In article , Form@C
wrote:
On Sun, 04 Jan 2004 17:32:20 +0000, Stewart Pinkerton wrote:
Sorry, it wasn't very clear was it? What I meant to say is that "high"
power amps are usually designed and set up to produce their best
results an appreciable way up to their rated power output.
No, they aren't.
Oh yes they are... You will almost always see distortion levels etc
quoted for a particular power output level. That power level is often
quoted at either full rated power or at least somewhere well above 5W or
so
I'd agree that it is quite common for magazines these days to only quote a
single value in this sort of way. However the main reason they took to
doing this is that for most amplifiers they found that the distortion at
lower power levels is nearly always *smaller* than at a 'high' power level.
Thus they stopped bothering to show the old THD/power plots.
However if you look at actual specs as quoted by makers, they have often
quoted in terms like:
1) less than 0.03 percent at 700 Hz, 0.1 percent at 10kHz. Any level up to
45W into 8 Ohms. (Quad 303).
2) less than 0.1 percent THD at any level up to 60 watt into 8 Ohms. 20Hz
to 20kHz. (Lentek integrated amp. early 1980's)
3) less than 0.02 percent at any level up to 200 W into 8 Ohms, for any
frequency in the 20Hz - 20kHz range. (Armstrong 732 power amp. Chose this
as I know the amp quite well. :-) )
Quad also used to show plots of TDH versus power with a 0.03% measurement
floor at a few watts.
I checked the above by looking again at some of the old literature I have
in front of me. The above were chosen as they were the nearest values to
hand, but I'd expect that most decent solid-state amps of the past 20 odd
years would provide similar results. In each case the THD tends to fall
away with the power level even at low powers where the measurement vanishes
into the noise floor.
Hence the way this is quoted in reviews, etc, is mainly for brevity. It
should not be taken to imply the distortion actually rises at low powers.
- very loud in most domestic rooms. (You may also notice that
manufacturers don't often quote exactly what the load was theat the amp
was driving when the measurements were taken.
In my experience 8 Ohms is ether quoted, or the standards assume it is 8
Ohms in the absence of any statement saying it is not.
Some amps measure well into resistive loads only.) Only the better amps
will quote specifications at low power levels.
It can be the case that THD can be higher into reactive loads in some
cases. However this effect is probably small in most cases, and also tends
to fall away as you reduce the power. Probably minimal in most cases
relative to the distortion levels in most *speakers*. :-)
You are correct, but that predetermined level can be very vague in many
cases. There is no way for the user to tell when the amp changes out of
class A without getting a scope out,
If the user can't tell, then perhaps it does not matter much. :-)
However my own experience was that the quiescent level was much the same,
or higher, in high power AB amps than lower power ones. Hence if you think
this matters for low power use, I'd go for high power amps (higher
quiescent) on this basis. That said, the levels at which the amp runs with
both devices nominally conducting is generally quite low, so I have my
doubts this matters much once an amp is set up and biassed sensibly.
so manufacturers often "bend" the quoted output to make the specs look
good! An amp can be classed as AB if it produces the first 50mW as class
A of course...
Never seen that put forwards as a formal specification. So far as I know,
AB just means that both sides conduct at low output power. Provided this is
done well, crossover artefacts should be efectively removed so far as THD
is concerned.
Class B amplifiers always cut off one of the devices at some point.
I would have defined this slightly differently. So far as I am concerned,
class B means that the positive and negative devices *never* conduct
symultaneously.
This is the primary cause of the "transistor sound" - a lot of
odd-harmonic distortion.
This was the case in the 1960's. I have my doubts that it is very relevant
these days. I would suspect that most decent solid state power amps during
the last 20 odd years do an effective job of removing any cross over
effects. We know more now than then, and modern devices are somewhat better
than the devices we had to make do with back then... :-)
Slainte,
Jim
--
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http://www.st-and.demon.co.uk/Audio/armstrong.html
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http://www.st-and.demon.co.uk/JBSoc/JBSoc.html
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