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post #1 of 637 Old 08-13-2013, 05:51 AM - Thread Starter
 
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Besides distortion, what else can influence the quality of power? If 10 amplifiers deliver 100 watts at 0.1% distortion then can one say the power is equally clean?

Is it really all about distortion or the lack of it, or is there some other variable that influences how clean the power is?
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post #2 of 637 Old 08-13-2013, 06:32 AM
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There are many characteristics of an amplifier design ranging from damping factor to slew rate. The only two that matter in terms of audibility are the distortion and the frequency response. As long as an amplifier has a frequency response that stays within a couple of db of flat in the audible range and as long as it has THD of less then 1%, it won't color the sound audibly. Virtually all modern solid state HiFi amplifiers are "clean." They don't have any affect on sonics. Some inexpensive tube amps have distortion high enough or frequency response bad enough to be audible. For the most part "cleanliness" is not something you need to worry about.
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post #3 of 637 Old 08-13-2013, 09:23 AM
 
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So when people say power is power or watts are watts, that isn't far from the truth?
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post #4 of 637 Old 08-13-2013, 10:17 AM
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So when people say power is power or watts are watts, that isn't far from the truth?
To a first approximation, yes.

Watts really are watts. But that doesn't mean that a "40 watt" amp will perform exactly the same as any other amp rated at 40 watts. For one thing, a proper power spec isn't a number; it's a set of curves. One 40 watt amp may clip at 41 watts. Another may not clip till 80.

But very few speakers require more than 40 wpc for normal use. So in the real world, those two amps will perform the same in most instances.
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post #5 of 637 Old 08-13-2013, 10:21 AM
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Originally Posted by Heinrich S View Post

So when people say power is power or watts are watts, that isn't far from the truth?

As long as you are talking about these watts being used to drive an actual loudspeaker, then I would say yes. In the 100's of discussions about the "sound" of amplifiers you will always see the disclaimer "at the same power". If you are just talking about rated power, make sure you are comparing amplifiers that are rated the same way. Budget components are rated at 8 ohm loads only and the rated power is the maximum output at several percent distortion. Most high end amplifiers are rated at 8 ohms and as low as 1 or 2 ohms and can deliver many times the rated power at lower impedance at very low distortion.

A budget 100wpc AVR and a high end 100wpc amplifier may both be rated to deliver 100w into an 8 ohm resistive load both with acceptable distortion, but what happens when the impedance of an actual loudspeaker dips to 2 ohms may make them behave quite differently. The AVR may not be able to supply enough current / power at that impedance which may cause distortion / overheating where the amplifier may be able to deliver well over its rated 100w at very low distortion with no issue. Many will argue about this subject, but I have experienced this first hand with a "difficult" speaker load - this was my experience:

My friend was driving very large speakers with his high end AVR - it sounded good, but at high levels it would overheat and shut down. I connected my external amplifier with the same wattage rating 100wpc at 8 ohms (and 300wpc at 2 ohms!) to the preamp outputs of his AVR and the difference was dramatic. At very low levels they sounded the same, but when the volume was turned up (my friend liked it LOUD) it was obvious that the AVR was not able to drive the speakers the same as the external amplifier. The clarity and the bass were dramatically better with the amp and the speakers could be played for hours at this level and the amplifier would never shut down. He wouldn't let me take the amp home and within two weeks he had convinced me to sell it to him. Just my opinion.

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post #6 of 637 Old 08-13-2013, 10:43 AM
 
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Quote:
Originally Posted by mcnarus 
Watts really are watts. But that doesn't mean that a "40 watt" amp will perform exactly the same as any other amp rated at 40 watts.

If that's true then what about Class A power? Everywhere I read I hear that Class A is the best type of power. So Class A sounds the same as any other topology?
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post #7 of 637 Old 08-13-2013, 10:48 AM
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Quote:
Originally Posted by Heinrich S View Post

Quote:
Originally Posted by mcnarus 
Watts really are watts. But that doesn't mean that a "40 watt" amp will perform exactly the same as any other amp rated at 40 watts.

If that's true then what about Class A power? Everywhere I read I hear that Class A is the best type of power. So Class A sounds the same as any other topology?

First, you should know a little about what the term "Class A" means.

Look here:

http://en.wikipedia.org/wiki/Amplifier#Class_A

If you have any other questions, post them in this thread referencing this article.
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post #8 of 637 Old 08-13-2013, 10:50 AM
 
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Frpm Wikipedia :
Quote:
Class-A designs have largely been superseded by the more efficient designs for power amplifiers, though they remain popular with some hobbyists, mostly for their simplicity. There is a market for expensive high fidelity class-A amps considered a "cult item" amongst audiophiles[9] mainly for their absence of crossover distortion and reduced odd-harmonic and high-order harmonic distortion.

So it would appear Class A has advantages over Class AB.
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post #9 of 637 Old 08-13-2013, 11:28 AM
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So it would appear Class A has advantages over Class AB.
And vice versa, obviously. But in neither case are those advantages typically audible, all other things being equal.
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post #10 of 637 Old 08-13-2013, 11:29 AM
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Originally Posted by Heinrich S View Post

So it would appear Class A has advantages over Class AB.

The advantage / differences are there and can be seen on the test bench, but may or may not be audible depending on who you talk to. And at these very low levels of distortion, the source recording and other factors can make much more difference than the advantages of the Class A amplifier. Personally, I don't believe that pure Class A is worth the heat / power usage / cost penalty and that modern Class AB amplifiers are excellent and a very acceptable compromise between Class A and other amplifier designs (Class D, Class G, etc).

In my personal experience comparing an AVR to a system of high end separates, there will be more differences in the noise floor and other factors than audible distortion which is harder to compare / quantify. The only thing that I've experienced that may be attributable to distortion is a better sound stage and more detail ("air") - but I've only heard this on the best recordings. A poor source recording (background hiss, muddy, poor dynamics, etc) and the differences between the two amplifiers will be lost - personally, I am only be able to tell the difference with excellent / very quiet recordings.

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post #11 of 637 Old 08-13-2013, 11:46 AM
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Originally Posted by Heinrich S View Post

So it would appear Class A has advantages over Class AB.

Not really. Crossover distortion is just one type of distortion, and in a properly designed class AB amplifier it will be too soft to hear. The enormous disadvantage of Class A for power amps is the huge waste of energy, plus the accompanying heat.

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post #12 of 637 Old 08-13-2013, 11:53 AM
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Quote:
Originally Posted by Heinrich S View Post

Frpm Wikipedia :
Quote:
Class-A designs have largely been superseded by the more efficient designs for power amplifiers, though they remain popular with some hobbyists, mostly for their simplicity. There is a market for expensive high fidelity class-A amps considered a "cult item" amongst audiophiles[9] mainly for their absence of crossover distortion and reduced odd-harmonic and high-order harmonic distortion.

So it would appear Class A has advantages over Class AB.

So, you didn't bother to even look at the disadvantage list? There is only one, but its a biggie, and its such a biggie that it falsifies a large proportion of so-called class A amplifiers that are on the market today.

The idling power usage of true class A power amplifiers can be huge. It's such a problem that there are almost no true class A power amplifiers. Even the ones that are called class A are often insufficiently biased to operate in class A at at high power and with low load impedances.

In contrast the listed disadvantages of class AB amplifiers are for all practical purposes: solved.

As a result the balance of practical advantages goes to class AB, not class A.
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post #13 of 637 Old 08-14-2013, 01:46 AM - Thread Starter
 
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I see many receivers aren't able to provide much more power into 4 ohm loads compared to 8 ohm. AV receivers have puny power supplies compared to some integrated amplifiers that can supply a lot more current into low impedance loads, so I would imagine sound quality can improve in those cases.
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post #14 of 637 Old 08-14-2013, 02:14 AM
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It depends on whether or not you exceed the amp's ability to supply current to the speakers. Lower impedances draw more current. That current generates more heat. As long as the amp can keep up, all is well. The amplifier impedance handling ratings are based on full output power. Most of us rarely or ever use full rated power and, if we do, we don't for long enough to heat up the output stage of an amplifier. My advice is to try it and continue feeling the receiver to see if it is overheating. If it is, then you have a problem. If it isn't, all is well. So the answer isn't so simple. I operate a pair of 4 ohm speakers quite successfully with an AV receiver. I get no overheating at all.
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post #15 of 637 Old 08-14-2013, 03:00 AM
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Quote:
Originally Posted by OllieS View Post

I see many receivers aren't able to provide much more power into 4 ohm loads compared to 8 ohm. AV receivers have puny power supplies compared to some integrated amplifiers that can supply a lot more current into low impedance loads, so I would imagine sound quality can improve in those cases.

Imagine what you will, but audio gear does not require big power supplies to effectively amplify musical and dramatic audio signals because of the high crest factor that these signals always have. Old school equipment with big heavy power transformers and heat sinks so that they are able to amplify high powered pure sine wave signals on the test bench are basically a waste of iron aluminum in actual use.
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post #16 of 637 Old 08-14-2013, 04:28 AM - Thread Starter
 
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How do you know audio equipment does not need big power supplies? Isn't that a generalisation on your part?
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post #17 of 637 Old 08-14-2013, 05:00 AM
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Originally Posted by OllieS View Post

How do you know audio equipment does not need big power supplies?

I know what I know based on statistical analysis of real world audio signals, practical experience, and theoretical analysis of audio signals.

I've laid this information out here numerous times, so the question is - do I have zero credibility with you or has the analysis been over your head? ;-)
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Isn't that a generalisation on your part?

Of course both those points are generalizations. Again this concept may have no credibility with you, but some well-qualified generalizations are relevant and even true!

This line of thinking started with me about a decade back with a very practical observation. I had the opportunity to bench test about 10 well-known higher end power amps. I came up with a test suite that was composed of classic pure sine wave testing, complex signals that were like music, and actual recordings of music and voice. I noticed that while my test bench was straining to support the tests and the equipment was showing signs of strain during the sine-wave tests, everything skated right through the tests involving music-like signals and actual music. It dawned on me that this equipment was built to amplify sine waves but 100% of its daily use was music.
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post #18 of 637 Old 08-14-2013, 05:11 AM
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Originally Posted by FMW View Post

It depends on whether or not you exceed the amp's ability to supply current to the speakers. Lower impedances draw more current. That current generates more heat. As long as the amp can keep up, all is well. The amplifier impedance handling ratings are based on full output power.

The more relevant concept is that most audio gear basic power ratings are based on amplifying sine waves, which in fact forms about 0% of their actual use.

The ability of a power amp to handle low impedance loads is based on the beef in the power supply and heat sinks, as well as the amp's output devices. The power supply and heat sinks are heavily loaded by sine waves but lightly loaded by music and drama because their failure is based on overloads over relatively long time periods (seconds and minutes). The output devices are loaded about the same by every kind of signal because their failure takes place based on overload over relatively short periods of time (milliseconds and seconds).

One of the areas of technology that has continued to improve over the decades since the introduction of SS to audio in the 1970s is the current carrying capacity of transistors versus their size and cost. This sets the stage for economical amplifiers that are better matched to the needs of the actual signals we use amplifiers for, as opposed to the unrealistic ways that they are bench tested.
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Most of us rarely or ever use full rated power and, if we do, we don't for long enough to heat up the output stage of an amplifier. My advice is to try it and continue feeling the receiver to see if it is overheating. If it is, then you have a problem. If it isn't, all is well. So the answer isn't so simple. I operate a pair of 4 ohm speakers quite successfully with an AV receiver. I get no overheating at all.

IME most equipment overheating is due to poor ventilation, not speakers with impedance curves that dip below 4 ohms.
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post #19 of 637 Old 08-14-2013, 02:33 PM
 
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Originally Posted by arnyk 
The ability of a power amp to handle low impedance loads is based on the beef in the power supply and heat sinks, as well as the amp's output devices. The power supply and heat sinks are heavily loaded by sine waves but lightly loaded by music and drama because their failure is based on overloads over relatively long time periods (seconds and minutes). The output devices are loaded about the same by every kind of signal because their failure takes place based on overload over relatively short periods of time (milliseconds and seconds).

Okay, but if I have speakers like the B&W CM9's that have a minimum impedance of 3.41 ohms, then surely I'll need quite a beefy amplifier to drive them with? Or does that not matter because we're talking music signals?
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post #20 of 637 Old 08-14-2013, 05:03 PM
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Originally Posted by arnyk View Post

most equipment overheating is due to poor ventilation, not speakers with impedance curves that dip below 4 ohms.
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Originally Posted by Heinrich S View Post

Okay, but if I have speakers like the B&W CM9's that have a minimum impedance of 3.41 ohms, then surely I'll need quite a beefy amplifier to drive them with? Or does that not matter because we're talking music signals?

I agree that most equipment overheating is due to poor ventilation, but certainly not all.

The last paragraph of post 5 was an actual experience that I had - my friend's $1500 Sony ES AVR driving his huge "6 Ohm nominal" floor standing speakers (don't remember the brand) while sitting on an open shelf rack in an air conditioned home overheating and shutting down. We checked, and the AVR was even set to "4 Ohms". When set to "8 Ohms" it shut down almost immediately, when set to "4 Ohms" from a cold start it would run for nearly 30 minutes before shutting down (this is playing music quite loud). I'm sure this isn't the case for all AVRs, but this was supposed to be a "flagship" component.

Personally, I am very suspicious of any solid state amplifier which requires the load impedance to be selected - but many AVRs, even high-end, still have impedance selection. To me this indicates that for lower impedance loads these AVRs cannot supply enough current or handle the generated heat (or both) without configuring the circuit. I didn't love my friend's huge speakers, but they certainly proved a point to both of us - the amplifier stages in his modern AVR were no match for those in my 15 year old amplifier.

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post #21 of 637 Old 08-14-2013, 05:29 PM
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Okay, but if I have speakers like the B&W CM9's that have a minimum impedance of 3.41 ohms, then surely I'll need quite a beefy amplifier to drive them with? Or does that not matter because we're talking music signals?
Well first of all, 3.4 ohms isn't really that tough a load. They're 8 ohms nominal, so surely above 4 ohms for almost all of the audible spectrum. I would guess that the vast majority of AVRs out there could drive 2 of them to decent if not thunderous volume. Some of those AVRs might choke on a continuous sine wave into 3 ohms, but I doubt your musical taste runs in those directions.

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post #23 of 637 Old 08-15-2013, 03:07 AM
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Quote:
Originally Posted by OllieS View Post

I see many receivers aren't able to provide much more power into 4 ohm loads compared to 8 ohm. AV receivers have puny power supplies compared to some integrated amplifiers that can supply a lot more current into low impedance loads, so I would imagine sound quality can improve in those cases.

Expose a bare electrical wire from the wall socket and connect it to your speakers! Plenty of current there even though it doesn't weigh much..! (No, don't do this, kiddies)

I drive my speakers with a couple of small NuForce STA-100 class-D amps that precisely double their output from 8 ohms to 4 ohms. They don't weigh a ton, they don't get hot, and they don't waste 50% of their power. It's kind of like connecting your speakers directly to the power in the wall socket, only with a switch quickly flicking on and off to vary the power.

For me now the idea of a large transformer making an audible humming noise in my listening room is so last year.
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post #24 of 637 Old 08-15-2013, 04:30 AM
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Quote:
Originally Posted by Heinrich S View Post

Quote:
Originally Posted by arnyk 
The ability of a power amp to handle low impedance loads is based on the beef in the power supply and heat sinks, as well as the amp's output devices. The power supply and heat sinks are heavily loaded by sine waves but lightly loaded by music and drama because their failure is based on overloads over relatively long time periods (seconds and minutes). The output devices are loaded about the same by every kind of signal because their failure takes place based on overload over relatively short periods of time (milliseconds and seconds).

Okay, but if I have speakers like the B&W CM9's that have a minimum impedance of 3.41 ohms, then surely I'll need quite a beefy amplifier to drive them with? Or does that not matter because we're talking music signals?

I ran these speakers plus a subwoofer at reference levels with no problems, driven by a relatively light, cheap Yamha RX-V371b AVR (ca. $200 street price) with an old school power supply that still doesn't hum. ;-):



Compare that to:

"Impedance reaches a minimum of 4.39 ohms at 17.2 kHz and a phase angle of –49.17 degrees at 98 Hz."

From: http://www.hometheater.com/content/bampw-cm9-speaker-system-ht-labs-measures

That's about as close of a real world comparison as I can come to without running out and spending a $grand out of my own pocket for speakers I don't want.

Both speakers have their minimum impedance around 120 Hz which is more than twice the power line frequency, so the power supply caps get recharged by the power line at about the same rate.

Within reason the need for beefy amps is relieved by the relatively high crest factor of music. The crest factor of music is generally far more than 6 dB, which means that you can run 8 channels of music with a power supply and heat sinks that can only run 2 channels of sine waves.
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post #25 of 637 Old 08-15-2013, 06:03 AM - Thread Starter
 
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Originally Posted by arnyk 
Both speakers have their minimum impedance around 120 Hz which is more than twice the power line frequency, so the power supply caps get recharged by the power line at about the same rate.

What is the power line frequency?
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post #26 of 637 Old 08-15-2013, 06:07 AM - Thread Starter
 
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Quote:
Originally Posted by arnyk 
Within reason the need for beefy amps is relieved by the relatively high crest factor of music. The crest factor of music is generally far more than 6 dB, which means that you can run 8 channels of music with a power supply and heat sinks that can only run 2 channels of sine waves.

Okay, but if music has a high crest factor then that means that dynamically you do need high current for dynamic portions in music. In sudden bursts you might need a lot more power, like 10x or 100x the power. Like you said, music has a high crest factor but that doesn't mean you don't need a beefy amp.
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post #27 of 637 Old 08-15-2013, 07:00 AM
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Quote:
Originally Posted by OllieS View Post

Quote:
Originally Posted by arnyk 
Within reason the need for beefy amps is relieved by the relatively high crest factor of music. The crest factor of music is generally far more than 6 dB, which means that you can run 8 channels of music with a power supply and heat sinks that can only run 2 channels of sine waves.

Okay, but if music has a high crest factor then that means that dynamically you do need high current for dynamic portions in music. In sudden bursts you might need a lot more power, like 10x or 100x the power.

Agreed.
Quote:
Like you said, music has a high crest factor but that doesn't mean you don't need a beefy amp.

Short term power doesn't require a fraction of the beef that is required by steady state power.

Let me give you a real world example. Once upon a time I experimnted with using amps designed for car audio at home. The obvious sticking point is the need for a 12-15 volt power supply with the ability to supply lots of current in short bursts.

I found that this power supply was required to avoid overloading ans shut down with musical peaks.:



It costs $150, and it weighs over 20 pounds. It will put out 36 amps all day long.

Laster on I was able to power the same amplifier through the same music at the same loudness with the same speaker with one of these:



Tiny, costs under $20, weighs less than two pounds, and has very fast acting current limiting that will shut it down in a heartbeat if you try to draw too much current.

Magic?

No. But I didn't tell you the whole story. I also added one of these:



Actually, these are far larger than the one I use (500,000 uF versus the 40,000 uF)..

The stiffening cap about matched the weight and cost of the $20 power supply above, but my system cost and weight was still only a tiny fraction of the lab power supply I used at first. Furthermore no way would the smaller lighter system handle pure sine waves but that was not what I wanted to listen to. BTW, the speakers I was using to audition and test were KEF Q10s that have a broad region with about 3 ohms in their impedance curve..
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post #28 of 637 Old 08-15-2013, 08:26 AM - Thread Starter
 
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Sorry Arny, what is the power line frequency you were talking about? I don't know what that means.
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post #29 of 637 Old 08-15-2013, 09:07 AM
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Originally Posted by Heinrich S View Post

Okay, but if I have speakers like the B&W CM9's that have a minimum impedance of 3.41 ohms, then surely I'll need quite a beefy amplifier to drive them with? Or does that not matter because we're talking music signals?

Not necessarily. It would depend on way too many factors. B&W is pretty good about being honest in their nominal impedance ratings. You can consider your speakers 8 ohms for purposes of amplifying them. Impedance varies with frequency for all speakers.
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post #30 of 637 Old 08-15-2013, 09:23 AM
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Sorry Arny, what is the power line frequency you were talking about? I don't know what that means.

The power line frequency is 50 Hz in most of the world and 60 Hz in the US. The way power supplies are built, this leads to power being delivered to the filter capacitors in audio gear either 120 or 100 times a second. If the signal being amplified is below the power line frequencies, the power to amplify the signal has to come from the filter capacitors, and there is more benefit to having large capacitors in the equipment power supply.

If you have a subwoofer that is crossed over at a frequency that is higher than the power line frequency, then the need for large power supply filter capacitors in the AVR is vastly reduced because only the subwoofer amplifier has to deliver signals at or below the power line frequency.
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