This particular paper is interesting, but very dated . . . and no disrespect to the author or his work in context of the time. But it's important to understand that this was published some fifteen years before that of Thiele and Small, and the T/S equations predict exactly every aspect of the driver/source-impedance interactions that he gives these approximate, experimental methods for optimizing.
It also bears mention that at this time, typical loudspeakers were designed with a very small back-enclosure and a horn (using Webster's equations), or a very large infinite-baffle . . . reflex designs were extremely rudimentary and typically more like an infinite-baffle as far as the system Q is concerned. Let's also not forget that liberal application of tone controls was also not frowned upon like it is today . . . a great way to compensate for all kinds of unanalyzed factors relating to speaker design. |
THaks Shadorne, that explasins what Bobby was talking about, with which he then says, which is why it works so well with tube amps, Atma-sphere among the best with my speakers. |
THaks Shadorne, that explasins what Bobby was talking about, with which he then says, which is why it works so well with tube amps Indeed, a critically damped acoustic suspension speaker does not need an amplifier with high damping factor as much as your regular underdamped boomy type designs that will get positively sloppy without amplifier control. |
Shadorne, a long time ago I remember reading(though for the life of me, I can't remember where or when), that one shouldn't use a DC coupled amp with ported speakers. I am under the impression that DC coupled amps are noted for their tight bass response. Not that I doubt you, but, if all that is correct (and it very well might not be), it would seem to disagree with your last post. My, how this thread has taken off on another path! |
Hi Unsound . . . it's not so much a matter of whether or not an amplifier is "DC coupled" so much as what its output impedance is -- Atmasphere manufactures "DC coupled" amplifiers with highish-to-very-high output impedances. I would summarize his position on this (sorry Atmasphere if I'm imprecise) as that speaker manufacturers have a responsibility to keep their impedance curves fairly smooth, and/or that the end user needs to be aware of what speakers produce good results with his amplifiers when making purchasing decision.
My point in the previous post is that with the mathematical tools available to the modern speaker designer, one need not use that very crude experimental methods outlined in the paper Atmasphere linked to . . . it's been possible for some time to accurately predict the loudspeaker "damping" behavior for any given amplifier output impedance. And of course different loudspeaker designers have different goals for this criteria.
BTW, there are very few amplifiers that could truly be called "DC coupled", and very few of these actually have a closed-loop frequency response that extends to DC (which IMO isn't necessarily a good idea anyway). Most conventional "DC coupled" amplifiers may indeed have the output stage DC-coupled to the positive side of the loudspeaker, but on the negative side, the speaker current is actually returned through the main filter capacitors . . . meaning that with the exception of any residual DC-offset current (or if the amplifier has a fault), they are effectively capacitively-coupled amplifiers, with the main filter caps inside the feedback loop. |
Another way to think of damping is that a high damping amp "shorts" the speaker woofer voice coil when the signal is "zero" - this means that the voice coil current is maximized (sees least resistance) in response to the movement of the coil through the magnetic field which creates the strongest possible back emf which opposes the coil movement and acts as a "damper" for as long as it takes the coil to come to rest. The more powerful the magnetic field (magnet size/weight) and the bigger the diameter of the voice coil and lighter the cone weight then the better an amplifier is able to tightly control a woofer. (This is why large 4 inch voice coils that are very short (light weight) and sit in a long powerful magnet gap are so desirable. These are extremely expensive as well with a quality 12 inch woofer costing around $1000 and weighing about 25 lbs or more.
Of course at resonance, the impedance of the voice coil rises and electrical damping is not as powerful and that is where the "acoustic suspension" design becomes critical to how the speaker will sound. |
These are extremely expensive as well with a quality 12 inch woofer costing around $1000 and weighing about 25 lbs or more. I would add that outside of pro applications these kind of woofers are few and far between - buyers will pay for veneer or cabinet work but not for honking great big magnet and voice coil that they cannot even see - of course manufacturers respond accordingly to the customer demand (customer is king). Without a good expensive woofer the best damping factor in the world is not going to put lipstick on the proverbial pig... |
My understanding is that a woofer's electrical damping is described by the electrical Q (Qes) and the mechanical damping is described by the mechanical Q (Qms). The Qes value given in a spec sheet assumes a true voltage source amplifier, and in practice is raised by the amplifier's output impedance. I think the equation is Qes' = Qes x (Re+Rampout)/Re, where Re = the voice coil's DC resistance and Rampout = the amplifier's output impedance.
Much as I like prosound woofers, most of them have too much electrical damping (too low Qes) to give good bass extension with a low output impedance (high damping factor) amplifier. In other words, most prosound woofers are overdamped for home audio usage when used with solid state amps.
Duke |
[/quote] In other words, most prosound woofers are overdamped for home audio usage when used with solid state amps. [/quote]
Good point - prosound woofers are often in a vice-like grip of the amplifier and excursions are tightly controlled and the bass is often tight and thin compared to the warmer and more extended sound of consumer designs. Partly this design approach helps protect the woofer from damaging over excursions. |
What are the virtues of a DC-Coupled amplifier like the Atma-sphere? |
Much as I like prosound woofers, most of them have too much electrical damping (too low Qes) to give good bass extension with a low output impedance (high damping factor) amplifier. Excellent point - they also tend to have lower mechanical mass for sensitivity, and lower compliance for better power handling, resulting in a higher resonant frequency. The problem with increasing the amplifier source impedance for this type of driver (especially in a domestic application) is that while this raises the Q and the bass output, it leaves the resonant frequency unaffected . . . the result is then simply wooly, boomy mid-bass and no increase in bass extension. An interesting exception is the JBL 2235/2245, which have mass-rings (weights) attached to the voice-coil former to both increase Q and to lower Fs. But this also makes them much more prone to over-excursion . . . so it's not uncommon for a reconer to remove the weights if there's a problem bottoming-out the voice-coil against the magnet, again raising Fs and lowering Qms in the process. |
Pubul57, there's no shortage of material on the web that contains Atmasphere waxing lyrical about his approach to building amplifiers . . . I suggest you look at his website or peruse his posting history on Audiogon if you haven't caught it by now. |
The problem with increasing the amplifier source impedance for this type of driver (especially in a domestic application) is that while this raises the Q and the bass output, it leaves the resonant frequency unaffected . . . the result is then simply wooly, boomy mid-bass and no increase in bass extension. I would also add that amplifier tight control is a good thing if one desires overall low distortion (THD). However, as Duke points out, this kind of precision response will be at the expense of bass warmth/extension and coloration. In the end it means that a large 12 inch pro woofer may only get you to 40Hz at 3 db point - whilst some consumer designs can eek out 25Hz to 30 Hz even with a 6 inch ported design (pleasant sounding and good value but with lots of added distortion of course). |
You're precisely correct Shadorne . . . and that's why in the pro world, a 12" driver is considered small for a woofer, and usually used for midrange/midbass. Even most pro 18" drivers have a pretty high Fs and low Q compared to a consumer 12" . . . and why most of those pro 18" bass drivers are in W-bins, sugar-scoops, tapped-horns, etc.
But also keep in mind that distortion is very SPL-dependent, and the onset comes on very quickly indeed! So for a smaller cabinet size and better bass extension in the typical domestic environment, having a very clear idea of the maximum intended SPL is crucial for the loudspeaker designer to achieve the requisite performance. |
Pubul57, Kirkus has been right on all his points but one- I've not really talked a lot about the DC coupling issue, even though I am a proponent of OTLs (the two are not always the same thing).
In a nutshell though, I think true DC coupling from input to output to be a bad thing, because every power supply has a low frequency pole, and if you exceed it (which a true DC coupled amplifier can) then you can modulate the power supply with audio from the amp, and that's bad- it gobbles bass impact and adds distortion.
So a frequency pole somewhere in the amp that limits it to a point at least an octave above the low frequency pole of the power supply is a good idea. We take it a step further by having a separate power supply for the driver, which is part of where we get our low IM distortion numbers from.
A few years ago I looked all over the web trying to see if anyone had scanned or posted that GE study, but so far it exists in print form only, no web. |
Kirkus wrote:
"The problem with increasing the amplifier source impedance for this type of driver [prosound woofer] (especially in a domestic application) is that while this raises the Q and the bass output, it leaves the resonant frequency unaffected . . . the result is then simply wooly, boomy mid-bass and no increase in bass extension."
With any speaker system designed to be "flat" with a voltage-source amp, there will be a bump in the bass region when that speaker is used with an amp that has a high source impedance. This is not uniquely limited to prosound woofers speakers. In many cases lowering the port tuning frequency will smoothe out the bump while extending the bass. |
Duke, which of your speaker designs is best for a room 11x18? |
Pubul57, I'd rather go offline with that to avoid taking the thread on a commercial tangent; I'll shoot you an e-mail.
Duke |
|
Kirkus, Shadorne, and Unsound, I really appreciate your input over the years. Inside of what I call the Voltage Paradigm, yours can usually be counted on to be accurate comments, and always done with decorum :)
I want to point you to Duke's comment about the port tuning dated 04/12. Here Duke is pointing out how the port is set up differently depending on whether you are using Voltage rules or Power rules. Its not the only thing I have seen in speaker designs, as in crossovers I have seen capacitive or inductive shunts used to get the right power response out of the amp so that the driver will roll off at the right frequency.
I guess what I am getting at here is that the Voltage rules are not the only game in town, and this has always been the case. However I find that if I am dealing with an individual who is educated with Voltage rules, that they will generally assume that there are no **other** rules. So I am interested in you thoughts regarding Duke's post. |
Atmasphere and Duke too, I think it's us (well at least I can speak for myself) that should be thanking you. Not many other manufacturers have been nearly as generous with their knowledge and expertise. It's encouraging to see such apparent enthusiasm even though you must be up to your eyeballs in this stuff each and everyday. I especially appreciate how difficult it must be to be financially vested, yet enthusiastic, while maintaining the level of decorum that you consistently do. |
However I find that if I am dealing with an individual who is educated with Voltage rules, that they will generally assume that there are no **other** rules. So I am interested in you thoughts regarding Duke's post. On the power rules front I think that provided a speaker is well designed smooth load with no large impedance swings and no low impedance dips (i.e. a decent speaker design) and provided it is well damped at resonance (again good practice - although not always followed - even by revered manufacturers such as Wilson)....then the speaker should sound great with a higher impedance amp (i.e. no need for a ridiculous high damping factor if the speaker is well designed). A damping factor of 10 is generally much more than enough and may be far preferable to an amp design with oodles of feedback, a damping factor of 500 and stability problems that result in high amounts of transient or IMD distortion (everyone knows that overly high gain and overly high feedback can bite you when playing real world music to a complex load - no matter what a speck sheet sheet implies). |
A bit off topic here, but, if a speaker manufacturer needs to veer from the criterion you use to describe a "decent speaker design" and needs use something other than a higher impedance amp to achieve better results, what's the harm? |
A bit off topic here, but, if a speaker manufacturer needs to veer from the criterion you use to describe a "decent speaker design" and needs use something other than a higher impedance amp to achieve better results, what's the harm? You mean like Infinity or a Totem Mani 2 or a MBL ...well it can be done and with the right amplification great results can still be achieved. However, a "decent speaker design" is NOT one that should present itself as a "stress test" to the amplifier, IMHO. |
Are "poorly" designed speakers by your definition, poor engineering, or an intentional tradeoff with gains in other areas? |
Unsound, I think the 'harm' comes in when the speaker requires that the amp use a lot of feedback to work right with the speaker. Since our ears use the 5th, 7th and 9th harmonics as a means to perceive sound pressure, and since negative feedback is known to audibly enhance (distort) these harmonics, the result is something that sounds more like electronics and less like music.
This harmonic distortion contributes to the amp sounding 'loud' or 'shouty', even though it might be nowhere near clipping. Now we get back on-topic: tubes, which generally use less feedback, tend to have more *usable* power as they tend to sound less 'shouty', and thus encourage you to turn the volume control up higher without stress. This assumes, of course, that the tube amp is able to drive the same speaker that the transistor amp can, for any kind of reasonable comparison.
IME if the speaker is has reasonable efficiency, and lacks weird phase angles and the like (IOW is not demanding of the amplifier), that **all** amps driving that speaker will sound better. BTW this is a strong argument for higher impedances, since even though a transistor amp might be able to drive a 3 or 4 ohm load without strain, that is not the same thing as saying it is sounding its best while doing so. |
It seems there is no single model or paradigm that can account for all possible combos of amp and speakers that work well together. Usually, two things work well together because they were designed to do so. So it all boils down more to the desires of the designer and how well that matches to those of the listener. There are certain engineering principles that can guide this process to a happy conclusion, however in the end no two designers or listeners are likely to share the exact same goals, which is why the search for a single absolute truth in audio is a fruitless one. |
A bit off topic here, but, if a speaker manufacturer needs to veer from the criterion you use to describe a "decent speaker design" and needs use something other than a higher impedance amp to achieve better results, what's the harm? Ah, Unsound . . . this is the crux of the matter. Atmasphere and I have actually beaten this horse pretty dead into the ground on other threads . . . again, I'll try to summarize: On one hand, I feel that one of the most important hallmarks of good engineering is to carefully consider the application, and design the equipment to work at its best within it. So a loudspeaker designer should consider the types of amplifiers that are likely to be driving the speaker, and design so as to acheive the most consistent and best results over the greatest possible number of situations. An amplifier designer should do likewise in consideration of the types of loudspeakers that are likely to be connected to it. If this approach is followed, then if a person buys a "great-sounding amp", and a "great-sounding pair of speakers" . . . then there's the highest likelihood of getting a "great-sounding system". I also personally feel that if an engineer's preferences for certain circuit topologies get in the way of these goals . . . then the engineer should maybe reconsider their preferences. Expecting a consumer to anticipate the effects of non-standardized technical equipment interface criteria is unreasonable, or at least very unlikely. On the other hand, Atmasphere feels that benefits of his particular circuit design approach are so great as to tolerate the fact that there will be some inconsistencies in performance depending on the particular application. To me, this is like making wines that can be wonderful and complex, but are inconsistent between vintages . . . the requirement is then on the purchaser to have some sommelier skills to compensate for it. And it's not that I necessarily have a great many circuit preferences that are contrary to Atmasphere's . . . I love the elegance of good vacuum-tube circuits, I think that there are good applications for well as open-loop topologies, etc. etc. It's just that I would never build and sell an amplifier with such a high output impedance, knowing that inevitabily a certain percentage of the time somebody would connect it to a loudspeaker that's largely incompatible . . . and I wouldn't be happy with the way it would perform under those conditions. |
Atmasphere, I'm not sure that many speakers actually necessitate that the partnering amplifier use feedback.
You say that the use of negative feedback contributes to the amp sounding "shouty". I find that ironic, in that "shouty" attribute is one of the most prominent ones that I find so objectionable in the speakers that are usually paired with tubes.
It would appear to me that in some case lower impedance would offer benefits that the speaker designer feels outweigh whatever negatives that tag along.
Again, I suppose this where we disagree. I would suggest that if a speaker manufacturer builds a superior speaker that requires an amplifier with a particular set of parameters to be effective, then so be it, make and use that amplifier. You on the other hand seem to suggest; that I can make a superior amplifier so long as the speaker works within it's parameters. As I feel the speaker/room interface presents the most challenges for the prospective system builder, I would propose; that the speaker be the determining factor in determining an amplifier/speaker interface. You, on the other hand would propose; that the amplifier(s) should be the determining factor in an amplifier/speaker interface. I suppose we have a conundrum, as to just who is seeing the forest from the trees?:-) |
Two different recipes that both can produce some very good soup.
Atmasphere's is a more unique recipe I'd say that might deliver some very special results if followed properly. |
In the last 20 years the use of SETs has really increased a lot. It has resulted in a lot of speakers that were simply not available 20 years ago. So these days its ten times easier to find a speaker that works with 'current source' amplifiers than it used to be.
Unsound, I can think of three speakers that if you put a tube amp on them, the result will be shrill, and all for the same reason: the amp will not double power as impedance is halved (or conversely, cut the power to 1/2 as the impedance doubles): the B&W 802, the Avalon Isis and the mbl101e. So if you are working with speakers that have similar impedance curves, I can totally see where you are coming from.
BTW the speakers on that short list are all examples of Voltage Paradigm technology, and its been my experience that when you mix Voltage and Power technologies, you will get a tonal aberration; 'shrill' is a common resulting aberration. Several others have been mentioned above, such as the 'wooly bass' that Duke was commenting too. |
Atmasphere, I wasn't talking about those kind of speakers. I was talking about horns and others that are supposed to be tube friendly. |
If I may steer this thread a bit back to the original question . . . there's are two characteristics typical of tube amps that I think make the perception common that "tube watts" are more powerful than "solid-state watts":
First, as others have alluded to . . . tube amps (as a group) have less offensive overload behavior, so it's possible for a slight bit of clipping to be much less noticeable than for a typical solid-state amp.
But there's also the fact that the overwhelming majority of both solid-state and tube amplifiers use unregulated power supplies, meaning that the voltage available to run the circuitry goes down the harder the amplifier is driven. And exactly how much it goes down is a function of the power supply's total capacity and its ability to store energy . . . in relation to the energy peaks required by the music being played.
And the particular set of energy-storage dynamics between a typical tube-amp power-supply and that of a typical solid-state amp are VERY different. ("Typical" here means push-pull outputs operating in Class AB or B, SS direct-coupled, and tubes transformer-coupled with C-L-C filtering.) The tube amp generally has significantly longer time-constants in its filtering in relation to the currents required by the output stage . . . meaning that whatever "dynamic-headroom" power is available (short-term peak power above the maximum available steady-state power) can be delivered over a longer period of time.
There are several mechanisms at work here. First, a push-pull tube amp reflects its impedance back to the power-supply as two full-cycles for every output cycle. Second, the output transformer primary inductance acts as effective energy storage for signal waveform asymetry. And third, the impedance transformation of the output transformer works backwards as well, drastically reducing the peak current demand on the power supply.
So a hypothetical 40-watt push-pull vintage tube amp may have 40uF of capacitance on the main plate supply and 5K transformer primary, and let's say we're using a 4-ohm output tap. 40uF seems chincy by today's standards, but this is equivalent to more like 100,000uF for a direct-coupled solid-state amp . . . and for comparison, a good quality 150w/4-ohm solid-state amp usually has something like 25,000uF. And while the SS amp does have two caps, since the output current is half-wave rectified (rather than frequency-doubled as in the P-P tube amp), their effective capacitances don't add together. And then the tube amp usually has another 40uF of capacitance and a choke in front of it, which probably gives about 2-1/2 times again the total energy storage for the power-supply.
So it's entirely possible that this hypothetical 40-watt tube amp may have similar (amount in dB) of dynamic headroom to the hypothetical 150-watt SS amp, but is able to maintin its dynamic power rating for ten times as long . . . let's say 50 milliseconds instead of 5 milliseconds. With a typical music waveform, this is a dramatic difference. |
Kirkus, Thanks! Your explanation of longer duration of headroom makes a lot more sense to my ignorant understanding of things than any of the other explanations offered so far.
I hate to use "my" current amplifier as a point of contention, but, my scope is somewhat limited. The manufacturer of my older cap coupled ss amplifier claims that it can double it's rated output for up to a couple of minutes at a time. Is that realistic? Is it due to it not being DC coupled design? Is it something all together different? What about ss amps like the Ayre that use chokes? I guess what I'm asking is whether tube amplifiers will always have this advantage over ss, or is it a matter of application?
It would appear to me that, while what you posted might very well be true, ss can usually offer more steady out-put power for the same dollar as most tube amplifiers. If so, wouldn't that negate some of the advantages you suggest for tube amplifier headroom duration? |
Kirkus, "the particular set of energy-storage dynamics between a typical tube-amp power-supply and that of a typical solid-state amp are VERY different." Thank you for your insight! Finally, we now have something that tangibly advances the conversation.
My position throughout this thread has been that we are clearly measuring the wrong things (the most obvious being WPC), and that we need SOMETHING to bring us into the world of calculus, as opposed to arithmetic/algebra. Again, the complex relationship of loudspeaker/amplifier/music is not a static or steady state, but a dynamic phenomenon. Distortion, another steady state parameter, is most definitely not the answer.
What Kirkus has laid out on the duration of dynamic headroom pushes the discussion into the promised land of calculus. For that, praise, admiration, and congratulations are in order. Again, I thank you! |
Do tubes have the same "advantage" in preamplifiers? |
"Do tubes have the same "advantage" in preamplifiers?"
That's a good question! For many like me with tube-unfriendly speakers, the pre-amp (and source perhaps) are the practical places to introduce tubes.
I love my ARC sp16 tube pre-amp, but I do wonder if a good SS pre-amp could also deliver for similar cost or less once the speaker/amp optimization has been achieved. |
I think they do. It would be really tricky to build a zero-feedback transistor preamp. With tubes you can do three stages of gain between the LOMC phono input and the line out, I doubt that the same can be said of transistors, at least, I've not seen any so far.
I have an additional comment about distortion that Joe mentioned- that of, shall we say, 'dynamic distortion'. Its my opinion that we need some sort of distortion test that uses a non-repeating waveform similar to what you see in real music. What Chaos Theory is saying about this is that sine waves represent a stable state in an amplifier circuit that might otherwise have chaotic response. BTW Norman Crowhurst pointed this out 50 years ago in his writings about negative feedback. Chaos Theory really seems to point to the idea that negative feedback is a destabilizing factor in amplifier circuit design, in fact, going so far as to call it non-linear.
This seems to fly in the face of traditional theory, where negative feedback occurs as something to increase linearity. But what we find by its application is that the energy of the distortion (nice calculus segue here BTW) is not changed at all- it is instead spread over the spectrum as a harmonic noise floor injected into the circuit's output. In addition, in-harmonic distortions are created due to inter-modulations at the feedback node.
The way the ear deals with this is interesting- our ears can penetrate natural noise floors like hiss or the wind blowing by about 20db, but can't do the same with this harmonic noise floor, which thus masks detail that exists below it. This is one reason why zero feedback circuits tend to be more spacious. |
There are many here much more qualified than I am to answer these questions, but, it would appear to me that it really wouldn't be an issue with preamp. I would think most pre-amps have more than enough steady state power necessary to drive a power amplifier. Head room and it's duration wouldn't be a necessary consideration.
Atmasphere, would you consider a passive pre a ss device? If so, where would the feedback be there? It seems as though some of your arguments are based upon opposing theories rather than laws. |
Trelja, thanks for the kind words.
Unsound, are you referring to the Threshold S/500? If so, there must be some inconsistencies around what is meant by "double its rated power for several minutes" . . . because I remember this amplifier having rather conventional headroom characteristics (but it's been at least 15 years since I've worked on one, so I may be wrong). To clarify, what I'm terming "dynamic headroom" will generally manifest itself as the ability to generate additional unclipped short-term voltage and current beyond the steady-state clipping power.
If "short term" means several minutes . . . then the limitation that keeps short-term capacity from being long-term capacity is almost surely thermal dissapation, not energy storage or output device current-limiting. The only SS amps that I can think of that exhibit this characteristic are what I would call "asymmetrical class H" operation, like the old NAD "power envelope" design. This works by having the amplifier operate from lower-voltage rails most of the time, but for large-signal peaks there is another set of commutating transistors (like conventional class G or H) that pulls the voltage up to a higher rail, and then keeps it there for a fairly long time-constant. The reason why it doesn't operate at the higher rail all the time is purely thermal . . . so I would actually consider it this design a higher-powered amp with thermal limitations, rather than a lower-powered amp with lots of headroom. But that's purely terminological I guess . . . |
Do tubes have the same "advantage" in preamplifiers? No. Tube preamp stages almost universally operate in Class A, so they have a steady-steady current draw regardless of the output level. Consumer preamps also usually operate into high-impedance loads (little current required), and virtually never have output transformers (to perform the power-supply impedance transformation). In a headroom sense, tube preamps do operate from much higher-voltage rails . . . and even though they tend to have more linearity problems as they approach their voltage limits, there's still usually a certain headroom advantage. But this is likely to only be an issue for something like a professional mic preamp, seeing the raw feed from a large-diaphragm dynamic mic that's three inches from a kick-drum head. Atmasphere wrote: I have an additional comment about distortion that Joe mentioned- that of, shall we say, 'dynamic distortion'. Its my opinion that we need some sort of distortion test that uses a non-repeating waveform similar to what you see in real music. I take it you're familiar with Matti Otala's 1972 AES paper that started the whole TIM measurement thing, and the huge number of papers and followup over several years in response? Do you feel that there are aspects of this issue that need further research? BTW Norman Crowhurst pointed this out 50 years ago in his writings about negative feedback. I take it you're referring to Crowhurst's two AES papers from 1957 and 1969. There are several historical things to keep in mind with how amplifiers were rated in those days, and most of what Crowhurst seems to have been concerned with in the 1957 paper is practical problems with the use of negative feedback as a method for reducing costs. He also makes limited to "regenerative distortion" (feedback making distortion of the distortion), but much of the theory here is very vague. If we look at the perception of simply how powerful the amplifier is . . . I think it breaks down over tube/solid-state lines much more than global-NFB/no-global-NFB lines, and I personally don't feel that negative feedback has a whole lot to do with it. Now for subjective sound quality and measured performance, feedback is a huge topic, and I'd love to get some discussion on these papers and their theory!! But let's start a new thread for it . . . |
@Ralph:
Found you comment about the MBL101e's interesting. Years ago, Michael Gindi had the original MBL, and the best sound I've ever heard from the MBL101. It was I think in part a large function of the size and dimensions of his room blending in with an omnidirectional speaker.
Now Michael had used several tube amps quite happily in those days with the MBLs including Ken Stevens original tube amps as well as the Jadis JA-500. Neither sounded shrill but one could still hear the sonic siggie of each amp.
Also, the impedance of estats drops too--as with my Martin Logans--to around 1 ohm or so in the upper octaves. Yet, the cj tube amps have always worked quite well with the ML (and I've tried many other ss and tube amps with them) despite their obviously not being able to increase power as the impedance drops (and there's the case of Quad 63 that sounded great with Futterman OTL because their impedance went up).
Cheers,
Myles |
Myles, the mbl has an impedance peak of about 8-9 ohms in the midrange driver. The designer is expecting the amp to reduce power by 3 db through this range. An amplifier with feedback will do it, one without will not. A good number of tube amps tend to sound shrill on this speaker even though otherwise they have plenty of power. So Michael was probably using an amp with a lot of feedback.
The problem here is that IMO, amps with feedback sound somewhat shrill out of the box, IOW its my opinion that a speaker that requires this will never sound like real music.
Kirkus, I know about the TIM articles but obviously amps designed to overcome that 'issue' were horrendous.
IMO the issue with feedback boils down to open loop propagation delay in the amplifier- IOW its a timing issue. The feedback signal simply does not arrive back at the input in time to make the correction. With a steady-state signal, the amp locks in pretty well over a few iterations, but with a constantly-changing waveform the amp will be chaotic. This is an interesting subject and I agree- a topic for another thread. |
What a incredible collection of very high-tech posts! You guys lost me at the first page of the string.I guess all the music I have been listening to while trying to follow along has put me into a brain overload.
I own Acoustat 2s with a really old tube preamp and a old ss power amp.This seems to be a pretty good combination, though I have to say I look forward trying out a tube mono block setup.
I will say that my experience is that tubes sound more alive and real to me.IMHO goes without saying,s
ince I have no where near the knowledge of you guys.I bow to your expertise.
Ralph: Will your 60watt OTLs work (with zeros) or do I need to save up more for your 140watt monsters?
e |
Always thought the Acoustats were designed around the "current" paradigm. I would be surprised if the M60s would do it, Zeros or not. Always seemed like SS was the way to go with that speaker, but.... |
I have heard them with tubes and they make magic.SS is surely the norm, but I have used a 60 watt Arcam SS amp and they were not bad,not as good as with the 120 watt Hafler but not bad. This made me wonder about the 60 watt tube OTL with zeros.
What I have found with these speakers is;the better you feed them (quality that is not quantity)the better they sound.
cheers
e |
Emorrisiv, I've heard the Acoustats sound great with a set of M-60s and a set of ZEROs. The guy that had them also had an ARC Classic 60, and he had to use the ZEROs with that amp also. They seemed to otherwise present a challenging load for a tube amp, but my guess is that they were doing what a lot of ESL manufacturers do, which is to set the impedance low to try to get transistors to work with them.
At any rate Acoustats and tubes are a great combo if you can overcome the impedance issue, and ZEROs do that quite well. |
Of course Acoustat made ss amps to go with their speakers. |
Yes, just as they also made a dedicated tube OTL-powered speaker. |
Yes, there were older Acoustats with dedicated OTL's, but the newer ones were recommended to be used with Acoustats own Transnova ss amps. Those old ss amps were pretty darn good for their day, especially for the then asking price. |
