"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.
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. 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 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.
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.
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.
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.
--
Trevor Wilson
www.rageaudio.com.au
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