Re: A Comparison
- From: "ScottW" <ScottW48@xxxxxxxxxxx>
- Date: 21 Oct 2005 15:45:37 -0700
Arny Krueger wrote:
> "ScottW" <ScottW48@xxxxxxxxxxx> wrote in message
> >> 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.
Its called margin.
> > 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.
BS... this doesn't happen until you have exceeded its current
> 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.
Then you have no valid explanation for the reduced sensitivity or
less than double 8 ohm load power output.
You just keep spinning yourself into a hole.
> 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?
Yeah.. I got it. and effective dynamic range is also reduced as the
noise floor is probably relatively fixed.
> > 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.
You measuring noise floor in power? I don't think so.
> >> 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.
So much for the drooping bandwagon you jumped on.
> >> 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
Lost track of who you're responding to again I see.
> > 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.
and you'd have a sample of 1. I'm sure QSC isn't report increased THD
into 4 ohm loads because they think it sounds cool.
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