The trick with Martin Logans is that quite often if you have a tube amplifier, you will want something like a set of ZEROs (http://www.zeroimpedance.com) to help the amp drive the really low impedance of the speaker, particularly at high frequencies.
ML has been trying to get their speakers to work with transistors; to this end they have kept their impedances low. The problem with this approach is that ESLs and tubes have been the match made in heaven since the 1950s- you put transistors on the speaker and brightness is the name of the game. So it you want to use tubes, you have to deal with the impedance and that is what the ZEROs are for. |
Bifwynne, if you installed a resistor to raise the impedance of the load it would certainly absorb power. That is why you use a transformer, in this case an autoformer.
The reason the ZERO is an autoformer is that it does not have to block DC, which makes it simpler and better performance (wider bandwidth). The ZERO has wider bandwidth than almost any amplifier made, tube or transistor.
The second thing about the ZERO is that the amplifier already has a lower output impedance, its not like 3000 ohms or something, instead we are simply going from 16 ohms down to 4, 3 or 2 ohms. With such a low resulting turns ratio again greater bandwidth is possible.
Now its a simple fact that there is no good argument for lower impedance (4 ohms or less) loudspeakers in high end audio; that is to say if sound **quality** is your goal. If sound **pressure** is your goal then you have a weak argument for lower impedances, if you have a solid state amp capable of supporting that lower impedance properly; not all do.
This is where the ZERO comes in- for those speakers that unfortunately are lower impedance due to poor design choices on the part of the designer (I added that last caveat because that is precisely the problem), the ZERO is a problem solver. Certainly it will help a smaller OTL drive such loads, but it will also help transistor amps that might seem perfectly comfortable on 4 ohms to actually sound better (although with less power). IOW, a transistor amp will sound smoother and more detailed when driving a higher impedance (due to making less distortion)- enough so that there can be a benefit to using the ZERO for that reason.
Paul Speltz, who makes the ZEROs, has a letter from Steve McCormick describing this.
I don't see the ZEROs as obviating the 'OTL sound' either. As I mentioned earlier, the ZERO has very wide bandwidth, wider than any amplifier; it would not be possible if it had to block DC or work with a high impedance input!
If speaker designers ever sort out how the load affects the distortion produced by the amplifier then we will see higher impedance speakers and no need for the ZERO. But I suspect that day will never come, so the ZERO will continue to be quite useful as a problem solver for those prefer sound quality over distortion. |
Tube amplifier power has always been more expensive (the tubes themselves cost more, the filament circuit to light them up costs something, so does the output transformer). It was the reduced cost of transistors that got the industry going in that direction in the first place.
Class D has added another order of magnitude to that. |
I take it Unsound, that you are an advocate of increasing distortion :) Because by disagreeing with me, that is exactly what you are doing. Let's try that on for just a moment. Usually in high end audio we are interested in getting rid of colorations and just having a neutral presentation; the fact that equipment does have colorations sparks a lot of conversation as we both know. We also know that the ear translates distortion into tonality- for example the 2nd order harmonics make tubes amps sound more lush. So what you seem to advocate is to use a lower impedance loudspeaker, as the amplifier will certainly be more distorted. Because that *is* what happens. You can see it in the specs of all amplifiers in existence. None have decreased distortion with decreased impedance. So the only conclusion available to me is that you seem to want the amp to sound more distorted. If you are all like 'no, that's not me, why are you putting words in my mouth?' then you and I are on the same page, which means that you do indeed want to get the impedance higher as that will reduce the distortion coming from the amplifier. George, take a take a look at the links on this page: http://www.zeroimpedance.com/zeroimpedance_002.htmPaul Speltz has a letter from Steve McCormick, stating that his amps (which can drive 4 ohms effortlessly) sound better driving 4 ohm speakers through the ZEROs. The reason is simple: distortion is lower. That translates to 'smoother, more detailed' as far as the ear is concerned. Bruce, what will happen in the scenario you describe is that the amplifier probably will have a problem on the higher impedances. Voltage Paradigm amplifiers often do. However, if that amplifier happens to also use tubes, it will do better into the higher impedances than a transistor amp will. |
George, you took the quote out of context- sounds like you didn't read the article to me. Unsound, I have a number of friends in the industry that make transistor amps. When Steve's letter turned up, I asked them about this and their response (paraphrasing) was: 'Just because the amp can drive a lower impedance with more power is not the same as saying it is also sounding its best.' Now there is no argument that supports your position that because the impedance of the load is lower, that this translates to extend frequency response, improve the linearity of frequency response, and as you've already noted can increase dynamic range. Of the latter point 'dynamic range' you are putting words in my mouth. But given your response (and apparently also that of George) it does indeed appear that the both of you prefer to have your amplifier add some distortion. So in that regard we do indeed differ- I prefer a lack of distortion if possible. |
on this very forum speaker builders have suggested that with a lower impedance they can more easily achieve deeper bass response (extended frequency response), a more linear impedance (improve linearity of frequency response)and as you've said a lower impedance can make a speaker play louder (increase dynamic range)(am I really putting words in your mouth?). To that let me add that I've yet to see an impedance plot from a speaker that claims to provide waveform fidelity and can back it up with an appropriate square wave response that doesn't also demonstrate an impedance plot that drops below 8 Ohms. You are indeed putting words in my mouth. What I said was that a lower impedance can sometimes get you more power out of some transistor amps. That is a long way from saying that is is increased dynamic range! For example the speakers I have at home are 98 db, go down to 20Hz, and are 16 ohms. They tend to have dynamic range because of their efficiency, which is where dynamic range actually comes from. Regarding the other point you made in the quote above, a link would be nice. I can't think of a mechanism that would give a speaker with lower impedance an advantage of better LF response. The two are unrelated: you can get exactly the same LF response from a speaker that is 16 ohms or 24- impedance has nothing to do with it. George, unlike Unsound, you are a troll. Your comments are unwarranted, without merit and uncalled for. |
Bruce, I am a moderator on another site that is unrelated to audio. In that regard they put me through some training so that I could spot posts that went beyond the pale. I don't see Unsound doing that- he attacks the argument, not the person (which is how its done). IMO debate is fine and healthy- and is what forum sites are for. In the end, the debate can help people gain education or at least viewpoint on a subject.
George is different that he attacks the person (look for the word 'stupid' in his last post) and he will often negatively engage in topics just so he can get a rise out of other posters. That is one of the definitions of trolling. (As a side note its also pretty evident that he does not know what he is talking about WRT the ZEROs.)
This site does not employ a 'Report' function so its pretty hard to alert moderators when this sort of thing is going on- they have to encounter it on their own.
Unsound- I see that you reposted some earlier comments I made and one from Duke; I don't see exactly why though. Could you elucidate? |
Unsound, I don't see the comments you found doing anything but supporting my position, and also this: http://www.atma-sphere.com/Resources/Paradigms_in_Amplifier_Design.phpThe bit about Jim Theil using drivers in parallel to get more current out of the amp at low frequencies is a classic Voltage Paradigm move. The problem is it also increases the distortion of the amplifier. As the other quotes you found also state (paraphrasing) - if sound **quality** is your goal then there is no argument for 4 ohms. If sound **pressure** is the goal then there is a weak argument for 4 ohms if you have a solid state amplifier that supports that operation. Now in the case of Jim Theil, by using two woofers in parallel, and assuming an amplifier that can produce constant voltage into most loads, the simple result will be that it makes twice as much power into the woofer array than it would if there was only one woofer. Its also likely that the woofers chosen may well be 3 db less efficient than the midrange and tweeter, if by any chance the latter are 8 ohms. Now in the case of a Martin Logan ESL panel, the panel impedance is not the result of the behavior of a driver in a box. It is the result of a capacitor, whose range of impedance is set by a matching transformer. As a result, its not to your advantage to see an increase in power at low impedances- the panel is as efficient at 10KHz as it is at 1KHz, even though the 10KHz impedance is much lower. It is for this reason that it is perfectly reasonable for one to use a set of ZEROs on the speaker, even though there is a matching transformer in the speaker, and perhaps another one in the tube amplifier employed. IOW, the 4 ohm tap on many tube amplifiers may not be enough to allow the amp to deal with the 0.5 ohm impedance at 20KHz that is common with many ML ESLs. Heck, a lot of transistor amps have trouble with that too- especially if a speaker cable is being used between the amp and speaker. |
^^ OK. Its my contention that reducing distortion in the reproduction chain leads to better fidelity/better sound, especially if the types of distortion we are reducing are the types to which the ear is particularly sensitive. In this regard I favor sound quality over sound pressure (volume); I think you will find that most audiophiles do. You can see in the specs of any amplifier (solid state, class D or tube) that the distortion is indeed higher driving a lower impedance load. If you think the distortion involved is negligible, its not, it usually is of the types that the ear cares more about. This can be so significant that the use of an autoformer to raise the load impedance to an otherwise very capable solid state amplifier can result in improved sound. Were this not the case, the insertion of the autoformer would have adverse effects. Here is the text of some communications that Paul Speltz (designer of the ZERO) has received from a well-known solid stage amplifier designer: Hi Paul -
I hope you had a nice weekend out in the woods - it's cold out there!
I'm still enjoying the autoformers very much, and they are continuing
to improve as they break-in. I wanted to comment on why I finally
decided to try them (and wish I had a long time ago!). In the past, I
had always thought of autoformers as a "speaker tweak" or a kind of
crutch for amplifiers that couldn't handle difficult loads. I figured
that my amps could drive anything, so why worry about adding extra
boxes that I didn't need. Over the past several years, though, I have
been working more and more with line-level coupling transformers (part
of my new VRE-1 preamp design) and I have seen some evidence that
suggested to me that something in the nature of the load with the
transformers made the source driving circuits "happy," to use the
scientific term ;-) Thus I came to be interested in the autoformers as
a possible *amplifier tweak.* (I guess maybe you see them from the
speaker's point of view, while I tend to see things from the amp's
perspective.) Whatever the reason, they certainly work well, and I do
feel that the amp is "happier" with the conjugate load. Maybe it's
nothing more than a more benign impedance, but I suspect there is
something more at work. I suppose it might be possible to try them
with a high-efficiency,16 Ohm speaker as a test of some sort, but I
suspect that the sound would still improve with the autoformers.
Anyway, whatever the truth of the matter is, I am a convert.
Thanks again, Paul.
Happy holidays,
Steve McCormack
SMc Audio |
BTW, as I read it; George didn't call you stupid, he called one of your suggestions stupid, and that was after you called him a "troll", which was after he posted a link where he recommend your product. This line of posting is somewhat out of character for you. Hi Unsound, yes it is, and one should consider that I do refrain from that sort of thing as much as I can. Recommending the ZERO is something I have done a lot, and not just for our amps and I will continue to do so despite George's remonstrations. This is for no other reason than the ZERO really does work. One other thing- to clarify, I am not stating that 4 ohm speakers are bad speakers. I am stating that any amplifier driving them will sound harsher and less detailed as opposed to the same amplifier driving the same speaker that was 8 ohms, were all other matters to be equal. Put another way, a simple method of making a speaker seem more transparent and easier to listen to is to make it be higher impedance. This might be all about how important it is to an audiophile to have increased transparency/detail, coupled with a smoother presentation. I like those things myself. Something I have not mentioned yet is the effect of the speaker cable on 4 ohm speakers- but it should be obvious that they are far more critical for a 4 ohm speaker than the cables are for 8 or 16 ohms. Damping factor of amplifiers is normally stated with respect to an 8 ohm load; that number is initially cut in half with a 4 ohm speaker. Add to that the seemingly low DC resistance of the speaker cable and the damping factor of the amplifier is reduced considerably more that one might intuit. RCA published a nomograph that showed this relationship several decades ago. |
So my sense is to consider the full impedance curve, not just a static number. Maybe that is where "all other matters" are not equal. That is certainly part of it! There is a reason some speakers are considered 'difficult loads' and that is never a good thing as far as the amp is concerned. The OP was about tube amps that can drive ML ESLs. The issue here is that ML has been really trying hard to make their speakers work better with transistors, which is not easy to do, because ESLs want constant power and transistors are constant voltage (see http://www.atma-sphere.com/Resources/Paradigms_in_Amplifier_Design.php. I'm pretty sure I explained all this earlier- a lot earlier: http://forum.audiogon.com/cgi-bin/fr.pl?aamps&1394129996&openflup&11&4#11But if you have a set of ZEROs, its pretty easy, then its just about whether you have enough power, not whether the amp is comfortable with the load. Seems to me 60 watts will make most MLs play reasonably well. |
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. |
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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...). |
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. |
Jandidden, keep in mind this thread is not about transistor amps. But more importantly you have to be careful here not not violate something called Kirchoff's Law. Actually you will find that such is impossible as it is a law of physics, and not part of legal code :) This law is known also as the law of energy conservation and simply states that the amount of energy in an electrical network is equal to the amount of energy going into that network. What feedback does not affect output impedance: Now if we take two output circuits, one with a high impedance, for example the single ended output of a tube preamp, and that of a transistor power amplifier we will see that due to the lower output impedance of the amplifier that it will drive a lower impedance. Now if what you and George say is true, that adding negative feedback lowers output impedance, it then follows that if we add feedback to the preamp circuit its output impedance will become so low that at some point we can drive a 4 ohm load effortlessly. But what we find is that is not the case. Now you may argue that the preamp output is not relevant, so let's take the case of an SET with a 10 ohm output impedance on the 8 ohm tap. If what you say is correct, its output power will increase if feedback is added when asked to drive a 4 ohm load off of that tap. But again, what we find is that the 4 ohm output power is unchanged. The reason for this is if negative feedback really did decrease the output impedance, the resulting circuit would have the increased current to drive a lower impedance. That of course would violate Kirchoff's Law. Of course, the real way to provide for greater current ability is more output devices, larger heatsinks, output transformers, power transformers and the like. IOW, what is happening is that the term 'output impedance' as used with the Voltage Paradigm is a charged term that actually refers to servo gain in the output circuit and not the actual impedance of the devices involved (all types of which have an impedance greater than zero ohms). One might state that the issue here is semantic- I point it out simply because its use in the context of teh Voltage Paradigm leads to a lot of confusion- but that is how the audio industry is set up. If you are having trouble following this, it is because you are operating within the Voltage Paradigm. The word 'paradigm' has to do with a platform of thought; quite often viewpoints outside of that platform are hard to think about or might be considered blasphemous. A further note- people have accused me of making up the two Paradigms (voltage and power) that I mention in the article at this link: http://www.atma-sphere.com/Resources/Paradigms_in_Amplifier_Design.phpIn case I run into that I refer them to this google search on the Fisher A-80 amplifier https://www.google.com/search?client=ubuntu&channel=fs&q=fisher+a-80+amplifier&ie=utf-8&oe=utf-8The very first hit you get at this link is a YouTube image of the damping control of the Fisher amplifier. It is marked 'Constant Voltage' at fully counter clockwise, 'Constant Power' at noon and 'Constant Current' at fully clockwise. This is because there was a time when not all loudspeakers conformed to the voltage model, despite George's and Bruce Rosenblit's remonstrations. Such speakers are still made today, and in increasing numbers. Any speaker than can be driven successfully by an SET will be an example. ESLs are also an example as it will be found that their impedance curve is not based on the resonant impedance of a driver in a box. If you could show that the impedance curve of the speaker (which in most ESLs varies by about 10:1) is also an efficiency vs frequency curve then you would have an argument that the speaker is a Voltage Paradigm device. IOW the ML ESLs are a low impedance Power Paradigm loudspeaker. |
^^ Ah- good.
And we now also see why the Power Paradigm is still around some 60 years after it was supposed to be supplanted by the Voltage Paradigm: and that is because amplifiers operating under the Power Paradigm tend to make less odd ordered harmonic distortion.
This is partially because nfb will add trace amounts of odd ordered harmonics, even when used in large amounts.
The human ear/brain system is very sensitive to these distortions, more so than anything else, as it uses higher ordered harmonics to determine how loud a sound is. As a result, nfb will make an amplifier sound brighter than it really should be (regardless of tube or transistor) because the ear/brain system translates distortion as tonality.
This is why two amps on the bench might have the same bandwidth but one can sound bright and the other doesn't. |
:) Yes- damping factor is different from "output impedance" in that it gives you an idea of the servo performance of the circuit, it it has any. Unsound, what you see from that graph is the woofer driver in a box, combined with the impedance curve of the ESL. The Aerius is a hybrid speaker. Interestingly, we also see that as ML speakers go, this one is less severe, dropping to a fairly manageable 2 ohms at 20KHz. If I can draw your attention to the letter that is appended to this article, you can see from it why the ZERO works as well as it does, effectively lowering the output impedance of a zero feedback tube amplifier into the range where it might conform with the parameters laid out in the letter. The response plots initially do seem to suggest that the speaker is Voltage Paradigm, since such amplifiers seem capable of producing flat frequency response. I feel its important to point out that the amplifiers in question all show a peak in response as I forecast, which seems to start up at about 7KHz, and increasing with frequency as the impedance of the speaker continues to drop. Since the ear can hear a change in a spectrum of frequencies much better that it can with a single frequency, we now see that there is a correlation between the measurements and subjective listening experience wherein the amplifiers are causing the speaker to have brightness on the top end. As I have mentioned before, this is a common complaint when combining ESLs and transistors. The whole point of the Voltage Paradigm when it was proposed way back in the late 1950s and into the 1960s was that it would eliminate tonal coloration due to frequency response errors. What we see from these measurements is that isn't happening. Now the Power Paradigm does not make any such guarantee. Instead, it seeks to keep the kinds of distortion to which the ear is most sensitive to a minimum. This is does because the ear translates such distortions to tonality, the idea being that there is a tipping point in the brain where the tonality of distortion can be more pronounced than actual frequency response errors. If this pans out correctly, it may well mean that the result will seem more linear, even when the frequency response does not seem to be quite as flat. IOW, this whole thing has to do with how we perceive sound as opposed to how we measure it. Its a classic argument, and as I pointed out in my article about the Voltage and Power Paradigms ( http://www.atma-sphere.com/Resources/Paradigms_in_Amplifier_Design.php) this argument has been around ever since the Voltage Paradigm was introduced. |
Wrm57, the easiest rule of thumb is ask the designer what sort of amplifier they designed the speaker for. |
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 8 Ohm wattage into 4 Ohms, and should again increase into 2 Ohms. 'Stable' means that the amp won't oscillate when presented with the load in question. Atmasphere, perhaps I'm not seeing the same things you are? Perhaps. Take a look at Fig 4 at the link you provided. |
Hvowell, said earlier:
Now is the reason this type of amp works is (assuming the speaker has a sensitivity (SPL) of 85db/1W into 8 ohms)
.25W into 32ohms = 85db
.50W into 16ohms = 85db
1W inot 8 ohms = 85db
2W into 4 ohms = 85db
4W into 2 ohms = 85db
so the SPL remains constant into all impedances?
The reason this is not so is because in the case of an ESL, the impedance curve is **not** an efficiency curve. The fact is that the impedance curve is based on a capacitor more than anything else. This makes ESLs fundamentally different from dynamic drivers in a box. In an ESL, the efficiency is fairly constant despite the impedance. Generally speaking, ESLs have an impedance curve that varies by about 10:1 over the range of the speaker, which makes any one of them tricky for all amps. The reason they work at all with solid state amps is due to the negative feedback loop in the amplifier which allows the amp to adapt somewhat (reduce its output at higher frequencies) to the lower impedance at high frequencies. Despite that its a common complain that a transistor amp will be too bright on an ESL (not the least of which is that negative feedback in and of itself can cause brightness; likely a topic for another thread). For example the Sound Labs, early Quads and Audiostatics are easily driven with tubes as the matching transformer in the speaker sets the bass impedance fairly high (usually over 16 ohms). OTOH, Martin Logan use the matching transformer to set the bass impedance at about 4-5 ohms, making the 20KHz about 0.5 ohms. This is why they want an amplifier that can double power reliably into low impedances, else they would simply not be getting any highs at all! Note: George is mis-using the word 'stability' and 'stable' in his comments (it may not be his fault; this is a common mis-use of the word). Stability in an amplifier is how well the amplifier resists *oscillation*. For example our smaller amplifiers, like the M-60 (which is an OTL) are perfectly stable driving 4 ohms even though they don't make as much power doing so. OTOH some transistor amplifier are unstable driving capacitive loads (IOW they can go into oscillation) even though they can double power as the load impedance is cut in half. We have customers driving Martin-Logan ESLs quite successfully using either our old Z-Music autoformers from years ago or the ZEROs. Both allow our amps to drive the ML ESLs with ease with no high frequency softness. I've heard one of these systems; if anything to me the combination might have been a little bright, but some of that may have been caused by the solid state preamp that was also in the signal chain. |
Yet they sound perfect and well extended when driven by my big current pushing solid state monoblock amps. 'Current pushing'? This sounds like a misconception to me. Again, Martin Logan is specific about the amplifier to use with their speaker because they have made the speaker so hard to drive. 0.5 ohms is hard on all amps, not just tubes! What you want to consider is how the amplifier distorts when presented with a low impedance- and in this case, **all** amplifiers, tube or solid state, will have higher distortion which will result in brightness. I suspect this is why Paul Speltz got a letter from Steve McCormick, wherein Steve described how even though his amps can drive 4 ohms effortlessly and with double the power of the 8 ohm load, the fact was that they sounded better (smoother, greater detail) when using the ZEROs to drive the same load. The simple fact is that you don't want an amplifier to work hard- it will have greater distortion which is to say it will sound harsher and with less detail. George, you should give a set of ZEROs a try with your Rogue amplifiers. I think you will find that by using them, they keep up with your solid state amps in the highs. That is the experience of many people using the ZEROs. The ZERO website ( http://www.zeroimpedance.com) has many comments in that regard. |
^^ Did you speak to Paul about it? Your experience is unusual (although much depends on the amp too). |
Hard to say Al, he's not said what phenomena he ran into with any specificity.
Paul Speltz is very supportive though and often has suggestions for how to sort things out should one run into problems. |
Transistors and tubes are the same speed (speed works out to bandwidth, so imagine how it was that color television was possible with tubes; color television requires some bandwidth!).
It is the output transformer of a tube amp that usually limits its speed. However, delivering the 'current' to play bass on an ESL is no problem at all for a tube amp, in fact generally speaking transistors are less able to deliver the power needed, simply on account of the fact that the bass region of most ESLs is also the region of the highest impedance, which is often ten times higher than it is at 15-20KHz. |
