How To Measure "Current" In An Amplifier?


.
I've heard lots of times that an amplifier needs lots of "current" to drive a low impedance load.  
Is there any measurement on a spec sheet that would measure current?  
A high watts per channel amp does not necessarily mean that the amp has high current.
.
Convert?fit=crop&h=128&policy=eyjlehbpcnkioje1mda4ntezmtasimnhbgwiolsicmvhzcisimnvbnzlcnqixx0%3d&rotate=exif&signature=b8cf1350cfef75968b2ff97f239f2d512c6eebfcb3abd6beb94c7e8c88fc2813&w=128mitch4t
What is probably the best indication of an amp's ability to drive low impedances and/or impedances that have highly capacitive phase angles at some frequencies (which would also increase the amount of current that has to be supplied) is if the amp has a continuous power rating into 4 ohms that is twice its rating into 8 ohms, and if it has a rating into 2 ohms (if that rating is provided or is indicated in the measurements section of published reviews) that equals or approaches twice its rating into 4 ohms.

Current specs that may be provided in manufacturer literature should be ignored in most cases. They usually represent how much current can be supplied into a dead short (zero ohms) for some unspecified number of milliseconds (thousandths of a second). And according to a post I recall that was made a while back by Atmasphere in another thread on this topic, that spec is commonly not even based on the short being placed at the output of the amp, but at the output of the power supply instead.

Regards,
-- Al
Mitch, @almarg is pretty much dead on.

There are a couple of other indicators. A square wave into a simulated speaker load, but that's usually only provided by Stereophile, as well as the damping factor. A high damping factor is an indicator of low output impedance, and, by inference, high current capabilities.

But these really are just indicators.

My personal view is that very few speakers justify being designed to be so difficult.  Electrostatics one of the few exceptions. Otherwise, I'd say steer clear of "discerning" or difficult to drive speakers as they usually have some other underlying issue which a simpler design would avoid.

Best,


Erik
Clarification!

Sorry, I forgot.  Yamaha used to, and may still, provide square wave signals in their manuals, indicating very good frequency and current capabilities into difficult loads.

Others may as well, but in the consumer market I haven't seen any manufacturer provide that level of performance detail.

Best,

Erik
Erik, re: the last paragraph of your last post, one could argue that other considerations mandated speaker designs with such an impedance, and that one might rather just avoid amplifiers that weren't robust enough to handle the task at hand.
@unsound

Yes, one could argue it. :) That is not a position I ascribe to very often any more though.

I can see a variety of choices in drivers, and integration between them mandating certain impedance profiles, but once I saw crossovers deliberately "juiced" to be low impedance, I stopped believing in them. That is, I stopped believing that these low impedance designs were in fact the best the manufacturer could do, but that instead they were making speakers deliberately for the crowd that feels a speaker that can tell the difference between a low and high current amplifier was more "revealing" and therefore better for music listening, which is nonsense. In fact, the less "revealing" speaker (i.e. higher impedance) design would play better under all circumstances and was only a better speaker if you wanted to hear amplifier differences.

Best,


Erik
The manual for the amp I have (Musical Fidelity M6si) indicates a current output of "45 Amps peak to peak". I get the "45 amps" portion of that, but I'm not entirely sure what the "peak to peak" is meant to signify.

Almarg, Erik I'd be appreciate of your interpretation of that.

Thanks.
Hi gdhal,

If you imagine a sine wave, you can measure it as peak to peak or RMS.  P-to-P is just that, looking at an oscilloscipe, you look at the peak and valley and measure the difference. RMS (root means squared) tries to calculate the area under the curve, and is what amplifier watt ratings are usually based on.

So what the maker is saying is that if you had a sine wave, at maximum  you'd get 22.5 amps in the + direction, and 22.5 in the negative direction.

I'm not aware of any particular standard for measuring current, so this spec is just as good as any. :)

Of course, what matters in high capacitance loads is the current at the zero voltage. That's why you will read reviewers talk about phase angle. The worst conditions for an amplifier are high phase angle (high capacitance) along with low impedance.

Best,


Erik
A high watts per channel amp does not necessarily mean that the amp has high current.
This isn't actually true.

There is something known as Ohm's Law, which, unlike a speed limit, cannot be broken without the creation of an entirely new branch of physics. From it derives the Power Formula.

Power is equal to Ohms times the amount of current in the circuit, squared.
(P=RxIsquared, P is Power, R is Resistance and I is current)

So the fact of the matter is that high watts requires current, and the current cannot exist if the watts are not there. Let's take the example of 500 watts into 4 ohms. 500 = 4xI squared, solving for I we get 11.18 amps.

So why do we see really high 'current' ratings on amplifiers? Let's take a common rating, 80 amps and see what we get. Giving the amp the benefit of the doubt, we'll use a 1 ohm load so we get maximum current...

Solving for Power we get 6400 watts! Obviously there is something amiss.

What is amiss is that the 'current' rating is not a measure of the amplifier's output abilities. In fact it is the current created in a 10Millisecond period when a direct short across the power supply is made. The output section of the amp has nothing to do with it. To give you an idea of what this is about, we make an OTL that makes 220 watts and is not comfortable driving a 1 ohm load although it does fine into 4 ohms. Its 'current' rating is also 80 amps! This is because we have a substantial filter capacitor bank in the power supply and that filter bank will dump that much current when shorted out. It will make a really loud explosive sound when this is done as well- you don't want to be around **any** amplifier when the power supply is shorted out!!

This is one of the bigger myths in the audio amplifier world, part of the horse race for more power when finesse is not a concern. IOW, the manufacturer **could** express the capacitance of the supply (which is in essence what is happening here) in the form of Joules stored (Joules being the raw number of electrons stored, a combination of capacitance and voltage) which would give you a more accurate number, but the horse race for more power means that it gets stated in current instead, precisely because that is so easily conflated with the power output of the amp.

Its marketing.
Erik, yes I agree that some speaker designers might add components to the cross-over to purposely lower impedance, but not for the reasons suggested, but rather for (as succinctly found in paragraph one here):
http://www.stereophile.com/content/thiel-cs35-loudspeaker-measurements#7ti3uFpWxwLfT1ci.97

Thank you Erik. The difference between plus and minus peak certainly makes sense. But then given what your saying that there may not be (or at least you are not aware) of a "standard" for measurement, then I imagine it should be difficult to compare current specifications among different manufacturers. Hard to tell if a spec should be considered "good or bad".

In my case the entire output specs read 220 Watts per channel into 8 Ohms, Voltage 42 Volts RMS, 20Hz to 20 kHz (onset of clipping), 120 Volts peak to peak, Current 45 Amps peak to peak, Damping factor 180, Output devices 2 pairs per channel.

So would one conclude that given that current rating of 45, it can drive a low impedance load, or can't drive a low impedance load?
@unsound

Thanks so much for using a very specific example, it makes discussion a lot easier. The example you’ve shown for a Thiel is quite benign and an example of thoughtful design.

Please note the minimum impedance is around 4.5 ohms (by eyeball). The Thiel CS3.5 isn’t lowering the impedance to 3ish ohms, but rather it’s lowering what would otherwise be pronounced impedance peaks around 12 Ohms or much higher to be more "normal." This type of network, while drawing a little more power, makes the speaker very tube friendly, while maintaining good performance with solid state amps. The frequency response of tube amps often track the speaker impedance so by flattening the impedance above the woofer's resonance Thiel was making sure the speakers would behave similarly regardless of amplifier used. Good for Thiel for thinking of this in advance.

That’s not the kind of charlatanism I’m talking about though. In fact what I'm describing is the opposite, where impedance is brought to 3ish ohms or less, and reviewers, like buyers, are amazed that the speakers can sense such a difference in amplifiers. The Thiel CS3.5 on the other hand would play very nicely with a variety of amplifiers.

Best,

Erik
@gdhal

So low impedance = low ohms.

8 is really high impedance, 4 starts getting lower. What you want to look at is how well it drives 4 ohms. It should be close to double the 8 ohm spec. So, 220 x 2 = 440 Watts at 4 ohms.

But more importantly, what is your amp and what are your speakers? :)

Best,

Erik
Erik, can you give examples?:-)
It's hard for me to understand why a speaker designer would want to discourage potential sales just to make a point. 
Sadly, these impedance issues don't often discouarage sales.  Instead they are sold at a premium for being more "discriminating."  with the idea that somehow they will reveal more in the music.

I did a thorough electro-acoustical analysis of a Focal Profile 918. And by this I mean I measured each driver electrically and acoustically as well as reverse engineered and simulated the crossover.

To simplify it, perhaps too much, the bass section had an array of 8 power resistors (8 x 10 = 80 watts) to deliberately leave the midbass with a very low impedance (3 Ohms).

Replacing these resistors and associated parts with a coil left the complex frequency response identical but raised the impedance to similar amounts as the Thiel you mentioned, around 4.5 ohms.

Since then I've noticed a wide variety of Focal speakers, regardless of woofer or woofer count seem to share the same impedance profile, though not as blatant an approach.

Best,

Erik
@unsound

Take a look at my blog post about it, and scroll down to the "Bogus Woofer Cap" section. The original minimum impedance for this speaker was 2.6 Ohms.

http://pqltd.blogspot.com/2016/01/focal-profile-918-ultimate-upgrade-guide.html

There is 1 cap and 4 resistors that are simply completely unnecessary. They are the same type of conjugate network Thiel used, but instead of being for the buyer, are for the seller.  It's designed to waste up to 40 watts of power in an area where it's easy to hear.



Best,

Erik
Erik_Squires 9-26-16
... they were making speakers deliberately for the crowd that feels a speaker that can tell the difference between a low and high current amplifier was more "revealing" and therefore better for music listening, which is nonsense.
Excellent point. As I've said in a number of past threads, the ability of a component or system to resolve musical detail and its ability to resolve hardware differences are two different things, and may even be inversely correlated in some cases.

Gdhal 9-26-16
So would one conclude that given that current rating of 45, it [the Musical Fidelity M6si] can drive a low impedance load, or can't drive a low impedance load?
Hi Hal,

Per my earlier comment, as well as Ralph's (Atmasphere's) further explanation, that rating is essentially meaningless, and should be ignored. FWIW, though, I'll mention that for a sinusoidal waveform a current rating of 45 amps peak-to-peak corresponds in the RMS terms that are more commonly used to about (45/2) x 0.707 = 15.9 amps.

I have no particular knowledge of how well the Musical Fidelity M6si would do with a speaker having difficult impedance characteristics. But despite the manufacturer's statement that its "combination of high power, high current and extraordinary stability means that it can drive any loudspeaker with ease," several factors suggest to me that it would not do particularly well with such a speaker, in terms of sonics. Those factors include: Lack of a specified 4 ohm power rating; its relatively light 36 pound weight ("relatively light" given its 220 watt/8 ohm power rating and that it is apparently a class AB amplifier, and also given that it includes a preamp, a phono stage, and a USB DAC), which suggests that it is not designed in an especially robust manner; and its ~$3K price, which seems modest given all of the functionality it provides.

Best regards,
-- Al
 
@unsound

You probably already know this, but take a look at my LM-1 speaker measurements. It is NOT tube friendly like the Thiel. Notice the 24 Ohm peak in the bass and 16 Ohm impedance peak where the woofer and tweeter response meet.

http://speakermakersjourney.blogspot.com/2016/05/lm-1-bookshelf-measurements.html

These peaks are what the Thiel crossover was trying to avoid.

On the other hand, a solid state design would play beautifully with the LM-1

Best,

Erik


Thanks Almarg. Understood. And Ralph's explanation was a bit "over-my-head". :)

Erik, in answer to your question, Musical Fidelity M6si amp and Golden Ear Triton One speakers. 
Ahhhh.....

The GE Tritons are electrically and acoustically complicated beasties and exactly the kind of speaker design I've learned to avoid. Make yourself happy of course, though.

As for the amp, I would say that since they are excluding 4 ohm measurements and damping factor they are not particularly proud of it's output into low impedances.

Not the best match, surely.



Erik


I would say that since they are excluding 4 ohm measurements and damping factor they are not particularly proud of it’s output into low impedances.
Erik, just as a point of info the M6si manual indicates a damping factor of 180.

I’ll mention also that once damping factor gets above say a few tens (in other words, away from tube amp territory) I would not expect much if any correlation between damping factor and performance into low impedances.

Best regards,
-- Al

Al,

Probably true! :)

Erik
It's worth pointing out though that damping factor is not just one number. It can rise considerably in higher frequencies, where the Triton has oddly low impedance numbers.

AMT tweeters do not inherently have low impedance by the way. Cheap one's do. :)

Best,

Erik
Erik, MF does include the damping factor in their specs, and I posted it earlier, 180. As you do not care for the overall match between MF and GET, I'll ask your opinion then on what speaker you believe would match to the MF, and what amp you believe would match to the GET.

What I can tell you is I do have pretty good hearing, and am a member of an audio club on Long Island so I have the good fortune to listen to quite a few VERY expensive systems (components like Bryston top-of-the line amps, Alta Statement Tower speakers with an MSRP of 200K, Magnepan speakers, many other kinds of amps, etc. Many years ago (without disclosing my age :) ) while Eddie Layton (organist for NY Yankees and musician) was still alive, I would regularly listen to his then top-of-the line McIntosh setup (amp, preamp, speakers, source component, etc. all MAC), which at the time (late 70's early 80's) was valued at over 100K. So after listening to numerous systems throughout my lifetime I can tell you besides the fact that I'm rather pleased with the overall sonic performance of the MF and GET combination, I have yet to hear a better sounding combination of amp and speakers at their combined price point of 8K..
Erik, thanks for the link, interesting, and I really do like your writing style! Much of this is technically over my head. With that said, and playing Devil's advocate: might it be possible that in that links reference to the original cross-over design when coupled with an appropriate amp; that the idea was to strengthen bass response?

When one can get measurements like this:
http://www.stereophile.com/content/dunlavy-audio-labs-signature-sc-vi-loudspeaker-measurements#k4pSB...
does it really matter than it drops to below 3 Ohms? I would think that there are plenty of amps that wouldn't be too bothered by it.
@erik_squires
It is NOT tube friendly like the Thiel. Notice the 24 Ohm peak in the bass and 16 Ohm impedance peak where the woofer and tweeter response meet.

http://speakermakersjourney.blogspot.com/2016/05/lm-1-bookshelf-measurements.html

These peaks are what the Thiel crossover was trying to avoid.
Actually your speaker looks a lot more tube-friendly than the Thiel.

The issue here is 'will the amplifier produce a flat frequency response with the given load?" and if the tube amp has a damping factor of 20 or so, which is easily achieved, the answer in your case is 'yes'. In addition, it will do it with lower distortion and greater power than it will with the Thiel. This is simply because the Thiel requires the use of the 4-ohm tap of the amplifier driving it, and the 4-ohm tap is not as efficient as higher impedance taps, has less bandwidth and greater distortion.  Your impedance curves strike me as benign insofar as tubes are concerned.

The amps we make are low or zero feedback, but I would give them much better chances at driving your speakers than the Thiels on account of the impedance. Its more important for impedance to be high than it is to be flat.

Regarding your amplifier, the specs don't add up as presented so there is more to the story. The 45 amp figure seems to refer to output. Converted to RMS as Al did (so it makes more sense) the 15.9amps does not gel with 220 watts. Into 8 ohms at 220 watts the current is 5.24 amps. The 15.9 amp figure makes more sense if the load impedance is slightly less than 3 ohms. At this point the amp would be making a bit over 660 watts. Does that sound right to you?
Atmasphere, If I may: wouldn't that variable impedance load provide for a less linear frequency response and resulting timber distortion than the steadier impedance load of the Thiel via tube amplification?
@atmasphere Thanks for the clarification. If you don't mind, I may quote you when I recommend the kit. :)

Since I don't build tube amps, I'll defer to your mastery of the subject.

By the way, the LM-1 is a free-to-build kit in honor of my hero, Dr. Marshall Leach Jr.  All the design docs are online and I make no money at all when someone builds the kit.


Best,


Erik
HELP!
I was following this thread, I thought, pretty well.
But, Damping factor came into play.
I read a Wikipedia page explaining it, and it sounded like a high damping factor allowed an amp to control a speaker cone better than a low one.
So, what makes having a high damping factor bad?
Sorry if this is something obvious...
Bob
Atmasphere, If I may: wouldn't that variable impedance load provide for a less linear frequency response and resulting timber distortion than the steadier impedance load of the Thiel via tube amplification?
You may! :)

The answer though is 'no'. The variable is how much feedback the amp has, not whether its tube or not.

Its not going to take a lot of feedback to get the voltage response needed for flat output on Erik's speaker. My guess in looking at the curves is that about 6 db is all that's needed. Most tube amps with feedback have more than that.
Damping factor is usually measured as (8 Ohms / output impedance). So an amp with 100 df has an output impedance of 0.08 ohms. 20 has around 0.4 Ohms.

The higher the apparent output impedance (i.e. the lower the damping factor) the more the Voltage response will track the speaker impedance chart, instead of reamining flat.

By the way, adding feedback essentially improves damping factor. The normal way to improve df without feedback is to use massive banks of parallel output stages.

Best,


Erik
Atmasphere, I don't understand. What does feedback have to do with linear power output into varying impedances? 

Very generally:

High DF = good = low output impedance.

Low DF = bad = high output impedance

Where did some one (maybe me) say high DF was bad? I may have meant to say high output impedance is bad.

@unsound Feedback improves DF = lowers output impedance = less variability with output voltage. :)

Output impedance can  be thought of as a resistor at the outputs of the amplifier. With a perfect amplifier, this resistor is zero, and the voltage at the speaker is always the voltage at the amp.

However, amplifiers are not perfect.  Batteries are like this too. Looking at a AA battery, you model it as a 1.5V source, but in reality it has a rather large output resistance. So whether your flashlight sees 1V or 1.3V has to do with how much current is drawn and how big that output impedance is.

The more old-school your flashlight, the more current, and the more V will be lost in the output impedance. But modern LED bulbs sip current, so most of the batterie's V will appear at the LED terminals.

Confused yet? :)

Erik
^Perhaps I should rephrase the question:
What does feedback have to do with linear powerput into varying impedances and accompanying variable sensitivity?

@unsound

With very high output impedance the frequency response of the amplifier looks like the speaker impedance. I.e. not flat! :)

With very low output impedance the frequency response of the amplifier stays flat.

The missing step:

Increasing feedback reduces the amplifier output impedance. That and lowering distortion are why most amplifiers have at least some feedback incorporated into the design.
@unsound

Take a look at the top chart from Stereophile:

http://www.stereophile.com/content/conrad-johnson-lp125m-monoblock-power-amplifier-measurements#ETgE...

Notice the squiggly black line (called grey in the text)? That’s what happens when an amp with high output impedance drives the varying load a speaker presents. The frequency response of the amplifier goes up where the impedance of the speaker goes up, and down where the speaker impedance goes down.

Compare this to an amplifier with very low output impedance:

http://www.stereophile.com/content/theta-digital-prometheus-monoblock-power-amplifier-measurements#Z...

There is still some variability, but it's MUCH less than +- 1dB. :)

Mind you, I make NO claims one amp is better than the other. :) This is a pure specmanship so far. I am not sure given the chance to own either which I would prefer.

Best,


Erik
Erik, I wasn't quoting you saying High DF was bad, only referring to Almarg's response that a high DF would wreak havoc on tube amps.
Though if you could elaborate why, I would appreciate it.
Thanks for your replies, but I am still a bit confused.
Ohms, watts, and voltage, I get, but this Damping factor still kind of eludes me.
OK, but even with that feedback, unlike many ss amps, the vast majority of tube amps can't double down/halve up with varying speaker impedance and the corresponding varying sensitivity, and I would assume therein deviate from linear frequency response and ergo(?) distort timber, no?

@unsound That doubling/halving is exactly dependent on output impedance. :)

So, more feedback = less deviation = less aparent output impedance. :)

And, that's what you see in the two charts. The tube amp has more frequency dependent variability than the solid state amp.

This is the sort of thing you get into when you think about speakers that are more or less tube friendly. :)  The higher, and flatter the impedance of the speaker, the flatter the amplifier's output.

Best,


Erik
Again, I defer to @atmasphere 's experience. If he says my speaker kit is more tube friendly than the Thiel CS, who am I to argue? :)
Erik, saw your last post as I was posting. It would seem to me that we are in agreement then:-)
OK, but even with that feedback, unlike many ss amps, the vast majority of tube amps can't double down/halve up with varying speaker impedance and the corresponding varying sensitivity, and I would assume therein deviate from linear frequency response and ergo(?) distort timber, no?
No. What happens, and this is the part where we see that tube amplifier power is more expensive, is that the amplifier makes its full power into the lower impedances, and makes less than that (for example, 1/2 power into 8 ohms as opposed to 4 ohms) into higher impedances.

With a solid state amp, we are used to seeing it double power as impedance is halved. The difference here is that a tube amp is going to make its normal power into the **lowest** impedance, while the transistor amp is going to make its normal power into the **8 ohm** impedance.

I can see how this could be really confusing, so the other way to look at it is that in both cases the amplifier has to act like a voltage amplifier, which is to say that it will be constant power into all frequencies presented. Since the tube amp can't double its power as impedance is halved, instead it cuts its power in half as impedance is doubled.

Either way you get flat frequency response.

The real question is, 'Is flat frequency response important to the human ear?' which is another question entirely and the answer is 'sometimes'. The problem is that the ear converts any form of distortion into some form of tonality. The brain has tipping points where if there is enough distortion it will favor that sort of tonality over actual frequency response (whether flat or not). This is why some amps can sound bright and others not even though on the bench they measure exactly the same frequency response. The brightness can be caused by a tiny amount of higher ordered harmonic distortion or by IMD.

This is why speaker/amplifier matching is still a thing, even though speakers are **supposedly** voltage-driven! IOW you still have to take it home and see how it sounds, even after all the measurements!
@atmasphere  "Regarding your amplifier, the specs don’t add up as presented so there is more to the story. The 45 amp figure seems to refer to output. Converted to RMS as Al did (so it makes more sense) the 15.9amps does not gel with 220 watts. Into 8 ohms at 220 watts the current is 5.24 amps. The 15.9 amp figure makes more sense if the load impedance is slightly less than 3 ohms. At this point the amp would be making a bit over 660 watts. Does that sound right to you? "

I agree at face value the numbers don’t add up. I have sent a question to Musical Fidelity, but wouldn’t hold my breath for a response.

To your point "IOW you still have to take it home and see how it sounds, even after all the measurements!"

Yes, I wholeheartedly agree! This is kind of what I was trying to convey to Erik earlier in this post. What remains so puzzling, amazing and interesting to me after so many years in this hobby and "knowing" so much LESS than so many members in this thread is how after all the reading is said and done, and the actual listening begins, how the ears belie so much of the experience one gathers during the reading of these threads.

As the Grateful Dead would say, "if you get confused, listen to the music play".
Atmasphere, Yes, confusing indeed.
Your saying that tube amps halve their power as impedance is doubled? Isn't that what ss amps do?
Your saying that tube amps halve their power as impedance is doubled? Isn't that what ss amps do?
Yes, it is.

Maybe the way to look at this is the 8 ohm power. With a tube amp that is 35 watts, it will also be 35 watts into 4 ohms, and if the impedance of the load is double will then be 17 watts.

With a solid state amp that makes 35 watts into 8 ohms, its output into 4 ohms might be 70 watts, so 35 watts into 8.

This is what I meant about tube amplifier power being more expensive.
Gdnrbob 9-26-16
I wasn’t quoting you saying High DF was bad, only referring to Almarg’s response that a high DF would wreak havoc on tube amps.
Hi Bob,

No, I didn’t say that. Or at least I didn’t mean that :-)

I suspect you are referring to this statement:
I’ll mention also that once damping factor gets above say a few tens (in other words, away from tube amp territory) I would not expect much if any correlation between damping factor and performance into low impedances.
Tube amps often have damping factors in the single digits (i.e., less than 10), and in some cases even less than 1. The highest damping factor I can recall ever seeing for a tube amp was in the vicinity of 25 or so. While of course solid state amps commonly have damping factors in the hundreds, and sometimes even above 1000 (especially in the case of many class D amps).
I read a Wikipedia page explaining it, and it sounded like a high damping factor allowed an amp to control a speaker cone better than a low one.
So, what makes having a high damping factor bad?
Speakers vary widely in the amount of damping that is optimal, not only in terms of "control" but with respect to the tonal effects resulting from the interaction of their effective output impedance (which is inversely proportional to damping factor, as Erik indicated) with the speaker’s variations of impedance over the frequency range.

The majority of speakers are of course designed these days with the expectation that they will be used with solid state amplifiers, and those speakers will perform well with amps having relatively high damping factors. However once the damping factor gets above a few tens (arguably as little as 20, but 50 or 75 so would seem to be a safe ballpark) further increases in damping factor will **in themselves** make little or no difference in terms of both control and tonality. I say that despite beliefs to the contrary that are prevalent among many audiophiles, and claims to the contrary that can be found in a lot of marketing literature.

Many other speakers, though, are equally happy or even more happy with tube amps having low damping factors. It depends on the intention of the designer, as Ralph has said in a number of past threads. And some speakers are equally happy with tube and solid state amps, and with high or low damping factors. In those cases the intrinsic sonic character of the particular amp can be determinative of which is most optimal.

So how can a high damping factor be bad?

1)It may be a poor match for the particular speaker, the result being too much damping and "control," and/or tonal balances that were not intended by the designer.

As Erik indicated, a low damping factor, corresponding to a high output impedance, will result in output voltages from an amp that may vary significantly as a function of a speaker’s impedance variations over the frequency range. However some speakers will sound best when that variation **is** significant, i.e., when the variation of amp output as a function of load impedance variations results in power being held closer to constant than voltage. See Ralph’s paper on that subject.

2)High damping factors (and also very low total harmonic distortion) are accomplished in some designs by means of heavy-handed application of feedback, which can result in objectionable amounts of transient intermodulation distortion (that is not normally specified, and for which measurement standards do not exist), and/or increases in certain harmonic distortion components that are particularly offensive (even though **total** harmonic distortion may be reduced).

Hope that clarifies more than it confuses :-)

Best regards,
-- Al

Atmasphere, forgive me but I'm still confused:-). Are you saying for instance that:
a ss amp rated as 100 Watts into 8 Ohms = 200 Watts into 4 Ohms and 50 Watts into 16 Ohms.
and
a tube amp rated as 100 Watts into 8 Ohms = 100 Watts into 4 Ohms and 50 Watts into 16 Ohms.
and that's why tube amps are more expensive?
Using the previously posted example (fig.4):
http://www.stereophile.com/content/conrad-johnson-lp125m-monoblock-power-amplifier-measurements#xxGh...
adds to the confusion.
Very high Damping Factor doesn't make much sense since most of the speaker's impedance is resistive.  For 8 ohm speaker it would be about 6 ohms.  This resistance limits actual DF to 1.3
In order to prevent amp from making it worse 0.8 ohm output impedance would be fine (10% change).

Max current specification may suggest amplifier current limit but it doesn't specify the load impedance and doesn't say what happens with supply voltage.  Even amplifier with very weak power supply might be able to deliver high momentary short circuit current.
AFAIK speaker impedance usually dips around few hundred hertz (first crossover frequency).  Music on the other hand carries the biggest voltage peaks at the lowest frequencies.
@almarg and @atmasphere

These reviews indicate the Musical Fidelity can do 440 watts into 4 ohms and 800 watts into 2 ohms. I’m not stating that is or is not true, but IF true, would that support the 45 peak-to-peak amperage claim (or the RMS conversion to 15.9)? And, to the point of this thread, that it can "easily" drive a low impedance speaker?

http://www.goodsound.com/index.php/component/content/article?id=153:m

http://www.stereofil.no/stereo/integrerte-forsterkere/musical-fidelity-m6si