On Sun, 04 Jan 2004 19:10:08 GMT, "Form@C" wrote:

On Sun, 04 Jan 2004 17:32:20 +0000, Stewart Pinkerton wrote:

On Sun, 04 Jan 2004 12:31:32 GMT, "Form@C" wrote:

On Sun, 04 Jan 2004 11:09:48 +0000, Ian Molton wrote:

On Sun, 04 Jan 2004 08:27:50 GMT
"Form@C" wrote:

It's no use turning down a
100-200W monster amp because you arn't running them at their best.

what?

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...

Oh no, they aren't!

Well, it *is* the panto season.................

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 - very loud in most domestic rooms.

Yes of course, since that's conventional, and tells you how much power
the amp can put out.

(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. Some amps measure well into
resistive loads only.)

Very true! :-(

Only the better amps will quote specifications at
low power levels.

But any well-designed amp will have low distortion at low levels,
usually well below the noise floor.

They won't perform at
their best at a very low percentage of that level.

Absolute garbage!

Maybe, but its a fact! :-)

No, it's not. You are thinking of those nasty mid-70s 'Jap crap'
designs with excessive NFB and poor open-loop bandwidth, using
slow-switching output trannies. Doesn't happen with any decent modern
amp.

I *know* that many are
*supposed* to be class A at low levels, but they are generally not
designed as class A amps, and are not really running as such (the output
stage quiescent current is usually too low to allow correct class A
operation on a class AB amp).

Excuse me? A class AB amp *always* runs in class A up to some
predetermined level, which may be less than 1 watt or as many as 50 watts.
I use an amplifier which runs in true class A up to 50 watts into 8 ohms,
and in class AB above that level, up to its rated maximum of 400 watts
into 1 ohm. It sounds just fine at very low levels.

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, 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...

Yes, and this is also *totally* irrelevant to how it sounds. I have a
low-bias Audiolab 8000P and a 'pure class A' Krell KSA-50 mkII, and
they are sonically indistinguishable into my pretty tough Apogee
duetta Signatures.

At 400W you are not
running an amp that I would class as average domestic. It will (should) be
running bloody hot too! (1) The majority of AB amps out there run the
no-load current at below optimum for usable class A use and don't apply
sufficient drive to push the output devices fully. It keeps the size &
cost of the PSU down and reduces the heat, making the equipment more
acceptable to the domestic market.

Yes, and it doesn't make a jot of difference to the sound quality.

Thus it is a mistake to judge low-power
audio systems by simply turning down the volume on existing "high" power
equipment & changing the speakers for more sensitive ones.

Depends on the amp......................

Oh yeah! There are a lot of really good amps, but you won't often find
them available through your local outlets. Even so-called "hi-fi" shops
tend not to carry the good stuff as stock - it costs far too much.

Nah, it starts with the Yanaha AX-592 and carries on from there.

(1) A note for thos who may not know what we are on about... Stewart will
know this already! Class A is always less than 50% efficient, typically
46%, so you should be producing well over 100W of heat, with no signal
present, to be capable of class A at 50W - not including valve heaters of
course! In class AB amplifiers one of the output devices is cut-off on
alternate halves of the waveform, giving a power saving. Class B
amplifiers always cut off one of the devices at some point. This is the
primary cause of the "transistor sound" - a lot of odd-harmonic
distortion.

Yes, my '50 watt' Krell draws 300 watts from the wall when idling. For
those of an enquiring mind, the theoretical efficiency limit for a
class AB amp is 78% at full popwer. Naturally, no *high quality* amp
has to worry about valve heaters............... :-)
--

Stewart Pinkerton | Music is Art - Audio is Engineering