[Bertie the Bunyip]

Eeyore wrote in
:



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



fjukkwit wannabe ****


Bertire