Experiences of Class A solid-state ?
 

"Eeyore" wrote in message
...


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 ?

**Yup. Self has provided convincing proof of this.


--
Trevor Wilson
www.rageaudio.com.au



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Experiences of Class A solid-state ?
 

"Eeyore" wrote in message
...


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.

**That's because there are essentially no Class A push pull designs
(anymore). They're all Class A/B with different bias currents.


--
Trevor Wilson
www.rageaudio.com.au



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Experiences of Class A solid-state ?
 

"Eeyore" wrote in message
...


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.

**It can but, IME, never is.



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.

**It is when discussing what happens to BJTs in an output stage and why
modern BJTs are so linear from a handful of ma up to several Amps.

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.

**Again, not under discussion. For several reasons:
* Pro amps do not operate in Class A.
* Matching output devices is MUCH cheaper than building a pure Class A
amplifier.



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.

**Modern output devices have excellent current linearity over a wide current
range. Design the amp to take that advantage into account.



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.

**How low? I cannot measure crossover distrotion in any decently designed
amp. All I can see is noise.



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 !

**ALL amplifiers which do not use regulated power supplies are rail
sensitive. Which is 99.99% of them.



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.

**And again: Matched devices, modest bias current and the problem is solved.



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.

**Self's description and mathematics are difficult to refute in this area.


--
Trevor Wilson
www.rageaudio.com.au



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Experiences of Class A solid-state ?
 

Trevor Wilson wrote:

"Eeyore" wrote in message
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.

**That's because there are essentially no Class A push pull designs
(anymore). They're all Class A/B with different bias currents.

That's where I plan to differ.

Now. To get back to the original question.... Have you heard a noticeable
difference between Class A and A/B ?

Graham

Experiences of Class A solid-state ?
 

"Eeyore"

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

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



** Transistor car radios made in the 1960s were class A.

They had a 2N301 or similar TO3 pack Germanium PNP power transistor for
output, fed with 12 volts via an iron cored choke.

The speaker connected directly across the choke - it made a small click
when you did.





........ Phil

Experiences of Class A solid-state ?
 

"Eeyore" wrote in message
...


Trevor Wilson wrote:

"Eeyore" wrote in message
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.

**That's because there are essentially no Class A push pull designs
(anymore). They're all Class A/B with different bias currents.

That's where I plan to differ.

Now. To get back to the original question.... Have you heard a noticeable
difference between Class A and A/B ?

**Not in a properly designed Class A/B amp. In a poorly designed Class A/B
amp, increasing bias will make it better.


--
Trevor Wilson
www.rageaudio.com.au



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

Experiences of Class A solid-state ?
 

Trevor Wilson wrote:

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

**That's because there are essentially no Class A push pull designs
(anymore). They're all Class A/B with different bias currents.

That's where I plan to differ.

Now. To get back to the original question.... Have you heard a noticeable
difference between Class A and A/B ?

**Not in a properly designed Class A/B amp. In a poorly designed Class A/B
amp, increasing bias will make it better.

So do tell me, what happens to the delta Vbe in an A/B output stage ? ( note its
relationship to current )

Graham

Experiences of Class A solid-state ?
 

Phil Allison wrote:

"Eeyore"

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

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


** Transistor car radios made in the 1960s were class A.

They had a 2N301 or similar TO3 pack Germanium PNP power transistor for
output, fed with 12 volts via an iron cored choke.

The speaker connected directly across the choke - it made a small click
when you did.

It *could* have been capacitively coupled had it been a problem of course.

I just discovered in r.a.t that this has been done for SET outputs.

I had something more modern in mind though.

Graham

Experiences of Class A solid-state ?
 

On 16 Oct 2006 17:30:57 -0700, "Bret Ludwig"
wrote:


Trevor Wilson wrote:
"Dave Plowman (News)" wrote in message
...
In article ,
Eeyore wrote:
Have you ever spent any time listening to a true Class A solid state
amplifiers ?

Yes. A mate drove his ESLs with a Sugden. One of the cleanest sounding
systems I've ever heard. Wouldn't shake the windows, though. ;-)

**Sugdens have never built pure Class A amps (whatever that means). They
have only ever built high bias Class A/B designs. And, of course, Class A is
only Class A when specified into a particular load impedance. Usually 8
Ohms. Given the fact that ESLs vary all over the map, impedance-wise,
suggesting that the amp is Class A is even less likely.

This is actually true.

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.

So-called experts, and their toadies and asskissers (such as
Jeboo-selling Randy Slone) have pounded on the idea that Class AB is a
poor design methodology because of "transconductance doubling".
Obviously this is nonexistant in the region of Class A operation.

Do you mean nonexistent in the region of class B operation? It is
class A that provides the transconductance doubling because there are
two active devices operating in parallel.

d

--
Pearce Consulting
http://www.pearce.uk.com

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Experiences of Class A solid-state ?
 

Don Pearce wrote:

"Bret Ludwig" wrote:
Trevor Wilson wrote:
"Dave Plowman (News)" wrote in message
Eeyore wrote:
Have you ever spent any time listening to a true Class A solid state
amplifiers ?

Yes. A mate drove his ESLs with a Sugden. One of the cleanest sounding
systems I've ever heard. Wouldn't shake the windows, though. ;-)

**Sugdens have never built pure Class A amps (whatever that means). They
have only ever built high bias Class A/B designs. And, of course, Class A is
only Class A when specified into a particular load impedance. Usually 8
Ohms. Given the fact that ESLs vary all over the map, impedance-wise,
suggesting that the amp is Class A is even less likely.

This is actually true.

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.

So-called experts, and their toadies and asskissers (such as
Jeboo-selling Randy Slone) have pounded on the idea that Class AB is a
poor design methodology because of "transconductance doubling".
Obviously this is nonexistant in the region of Class A operation.

Do you mean nonexistent in the region of class B operation? It is
class A that provides the transconductance doubling because there are
two active devices operating in parallel.

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

Graham