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Topic: Amp power AVR vs Power amp (Read 28225 times) previous topic - next topic
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Amp power AVR vs Power amp

Reply #50
It has been determined in ABX tests, overs the past several decades, that people cannot tell any difference between the most expensive amplifiers and the least expensive that meet minimum hifi specs (essentially everything approaching $100 on up). There are many in the audiophile circles that like to dispute these tests, mainly on emotional considerations, but that doesn't change the results.


I would believe that you're correct - but do you have any references for more information (for the ABX tests and what meets 'minimum hifi specs')?  I'be been out of the game a long time; looking for equipment that meets some basic standards .... aaaaaaand... that the wife would accept having to look at on the shelf every day... 

Amp power AVR vs Power amp

Reply #51
I'm coming in late as I've only just seen this thread. It seems to me that nobody has given a really simple answer to the original question: why high current delivery capability might sometimes be beneficial. I'd like to have a bash at it:

Let's pick a power rating of 72W, for no better reason than all of the sums yield whole numbers. Let's suppose you have an amp with this spec: power output 72W into 8 ohms. What this means is that when driving an 8 ohm load, it can deliver 72W. The voltage is 24V and the current is 3A. So far, so good.

But if the impedance of the speaker being driven happens to dip to 2 ohms at some frequency (not at all uncommon), then when the amp is trying to send 24V to it, the speaker will demand 12A of current (current = voltage / resistance). Which means the amp now has to be producing 288 watts. But if the amp can't deliver more than 3A of current (still within its published spec), that's all the speaker will get, and the voltage will drop to 6V (voltage = current * resistance), resulting in a power output of just 18W.

This is an extreme example designed to illustrate the principle. I very much doubt that any real world amps actually behave this badly.

Amp power AVR vs Power amp

Reply #52
It has been determined in ABX tests, overs the past several decades, that people cannot tell any difference between the most expensive amplifiers and the least expensive that meet minimum hifi specs (essentially everything approaching $100 on up). There are many in the audiophile circles that like to dispute these tests, mainly on emotional considerations, but that doesn't change the results.


I would believe that you're correct - but do you have any references for more information (for the ABX tests and what meets 'minimum hifi specs')?  I'be been out of the game a long time; looking for equipment that meets some basic standards .... aaaaaaand... that the wife would accept having to look at on the shelf every day... 



Try this:
https://web.archive.org/web/20101214121953/...o/Amp_Sound.pdf

Amp power AVR vs Power amp

Reply #53
I'm coming in late as I've only just seen this thread. It seems to me that nobody has given a really simple answer to the original question: why high current delivery capability might sometimes be beneficial. I'd like to have a bash at it:

Let's pick a power rating of 72W, for no better reason than all of the sums yield whole numbers. Let's suppose you have an amp with this spec: power output 72W into 8 ohms. What this means is that when driving an 8 ohm load, it can deliver 72W. The voltage is 24V and the current is 3A. So far, so good.

But if the impedance of the speaker being driven happens to dip to 2 ohms at some frequency (not at all uncommon), then when the amp is trying to send 24V to it, the speaker will demand 12A of current (current = voltage / resistance). Which means the amp now has to be producing 288 watts. But if the amp can't deliver more than 3A of current (still within its published spec), that's all the speaker will get, and the voltage will drop to 6V (voltage = current * resistance), resulting in a power output of just 18W.

This is an extreme example designed to illustrate the principle. I very much doubt that any real world amps actually behave this badly.


Any number of improbable events are described above.

(1) The amp is being driven to just below clipping is improbale.  In fact music is constantly varying and achieves maximum peak value only a tiny part of the time even in the extremely improbable situation where it is being played this loud. 72 watts corresponds to 108 dB SPL with an average 90 dB/Watt speaker.

(2) It requires that all energy in the music being amplified be contained in a pure tone in the frequency range where the speaker has an impedance of 2 ohms. In fact music is not well-modeled by a single pure tone - musical sounds are complex waves with components spread across the audible spectrum.

(3) Reference these plots of the spectral contents of common music:



Not only does all of the energy in the music be concentrated at one frequency, but that frequency has to be some very low frequency because sounds that large don't exist at most frequencies.

The scenario above is similar to asking that all automobiles to survive collisions with meteorites the size of Manhattan Island! ;-)


Amp power AVR vs Power amp

Reply #55
Any number of improbable events are described above.
[snip]

Yes, Arnie, that's all very well, but I was trying to give the simplest possible theoretical explanation as to why current delivery capability matters, which I believe is what the OP asked for. I was specifically trying to avoid the additional complexities you (and many others) have mentioned.

I agree that in the real world, with music signals driving typical speakers, it's not a big issue. But there are some extreme examples where it does matter. Try driving something like a pair of Apogee Duettas or Quad ESLs with a cheap low powered AVR and see how far you get!

Amp power AVR vs Power amp

Reply #56
^Or more common than that, simply using the "Speakers A+B" function found on nearly all receivers and most typically wired in parallel. Suddenly those current production, "8 ohm" KEF speakers which dip down to around 3 ohms at a certain bass frequencies becomes 1.5 ohm. Ouch.

Amp power AVR vs Power amp

Reply #57
Any number of improbable events are described above.
[snip]

Yes, Arnie, that's all very well, but I was trying to give the simplest possible theoretical explanation as to why current delivery capability matters, which I believe is what the OP asked for. I was specifically trying to avoid the additional complexities you (and many others) have mentioned.


Unfortunately I see an explanation  supported in an unqualified way by improbable circumstances.  An explanation based on improbable occurrences is not that much of an explanation.

Quote
I agree that in the real world, with music signals driving typical speakers, it's not a big issue. But there are some extreme examples where it does matter. Try driving something like a pair of Apogee Duettas or Quad ESLs with a cheap low powered AVR and see how far you get!


Here's the impedance curve of Duettas:

http://www.stereophile.com/content/apogee-...er-measurements




Nothing particularly scary there.

Here is the impedance curve of Quad ESL 63s:



Nothing particularly scary there 20-20 KHz.

Here is the impedance curve of original Quad ESL 57s:



Things get a little strange in the range 10 KHz - 20 KHz, but there's very little energy there in typical recorded music:






Amp power AVR vs Power amp

Reply #58
^Or more common than that, simply using the "Speakers A+B" function found on nearly all receivers and most typically wired in parallel. Suddenly those current production, "8 ohm" KEF speakers which dip down to around 3 ohms at a certain bass frequencies becomes 1.5 ohm. Ouch.


I dispute your claim that "Speakers A+B" switches exist on nearly all modern AVRs.  I think that if you check your own claim with modern data you will join me in the dispute! ;-)

An easy way to do this is to search google for AVR receiver  and select images. Looking at the first 10 images of AVR back panels I find zero provisions for a second set of speakers.  This makes sense due to the complexity of actually providing for a second 5.1 set of speakers.  Stereo receivers are largely obsolete - like CD players.

Amp power AVR vs Power amp

Reply #59
There are two reasons why an amplifier's peak power can be much higher than its average power:

> The heatsink for the output transistors is inadequate to dissipate the heat generated by high output power over an extended time period. It is, however, able to handle brief periods of high dissipation.

> The power supply is not able to supply the voltage and current required for high output power continuously for an extended time period. However, large bypass capacitors in the power supply allow it to meet these needs for short time periods.

Expensive amplifiers may provide for higher heat dissipation in the heatsink and higher continuous power from the power supply, increasing their average power rating even if it does not increase their peak power rating. This would make sense if we listened to continuous sine waves. However, since most people listen to music instead, average power is a much less important rating than peak power.

Amp power AVR vs Power amp

Reply #60
I've actually never owned a receiver, strictly stereo or AV, that didn't have speakers A+B. I certainly agree it isn't universal and is much less common in 2014 production lines, especially not stereo ones, than it was a decade or so ago, but many of those older receivers are still in use today and there certainly are "speakers A+B" receivers, even of the AVR variety, in current production. My main point was that Speakers A+B driving low impedance pairs of speakers is a more common scenario than, let's say, 1 ohm Apogee Scintilla use.

I retract that the feature is on "nearly all" receivers, however I stand by the notion that it is a common feature on actual receivers being used in the field. Much more common than difficult speakers, like "1 ohm" ones.

Amp power AVR vs Power amp

Reply #61
Here's the impedance curve of Duettas:
[snip]
Nothing particularly scary there.

Apologies. I think it's the Scintillas that have a sub-1ohm impedance dip. I was dredging up old memories and got the model wrong.

If in practice there's no need to worry about an amplifier's current delivery capability, then there was little point in the many complicated responses to the OP's question, and also little point in my attempt at a very simplified explanation. We could have all just said "no need to worry about it" and left it at that.

As it happens I *don't* think that you can just forget about an amp's current delivery capability in all circumstances. But perhaps we'll just have to agree to differ.

 

Amp power AVR vs Power amp

Reply #62
It seems this discussion was actually about whether someone should replace a specific amp because it doesn't match the power rating of a specific set of speakers even though no problems have been identified with the sound quality.

Talking about theoretical issues that may never occur and likely won't ever occur with the OP is not doing him any favors. 

Must we talk him off the cliff again?