

That's not what I said. That is not at ALL what I said.

**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.

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.


--
Trevor Wilson
www.rageaudio.com.au



--
Posted via a free Usenet account from http://www.teranews.com

Experiences of Class A solid-state ?
 

Scott Dorsey wrote:

Trevor Wilson wrote:

**There are plenty of reasons NOT to go pure Class A and very few to do so.

Other than size and heat?

That's what I'm working on you see !

Graham

Experiences of Class A solid-state ?
 

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.

Graham

Experiences of Class A solid-state ?
 

Trevor Wilson wrote:

"Scott Dorsey" wrote
Trevor Wilson wrote:

**There are plenty of reasons NOT to go pure Class A and very few to do
so.

Other than size and heat?

**Cost, reliability and the fact that high bias Class A/B is lower in
distortion.

Lower ?

Graham

Experiences of Class A solid-state ?
 

Trevor Wilson wrote:

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.

You're assuming they all start as Class AB output stages there.

Graham