No offense, but the whole "doubling down" discussion wins my award for the Most Useless Bit Of Armchair Technical Analysis . . . or the Most Misconstrewed Measurement Comparison . . . something that really shouldn't be paid hardly any attention to.
In this test, what is being compared is the the maximum power output of a given amplifier, before clipping, into a few various resistive load impedances. But there are several mechanisms by which an amplifier can be driven into clipping, so unless you know exactly how the given circuit is behaving for the test . . . the information is useless. It's even more useless when comparing two amplifiers of very different topologies (i.e. OTL tube vs. conventional emitter-follower Class AB SS), because they can behave so differently when overdriven.
Virtually all amplifiers can be driven into clipping by excessive voltage - that is, the output device(s) approach the power-supply rail(s) to the point where there simply isn't any more voltage available to transfer to the speaker. This is the case when an amplifier sees a high-impedance or open load.
But since the overwhelming majority of amplifiers use unregulated power supplies, as the load impedance to the amplifier is lowered, the power-supply rail(s) sag, which reduces the amount of voltage available. So even if the amplifier circuit was perfectly lossless, there still wouldn't be as much voltage available as the load decreased . . . thus there will be less than double the amount of power available if the load impedance is halved. THIS is how the whole "doubling-down" comparison started -- as a way to compare the power-supply robustness between similar solid-state amps.
A typical solid-state feedback amplifier has a very low output impedance, so into a "realistic" speaker load, it will indeed be the power-supply that determines its clipping output. But since we're basically testing the amplifier into two different resistive loads . . . this says NOTHING about the performance of the circuit itself into a "difficult", or reactive load.
But suppose instead of a "realistic" load . . . we're talking about a "super-demanding" load, maybe 1.5 ohms or so. For most conventional amplifiers, to drive such a load all the way to the power-supply rails puts the output devices into danger zone, so a current-limiting mechanism kicks in and makes the amplifier clip earlier. But this is still a resistive load, so we still know very little about how the circuit handles a "difficult" loudspeaker - the best we can infer is how much money was spent on the power tranformer, filter capacitors, and output silicon.
Now let's look at a single-ended Class A circuit, a la Nelson Pass. It can still voltage-clip at the supply rail with light loading, but as the load impedance is lowered, the amount of voltage available is ultimately limited by the standing current available in the output device(s) (by Ohm's Law), even assuming the power supply is perfectly regulated. So it's the "sag" in the amplifier circuit itself (that is, its high output impedance), that determines the clipping-power-vs.-load-impedance picture. And this amplifier will of course deal with a complex speaker load very differently than the conventional amplifier, but the "doubling-down" test reveals nothing about this.
The situation is very similar with a push-pull OTL amp, here (provided no feedback) the main limiting factor as the load decreases is the output tubes' plate resistance. But again, the "doubling-down" test can't reveal whether this is the result of a sagging power-supply or output stage, and says nothing about its performance into a loudspeaker load.
In a transformer-coupled tube amp the designer actually can make some trade-offs in this area - the load to the output tubes is commonly chosen to be a "sweet spot" to maximize power efficiency, while the output impedance is lowered by use of negative feedback. Thus, it will put out maximum power into the load impedance (or impedances, in the case of multiple taps) chosen by the designer. In this case, the "doubling down" test simply reveals to which output transformer tap the output leads are connected.
It seems that the real points of the Atma-Sphere "white paper" are about the disapproval of negative feedback, and tolerance of high output impedance. These are long-debated topics in audio, but the "doubling-down" test, and its discussion, is a very crude and inaccurate way of analyzing these characteristics and their effects. |
in the case of the B&W 802 you need to have 3 db more power into the 4 ohm woofer load as you do into the mids and highs because the woofers are 3 db less efficient. I am ignoring the effects of feedback here and doubtless there are amps that don't quite double power that can do OK on this speaker. Atmasphere, it still seems that you have confused the fact that the maximum clipping power of an amplifier is simply not any kind of indicator of its output impedance. And as you well know, as long as a given amplifier is below clipping, it is the output impedance, NOT the maximum clipping power, that determines how much a given (complete) loudspeaker's frequency response will vary simply as a result of its non-linear impedance characteristics. Incidentally in a loudspeaker design, it is NOT the efficiency and impedance of the drivers that are the biggest factor in determining how much impedance variation there is across the spectrum. In fact, most older (i.e. 1960s) loudspeakers have far more than 3dB efficiency difference between the direct-radiating bass/mid-bass driver(s) and their horn mids and highs. And these are speakers that you would consider to have relatively flat impedance curves, and are very "OTL-friendly". Rather, it is the relationship of the bass drivers to the cabinet, and the choices made in crossover design, that are the key factors in determining the impedance curve of a complete loudspeaker. Which also of course has nothing to do with Paradigms of Philosophy or Increasing Cheapness on the part of loudspeaker designers, and especially has nothing to do with the Rise of Negative Feedback . . . It's true that loudspeaker designers over the past several decades have indeed moved toward designs that rely less on mechanical damping within the drivers themselves, and more in the combined behavior of loudspeaker cabinet. But this shift is attributable to the monumentally influential work of the Austrialian A. Neville Thiele in the 1960s, and the continuation/publishing of his work by Richard Small in the JAES in the 1970s - for which we have the equations and parameters that bear their two names. Also hugely important was the work of Edgar Villchur in acoustic-suspension loudspeakers and dome radiators, and the huge advances in materials science . . . These and many other truly scientific efforts are what we have to thank for the amazingly high level of sound reproduction that we enjoy today . . . so you'll forgive me if I bristle at a crumudgeonly, dogmatic analysis that attempts to force it into those old unsolveable harps of "tubes-vs-transistors" and "feedback=bad". |
Sorry Unsound, it's been way too many years since I've had a Threshold amp on the bench, or looked at a schematic of one, to comment intelligently. I seem to remember the Stasis output stage resembling the complimentary-feedback-pair configuration . . . which is pretty similar in behavior to a conventional emitter-follower arrangement . . . but I could be completely wrong.
Magfan, if we're going to actually go down the road on amplifier stability . . . then the main thing is for the designer to accurately analyze the phase margin across the ENTIRE bandwidth of the amplifier - and this can be a challange with one (i.e. the vast majority) that uses a tranresistance amp for voltage gain, as this is the one configuration of a bipolar transistor where the gain is highly beta-dependent. Usually the open-loop gain follows a first-order slope over most of the audio spectrum, making the phase margin 90 degrees into a resistive load. So it's usually a capacitive load that causes the alarming decrease in phase margin, hence the use of an output inductor (and frequently a parallel Zobel network) within the amplifier. |
The Power paradigm term for this is 'power amplifier' since such amplifiers attempt to make constant power into all loads rather than constant voltage. Er, no, unless you're inventing your own terminology here. A voltage amplifier amplifies voltage, as in a small-signal transconductance stage. A current amplifier amplifies current, as in a cathode- or emitter-follower. So . . . a "power amplifier" amplifies both, as in the output stage of a typical transformer-coupled tube amp. As a complete device, an audio "power amplifier" of course must amplify both in order to be of much use, hence its name. |
First, I'm not talking about clipping here- at all. I am well aware of the significance of the woofer in the box! Atmasphere, clipping power is precisely what this thread, and "doubling-down" is all about. What I'm confused about is why you seem to be discussing clipping-power specifications and output-impedance specifications as if they're interchangable . . . or at least a common debate. They're not. What is important to note here is the word paradigm. If you are operating solely within a paradigm, anything outside that paradigm can be construed as blasphemy.
The Power Paradigm amplifier is a 'power source', i.e. it will make constant power into any load. That is the voltage and current will both vary. I don't know of an amp that does this but that is the ideal, just as there are no true 'constant voltage' amplifiers out there either- that is the ideal. Does this clarify things? Blasphemy is for the dogmatic, and I think it's best not to look at audio this way :). A typical high-quality conventional solid-state amplifier is pretty damn close to a pure voltage source. There's no reason why one couldn't build one that was almost a perfect constant-current source as well (except that it would severely alter the frequency response of the attached loudspeaker). But if it's a perfect "power source" you want, then you simply need to build a passive network that inversely approximates the impedance of the speaker, and use it in series with an amplifier that has a low output impedance. Before the Voltage paradigm was proposed (MacIntosh and EV were two proponents in the 50s and 60s) the Power paradigm was the only game in town. I have had to create the terms 'Voltage Paradigm' and 'Power Paradigm' simply because the industry is mum on this subject in general- its inconvenient. I think the industry is mum because these Paradigms only really exist on the Atmasphere website. Exactly who proposed this 'Voltage Paradigm'? Can you cite it as a bibliographical source, like one should in a proper scholarly paper? I know we've been down this road before . . . but if you actually measure the output impedance of most hi-fi amplifiers from the 1950s and 1960s (McIntosh, Marantz, Scott, Dyna, Fisher, Quad, Leek, Citation, Eico, etc. etc.), they have a reasonably low output impedance . . . low enough to keep the impedance-related response variation of even a modern loudspeaker within +/- a dB or so. Did these manufacturers jump the gun on the Voltage Paradigm, not realizing it shouldn't be in effect until . . er . . Thiele and Small had ratified it? Of course not. Atmasphere, I understand and respect that you design your products (and analyze their measured performance) in a way that meets your specific technical goals and personal preferences. But as for these "white papers" that you present on your website and promote on these forums . . . they simply don't pass muster in terms of technical or historical accuracy, or good scholarly form . . . the Grand Conspiracy overtones being particularly tiresome. |
Best Thanksgiving and holiday wishes to all . . . I am not discussing clipping at all, nor do I see clipping power as interchangeable with output impedance- no idea where you got that. Atmasphere, here's where I got that . . . quoting from your "white paper" that started this whole thread: Let's say you have a high quality 150/channel transistor amp. 150 watts into 8 ohms, a reasonable amount of power, but if you have a four Ohm speaker its 300 watts. Nice. Into 2 Ohms, if the amp doesn't blow up or current limit, 600 watts. So what does the amp produce driving 16 Ohms? 75 watts. Into 32 Ohms its only 35 watts! . . .
. . . This is what the right OTL can do into these impedances: 150 watts into 8 ohms, 145 into four (less than 1/2db difference), about 80 watts into 2 ohms, but into 16 we have 149 watts, into 32 ohms 145 watts . . . Are you not, in all the wattages above, referring to the maximum power available, BEFORE CLIPPING, into various resistive load impedances? If not, to what are you referring? And from these specifications, you conclude: Thus there is no way that a transistor amp can be described as linear if it is subject to these problems and that is one of the reasons why transistor amps produce so many amusical colorations. The reason has to do with the vanishingly small output impedance of the transistor amp In your Power Paradigm "white paper", the same conclusions are made: Let's say you have a high quality 150/channel transistor amp. 150 watts into 8 ohms, a reasonable amount of power, but if you have a four Ohm speaker its 300 watts. Nice. Into 2 Ohms, if the amp doesn't blow up or current limit, 600 watts. So what does the amp produce driving 16 Ohms? 75 watts. Into 32 Ohms its only 35 watts! This could result in serious problems were the speaker a typical electrostatic, where such impedances are common in the bass frequencies. This explains why transistor amplifiers are usually such a poor match for electrostatic speakers. No, these power ratings say absolutely NOTHING as to why an amplifier may be a good or a poor match for an electrostatic speaker. Or do you mean output impedance here as well? Can you not see that you use the concepts of maximum clipping power and output impedance interchangibly, or you feel that one is an accurate indication of the other? Kirkus, its clear to me that your perspective is that of the Voltage Paradigm Actually, I don't feel personally polarized on any of these issues . . . I find it far more interesting to try to work to understand the actual correlations between circuit design, measured performance, and perceived quality of sound reproduction. And to this end, it seems obvious to me that factors such as output impedance, maximum power vs. load impedance, type of feedback employed, and circuit topology are best considered and analyzed individually, rather than as a group or belief system. |
Unsound, thanks for the link to the Threshold Stasis article. From the description given, there are two main differences between the Stasis circuit and a conventional solid-state amplifier.
First the output stage -- a standard bipolar emitter-follower output section can be thought of as using 100% local voltage feedback, purely a function of the transistors' exponential Vbe/Ic characteristic, which delivers very good linearity and low output impedance (even before global feedback) as a result of the bipolar transistor's high transconductance. It seems that the Stasis circuit instead uses local current feedback to effectively bootstrap the output transistors to the voltage amp, and its linearity and output impedance will be a result from the particulars of the "current sensing" circuit employed (which I assume to be quite effective).
The other main difference isn't so much a Stasis thing, but it's the fact they use FET transconductance voltage amps, rather than a bipolar transresistance voltage amp. This means there's a fraction of the raw open-loop gain available. According to the text, there's no overall feedback, so I'm not quite sure about the hows and whys of the differential input stage, but suffice it to say that a low-gain FET voltage amp is a symbiotic choice when paired with a low- or zero-global-feedback design.
But in terms of clipping, I think the overall output impedance is pretty low, so it should still behave very similarly to a conventional amplifier as far as "doubling-down" is concerned. |
Atmasphere, we're definately on the same page as far as the system-matching aspect of different amplifiers being optimum for different loudspeakers, especially when comparing among various speakers and amplifiers that use very different technologies and design approaches. But, er, well take this statement: Power Theory (or Power Paradigm) is where the amp seeks to make constant power into all loads. It will not succeed, but that is the goal. The Dynaco ST-70 is a good example, 4,8,16 ohms its 35 watts. Our own MA-2 is another, 4,8,16 ohm 220 watts. Some transistor amps fall into this category. I'm sorry, but the reasons why a ST-70 and a MA-2 both have constant power ratings into 4,8, and 16 ohms are completely different from each other! Further, a ST-70 has a much lower output impedance than that rated for the MA-2, and thus will interact with the loudspeaker impedance to a much different degree. To use your B&W 802 example . . . according to Stereophile's measurements, this loudspeaker varies from a low of about 3 ohms in the mid-bass, with a peak of over 21 ohms in the midrange. Looking at my notes from the last time I measured a stock ST-70, it had an output impedance of about half an ohm over most of the audioband, from the 8-ohm tap . . . so I'll take an educated guess and assume it's about 0.29 ohms from the 4-ohm tap, which is what you would use for the 802. So a ST-70 driving a B&W 802 would thus have about a 0.68dB response peak in the midrange due to the interaction of the amplifier output impedance and the speaker's impedance curve. For comparison, a hypothetical traditional solid-state amplifier (0.05 ohm output impedance) would have a 0.12dB response variation. But the MA-2 (published 1.75 ohms output impedance) would have a peak of 2.3dB! And for the MA-1 and M-60, the deviation is even greater . . . 4.04dB and 5.93dB respectively. Now we're in complete agreement that the B&W 802 is a bit of an extreme case, and also that it's a poor match to an Atma-Sphere amp. And I'm not suggesting this is an inherently bad thing about Atma-Sphere amps (just a "system misapplication" if you will), or that the Dyna is a stellar match either . . . I'm personally a fan of the ST-70 but am also very familiar with its myriad shortcomings. But the ST-70 can hardly be considered a "Power Paradigm" amplifier because of its comparatively low output impedance. Let me again re-iterate my original point: The suitability of an amplifier for driving a particular speaker simply CANNOT be inferred from looking at its clipping-power ratings into various load impedances! Ever. Period. It's output impedance that makes the difference. Two secondary points: First, historically, there has been no Paradigm Shift - common practice loudspeaker design has ALWAYS been about constant voltage with frequency . . . as most vintage hi-fi tube amplifiers (such as the ST-70) have low output impedances, especially when compared to loudspeaker impedance curves of the day. Second, this is NOT a tube/transistor thing . . . there are many examples of solid-state amps with high output impedances in addition to tube amps with low output impedances. Incidentally I actually commend Atma-Sphere for publishing their output-impedance specifications . . . |
