Amplifiers affecting speaker frequency response - AVS Forum
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post #1 of 362 Old 11-05-2013, 12:29 PM - Thread Starter
 
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I've heard many people tell me that amplifier measurements into a resistive load don't you tell how that amplifier will behave with a real loudspeaker.

 

Is that true? It would seem to make sense on the surface. I defer to your expertise on the matter.

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post #2 of 362 Old 11-05-2013, 12:39 PM
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The resistance of a speaker is called "impedance" and is expressed in ohms but is an average of the "reactive" resistance because the value changes with frequency. There are other factors involved besides, those being the inductive resistance of the voice coils and the capacitive resistance of the circuit elements. This all has an affect on the resultant potential reaching the speaker coil across its operating frequency range of the speaker. The reactive elements can both enhance and reduce, distort and even elimnate certain points across the audio spectrum. Manufacturers spend lots of time getting things right.
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post #3 of 362 Old 11-05-2013, 01:06 PM
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I've heard many people tell me that amplifier measurements into a resistive load don't you tell how that amplifier will behave with a real loudspeaker.

Is that true? It would seem to make sense on the surface. I defer to your expertise on the matter.
It's true, but it's one of those truths you'd be better off not knowing. The standard way of measuring amplifier power is far more demanding than anything you are likely to throw at them at home. Measurements use continuous power. At home, the only time your amp is likely to be stressed in any way is from an instantaneous peak, or transient. On a continuous basis, your amp is probably putting out no more than a few watts, whatever its rating.

The makers of high-end (read: high-priced) amps would like you to believe that there's something the spec sheets aren't telling you, and that their 50 watts is somehow more powerful than some Japanese receiver's 50 watts. That's hooey.

If you can't explain how it works, you can't say it doesn't.—The High-End Creed

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post #4 of 362 Old 11-05-2013, 01:35 PM
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Hi Shaun,

Since the title of your thread mentioned frequency-response, I will try to address the effects of the speakers on the frequency-response.

An amplifier might measure flat using an 8 ohm resistor as a load, and it might also measure flat into an 4 ohm resistor. But the output power through 4 ohms will be considerably higher than through 8 ohms at the same volume setting.

So imagine a speaker whose impedance wanders between 4 ohms and 8 ohms as the frequency changes. The speaker would output more power at the frequencies of lower impedance, and therefore the frequency-response would no longer be flat. Since the amplifier manufacturer has no control over the speakers, the resistor is the best they can do.

As Mjd420nova said, speaker manufacturers spend a lot of energy trying to get things right, but they are never perfect. The crossover design goes a long way to addressing the problem, but in my opinion the most flexible solution is to compensate with a DSP.
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post #5 of 362 Old 11-05-2013, 01:57 PM - Thread Starter
 
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Sorry if my question was poorly formed, but the reason for my question was because some people claim amplifiers do not sound different. Some people claim that amplifiers hooked up to resistive loads don't tell the whole picture, especially when the frequency response shows a flat line.

 

I've heard people tell me that when hooked up to a real speaker, the frequency response is no longer flat, hence resistive load measurements don't tell then whole story. This is what I want to know. I own a Krell that I'm thinking of selling as it's too big, and consumes too much power for something more cost effective.

 

I just would like to know the facts here. If an amplifier measures flat using a resistive load, using a speaker it will not be flat like with the resistive load? Or is there something I am missing?

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post #6 of 362 Old 11-05-2013, 02:13 PM
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Quote:
Originally Posted by Shaun B View Post

Sorry if my question was poorly formed, but the reason for my question was because some people claim amplifiers do not sound different. Some people claim that amplifiers hooked up to resistive loads don't tell the whole picture, especially when the frequency response shows a flat line.

I've heard people tell me that when hooked up to a real speaker, the frequency response is no longer flat, hence resistive load measurements don't tell then whole story. This is what I want to know. I own a Krell that I'm thinking of selling as it's too big, and consumes too much power for something more cost effective.

I just would like to know the facts here. If an amplifier measures flat using a resistive load, using a speaker it will not be flat like with the resistive load? Or is there something I am missing?

for like a couple of decades Stereophile has consistently tested amps into both resistive loads and a speaker simulator that has what they think is a tough impedance to drive. You always see some departures from flat, usually less than two tenths of a dB. Maybe half a dB for a few amps. Stereophile.com, equipment reviews, speakers (there's always a shortcut to measurements for full reviews)

Basically output impedance of the amp determines this, and lower output impedance is better. But also it's highly unlikely that the little variations you see are audible. Of course a single ended triode tube amp will have a higher output impedance and will have a rolloff in the bass, usually, that may be audible.
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post #7 of 362 Old 11-05-2013, 02:20 PM
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Quote:
Originally Posted by Shaun B View Post

Sorry if my question was poorly formed, but the reason for my question was because some people claim amplifiers do not sound different. Some people claim that amplifiers hooked up to resistive loads don't tell the whole picture, especially when the frequency response shows a flat line.

I've heard people tell me that when hooked up to a real speaker, the frequency response is no longer flat, hence resistive load measurements don't tell then whole story. This is what I want to know. I own a Krell that I'm thinking of selling as it's too big, and consumes too much power for something more cost effective.

I just would like to know the facts here. If an amplifier measures flat using a resistive load, using a speaker it will not be flat like with the resistive load? Or is there something I am missing?

What you say is true. But the effect will express itself on any amplifier. Amps don't all measure the same. But in the case of modern solid state amplifiers, those differences in measurements are below the threshold of audibility. So the amps indeed measure differently and yet sound the same.
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post #8 of 362 Old 11-05-2013, 02:29 PM
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Originally Posted by JHAz View Post

for like a couple of decades Stereophile has consistently tested amps into both resistive loads and a speaker simulator that has what they think is a tough impedance to drive. You always see some departures from flat, usually less than two tenths of a dB. Maybe half a dB for a few amps. Stereophile.com, equipment reviews, speakers (there's always a shortcut to measurements for full reviews)

Basically output impedance of the amp determines this, and lower output impedance is better. But also it's highly unlikely that the little variations you see are audible. Of course a single ended triode tube amp will have a higher output impedance and will have a rolloff in the bass, usually, that may be audible.

+1. If amps were ideal and had zero source impedance, then their frequency response would be load independent (ie, it wouldn't change regardless of what the load impedance is). But real-world amps are not ideal. They have a (very small) source impedance, and that means that their output frequency response does vary with real-world speaker impedances, which are not just simple 8 Ohm or 4 Ohm loads.

Nevertheless, in practically any modern solid state amp (class a or a/b), the source impedance is sufficiently low enough that the resulting changes from flatness in frequency response are so small as to be inaudible.

In tube amps, the source impedance from the amp can be higher - high enough to make audible differences in frequency response.

In class D amps as well, the source impedance can sometimes be high enough to lead to audible differences in frequency response with certain speaker load impedances.

Stereophile does indeed include plots for amplifier frequency response vs ideal 8 Ohm loads and a simulated real-world speaker load. The amps that have good (low) source impedances show very little change in frequency response when powering the real-world speaker load. The amps that have high source impedances show significant changes in frequency response when powering the real-world speaker load. It's one of the things they do right - and it's a nice resource they provide. I just wish they'd cut all the other nonsense they publish.

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post #9 of 362 Old 11-05-2013, 02:32 PM
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Here is a link to the Stereophile article about their dummy speaker load. For an example of extremely good performance in this regard, see figure 1 of the Parasound Halo JC-1 review. For an example of extremely poor performance, see figure 1 of the $350,000 Wavac SH-833 amplifier review. biggrin.gif
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I just wish they'd cut all the other nonsense they publish.

You got that right! smile.gif
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post #10 of 362 Old 11-05-2013, 02:42 PM
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Hi Shaun,
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Originally Posted by Shaun B View Post

. . . Some people claim that amplifiers hooked up to resistive loads don't tell the whole picture, especially when the frequency response shows a flat line.
It might not tell you the whole picture, because a good part of the picture is missing: the speakers. It does tell you what you need to know, and given the impedance curve of the speakers, can accurately predict what the result would be. That science is pretty trivial. The resistive load is as far as the amplifier manufacturer can go without taking a particular speaker into account.
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. . . especially when the frequency response shows a flat line.
I would then ask some people what they would like to see, if not a flat line. The idea of a flat line is that the output of the amp matches the input. Despite what a salesman might tell you, and amplifier should not color the sound, as a non-flat frequency-response would do.
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post #11 of 362 Old 11-06-2013, 12:34 PM - Thread Starter
 
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Stereophile does indeed include plots for amplifier frequency response vs ideal 8 Ohm loads and a simulated real-world speaker load. The amps that have good (low) source impedances show very little change in frequency response when powering the real-world speaker load. The amps that have high source impedances show significant changes in frequency response when powering the real-world speaker load. It's one of the things they do right - and it's a nice resource they provide. I just wish they'd cut all the other nonsense they publish.

Okay, so you are saying that low source impedance means that the speakers impedance won't affect the frequency response, and a high source impedance will affect the frequency response according to the speakers impedance?

 

Is there any evidence for that? I see graphs on Stereophile but it's all dummy loads. How do we know that low source impedance won't change the frequency response by audible amounts?

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post #12 of 362 Old 11-06-2013, 01:07 PM
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Hi Shaun,

Most of it is ohms law. Here is my attempt at an explanation:


An amplifier has a source impedance, which is a fixed number for a well-designed amplifier, typically under 4 ohms. There are excellent amplifiers with higher source impedances (my Sansui is somewhere under 8 ohms), and they will work fine as long as the load impedance (the speaker) does not drop below the source impedance. The source impedance is what the amplifier can drive, the load impedance is what determines how much of that the speaker will draw.

The problem is that, while the source impedance is normally constant across frequency, the speaker's load impedance is not. At those frequencies where the load impedance drops, the power-draw will increase, and therefore the volume as well. So the speaker is what makes the amplifier's "flat" frequency-response "unflat" coming out of the drivers. That is one reason why the speakers are so important.

Now, that fluctuation in power with frequency through a particular speaker will be identical between two well-designed amplifiers, as long as the load impedance never drops below the source impedance in either amplifier. If it does, as if I tried to drive 4 ohm speakers with my 8 ohm Sansui, bad things could happen, ranging from a little distortion to blown output transistors.

By using only ohms law, you can accurately predict the frequency-response of the system, given the frequency response of the amplifier and the impedance curve of the speaker, assuming that the source impedance is less than the load impedance across all frequencies.
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post #13 of 362 Old 11-06-2013, 01:41 PM
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Quote:
Originally Posted by mjd420nova View Post

The resistance of a speaker is called "impedance" and is expressed in ohms but is an average of the "reactive" resistance because the value changes with frequency. There are other factors involved besides, those being the inductive resistance of the voice coils and the capacitive resistance of the circuit elements. This all has an affect on the resultant potential reaching the speaker coil across its operating frequency range of the speaker. The reactive elements can both enhance and reduce, distort and even elimnate certain points across the audio spectrum. Manufacturers spend lots of time getting things right.

No, resistance is resistance. Impedance is the vector sum of resistance, inductive reactance and capacitive reactance.
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post #14 of 362 Old 11-06-2013, 02:09 PM
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Originally Posted by MarkHotchkiss View Post

It might not tell you the whole picture, because a good part of the picture is missing: the speakers.

Exactly - if you are worried about frequency response less than 1% of any problems you are going to have are going to be related to the frequency response of a good, modern solid state amplifier and 99%+ is going to be related to the frequency response of the speakers and how they perform in your particular listening room.

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So imagine a speaker whose impedance wanders between 4 ohms and 8 ohms as the frequency changes. The speaker would output more power at the frequencies of lower impedance, and therefore the frequency-response would no longer be flat.

Or more accurately how much power a speaker draws at a particular frequency. At certain frequencies the impedance will dip and the speaker will draw a lot more power. At other frequencies the impedance will peak and the speaker will draw very little power at that frequency. It is the job of the speaker designer (by using the drivers, cabinet, and crossover) to minimize these dips and peaks presented to the amplifier and even more importantly to make the acoustic output as constant as possible through the entire frequency range.
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post #15 of 362 Old 11-07-2013, 12:17 AM
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Originally Posted by MarkHotchkiss View Post

An amplifier has a source impedance, which is a fixed number for a well-designed amplifier, typically under 4 ohms. There are excellent amplifiers with higher source impedances (my Sansui is somewhere under 8 ohms)
Good grief. The Zout of an SS amp will typically be milliohms with the slight exception that many class D units have a rising Zout due to the output filter; usually fairly narrow band at or above 20kHz.
For most tube amps like the ST70 (and similar topologies) it will be about an ohm, and for zero gNFB units it will be two or more.

Unless speaking about a poorly designed tube unit like the WAVAC beav linked earlier, the Zout of modern AB SS amps is so small as to not be worth thinking about, and has been for a long time.
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post #16 of 362 Old 11-07-2013, 03:56 AM - Thread Starter
 
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Can anyone explain why resistive loads on amplifier bench test results are more stressful on amplifiers than real loudspeakers?

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post #17 of 362 Old 11-07-2013, 04:03 AM
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Quote:
Originally Posted by Shaun B View Post

Can anyone explain why resistive loads on amplifier bench test results are more stressful on amplifiers than real loudspeakers?

Sure, a resistive load draws a constant level of current while speakers vary constantly. There is no let up with the resistive load. Speakers playing music have periods with little current draw. At a given volume level, the speakers will draw less current over time.
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post #18 of 362 Old 11-07-2013, 05:27 AM
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Quote:
Originally Posted by Shaun B View Post

Can anyone explain why resistive loads on amplifier bench test results are more stressful on amplifiers than real loudspeakers?

The resistive load has the same resistance at any audio frequency, and probably well beyon.

The speaker load's impedance is somewhat reactive which means that only part of the load it presents dissipates power, and it is generally higher than its specified impedance over most of the audio range.

Here is the impedance curve of one of the superficially scary speakers on the market:



People report being able to drive it with a good AVR, but given the < 2 ohm impedance above 10 KHz at first thought that almost seems impossible.

However, the speaker's impedance is > 4 ohms below 3,500 Hz, > 6 ohms below 2,500 Hz, and rising above 12 ohms below 200 Hz where most of the energy in the music is. If the speaker is used with a subwoofer then it presents no load at all below 60-80 Hz or whatever the subwoofer's crossover frequency is.

A 4 ohm resistor used in bench test would show up o this chart as a line all the way across the diagram from 10 Hz to 50 KHz (and outside that range as well) @ 4 ohms, which is well below that of the speaker below 3.5 Khz.

Most of the energy in music is at lower frequencies, So from the standpoint of playing music the 4 ohm load is a far tougher load than even this very tough-seeming speaker over most the range of frequencies where most of the energy in music actually is.

Now for a more typical loudspeaker:




This speaker barely goes below 8 ohms at any frequency, and so it is an easier load over the audio band on the average than even an 8 ohm resistor.
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post #19 of 362 Old 11-07-2013, 05:38 AM
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Quote:
Originally Posted by Shaun B View Post

Quote:
Stereophile does indeed include plots for amplifier frequency response vs ideal 8 Ohm loads and a simulated real-world speaker load. The amps that have good (low) source impedances show very little change in frequency response when powering the real-world speaker load. The amps that have high source impedances show significant changes in frequency response when powering the real-world speaker load. It's one of the things they do right - and it's a nice resource they provide. I just wish they'd cut all the other nonsense they publish.
Okay, so you are saying that low source impedance means that the speakers impedance won't affect the frequency response, and a high source impedance will affect the frequency response according to the speakers impedance?

Is there any evidence for that? I see graphs on Stereophile but it's all dummy loads. How do we know that low source impedance won't change the frequency response by audible amounts?

The Stereophile dummy load is a bit of a free ride - last time I did the analysis about half the speakers they tested had more challenging impedance curves than their speaker simulator.

Even so they test amps that give 2.5 dB or more frequency response variations with it, and over an octave or two that kind of a dip or peak can be heard in an ABX test.

There is a ton of evidence for the idea that an amplifier's source impedance can affect its frequency response. Sterreophile's tests form a good part of it, if you know what they mean and how they relate to the real world.

First off, a good SS amp has a source impedance of well under < 0.05 ohms which gives a damping factor of 80 for a 4 ohm load and 160 for an 8 ohm load.

A typical speaker might have a minimum impedance of 8 ohms in which case about 1/160th of its output will be dropped across this internal equivalent resistance inside the amp. That is well under a 0.1 dB loss. At other frequences the speaker might have a maximum impedance of 100 ohms, in which case almost no voltage is dropped across the internal reistance. Therefore the internal resistance is contributing a frequency response of less than 0.1 dB which negligible.

Now we hook up one of the better tubed amps with relatively low internal source impedance (a McIntosh, not a SET) and its internal resistance might be the equivalent of less than 0.5 ohms. With the minimum 8 ohm load the voltage drop inside the amp is a maximum of 1/16th of its output which causes a loss of about 0.6 dB which can be barely audible at its upper limit but not audible at an average value of 0.25 ohms.. If we look at the SET its internal resistance might be 2 ohms which is dropping 1/5 of the output of the amp, for a loss of > 2 dB which can often be easily heard..
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post #20 of 362 Old 11-07-2013, 05:45 AM
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Originally Posted by andyc56 View Post

Here is a link to the Stereophile article about their dummy speaker load. For an example of extremely good performance in this regard, see figure 1 of the Parasound Halo JC-1 review. For an example of extremely poor performance, see figure 1 of the $350,000 Wavac SH-833 amplifier review. biggrin.gif
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I just wish they'd cut all the other nonsense they publish.

You got that right! smile.gif

+1

The problem is that their often credible technical tests give credibility in many audiophile's eyes for the not-so-credible ones, for example those related to jitter. They also lend credibility to their listening tests that are almost always sighted and thus highly suspect when the equipment involved isn't a loudspeaker.
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post #21 of 362 Old 11-07-2013, 09:17 AM
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Quote:
Originally Posted by Shaun B View Post

Can anyone explain why resistive loads on amplifier bench test results are more stressful on amplifiers than real loudspeakers?
That comment is made in the context of a bench test with a resistive load being made at maximum power as opposed to listening to music (with speakers) which would be at lower levels. It is not correct at all with respect to type of load. Resistive loads in electronics are the easiest to drive. Inductive/capacitive loads are harder for a number of reasons, one of which would be interfering with the feedback loop of the amp and cause oscillations. "Well-designed" amps can tolerate such varying loads but what makes them "well-designed" is the requirement for dealing with a more difficult load of a real speaker.

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post #22 of 362 Old 11-07-2013, 11:18 AM
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You have to go pretty far off the reservation to get poor speaker/amp interaction to the point of audible frequency response deviations. It's not really a practical problem unless you have a fetish for fire-bottles.
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post #23 of 362 Old 11-07-2013, 11:24 AM
 
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Originally Posted by amirm View Post

"Well-designed" amps can tolerate such varying loads but what makes them "well-designed" is the requirement for dealing with a more difficult load of a real speaker.
Yes, but do those "well-designed" amps make audible difference?
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post #24 of 362 Old 11-07-2013, 01:26 PM
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Originally Posted by Wayne Highwood View Post

You have to go pretty far off the reservation to get poor speaker/amp interaction to the point of audible frequency response deviations. It's not really a practical problem unless you have a fetish for fire-bottles.

Most SS amps have pretty low output impedance and I would agree. Not true for tube amps due to their much higher output impedance.

No idea what the fire-bottle fetish is about, pretty sure I don't have it... smile.gif

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post #25 of 362 Old 11-07-2013, 02:12 PM - Thread Starter
 
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Originally Posted by mcnarus View Post

The makers of high-end (read: high-priced) amps would like you to believe that there's something the spec sheets aren't telling you, and that their 50 watts is somehow more powerful than some Japanese receiver's 50 watts. That's hooey.

 

Just to touch on this. I've seen some 50 watt amplifiers that have large stiff power supplies and can deliver high current. I don't think you can compare a 50 watt generic Japanese receiver that typically have very puny power supplies, that can't deliver high current, to some of these more robust units.

 

I don't think you can say that 50 watts is 50 watts - what if one amp has greater current x voltage to equal 50 watts?

 

Here is a Naim Nait 5si :

 

 

Look at the size of that transformer? You think a typical Japanese amp like an Onkyo at 60 watts is going to be able to deliver high current like this? 

 

If you look at Stereophile I'm sure you'll find these "low powered" amps on the surface that actually have current reserve in spades, and if you take off the top lid you'll see how robust they are. The same can't be said for an entry-level Onkyo or Denon.

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post #26 of 362 Old 11-07-2013, 02:31 PM
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Just to touch on this. I've seen some 50 watt amplifiers that have large stiff power supplies and can deliver high current. I don't think you can compare a 50 watt generic Japanese receiver that typically have very puny power supplies, that can't deliver high current, to some of these more robust units.
Sure you can. And when you do, you find that there are certain conditions when the better-built unit will outperform the other, but those conditions turn out to be few and far between in the real world. In the real world, when both amps are typically pumping out only a couple of watts most of the time, with only very brief transient peaks, the better power supply will make no audible difference.
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I don't think you can say that 50 watts is 50 watts - what if one amp has greater current x voltage to equal 50 watts?
No, 50 watts really is 50 watts, unless you live in some alternate universe. But just because an amp is specced at 50 watts doesn't mean it will perform identically to another 50-watt amp under all conditions. Partly that's because what's on the spec sheet is partially a marketing decision. And partly it's for technical reasons such as the ones you raise. But again, "under all conditions" covers far more ground than your home audio system ever will. The conditions under which that beefier power supply matter will probably never apply to you, unless you are trying to fill a barn with an underpowered amp.

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post #27 of 362 Old 11-07-2013, 02:34 PM
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No idea what the fire-bottle fetish is about, pretty sure I don't have it... smile.gif

Seems to me like you understood perfectly.
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post #28 of 362 Old 11-07-2013, 02:57 PM - Thread Starter
 
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Sure you can. And when you do, you find that there are certain conditions when the better-built unit will outperform the other, but those conditions turn out to be few and far between in the real world.


So you are saying that a big stiff power supply isn't necessary in the real world? What if a speaker has a low impedance?

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post #29 of 362 Old 11-07-2013, 03:03 PM
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Power = voltage * current. An amplifier with ten times the current output capability will still put out the same current as a "weaker" amp at the same power level. If you control the water flow at the tap to some modest value it does not matter if it is hooked to a garden hose or a fire hose.

Only a few times have I seen (as in measured and heard) benefits from a massive increase in power supply capacity, and the conditions were pretty different than what the average listener will ever see.

"After silence, that which best expresses the inexpressible, is music" - Aldous Huxley
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post #30 of 362 Old 11-07-2013, 03:50 PM
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So you are saying that a big stiff power supply isn't necessary in the real world? What if a speaker has a low impedance?
If you have a low-impedance speaker, then you need an amp capable of driving a low-impedance speakers. But an amp that claims to have a "big, stiff power supply" won't necessarily drive them any better than one that doesn't.

If you can't explain how it works, you can't say it doesn't.—The High-End Creed

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