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post #271 of 637 Old 10-30-2013, 08:56 AM
 
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post #272 of 637 Old 10-30-2013, 09:30 AM
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Originally Posted by Shaun B View Post

Just recently joined after reading through this interesting thread. I have a few questions though.

I had a Krell KSA200s with power/bias indicators. Driving large 1.5 long ribbon at moderate sound levels easily pushed the amp easily into full 200 class A mode. That despite the fact that the ribbon had a flat impedance of 5 ohm.  I would guess the sound level was about 10 watt.

Why would the amp bias itself to such high levels if the peak power demand were so low?  Would it have been safe to use a far less powerful amplifier in this case? Or are the bias/power indicators not accurate or reliable?

I've found at least one other post that seems to question the operation of the Class A mode indicators such as this one:

http://www.diyaudio.com/forums/markaudio/153141-marks-new-alpair-12-a-6.html

"On peak occasions my Krell KSA200 biasing indicators lit up to 200 watt class A biasing without playing excessively loud into my ribbon speakers."

I find no independent confirmation of the correctness of its operation.

I am prone to question the reliability of the indicator's operation.
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post #273 of 637 Old 10-30-2013, 09:57 AM
 
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I find no independent confirmation of the correctness of its operation.

I am prone to question the reliability of the indicator's operation.

 

So it could be reporting something that isn't really there?

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post #274 of 637 Old 10-30-2013, 10:37 AM
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Originally Posted by Shaun B View Post

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I find no independent confirmation of the correctness of its operation.
I am prone to question the reliability of the indicator's operation.

So it could be reporting something that isn't really there?

Could be.
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post #275 of 637 Old 10-30-2013, 10:56 AM
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Quote:
Originally Posted by Shaun B View Post

So it could be reporting something that isn't really there?

You were asking about using a smaller amp. The typical 100 watt per channel amp would have only 3 db less overhead so the difference between the two in terms of a clipping level wouldn't be all that much. In other words, if 200 watts per channel handles things, 100 watts per channel are likely to be able to handle it as well.
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post #276 of 637 Old 10-30-2013, 11:03 AM
 
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You were asking about using a smaller amp. The typical 100 watt per channel amp would have only 3 db less overhead so the difference between the two in terms of a clipping level wouldn't be all that much. In other words, if 200 watts per channel handles things, 100 watts per channel are likely to be able to handle it as well.

 

More concerned about the bias/power indicators than anything else - if they're reporting something false then that would give me pause to worry. I thought those types of indicators were generally accurate, but perhaps they're not.

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post #277 of 637 Old 10-30-2013, 07:41 PM
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Originally Posted by Shaun B View Post

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You were asking about using a smaller amp. The typical 100 watt per channel amp would have only 3 db less overhead so the difference between the two in terms of a clipping level wouldn't be all that much. In other words, if 200 watts per channel handles things, 100 watts per channel are likely to be able to handle it as well.

More concerned about the bias/power indicators than anything else - if they're reporting something false then that would give me pause to worry. I thought those types of indicators were generally accurate, but perhaps they're not.

Frankly they may have no practical purpose other than eyewash. A good amp like the Krell should sound equally good whether it is running Class A or Class AB.
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post #278 of 637 Old 10-31-2013, 01:02 AM
 
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Frankly they may have no practical purpose other than eyewash. A good amp like the Krell should sound equally good whether it is running Class A or Class AB.

 

Why would Class AB sound the same as Class A? Surely Class A has benefits over AB, otherwise what is the point? better?

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post #279 of 637 Old 10-31-2013, 04:14 AM
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Originally Posted by Shaun B View Post

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Frankly they may have no practical purpose other than eyewash. A good amp like the Krell should sound equally good whether it is running Class A or Class AB.

Why would Class AB sound the same as Class A?

As long as distortions of all kinds are low enough, an amp adds (or subtracts) nothing audible to the signal it amplifies. The amp will make slight changes to the signal, but the changes are too small to hear.

Class AB done well is more than sufficient to meet that goal. It is not the least unusual for class AB amplfiiers to have far less than 0.01% distortion, which is ten times or more less than what the ear can detect under the most ideal of possible conditions.

Class AB dates back to the late 1920s or early 1930s when push-pull (tubed) amplifiers were invented. Needless to say this technology has been sufficiently developed in the intervening 80-90 years.

It is ironic that during the heyday of tubed amplifiers (1950s-1960s), single ended Class A amplifiers were relegated to very cheap amplifiers such as those in cheap AM radios, low-powered phonographs, etc. Class AB push-pull amplifiers were reserved for higher end products. Single ended amplifiers were reintroduced to audio's high end marketplace when tubes fell out of the mainstream and became niche, boutique products. They hadn't been taken seriously for the previous 30-40 years.
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Surely Class A has benefits over AB, otherwise what is the point? better?

Class A wastes power and reduces the amount of undistorted power that can be obtained from a given power supply, set of output transistors or tubes, output transformers, heat sinks, etc. The wastage is a minimum of 2:1. That's why most so-called "Class A" amplifiers have some kind of mechanism to shift out of Class A to reach their upper power levels.

Class A is required for undistorted amplification from amplifiers that are single-ended or in other words, too small to warrant being push-pull.
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post #280 of 637 Old 10-31-2013, 10:09 AM
 
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I don't know how to argue with that. I've always been led to believe that Class A power was better quality than AB... if it's just a marketing trick and nothing more then I probably could have saved a lot of money!

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post #281 of 637 Old 10-31-2013, 10:14 AM
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How can you argue if you've only been led to believe? smile.gif

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post #282 of 637 Old 10-31-2013, 10:47 AM
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Quote:
Originally Posted by Shaun B View Post

I don't know how to argue with that. I've always been led to believe that Class A power was better quality than AB.

Everything Arny said is correct. If you don't want to bother learning what all that really means - and I don't blame you if you don't! - you can trust what Arny says more than all the hi-fi writers and reviewers combined. In this case "better" is about distortion low enough to not be heard. Modern class AB power amps easily achieve this.

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post #283 of 637 Old 10-31-2013, 10:59 AM
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Originally Posted by Ethan Winer View Post
 
Quote:
Originally Posted by Shaun B View Post

I don't know how to argue with that. I've always been led to believe that Class A power was better quality than AB.

Everything Arny said is correct. If you don't want to bother learning what all that really means - and I don't blame you if you don't! - you can trust what Arny says more than all the hi-fi writers and reviewers combined. In this case "better" is about distortion low enough to not be heard. Modern class AB power amps easily achieve this.

--Ethan

 

+1. (And from someone who still uses a decades-old 55 watt per channel Class A amp in his 'music system'). Arny forgot to mention one benefit of Class A amps: they double as very effective room heaters :)

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post #284 of 637 Old 10-31-2013, 11:37 AM
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Don't forget that class AB amps do operate in class A for small signals, only moving to class B at higher power levels to save heat and expense. They then transition in and out of the class A region during normal operation. Class A is used to eliminate, or at least greatly reduce, crossover distortion that happens as the signal changes polarity, and with limited negative feedback (and lower loop gain) in the design. (The pros and cons of feedback is another common debate topic, natch.) I bet many people with class AB amps operate in class A much of the time anyway and could not tell when the transition to class B happens -- it is not audible.

Old MacIntosh tube amps used to be class B, not sure about any other audio amplifier designs.

Eventually any amp will shift out of class A given a large enough signal or small enough load. There are practical and physical limits to everything.

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post #285 of 637 Old 10-31-2013, 02:14 PM
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Quote:
Originally Posted by Shaun B View Post

I don't know how to argue with that. I've always been led to believe that Class A power was better quality than AB... if it's just a marketing trick and nothing more then I probably could have saved a lot of money!

Yes, Class A it is classic marketing hype. It has such a nice ring to it. ;-)
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post #286 of 637 Old 10-31-2013, 07:00 PM
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Originally Posted by Shaun B View Post

So it could be reporting something that isn't really there?

The Owner's Manual describes the function in non techincal terms.

Apparently the amp uses what Krell dubs an "Anticipator" circuit to monitor the input signal and predicts bias needed to remain in Class A, then reacts accordingly.
Quote:
The KSA amplifiers utilize Sustained Plateau Biasing, a proprietary KRELL
design. The Bias display meter on the front panel of the amplifier displays
which level of bias the amplifier is utilizing. Unlike sliding bias designs,
Sustained Plateau Bias only changes level when the demand situation
relative to input and source material changes. The signal passes through a
section called an Anticipator circuit. Once the level of bias is determined by
the Anticipator circuit, it automatically biases the output ~f the amplifier
before the signal is passed through the output stage. The Anticipator circuit
is several times faster than the output stage itself. This technique insures
all signal is passed through the amplifier in pure Class A operation. If the
amplifier only requires a low level of Class A bias, the amplifier will bias itself
to use only the amount of Class A power necessary to the demand
parameters. Should the demand parameters exceed the bias level the
amplifier is presently working under, the amplifier will increase its bias to
the next level to allow all signal to come through in Class A operation. If the
demand parameters are less then the bias level indicates for more than 15-
20 seconds, the amplifier will drop to the next lowest bias level.
The bias levels are indicated by appropriate LEDs in the faceplate meter

Just because there is a knob doesn't mean you should turn it.
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post #287 of 637 Old 10-31-2013, 10:27 PM
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Originally Posted by whoaru99 View Post

The Owner's Manual describes the function in non techincal terms.

Apparently the amp uses what Krell dubs an "Anticipator" circuit to monitor the input signal and predicts bias needed to remain in Class A, then reacts accordingly.

Of course one of the claims of Class A amps is that they have less crossover distortion, but good Class AB amplifiers have distortion + noise less than 0.005% which is so far below the threshold of audibility that it can be ignored. I thought that the other claim of Class A amps was a simpler circuit topology - that Krell circuitry certainly doesn't sound simple.

One question that I have is about crossover distortion is "fully balanced output" amplifiers or "bridged" amplifiers - where one amplifier drives the signal positive to the positive output terminal and another amplifier drives the signal negative to the negative output terminal - both amplifiers idle at zero and each drives half the voltage in opposite directions when amplifying a signal - this type of amplifier wouldn't have crossover distortion either - would it?

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post #288 of 637 Old 10-31-2013, 11:24 PM
 
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Originally Posted by whoaru99 View Post


The Owner's Manual describes the function in non techincal terms.

Apparently the amp uses what Krell dubs an "Anticipator" circuit to monitor the input signal and predicts bias needed to remain in Class A, then reacts accordingly.


But that doesn't mean the circuit is reliable? Otherwise why would the amp bias itself to such high levels if my peak SPL levels were so low? I was listening at moderate levels. Seems strange.

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post #289 of 637 Old 10-31-2013, 11:26 PM
 
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So what I've gathered reading this thread is that if you amplifier is not clipping, you don't need a bigger amplifier. But an amplifier can clip in moderate amounts, so wouldn't it still be useful to get a more powerful amp? Perhaps the clipping is not obviously audible, but takes on another character. Making sound compressed, or slightly hard.

 

Is that a valid reason to consider a more powerful amp?

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post #290 of 637 Old 11-01-2013, 04:40 AM
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Originally Posted by Shaun B View Post

So what I've gathered reading this thread is that if you amplifier is not clipping, you don't need a bigger amplifier.

Hold that thought!
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But an amplifier can clip in moderate amounts,

Meaning what? Clipping is an all-or-nothing thing. Either the signal is clipped or it isn't clipped.
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Perhaps the clipping is not obviously audible, but takes on another character. Making sound compressed, or slightly hard.

This of course can happen. But it does not violate the rule that clipping either is or it is not.

Clipping is 100% measurable and is one of the most obvious of all distortions when you are monitoring an amplifier with technical equipment.

Parts sufficient to add a highly effective clipping indicator to a power amp involves no more than 6 parts that run less than a dime a piece in single quantities. In typical high volume product production quantities adding them to AVRs might add a dollar or less in terms of parts and production costs.

One of my frustrations is that AFAIK no consumer power amps seem to have clipping indicators. They are common in amplifiers that are built for professional use which also generally perform as well if not better and cost far less per watt than consumer power amp "separates".
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Is that a valid reason to consider a more powerful amp?

The basic idea of your post, this strange and undocumented "Moderate clipping", seems to be a fictional creation. If you could explain it better, perhaps there would be something more to say, but at this point it seems to be an oxymoron.
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post #291 of 637 Old 11-01-2013, 04:59 AM
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Originally Posted by Shaun B View Post


But that doesn't mean the circuit is reliable? Otherwise why would the amp bias itself to such high levels if my peak SPL levels were so low? I was listening at moderate levels. Seems strange.

Based on re-reading your post you don't really know how much power was being used, nor is "moderate" listening level universal enough to get a reliable feel that way, and sensitivity of the speakers is also a key factor. So, to speculate there is some sort of problem with the bias indicators or the Anticipator circuit is a moot point.

Just because there is a knob doesn't mean you should turn it.
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post #292 of 637 Old 11-01-2013, 05:09 AM
 
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The basic idea of your post, this strange and undocumented "Moderate clipping", seems to be a fictional creation. If you could explain it better, perhaps there would be something more to say, but at this point it seems to be an oxymor

But not all amplifier clipping will result in gross audible distortion, surely? What if an amplifier clips, but makes sound a bit edgy or hard. It's clipping moderately, which would result in this character and not grossly, which would probably result in a horribly distorted sound. Is that not a scenario that can exist?

 

All I'm saying is that does an amp have to be full out grossly distortion in order for a more powerful amp to have merit, or is it possible for an amp to be distorting, in moderate amounts, that would still warrant the use of a more powerful amplifier under such conditions?

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post #293 of 637 Old 11-01-2013, 05:10 AM
 
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Originally Posted by whoaru99 View Post


Based on re-reading your post you don't really know how much power was being used, nor is "moderate" listening level universal enough to get a reliable feel that way, and sensitivity of the speakers is also a key factor. So, to speculate there is some sort of problem with the bias indicators or the Anticipator circuit is a moot point.


Exactly. I have no idea! I'm only going based on what the indicators suggest. Which is why I said I found it strange. If my listening levels are not exactly loud then there shouldn't be much activity going on.

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post #294 of 637 Old 11-01-2013, 05:21 AM
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Originally Posted by Shaun B View Post

So what I've gathered reading this thread is that if you amplifier is not clipping, you don't need a bigger amplifier. But an amplifier can clip in moderate amounts, so wouldn't it still be useful to get a more powerful amp? Perhaps the clipping is not obviously audible, but takes on another character. Making sound compressed, or slightly hard.

Is that a valid reason to consider a more powerful amp?

As Arny says, the amp is clipping or it is not clipping. Perhaps more to your point though, audibility threshold of clipping is a different matter than whether or not the amp is technically clipping.

I think there is a misconception about what clipping sounds like, as if the sound is going to cut out or some huge dramatic effect all of a sudden happens. The effect of clipping begins all but undetectibly except by instruments and gets more and more pronounced as the level of clipping increases. If you hear the symptoms you describe and it's attributable to clipping then, yes, considering a bigger amp might be appropriate. That said, it takes a much bigger amp to get you a meaningful improvement in that regard. I think if you don't at least double power you're just wasting your time. But, then, you may be up against other concerns like speaker capability.

Just because there is a knob doesn't mean you should turn it.
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post #295 of 637 Old 11-01-2013, 05:30 AM
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Originally Posted by Shaun B View Post


Exactly. I have no idea! I'm only going based on what the indicators suggest. Which is why I said I found it strange. If my listening levels are not exactly loud then there shouldn't be much activity going on.

I think you're not catching my drift. The indicators are most likely indicating properly according to the design intent but you're underestimating the demand, or at least the anticipated demand.

Again, there aren't enough facts to draw any truly meaningful conclusion.

Just because there is a knob doesn't mean you should turn it.
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post #296 of 637 Old 11-01-2013, 05:33 AM
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Originally Posted by arnyk View Post

One of my frustrations is that AFAIK no consumer power amps seem to have clipping indicators. They are common in amplifiers that are built for professional use which also generally perform as well if not better and cost far less per watt than consumer power amp "separates".

I haven't seen nearly as much equipment as you have, but I have seen quite a few with clipping / distortion indicators. Of the four different vendor's amps that I own right now, two have clipping / distortion indicators (Adcom and Bryston) and two do not (Outlaw / ATI and Parasound):

(from owners manuals)
Adcom GFA-7000 - Independent distortion LEDs for all 5 channels (indicating transient clipping - 1% THD)
Bryston 4BSST2 - Each channel has a LED indicator to monitor the following conditions:
Unlit - indicates channel has no power.
Red - indicates channel is muted (power-up)
Green - indicates channel operation is normal.
Flashing Red - indicates channel clipping.
Orange - indicates channel thermal shutdown.

I had to turn the volume to insane levels to even see a flicker (in the dark) on these and promptly turned the volume back down - but then I have always had fairly sensitive speakers.

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post #297 of 637 Old 11-01-2013, 05:58 AM
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A big issue in all of these sorts of discussions is that one persons moderate level is another persons insane level, and vice versa.

Just because there is a knob doesn't mean you should turn it.
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post #298 of 637 Old 11-01-2013, 06:14 AM
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Quote:
Originally Posted by Shaun B View Post

Quote:
The basic idea of your post, this strange and undocumented "Moderate clipping", seems to be a fictional creation. If you could explain it better, perhaps there would be something more to say, but at this point it seems to be an oxymor

But not all amplifier clipping will result in gross audible distortion, surely?

Agreed - the audibility of clipping "depends on". IME the two major things that the audibility of clipping "depends on" is the amount of the signal that is clipped off, and the percentage of the time the signal is clipped off. These two influences tend to go hand-in-hand due to the dynamic nature of music and drama.

http://www.peavey.com/support/technotes/concepts/clippingrevisited.cfm

"The audibility of clipping depends upon the degree."

"In general, the more complex the sound, the less audible the clipping. Light clipping of transients will in fact increase the apparent high-frequency content and give the sound a little more edge. One obvious exception is the human voice. While the voice is a very complex signal, our hearing is highly attuned to voice frequencies, and we have daily reminders of what undistorted speech sound"
Quote:
What if an amplifier clips, but makes sound a bit edgy or hard. It's clipping moderately, which would result in this character and not grossly, which would probably result in a horribly distorted sound. Is that not a scenario that can exist?

Of course that can happen.
Quote:
All I'm saying is that does an amp have to be full out grossly distortion in order for a more powerful amp to have merit, or is it possible for an amp to be distorting, in moderate amounts, that would still warrant the use of a more powerful amplifier under such conditions?

I think you've answered your own question. As long as we think that we are doing high fidelity, anything that audibly changes the signal is undesirable, and that includes "Light clipping of transients will in fact increase the apparent high-frequency content and give the sound a little more edge." The Hoi Palloi may prefer the music with the added edge, but a true audiophile will be repulsed by the artificial sound.

Light clipping is used with hypercompressed recordings to restore the sense of liveness that is lost due to the compression.

It has been my fundamental belief all along that any clipping is bad clipping, and my analyses have been based on that principle. Good SS amps are cheap enough that there's no economic need to tolerate clipping.
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Originally Posted by whoaru99 View Post

A big issue in all of these sorts of discussions is that one persons moderate level is another persons insane level, and vice versa.

Agreed and this issue is made more divisive by the relatively high percentage of the population that has Hyper-hearing or Hyperacusis.

http://www.aitinstitute.org/hypersensitive_hearing.htm

IME as an operator of live sound systems serving audiences with hundreds of people who tend to be vocal about their preferences (churches) is that the percentage of the population especially above 50 that has Hyper-hearing or Hyperacusis.is not small.

If you have this condition average levels of 80 dB might be fine, but 85 dB might cause serious discomfort. This contrasts with people with normal hearing where 80 dB might be their preferred listening level but a little more is merely louder and the threshold of actual pain is well above 110 dB.
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post #300 of 637 Old 11-01-2013, 06:30 AM
 
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I think you've answered your own question. As long as we think that we are doing high fidelity, anything that audibly changes the signal is undesirable, and that includes "Light clipping of transients will in fact increase the apparent high-frequency content and give the sound a little more edge." The Hoi Palloi may prefer the music with the added edge, but a true audiophile will be repulsed by the artificial sound. Light clipping is used with hypercompressed recordings to restore the sense of liveness that is lost due to the compression.

 

That's one of the reasons I didn't want to worry about it. I bought an amplifier that according to Stereophile, measures pretty well and can deliver more clean power into very low impedance loads. I probably have more power than I need, but I'm okay with that. I also always wanted a Krell, and I know they're pricier than other brands and models, but it's one of those desirable purchases, at least to me.

 

I do understand there are far cheaper alternatives out there.

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