Al, perhaps the EPDR article I referred to above might put a little gloss on what you said. The underlying thesis of the EPDR article is that a speaker's "nominal" impedance is further reduced if it is coupled with a negative phase angle at the particular frequency in question. So, if the "effective" impedance, or EPDR, is even lower, I can only assume that the "db delta" between a SS amp and a tube amp, even with a "low'ish" output impedance, would likely be greater. And I believe you surmise the same in your last post.
I think the take-a-way for us is that we may not be able to tell whether a particular amp/speaker combo will be a good match by looking at specs alone. But we should be able to identify (or at least be wary of) a combo that may be a bad match. Of course, if someone likes sonic coloration -- hey, all the power to sonic coloration.
Yeah ... I agree with the comments above, either the manufacturers and/or the dealers need to do a better job of helping customers understand this stuff before pushing product.
Thanks again Al. |
It's only cruel to an amplifier if it's not up to the task. |
I was just giving you guys a hard time. That doesn't come through in print very well. We get too serious sometimes.
Of course not every amp can handle the job when you've got a speaker that has an impedance of .5 ohms at 20khz. But you don't need a lot of power to reproduce most musical content at 20khz either. The highest fundamental on a guitar is less than 1khz. Overtones are very important but don't need a lot of power to reproduce.
Robert Deutsch had no problem with the 40wpc Prologue Premium Integrated and the ML Montis.
Here is what Martin Logan says in the manual for their current hybrids:
What size amplifier should I use?
We recommend an amplifier with 100 to 200 watts
per channel for most applications. Probably less would
be adequate for our smaller hybrids or when used in
home theater where a subwoofer is employed. Our
hybrid designs will perform well with either a tube or
transistorized amplifier, and will reveal the sonic character of either type. However, it is important that the
amplifier be stable operating into varying impedance
loads: an ideally stable amplifier will typically be able
to deliver nearly twice its rated wattage into 4 Ohms
and should again increase into 2 Ohms.
And:
Could you suggest a list of suitable electronics and
cables that would be ideal for Martin Logan speakers?
The area of electronics and cable choice is probably
the most common type of question that we receive. It
is also the most subjective. We have repeatedly found
that brands that work well in one setup will drive
someone else nuts in another. We use many brands
with great success. Again, we have no favorites; we
use electronics and cables quite interchangeably. We
would suggest listening to a number of brands—and
above all else— trust your ears. Dealers are always the
best source for information when purchasing additional
audio equipment.
So, you don't want an SET but you shouldn't be too worried about that .5 impedance either. ML did not say the amp has to double wpc with each halving of impedance, not even close. I do not think that a company that has lasted as long as ML has in the high-end world could get away with advising customers to use amps that make their speakers sound bad.
So, We need to be aware of specs but Trust your ears and Enjoy your music! |
Tom, you quote the ML manual as advising that "an ideally stable amplifier will typically be able to deliver nearly twice its rated wattage into 4 Ohms and should again increase into 2 Ohms."
My take-a-way is that ML recommends using a rock-solid stable SS/Voltage Paradigm amp, which as discussed above has a very low output impedance. Stated differently, the ML ESLs in question were likely voiced to be driven by a SS amp.
Hence, based on what Al explained, using a tube amp which has a "high'ish" output impedance will likely result in sonic coloration to some degree. As Al also said, some may not find this to be a problem and that's ok. :)
So ... after all that has been written, I think the best answer to the OP's Q is that if he/she really wants to use a tube amp to drive his/her ML ESLs, consider a low output impedance BEAST that can deliver serious current.
Frankly, I would be biased (pun) in favor of using a high quality/high power SS amp e.g., a Pass, Ayre, Bryston, inter alia.
Regards,
Bruce |
03-31-14: Tomcy6
Of course not every amp can handle the job when you've got a speaker that has an impedance of .5 ohms at 20khz. But you don't need a lot of power to reproduce most musical content at 20khz either. True, but if the amp cannot keep constant voltage at 20khz it will result in the treble being shelved down no matter what volume level you are listening to, resulting in a softened treble, some unexperienced might call this smoother, but their having themselves on. (as the sterophile test reports state) Quote Sterophile: " The shape of the impedance trace will result in the Montis's top octaves shelving down when the speaker is driven by a tube amplifier having a high source impedance. This is why Robert Deutsch found that his Audiopax amplifier sounded too soft and lacking in definition." Cheers George |
"Hence, based on what Al explained, using a tube amp which has a "high'ish" output impedance will likely result in sonic coloration to some degree."
Yep. The specs give you guidelines but cannot answer what sounds best in each case. The key part is "to some degree". The question is always which colorations and to what degree. The answer will be different likely in each persons case though the specs might indicate certain approaches to yield the best results overall. |
Its funny how there are always two ways to look at good sound.
1) technical
2) practical/case by case
Nothing guarantees each perspective will resulting the same choices/solution. Why should they each case is different even if for no other reason than room acoustics and how each person hears. Would we expect two people to choose the exact same pair of shoes? No. But we all kinda know what makes some shoes better than others. That's the technical perspective and the only one of global value. But then , the shoe gotta fit.... |
"In otherwords it will do the classic thing that can happen with low current or high output impedance tube amps "behave like a fixed tone control" instead of being flat into all loads."
That's a very interesting analogy!
I suppose, its a question of who is in charge, ie which component is going to dictate the parameters of the overall system that works best.
It makes sense that from the perspective of the amplifier or specifically the designer of a tube amplifier, they want their baby to be the one in charge.
The speaker designer trying to make a compact quality speaker with extended flat bass response that most might be willing to put in their rooms has a different perspective. The price paid in this case is more demand on the amplifier to help deliver the desired results.
Two different paradigms/ways of approaching things as Ralph likes to point out, each with unique strengths, weaknesses, and tradeoffs. Its good to be able to have the choice. |
Someone should sign up to do a "paradigm" shootout, ie the best of voltage versus the best of power paradigms. Of course, finding an unbiased doer or doers for something like this could be the hardest part.
I would volunteer someday, perhaps after I retire eventually, although I doubt I will have the financial resources needed to put into two complete and alternate SOTA systems that could both truly push things to their limits. :-) |
Or ... are you saying the relationship between an amp's output impedance and an ESL's varying impedance plot is simply not a relevant consideration to the ESL's sonic output because of some unique or different electrical characteristic peculiar to ESLs?? As I have pointed out before, with all ESLs, the impedance curve is not a function of a driver in a box. In addition, the impedance curve is also not the efficiency curve. IOW, the speaker has the same efficiency at all frequencies. Note that I used the word 'efficiency' as opposed to 'sensitivity'. I am restating my second paragraph above slightly, but if I had used 'sensitivity' what would be happening is that the impedance plot would also be a sensitivity plot. The reason is that the speaker's impedance plot is based on a capacitor. The result is that an amplifier that operates in the Voltage Paradigm will make too much power at high frequencies due to the low impedance, and usually not enough power at low frequencies for exactly the same reason. For this reason you will find that a lot of ML users that have solid state amps likely have them fairly close to the wall, which gets some bass reinforcement. Most ESLs, like any panel speaker, should be 5-6 feet from the wall. I think now you see why the ZERO is so useful when dealing with this speaker. But I will explain: if you use a solid state amplifier one of the really likely results is that while detailed, it will also be noticeably brighter than reality. I've experienced this first hand many times. Now if you use a Power Paradigm amplifier, as Al has pointed out, the 'output impedance' of the amp will interact with the low impedance of the speaker at high frequencies and you will experience a roll-off. This is why many tube amplifiers (in fact most tube amps) sound a little dark on that speaker if not outright rolled off. Those with negative feedback will of course sound less so (but of course negative feedback causes all amplifiers to sound bright due to trace amounts of odd ordered harmonics being introduced). Now if you just want things to sound natural with no roll off and no brightness, what to do?? That is where the ZERO is handy. It raises the impedance of the load enough that a tube amp can begin to operate within its constant power region. In this way you get high frequency speed and extension, while at the same time getting bass extension and impact. A long time ago ML made a speaker called the CLS I. Unlike the newer iterations, it had a more traditional ESL impedance plot (meaning it was higher impedance like the older Quads) and it was a breeze to play it with a tube amplifier. As we all know (or should know) transistor amps don't make a lot of power into higher impedances (+16 and +32 ohms), IOW it may well be that such an amp might only express about 1/4 of its 8 ohm power. It is for this reason that a 100-watt tube amp can keep up with a 400-watt transistor amp on Sound Labs BTW. Anyway, ML realized that if they were to sell speakers to the solid state market, which at the time was about 90% of the amplifier market, they were going to have to do something. The result was that succeeding speakers had much lower impedances, with the 0.5 ohm 20KHz impedance you now see. The ZERO corrects this issue so tubes can be practical on the speaker. |
You'd be far better off saving the $1140us for the Zero's and putting towards the right amp or speakers, instead of masking the problem with the Zero's, which itself has it's own set of problems.
Cheers George |
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I have been following this thread and feel itÂ’s time to post, seeing you have now called George a troll for the second time.
George has nothing to gain from these posts, as he does not build amps or speakers, AS YOU DO! He has presented nothing but facts, and left out statements of linked reviews you presented.
And as for my own experience with the Zero Autotransformer, I’ve tried them many times, and on a matched amp/speaker combo’s, they are a big step backward in sound quality IMO. They are as George puts it “a band-aid fix”, for people who have a mismatched amp/speaker combo that cannot be driven correctly - yet have no desire to change either to correct the mismatch and fix the underlying problem.
Sure, they can live with what they have using the Zero, but that will introduce another set of problems that then affect the sound quality. Is it worth $1140 for a band-aid fix??????? Or should they fix the mismatch problem, THAT is the real question???? |
I used Curcio modded Dynaco MK111's, on the stat panels, and solid state on the woofers, on ML Sequel 11's, with great results. |
I have been following this thread and feel itÂ’s time to post, seeing you have now called George a troll for the second time.
George has nothing to gain from these posts, as he does not build amps or speakers, AS YOU DO! He has presented nothing but facts, and left out statements of linked reviews you presented
And as for my own experience with the Zero Autotransformer - I’ve tried them many times, and on matched amp/speaker combo’s, they are a big step backward in sound quality. They are, as George puts it, “a bandaid fix”. For people who have a mismatched amp speaker combo that cannot be driven correctly, yet have no desire to change either and fix the mismatch correctly.
They can live with what they have using the Zero, but they will then introduce another set of problems that affect the sound quality. Is it worth $1140 for a bandaid fix??????? Or should they fix the mismatch problem, that is the real question???? |
I'm seeing mostly some well defended differences in perspective here. |
Tradeontheweb, the ZERO does not affect the sound quality as you and George suggest.
How it actually works is that many tube amps have problems on ML speakers. In such cases, they are not going to work very well. The ZERO solves that, plain and simple.
Nor is it a bandaid- it is however a problem solver and this is a classic case of the kind of problem it was designed to solve. And just FWIW, I don't make a dime when Mr. Speltz sells a set of ZEROs and he sells them to more than just customers of ours.
This thread was established to ask the question of what tube amps will be suitable for ML speakers. A few will work fine without the ZERO, but many will not, and the ZERO represents a practical solution so that any amp with enough power otherwise can do the job.
George keeps pulling the thread off-topic by suggesting that a powerful transistor amp be employed; he also said that my suggestion of the ZERO was 'stupid', and did so without any experience whatsoever with their use.
Its true that I sell amplifiers, and its also true that I have a lot of experience and feedback from people who use the ZERO. I hope its OK if I walk the talk. We can be pretty sure that George does not- he is just here so far as I can tell to derail the topic. That is one of the ways you can identify the activity of a troll.
Now if you think that I should not be posting here because I sell amplifiers, that's fine, again just FWIW audio happens to be a hobby of mine and I like it. That is the reason I am here- and I have tried my best to help out people whenever I have found that my experience was useful (I started my career in 1974 repairing consumer electronics at the local Allied Radio service facility, where I got a lot of troubleshooting experience repairing all manner of tape machines, car stereos, receivers, tuners, shortwave radios and the like- that was how I put myself through school.)
So to answer your question, In A Perfect World, of course its 'way better to start with a good match, in that way you and I totally agree. But here is a little experience tidbit for you: over the last 30 years or so of business, I have found that you can recommend that till you are blue in the face and people will still go right ahead and put an amp and speaker combo together where it is really obvious that it is not going to work, for the simple reason that they like the speaker, and they like the amp and they want both at the same time.
People wanting to put a tube amplifier on a ML ESL is an excellent example of that.
**That** is why the ZERO is handy- you can have your cake and eat it too, and it allows for excellent performance (for example the ZERO has bandwidth from 2Hz to about 2MHz, which is wider bandwidth than most amplifiers...). |
I recently auditioned a set of Manley Snappers with my ML Aeon i's and the combo with my ModWright SWL 9.0SE sounds absolutely superb! The Snappers are 100 WPC monoblocks that run in ultralinear mode - they are very tight and detailed in the lows and highs with no boom or grain and the midrange is to die for. They pair exceptionally well with the ModWright which is a very transparent tube-hybrid design. I have a fairly small-sized listening room and the Snappers have plenty of power to drive the Aeon i's - even for demanding rock and full orchestra pieces. Depending on your listening levels, I would highly recommend the Snappers as being a great match for MLs. |
Again two different perspectives on how to get a good amp/speaker match.
To me the point is zeros are the best solution for ML with many tube amps. If you match up amp and speakers directly, zeros not needed.
Perhaps an "adaptor" would be a better technical term for what the zeros are. They would make for some fairly pricy "band aids", though the argument could be made that an adapter is a technical form of "band aid". Its mostly just semantics, some more agitating to some than others. |
I certainly did not derail the topic Atmasphere, the OP asked for suggestions for tube amps to go with his Martin Logan speakers. I simply mentioned in my first post that they are a difficult load, no mention ever of him getting a SS amp ever, you are seeing things.
It was you Atmasphere that started spruiking on about the Zero from your first post, instead of guiding the OP correctly to a suitable amp.
It was also you Atmasphere that said, "that others said", (never implicating yourself) that the Zero is beneficial to all amps "even good solid state amps" into these kinds of loads.
You call me a troll twice now. Be careful Atmasphere, It almost looks like your shilling for the Zero, because your amps are the ones that need them the most, and visa versa, the Zero's need your amp's the most. as this quote from Paul Speltz from a TNT review of the Zero's
"Paul Speltz, a devotee of OTL amps, decided that he wanted to use his favourite DIY speakers with his OTL amp, but the nominal speaker impedance of 4 ohms wasn't a good match for his amp. With these thoughts in mind, Paul designed the ZERO Autoformer"
And then this quote from the TNT reviewer on the Zero's with amps that don't need them.
" The most important thing to remember about the ZERO Autoformers is: if there is no significant impedance mismatch, then there will be no real benefit to using the ZEROs. For example, when the Autoformers were connected as an impedance multiplier to a pair of B&W DM602 speakers there was little benefit for most tested amplifiers. In fact, the Le Amps and B&W combination sounded a little too bright with a strident sibilance that was distinctly irritating."
As I've always stated the Zero's add their own set of problems that can be clearly heard when the amp/speaker combo does not need them. (as the above reviewer found)
And to suggest they are beneficial for all amps into hard loads is an absolute misguided quote.
They are a bandaid fix for owners that do not want to change their amp or speakers so they are compatible.
Cheers George
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Capdek, glad to read that your Manley amp does the job. Just curious, do you know what the output impedance of the amp is? Does the amp have different taps? I ask about taps because you mention it's ultra-linear. |
Atmasphere, 04-03-14: "...George keeps pulling the thread off-topic by suggesting that a powerful transistor amp be employed; he also said that my suggestion of the ZERO was 'stupid', and did so without any experience whatsoever with their use...."
Georgelofi, 03-19-14:"...I have Ralph, and I stand by what I said, I have even put them on my system which definitely doesn't need them, and the sound took a big step backward...." |
Bifwynne, the Manley Snapper from the website looks as though it has 1 set of 8ohm speaker terminals?.
It states in the specs that it's damping factor is 4.7 (into 8ohms?) which would mean it's output impedance is around 1.7ohms. If it's a 4ohm tap which I doubt, then the output impedance is .85ohm.
Cheers George |
Atmasphere. I have indeed used the Zero's on many occasions, as I mentioned in my post. So my opinion IS based on experience, not on any wish to derail this thread, as you insinuate. My experience with them was as I indicated - yes, they allow amps that would otherwise struggle, to drive speakers that present too hard a load for them. An analogy would be saying that your standard 4 cylinder commuter car is capable of 200+mph. But only when towed by a Nascar ! Then claiming that there's no difference in the experience vs not being towed. By their very nature, the Zero's must add/subtract something from the sound. I'm sure their are many happy Zero owners out there. There must be, as they sell. But, I suspect those owners would be better off educating themselves, then spending the money on a better matching between components in the first place. I have yet to hear a system that benefitted (sound-wise) from their use. Your assertion that they "solve a problem" is correct, technically. But at the cost of sound quality. If sound quality is not a priority, then sure, buyers have indeed got their mismatched components to "work" together. But only that. The degradation of sound quality is obviously secondary to them. As a manufacturer, I would have thought your emphasis would be on making sure your amplifiers were matched with speakers that show them at their best ? Surely you're doing your products a disservice, by suggesting the Zero's as a bandaid panacea for buyers ? Just my 2c worth ! |
I could be very wrong or just confusing the Manley with VTL, but I seem to remember that they came with 5 Ohm outputs. Caveat, I seem to remember that my memory was once better, but I could be mistaken about that too.:-) |
Unsound: I just looked at the Snapper owners manual, and it states the output impedance is 1.5ohm, so the tap is less than 8ohms but more than true 4ohms.
This 1.5ohms output impedance was also measured and verified by Sound and Vision review of the Snapper.
Cheers George |
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Hi Unsound,
The Manley Snapper has a single output tap, which is described as having been optimized for a 5 ohm load. Its output impedance is specified as 1.5 ohms, as George indicated. Its specified damping factor of 4.7 is slightly inconsistent with that (8 ohms/4.7 = 1.7 ohms; 5 ohms/4.7 = 1.06 ohms), but is in the same rough ballpark.
You might be recalling having read something in which the author confused the output impedance of a tap with the load impedance the tap is optimized for. A not uncommon error.
Best regards,
-- Al |
Thanks Al. You are too generous, the error was probably all mine.:-) |
Almarg/Unsound/Bifwyne: Looking at these figures again the Snapper's speaker termianls are a 7ohm tap to be pedantic about it.
Cheers George |
George, if I may be a bit pedantic as well, although multiplying the 1.5 ohm specified and measured output impedance by the specified damping factor of 4.7 ohms equals 7 ohms, both the manual and this page at the Manley website clearly state that the output is optimized for a 5 ohm load. As I indicated the numbers are not quite consistent, but I would be inclined to think it more likely that the damping factor spec is inaccurate than the 5 ohm figure. In part because I note that the damping factor number does not appear in the manual, but the 5 ohm figure does. And also because maximum power ratings are provided only for 5 ohms and 8 ohms, not 7 ohms, with the 5 ohm capability being higher than the 8 ohm capability. Regards, -- Al |
Thanks George. I looked at the review and picked up the following concluding statement that "[t]he Manley's output impedance [1.5 ohms], while not unusually high for a tube amp, is still high compared with typical solid-state designs, and may therefore have some effect on the frequency response of the speaker-amp combination.—Thomas J. Norton"
The reason I asked the Q is because Capdek said that the Manley does a good job with his ML ESLs. I'm sure Capdek has good reason to be happy with his Manley/ML combo. But I suspect that the treble may be a tad shelved and the bass a bit augmented -- just based on what Thomas Norton said above.
Obviously, unless one measured the ML's frequency response in a controlled environment, it's difficult to know what's really going on with Capdek's rig. So, I'll leave it there. If Capdek is happy -- then that's all that matters.
This has been an interesting thread. I think all of us have leaned a lot. |
Correction to my previous post: delete the word "ohms" which appears immediately following the number "4.7"
Regards,
-- Al |
OK Almarg, I'll split hairs with you.
With a measured damping factor of 4.7 and an output impedance of 1.5ohms it's closer to an 8ohm tap than a 4ohm tap.
But all this can change with a bit more global feed back to raise the damping factor and lower the output impedance.
But then Audiophiles don't like the idea of too much feedback, so throw a Zero on it for difficult loads like these speakers the OP has and maybe be happy???
Cheers George |
George, where do you see an indication of a MEASURED damping factor of 4.7?
Regards,
-- Al |
From here Al. At the The Manley website. http://www.manley.com/msn.php Features and Specifications Manley Snapper Damping Factor: 4.7 Cheers George |
George, the 4.7 number appearing on that page is a specification, not a measurement. On the same Manley page an output impedance of 1.5 ohms is specified, which is confirmed by TJN's measurements in the review you linked to. On the same Manley page, and also in the manual, the statement "optimized for 5 ohms" appears. Maximum output power, input sensitivity, and frequency response are all specified for a 5 ohm load. The 5 ohm power rating is higher than the 8 ohm power rating, for the same distortion level.
I see no reason to assume that the 4.7 number is accurate and the 5 ohm number is not. As I said, the converse seems much more likely.
Regards,
-- Al |
All depends on which way you look at it Al.
Saying it's "optimised" for a 5ohm speaker does not mean it's a 5ohm tap.
But giving figures as exact as in 1/10th's, 4.7 for damping and 1.5ohm for output to me is far more exacting.
And from these two figures the tap is closer to an 8ohm tap than 4ohms.
Whatever the case, the way your seeing it, or me, it's not the ideal amp for what the OP has.
Cheers George |
With a measured damping factor of 4.7 and an output impedance of 1.5ohms it's closer to an 8ohm tap than a 4ohm tap.
But all this can change with a bit more global feed back to raise the damping factor and lower the output impedance. George, this may come as a surprise but increasing feedback does not affect output impedance. It stays the same, and the proof is simple. Your assertion that they "solve a problem" is correct, technically. But at the cost of sound quality. If sound quality is not a priority, then sure, buyers have indeed got their mismatched components to "work" together. But only that. The degradation of sound quality is obviously secondary to them. As a manufacturer, I would have thought your emphasis would be on making sure your amplifiers were matched with speakers that show them at their best ? Surely you're doing your products a disservice, by suggesting the Zero's as a bandaid panacea for buyers ? Tradeontheweb, sounds like you and I are more on the same page than different. Whenever someone calls me and is interested in one of our amps I always ask about their loudspeakers to see what I am up against. Sometimes they have speakers that they don't want to sell that are IMO not all that compatible. Sometimes they won't work even **with** the ZEROs. But if they will work, I suggest them. Quite often they ask 'doesn't that defeat the OTL aspect of the amplifier?' to which the answer is 'No.' That is because the ZERO has such a low turns ratio due to its only going from 16 ohms down to 4 ohms or so. It also does not have to block DC; between the two that gives it more bandwidth than most amps. You have to start with an amplifier that already has a low-ish output impedance and puts out no DC... People also ask if it degrades things as you allude to, the answer to that is "The ZERO is a problem solver and there will be no tradeoff whatsoever, if not it is either not being used properly or does not need to be used at all." Now its the former in that last statement that needs some looking into. For example, you can set the ZERO up to run only a certain portion of the loudspeaker. An example might be a speaker that has a 4 ohm woofer array but is 8 ohms in the mids and highs, and the speaker can be bi-wired. In that case you can recover some of the lost impact in the bass by using the ZERO only on the woofers. There are those cases where a person has a speaker that is so inefficient and/or with an adverse impedance curve that there is nothing for it; then I let them know that:
If you want a tube amplifier, your tube investment dollar in whatever amplifier that is will be best served by a speaker that is higher impedance and/or higher efficiency.
The ZERO only comes in when it is obvious that a person is simply not going to budge on their loudspeakers and that is that. Again, the ML ESLs have a very low impedance as they want more of the solid state market; to use them successfully with tubes the ZERO is an excellent option. The reason for this is simple- a tube amp using a set of ZEROs still has an excellent chance of sounding better than transistors for all the reasons that have been debated about tubes and transistors in the last 50 years. The ZEROs don't change that. |
If one looks at the Zeros as a type of adapter, its not that complicated to determine the value case by case, which will vary widely.
Some cases they will add value, but only when an adapter is what is needed to make two devices not designed to work well together work well together.
But clearly if starting from scratch, in most cases, one will be better off NOT using an adapter, especially one that costs big bucks in order to bridge a gap.
FRom the perspective of an amplifier vendor whose product plays by a different set of rules than most, there are surely many good applications of the adapter.
However, if speaker maker and amp maker play by the same set of rules, there should be no need for an adapter if they dd their job well. IF they didn't, they will probably go out of business and no adapter will help save them. |
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Output impedance does go down with nfb - that's a basic tenet. Easy to check: take a non-nfb amp and change the load d
from 8 to 4 ohms. You will see that the output voltage drops a bit. Now apply nfb, do the same thing and you will see that the drop is less, showing that the output impedance has dropped. In fact, it drops exactly with the feedback ratio. Apply 20dB nfb and it drops by a factoe of 10.
Of course we all know that Zout is not physically a resistor or sumptin'....
Jan |
Ralph and I and others discussed his views on the relation between feedback and output impedance in this thread. It essentially comes down to a matter of terminology. Regards, -- Al |
Al and Ralph, I read the article that George kindly provided. I also re-read the posts at the link Al provided. I think Al correctly states that there is a bit of friction caused by the use of terminology.
I was looking for, but didn't find the post where I coined the terms "synthetic" or "apparent" constant voltage source where negative feedback (NF) is used. I think that using such terminology is useful insofar as it helps one to better appreciate whether an amp becomes a true constant voltage source if NF is used or just acts like a constant voltage source to some extent.
Let me explain. My take-away from George's article is that if an amp has an output impedance that is greater than Zero, some part of its power output is being consumed internally, resulting in a voltage drop at the output taps when driving an external load.
The author illustrated this effect with some simple examples using an external speaker load of 8 ohms and two hypothetical amps -- one having an output impedance of: (a) .1 ohms; and (2) the other having an output impedance of 2 ohms. The math then follows illustrating how the amp's output voltage and corresponding power output changes under each scenario.
I surmise that the terminology confusion comes into play because of what NF does ... and does NOT do. That is, if an amp has an output impedance of say 2 ohms, then as Ralph says, that is its output impedance ... period. But if NF is used, the amp compensates for the voltage drop by increasing output voltage to comepensate, thereby causing the amp to perform like, but not actually be, a constant voltage source amp.
As such, with output voltage adjusted to offset the voltage drop at the output taps resulting from the interaction of the amp's own internal impedance and the speaker's impedance, the amp functions as though it was a constant voltage source amp, or like one to a greater extent.
But there is always a BUTT. This picks up on what Al, Ralph and the author of the EPDR article have been saying over and over again. Using NF cannot create energy. That is ... an amp using NF will be able to generate power only to the extent it is capable of doing so based upon its internal architecture. Or, as the EPDR article said, within its SOA (safe operatwing area). If the amp operates outside its SOA, it will cut back its power output, clip, become unstable, produce distortion, etc.
I said this at the beginning of this thread and I'll repeat it again. I think we are all agreeing with each other much more than not. Butt, I think we are talking past each other.
IMO, given that **most** (not all) speaker manufacturers seem to be making speakers with roller coaster impedance and phase angle plots, erring on the side of am amp having lots of SOA headroom and low output impedance is a safe play. That generally equates to a SS beast that can deliver lots of current/power.
OTOH, as I said here and in other threads, to illustrate my understand, my ARC tube amp functions within a pretty wide operating corridor like a SS amp, but to do so, requires NF, and a massive power supply. Or using my own terminology, my amp performs like a "synthetic" SS amp, to some extent within a large "SOA."
Hope this helps ... butt it probably doesn't. |
Excellent post, Bruce, with which I agree. I also agree with the first two-thirds or so of the paper George referenced, although I would point out that the author has taken the reasonable step of simplifying the circuit analysis he presents by only taking into account the resistive component of the load impedance. I would take exception, however, to much of the last part of his paper, beginning at the point where he states that: Loudspeakers are not constant power devices. Loudspeakers must have a constant drive voltage to provide a constant acoustic output with changing frequency. IMO (and I feel safe in saying in Ralph's opinion as well), that statement is incorrect and misleading. I would agree with it if it referred to the majority of loudspeakers, rather than being expressed as applicable to all loudspeakers. Many electrostatics being notable exceptions to his statement, IMO. The fundamental error he makes, IMO, is that he assumes that with all speakers flat frequency response in (in terms of voltage) corresponds to flat frequency response out. That will be true, at least approximately, for the majority of speakers these days. However it will not be true for a substantial number of speakers, including many electrostatics. As Ralph has pointed out many times, it comes down to the intention of the designer. Consider the classic Quad ESL-57 George referred to earlier, which is revered to this day (impedance curve shown here). It was designed before solid state amplifiers existed! And it is very successfully used with tube amplification by many audiophiles to this day. Although in fairness I'll say that I recognize that **at least among solid state amplifiers** the vintage Mark Levinson ML-2 which George referred to is recognized as being a particularly synergistic match, and that combination (with two stacked Quads per channel) was the heart of the highly regarded HQD system produced by Mark Levinson's original company (together with a large Hartley woofer and a Decca supertweeter, with a total of six ML-2 monoblocks driving the two channels in a triamped configuration). Regards, -- Al |
Thanks Al. My apologies for the numerous typos in my last post. I was typing with my laptop literally on my lap, drinking coffee with one hand and typing with the other hand. Truly a multi-handed effort.
That aside, I wanted to say that this has been one of the best threads to get posted in a long time. So kudos to all of the contributors.
I believe that if and when I ever change out my speakers or my amp, the terrific comments posted in this thread will make selecting a good amp/speaker combo a less haphhazard (sp) exercise. At the very least, I'll be able to ask better questions ... and know what to expect in terms of getting better answers.
Thanks again to all, including of course Ralph, Al, George, Mapman, Unsound and Judy.
Cheers,
Bruce |
Just a little background on my experience with the Manley Snappers.
I really think what can make a difference is how hard you're pushing the Snappers.
I was very concerned about the Snappers - given the specs - which is why I insisted on auditioning them before purchasing them. I had read that they were a tad more sweet in the midrange than the Rogue Audio M-150s (which is what I was planning on after reading some very positive reviews), but I suspected they wouldn't have the power to drive my Aeon i's - especially in the higher frequencies where the Aeon i's can drop down to just under 2 Ohms. And I should tell you that I am also using a pair of subs to augment the Aeon i's since they are totally passive and can only get down to 43Hz. But a practically brand new pair of Snappers came in at my local dealer's shop and he called me up and asked me if I wanted to try them out for a week. I decided to give them a try after speaking with Paul over at Manley Labs - being cognizant of the fact that my listening room is quite small and I would not likely be pushing the Snappers too hard.
All I can say is they sounded amazing through the Aeon i's. I brought over two audiophile friends - call them P and W. P is in his late 20's and can easily hear frequencies above 15kHz (which is actually pretty rare) and both were blown away by the sound. We we're comparing the Snappers with my Parasound HCA-3500 which has ZERO problem driving my ML's and the Snappers outperformed the 3500 in practically every way - especially on musicality and sound stage depth. What really shocked me was when W said he thought the Snappers had more slam than the 3500 - especially since slam is what the 3500 is really known for. P indicated that the highs were a bit more subdued with the Snappers, but they sounded much more pleasant in his mind. For W, there was no comparison - the Snappers sounded MUCH better in his mind - even to the point that he has now decided to move over to tubes. And we listened to all types of music - classical (full orchestra and smaller ensemble works), jazz (traditional and modern) and classic rock. Female vocals sounded especially life-like through the Snappers.
But keep in mind, the caveat to all this is the levels we were using for the audition given my room size. If you have a large room that needs some serious volume, then all the data does suggest that the Snappers will probably run out of steam and you might be better off with something like the M-150s or M-180's. But in my room at least, the Snappers had plenty of power to drive my Aeon i's with a level of quality that was leaps and bounds above the 3500.
Would a pair of M-150's or M-180's do better? Or one of the top-end Pass amps? Maybe. But all I can say is the Snappers were available, they were less than 8 months old and in like-new condition and they were priced at $3600 for the pair (they now retail new for $7200). So in my case at least, getting them was a no-brainer.
Specs are certainly important but they are not the end-all-be-all. At the end of the day, you really need to listen and decide for yourself how a component integrates with your existing system and room topology. For me, the Snappers were a major step-up and I'm totally happy with them. |
Jan, please re-read my non-tech take 2 or 3 posts north of this post. What you describe is exactly the effect of using NF -- the amp compensates for the output voltage drop based on the amount of NF applied to increase the output voltage to compensate for the interaction of the amp's internal impedance load and the external load of the speakers.
The effect is more precisely described as the amp is behaving as though it has low output impedance. So ... the NF circuit does not really change the amp's internal impedance. Instead, it acts like a servo to cause the amp to compensate for the Ohm's Law effects.
Of course, as Al, Ralph and others have said, the amp can only do this if it operating within its power delivery limits -- i.e., SOA (safe operating area). Also, there are rarely any free rides. Hence, NF has its own problems, e.g., increased odd ordered harmonic distortion ... and its detractors, e.g., Ralph. IMO, amp designers make trade-offs in order to design and build amps that perform at a certain level at a certain price point.
Probably still not saying this correctly, but it goes to show how the terminology can get in the way even though we are all really saying the same thing. |
Byfwinne, yes I agree. The internal amp Zout is not changed of course.
Re: increased 3rd with nfb: that has been laid to rest many years ago. Most people know the (in)famous Baxandall graph showing increased harmonics (even harmonics that were not there without nfb) with increasing nfb. Then, when you continue to increase nfb, the harmonics eventually became invisible again - all of them.
There are two sides to this. One, not everybody realised that the worst you could do in this situation, was to use moderate nfb. For lowest harmonics, in this situation (see below), use none, or use a lot!
Second, this was a single, crummy FET stage. It has been shown that if you look at this with a whole amplifier rather than with a single crummy stage, the effect is not seen. If you start with a reasonable amplifier, applying nfb decreases ALL harmonics. |
