Crest, Crown, or QSC? - Page 2

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Arny Krueger wrote:

< snip >

> forget about whether or not it puts exactly Q/2 watts into Z*2 ohms.

Which is unachievable in a practical design anyway !

Graham

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On 10 Aug 2005 18:41:32 -0400, kludge@panix.com (Scott Dorsey) wrote:

>Don Pearce <donald@pearce.uk.com> wrote:
>>
>>No. The output impedance of the power supply can be just fine. What
>>the power change with load tells you about is the current delivery
>>capacity of the supply. That is what makes it sag with lowered load
>>impedance.
>
>Again, how is this different?
>
>>The average amplifier these days has an output impedance of perhaps a
>>milliohm, so that will cause essentially no power loss at lowered
>>impedance.
>
>It might at 1 KHz, but I bet it doesn't at 10 Hz.... because of the
>feedback, the effective series resistance of the output stage might
>be zero, but the power supply is still in series with the whole thing
>and the impedance of the supply is still nonzero at low frequencies
>even though (due to lots of decoupling) it could be very close to zero
>at high ones.
>
>These are two ways of looking at the same thing, I think.
>--scott

You are right, but think for a moment about how the power supply
works.

You have a big electrolytic cap while holds up the rail voltage. It
also passes the signal current to the speaker, and thus needs to be of
really low impedance. The job of the rest of the power supply is to
keep that cap topped up to the correct voltage, and will be turning on
and off, not only once over 50th (60th) of a second, but depending on
the instantaneous load, maybe at longer intervals. So sometimes the
power supply will be low impedance, but mostly it will be high
impedance, with no current flowing at all. Obviously none of this
carry-on must affect the sound, so it is clear that the big cap is
doing essentially everything by way of carrying signal current and
providing the low impedance.

As for the effect of low speaker impedances on power output, if you
attach a speaker that is too low for the amp, the power supply will be
unable to supply the necessary current, so the voltage across that big
cap will drop. This reduced rail voltage is what causes the amp to run
out of headroom prematurely.

d

Pearce Consulting
http://www.pearce.uk.com

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"William Sommerwerck" <gizzledgeezer@comcast.net> wrote in
message news:nLCdnaDmFtTvUWffRVn-gA@comcast.com

>>> Try to find an amplifier whose power output varies
>>> directly with the load impedance. You'll have trouble
>>> finding any.

>> Including Krells. Actual measurements on Krell amps show
>> much more than rated output with high-Z loads. IOW they
>> lied about the output into hi-z loads to make the actual
>> performance into low-Z loads look better.

> I have no trouble with that. I don't mind lies if they
> make the amp look worse than it really is.

A scam is a scam.

>> The moral of the story is that you want an amp that puts
>> Q watts into Z ohms then buy one that does that, and
>> forget about whether or not it puts exactly Q/2 watts
>> into Z*2 ohms.

> The rationale, however, is that we want an amplifier that
> looks like a "perfect" voltage source, as load
> interaction is once source of differences in amplifier
> "sound". (Pace, Arny).

That means one thing - low source impedance.

There's even an irony here. My tests suggest that power amps
with really stiff power supplies aren't necessarily the best
idea when it comes to driving real-world speaker loads, even
agressive ones like mine (see below).

> Stereophile uses what is supposed to be a plausible
> speaker load.

IME it's way too easy, I suspect they keep it light because
it can't be allowed to embarass the SET trash SP's
advertisers pump. For example its minimum load is 6 ohms,
but many speakers go down to 3 ohms or so.

> I suspect it's downright pathological, because even
> expensive sold-state
> amplifiers (including Krells) show some interaction with
> it.

IME its not pathological enough. I designed my own for my
amp tests, shown here:

http://www.pcabx.com/product/amplifiers/index.htm

Compare that to:

http://stereophile.com/reference/60/

> What it does to tube amps is downright horrifying.

Good tubed amps don't suffer too badly. Then there's the
other stuff - most if not all SETs.

Archived from groups: rec.audio.pro (More info?)

I own a small retail shop in South Africa where our money is worth nothing
compared to the Dollar, Pound and Euro. my Main seller in Power-amps is
BEHRINGER EP1500 and EP2500 because they are affordable for your average
musician.I've sold them to anyone from 1 man bands, to 5 - 6 piece Rock
bands to Club and Pub Installations and haven't had a single comeback.

I have been using 2 QSC RMX 2450's in my rig since approx. 1999 and have
NEVER had any problems with it. They are real workhorses. I've used it to
drive my top end and subs, and both have handled perfectly.

So i can highly recommend either the RMX2450 or the EP2500

K.C

"John" <are.you.crazy@sendmenomail.com> wrote in message
news:are.you.crazy-EC2709.16250908082005@news.supernews.com...
> In article <H8mdndVZCrJCVWrfRVn-3g@comcast.com>,
> "Arny Krueger" <arnyk@hotpop.com> wrote:
>
> > If you are interested in
> > stretching a buck, but still getting something that has a
> > decent reputation, there are always the Behringer Europower
> > EP2500.
>
> i'm not certain that Behringer and "decent reputation" belong in the
> same sentence.
> --
> Digital Services Recording Studios
> http://www.digisrvs.com