"ScottW" wrote in message
oups.com

You've jumped to a conclusion that's not borne out by
the figures. The power output into a variety of load
impedances is simply part of the spec. It does not mean
that the power output of the Krell will double just by
changing the load from 8 to 4 ohms.


BS. Power output is either current limited or voltage
limited.

You're one young naive puppy, Scotty.

When you're talking spec sheet power output, the operative
limit is in the mind of the guy writing the spec sheed.

Obviously into an 8 ohm load the amp is voltage
limited.

If you put them on the bench, you'll no doubt find that they
are spec sheet limited.

*every* power amp I've ever tested in the past 10 years beat
its specs on the test bench.

The QSC is also voltage limited into 8 ohms but
becomes current limited into 4 ohms.

If you're talking about what happens on the test bench then
yes, that could be said. OTOH, if you want to be pedantic,
the power amp is voltage limited in both cases. It's just
that with the lower load impedance, the voltage limit is
lower because the power supply puts out less voltage when
more current is drawn from it. The amp is still clipping
because the output stage is attempting to exceed an internal
voltage limit. IOW, the output transistors are saturating
because their VCE is too low.

This is actually a real-world distinction because power amps
typically have some kind of current limiting. With 4 and 8
ohm loads its exceedingly rare for the current limiting to
come into play. The chances that the QSC's current limiters
are being activated in a standard bench test at rated power
with a 4 ohm resistive load is approximately zero. Its only
slightly more probable that current limiting is being
activated with a 2 ohm load.

IF the QSC is driven to max output into 8 ohms with 1.9 V
rms input it will suddenly find itself current clipping
if that load is increased to 4 ohms.

Exactly. Thus, the input voltage required to drive the amp
to a lower rated output voltage is lower. Get it now?

The max input is now only 1.7 V rms. Since the gain
is the same the
useful dynamic range of the amp is reduced.

Depends on how you talk about dynamic range. The usual
convention is to rate the dynamic range of a power amp based
on power output into the stated load. Since the amp can put
out more undistorted power into the lower load impeance,
most people would say it has more dynamic range.

In other words, it has nothing to do
with the "stiffness" of the power supply.

I guess we differ on the meaning of "stiffness". In my
opinion... the power (current and voltage) capability of
an amp is very dependent on its power supply.

Not always. Since you mentioned current limiting, we've got
to remember that current limiting circuits operate pretty
much independent of the power supply.

What it DOES mean, on the other hand, is that the Krell
is a 1400W amplifier designed for optimum performance
into a load of 2 ohms or less.

More correctly, its designed for acceptable performance into
a load of 2 ohms or less.

Since the output is
voltage limited, it's an inefficient amplifier choice
for an 8 ohm speaker.

Compared to what? All good amps are generally voltage
limited.


Excess current capacity may bother you, I like it. It
makes the amp more versatile and acceptable for use with
virtually any speaker.

The QSC is rated to put out more current into low impedance
loads than the Krell because, because of its higher power
rating - 1700 watts versus 1400 watts. I suspect that on
the bench the same ordering continues.

The QSC suffers degraded dynamic
range with load and the specs also show it suffer
increased THD into 4 ohm loads as well.

Actually there's no evidence to base this conclusion on. Any
comparison of the two spec sheets is a comparison of the
fantasies or marketing guys. You'd have to measure them
yourself to know whats *really* happening.