[Bertie the Bunyip]

Eeyore wrote in
:



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.


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

fjukkwit netkkko[


Bertie