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