Double down, good or bad?


I came across this article on Atma Sphere's website:

http://www.atma-sphere.com/papers/myth.html

In short, Atma Sphere believes having a power amp that is capable of doubling its power when impedance is half is not necessarily a good thing because speakers in general do not have a flat impedance across all freq range.

On paper, it does make sense. Though I am sure speaker designers take that into consideration and reduce/increase output where necessary to achieve the flatest freq response, that explains why most of the speakers measured by Stereophile or other magazines have near flat responses.

But what if designer use tube amps to design his speakers, mating them with solid state should yield higher bass output in general? Vice versa, tube amps yield less bass output at home?

I have always been a tube guy and learned to live with less bass weight/impact in exchange of better midrange/top end. Will one be better off buying the same exact amp the speakers were "voiced" with, not that it will guarantee good sound, at least not to everyone's ear.
semi
I always thought that doubling down was the sign of a technically correct amp that could handle the "WICKED LOADS" (without oscillating or otherwise becoming unstable) rather than a requirement.
I would never go so far to say it should be avoided. Ralph knows a lot about amps. By his own admission he pushes the technical envelope and is quite a design radical. His amps don't double down. I am sure he has had to explain that to a lot of potential cutomers.
Read Nelson Pass's discussions-see passlabs.com. Nuff said.
I'll take it a step further- there aren't **any** tube amps that double power, yet the tubes vs transistors debate has been here for 40 years... what does that tell you? The tubes failed to be the 'obsolete' technology that they were supposed to be- people keep listening to them.

The simple answer is that they must be doing something right!

The ability to double power as impedance is halved has been entirely a solid state thing. The idea behind it is that the amp is considered 'load impervious' if it can make constant voltage output into all loads, IOW double power as impedance is halved (conversely halve the power as impedance is doubled), so you can get flat frequency response from a speaker that has a box resonance. But what if the impedance curve of your speaker isn't based on a box resonance, like an ESL, magnetic planar or horn system? The model starts to fall apart.

So here's where that leads:

http://www.atma-sphere.com/papers/paradigm_paper2.htm

Its not really a tube vs transistor thing after all- its a bit bigger than that, just as it is not about objectivist vs subjectivist, and we are talking about equipment matching too. Its all those things, but also about the fact that the industry has a dilemma: make it look good on paper to make lots of dollars, or make it to sound good to the ear to make lots of good sound?

Its my contention that this has been going on a long time and the industry does not like to talk about it (doing so reveals the purely monetary angle), resulting in a ton of confusion (and often bad sound) in the audio community.
Swanny definitely has too much faith in 1 person. I work in leading edge semiconductor industry and over 10 years ago people thought we could not follow Moore's law any more, guess what? If we stop researching and understanding the technology, we will not progress. Still think it's nuff said?

I am curious about amp/speaker impedance interaction, I have owned numerous speakers and amps in my life, in fact I also owned several Pass amps. Nothing is perfect and there is not one amp or speaker that is superior than others, it's always a compromise game. But learning where to compromise is the fun part of this game.
Semi, I worked in a FAB for 30+ yrs. Yep, moore's law SEEMS to apply. Any new technology will make such rapid progress at the beginning. I've worked from submicron to power devices, many places in between, started with evaporation and saw the start of semiconductor sputtering. Etch from all wet to dry. You name it.
As I'm sure you well know, there are theoretical limits to speed, density and even memory limits. What do you do after you have junctions a few angrstroms thick with linewidths to match? How do you implant that and not either overdrive and still have l effective?
Certainly there will be opportunities in the future. nano devices, and even superconductors may play a part, but once the 'low hanging fruit' is gone, the pace of density increase must slow, if not reach saturation.

To help you close your 'amp/speaker impedance interaction' problem, please look up Power Factor on google. The Wiki article will explain much.
\
Also, since you are a semiconductor process (I hope) guy, you should be able to adapt over to the Smith Chart.


http://www.hometheaterhifi.com/component/content/article/228.html
Ralph, As much as I respect your opinions, I don't agree with the suggestion that amps that measure well don't sound good, and amps that sound good don't measure well. While I'm not suggesting that we have all the neccesary measurements to guarantee terrific sounding amps yet, I do suggest that we currently have the neccesary measurements measurements to guarantee that we don't make terrible sounding amps. Obviously the same token that keeps tube amps a marketable comodity, does the same thing for solid state amps.
We are not here to argue, we are here to learn from each other's knowledge.

Magfan, glad to see another fab guy. I am a yield guy because of my EE/device physic background, but know process well enough to do my job. As we shrank from 0.5um, we added LDD, halo, and now HKMG to deal with leakage. On top, both N & P stress are going wild and we have far exceeded Moore's law prediction in transistor drive current from linear scaling! All thanks to brilliant minds that work endlessly to produce faster and smaller chips :)

I think amplifier & speaker interaction is still an area that's not well understood when coupled with psycho-acoustic, human do not hear like machine for sure. If there is a perfect amp, Nelson Pass would not have changed and optimized his design constantly. Didn't he think X series amp was perfect when it was introduced several years back, why the X.5 update?
I have a Pass XA.5. Its output doubles as impedance is halved. It replaced a Pass XA amp. Its output was constant. In my system, there is no comparison between the two. The XA.5 sounds much better. There is now much better control of the lower frequencies. Bass is more articulate, fuller and deeper. I'm sure results will be different for different amps and different speakers.

I wasn't aware that Nelson Pass thought his X series amp was perfect. That is a very interesting statement, er, question you pose.
As Atmasphere says.....it's not really a tube vs transistor thing.
The Halcro amps do not double power as the the impedance halves and Bruce Candy is a 'disbeliever' in very low impedance speakers thus his amps do not perform well into 'difficult' loads.
Does he lose a heap of sales because of that?......probably.
Unsound, I agree with you. I should point out that what you said in your last post is not anything I was suggesting. I think its nice if an amp measures well, if it also sounds good to the human ear.

What I have found is that the physics of making an amp sound good has to do first with understanding the rules of human hearing, then applying the physics to get the amp to follow those rules.

Negative feedback is a primary violator of one of the fundamental rules of human hearing (its use enhances the harmonics that the ear uses to determine volume of a sound) and a lot of amps use feedback to reduce distortion i.e. look good on the bench. Amplifiers that have good bench specs and no feedback are rare but they exist. Interestingly enough, most of those amps also have a reputation for good sound too.
Semi,
did you look up the Smith Chart?
That will help in your quest.
We can conclude then that a technically correct amp is good. Doulbe down if done right is good. See(hear?) Bryston amps. It depends on where you are going and how you want to get there.
If you pass on Ralphs' amps merely because they don't double down you are making a mistake. On the other hand there are speakrs that should not be mated with Atmasphere.
If you pass on Ralphs' amps merely because they don't double down you are making a mistake
I agree. BTW, as Ralph's amps use tubes they do NOT double down. It's not a characteristic; They can do other things (like, operate safely without feedback). Indeed, some tube circuits half down!

Some one once noted that if an amp sounds good & measures bad, you're measuring the wrong things.
I've got an amp that's rated at 250 watts into 8 ohms, 400 into 4....

suppose tomorrow the manufacturer decides to re-introduce this identical amp...new fancy face plate..absolutely identical electrical design. However...

it's new ratings are now 100 watts into 8 ohms, 200 watts into 4 ohms, 400 watts into 2 ohms...etc.

does it now sound better? This "double down" can be as much specmanship as anything else. IMHO of course....
The virtue of an amps ability to double down just might depend on the speakers on which the amp is to be used on and the amp budget of the owner of those speakers.
Rayoo makes a good point. Of course, and though it has been done before, one could argue that understating an amps power input into higher impedances is a misrepresentation and could potentialy void the warranty on a given speaker.
The virtue of an amps ability to double down just might depend on the speakers on which the amp is to be used on and the amp budget of the owner of those speakers
No, no, it's just a matter of the circuit. SS amps, by design (constant voltage), act like that. The ability is just a characteristic of the circuit.

Anyway, most SS amps do not DOUBLE down -- only their specs do: take an amp that measures 100W into 2ohms (not a bad amp that!), then spec it 50W into 4ohms & 25 into 8 ohm. This amp's selling spec is: 25W/8 ohm.
Gregm, perhaps I used the wrong words, what I meant was that the benefit of having and amp that's able to double down, just might depend on the speakers on which the amp is to be used on and the amp budget of the owner of those speakers.
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.
Kirkus, it seems you got my point in your last paragraph. We could easily build an OTL with an output impedance of 0.1 ohms with enough feedback, but it would never sound right, and the lower output impedance would say nothing about what loads it could effectively drive.

The other aspect of the conflict of the Voltage and Power paradigms that is important here is equipment matching- any time you mix the two, you get tonal anomalies: a transistor amp driving horns for example will be shrill, a tube amp driving a set of B&W 802s will have insufficient bass.

In the case of the horns, the drivers are very reactive and the back EMF wreacks havoc in the feedback loop of the traditional transistor amp, making the presentation shrill, in the case of the B&W, the woofers are in parallel (4 ohms) and are 3 db less efficient than the 8 ohm midrange and tweeter; if a transistor amp is used it will have double the power into the 4 ohm portion of the impedance curve, thus driving the woofers with the 3 db extra needed for them to operate with the midrange and tweeter. The speaker cannot be driven properly by any amp that does not double power as impedance is halved.

There are many other examples!
Awesome and informative post, Kirk! Thanks.

Best regards,
-- Al
In the case of the B&W, the woofers are in parallel (4 ohms) and are 3 db less efficient than the 8 ohm midrange and tweeter; if a transistor amp is used it will have double the power into the 4 ohm portion of the impedance curve, thus driving the woofers with the 3 db extra needed for them to operate with the midrange and tweeter. The speaker cannot be driven properly by any amp that does not double power as impedance is halved.
Ralph, re "the speaker cannot be driven properly by any amp that does not double power as impedance is halved," you may recall that we discussed a similar point in another thread a couple of months ago. Wouldn't it be more correct to say that to be able to drive this impedance characteristic properly the amp has to have a low output impedance (relative to 4 ohms), and power capability into 4 ohms that is adequate to support the required volume? In other words, a solid state amp rated at say 200W into 8 ohms, but only 300W into 4 ohms, will do just as well with this speaker (IF all else is equal, and if the output impedance and 4 ohm output power criteria are met) as a solid state amp rated at 200W into 8 ohms and 400W into 4 ohms.

Regards,
-- Al
Kirkus, how do the current dumping type amps like the Threshold Stasis designs fit into your thinking?
Al, 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. I point it out though, because your typical tube amp is not going to work right on this speaker- it literally is designed with the constant voltage model -the Voltage Paradigm- in mind.
voltage source / current source?

I wish I could find the link to the answer to Kirkus post.
This page called out an amp test to determine true output into real loads.
4 ohms / 8 ohms and 45degree inductive / 45degree capacitive.

The area under the square, when graphed properly will show load compatibility of the amp in question.

Most amps, except HT stuff and lowest tier consumer stuff do at least well into a resistive load. Throw in a reactive load to see what you really have.
Magfan, voltage source / current source are Voltage Paradigm terms. A Current Source amplifier is one with a high output impedance. The Power paradigm term for this is 'power amplifier' since such amplifiers attempt to make constant power into all loads rather than constant voltage.

The two approaches require different speaker design considerations! Neither requires a flat impedance curve, but it you want to compare the two types of amplifiers, you need a speaker with a flat impedance curve and an adequate minimum impedance (8 ohms as opposed to 4 for example).
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.
Kirkus, you are right in most of your posts and apparently either did not read something somewhere in mine or misinterpreted it. First, I'm not talking about clipping here- at all. I am well aware of the significance of the woofer in the box! The very point of a woofer in the box is that there is a resonance. Voltage paradigm amplifiers ('voltage source' amps) will throttle back their power as they encounter the resonant peak of the box, and so not exacerbate that resonance. That is the basis of the theory of the idea of making an amp that is a 'voltage source'.

Maybe you missed the link, so here it is again:
http://www.atma-sphere.com/papers/paradigm_paper2.html

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.

Yet the web audio forums are full of people that are constantly harping about aspects the the conflict between these two paradigms- tubes vs transistors is one of those debates that seems to arise from this conflict. But if you think my posts are *about* tubes vs transistors you have missed the point entirely!

The Voltage Paradigm uses a sort of short hand nomenclature that leads to confusion and I think this is happening for you in this case. Of course all amplifiers are power amplifiers, what the Voltage Paradigm seeks to do is call an amplifier a 'voltage source' when it refers to an amplifier that makes constant voltage into any load (i.e. doubles its power as the load impedance is cut in half). Amplifiers that have a high output impedance and make (or attempt to make) constant power are termed 'current source' amplifiers.

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?
What the Voltage Paradigm seeks to do is call an amplifier a 'voltage source' when it refers to an amplifier that makes constant voltage into any load (i.e. doubles its power as the load impedance is cut in half).
For the benefit of others who may read this, and along the lines of my earlier post in this thread, I think it is important to clarify here that "doubles its power as the load impedance is cut in half" in this context does NOT necessarily mean that the power RATING doubles as the load impedance is cut in half.

It means simply that a voltage paradigm amplifier, as a consequence of its very low output impedance, WILL deliver twice as much power into a 4 ohm load as into an 8 ohm load, provided that it is not called upon to deliver more power than it is capable of delivering (e.g., it reaches the clipping point). And that maximum power capability will in some cases correspond to significantly less than twice the number of watts at 4 ohms as at 8 ohms.

Regards,
-- Al
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.
Kirkus, does this description help?:
http://www.stereophile.com/solidpoweramps/16threshold/index.html
I am not a techie on the level of Nelson pass or Atmapshere (still learning), but I do have some experiences of my own, using a well built tube amp, one can use almost any speaker regardless of load, if the amps are built correctly. For example, many folks like to state that B&W or Magneplanar speakers are very hard to drive and require at least 200 watts or more. The tube amps I use drive these types of speakers easily and make wonderful music. I have been on the tube rolling circus and also discovered with the right combination, I can turn off my massive 15" subwoofer as these amps deliver deep enough to not have a sub. On some really stuff, such as deep pedal organ's, well I have to have the sub on, but the slam of the kick drum is all there.

From the reports that are starting to fill in on these amps, I think that those that state that tubes doen right are great with almost any speaker load. My amps are rated to handle loads as low as 2ohms.

http://www.octave.de/english/products/PowerAmplifiers/INFO-MRE130.htm

http://www.sixmoons.com/audioreviews/octave3/octave.html

Ciao,
Audioquest4life
Audioquest4life, with all due respect you are indeed "(still learning)". It would take a hell of a tube amp to appropriately drive some Apogee's.
Unsound, we got a Golden Ear Award for our MA-1 driving a set of Apogees. This is a zero-feedback tube amp BTW.

Kirkus, its clear to me that your perspective is that of the Voltage Paradigm. As to papers on the subject, one was written by one of the designers at EV. The way tube amp manufacturers were getting their 'output impedance' down was by adding feedback- to the detriment of the resulting sound quality. The HK Citation 2 is a good example- a fair amount of feedback, used to reduce the distortion imposed by the AB pentode-based transformer-coupled output section. Despite that though, the amplifier fails to double power as the load impedance is halved. And we are not talking about clipping!!

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.

I am not discussing clipping at all, nor do I see clipping power as interchangeable with output impedance- no idea where you got that.

FWIW the 'Grand Conspiracy' thing you mention seems to me an example of adding meaning where none existed prior. All I am pointing out is what is causing the tube/transistor debate, the objective/subjective debate and the equipment matching conversation- they are all the same thing. There is a secondary conversation regarding the rules of human hearing/perception, wherein I contend that it is important to understand those rules and adhere to them as design principles. FWIW, that, for the most part, is not happening in audio.
Ralph, as I said, "it would take a hell of a tube amp...":-) Now if they were Scintillas, I'll recant my earlier post.
Unsound, I've heard a set of our MA-1s driving the old Full Ranges, which were one ohm (we used a set of autoformers that had one ohm taps). There is an old TAS review which mentions that from the early 90s as the speakers were owned by a TAS reveiwer.

Seems to me Paul Bolin used Duetta Signatures.
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.
Ralph & Kirk,

I think that the confusion (or lack thereof) between power at the clipping point and output impedance comes down to what I said in my post dated 11/24. It is important to clearly maintain a distinction, in discussions such as we have in this thread, between doubling down of an amplifier's RATED power output into 4 ohms vs. 8 ohms (which is what this thread was originally about, and which amplifiers having very low output impedance may or may not be able to do), and the fact that ANY amplifier having very low output impedance WILL deliver double the power into a 4 ohm speaker impedance vs. an 8 ohm speaker impedance, provided that it is not driven to the point where its output is limited by clipping or other factors.

Best regards,
-- Al
Ralph, sorry but to my mind that's cheating. Were the tube amps driving auto-transformers or Apogees?
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.
Kirkus, thank you for sharing your insight.
This is a very enightening thread. I have followed along as I used to be real SS die hard, considering even to delve into ultra ML or Classe stuff until I heard a pair of excellant tube amps that were designed to operate on 4 ohms, and drive low impedence speakers:

Push-pull mono output stages with new driver electronics for the output tubes. This all-new design improves the transient response with complex loads and reduces distortion - particularly when low impedance speakers are connected. Ideal for critical loudspeakers such as Martin Logan, Avalon, Audio Physik, Caldera, Isophon Vertigo and Europa.

Clearly, I can agree with Atmasphere on how a design is capable of perforimng such tasks as driving low impedance speakers, it is most likely the topology in that implementation that needs to be considered. My amps, the Octave MRE 130 are rated at 130, tested and reviewed at 140, but 100 watts into 8 ohms.

All I can say is with technology like that, I can drive easily a pair of B&W 800's in my 25x18.5 listening room with thunderous bass and more than enough volume to make it concert level. The addition of the so called "Super Black Box" and their promised benefits in bass control and 2-ohm stability. The Super Black Box has an additional 100 plus amps of current and capacitance. I am not sure how other companies would implement such a feature, perhaps larger power supplies, but this concept works with these amps.

I believe that that tube amp builders who design amps capable of driving low impedance speakers from the start are actually doing the audio community justice.



Ciao,
Audioquest4life
Audioquest4life:
I see a few posts back that you state the Magnepans are a difficult load to drive.
Not So. The impedance is pretty flat and at least for the 1.6s, the phase angle is moderate at worst. The only fly in the ointment is they seem to like current.
That being said, even my old Rotel RB1070 of 130x2 into 8 with NO 4ohm spec did well until pushed.

Stereophile measures phase / impedance but few manufacturers publish such data.
Magfan, I also find the Magnaplanars easy to drive.

Unsound, the TAS review using the Duettas did not use the autoformer, but in the case of the Full Ranges, without an output transformer of some sort, driving 1 ohm is difficult, even for transistors. Its not 'cheating'- its simply a tube amp that was able to do the job. The speaker otherwise is easy to drive.

Kirkus, I see now why we were not on the same page- I was referring only to my posts. You are right- the article is over-simplified. So I think we are really on the same page here. With regards to 'belief systems', what is happening in high end audio is tubes are effectively competing with transistors, and there are speakers (like the Wilson Watt/Puppy) that are more effectively driven by tubes than transistors and other speakers (like the B&W 802) that are more effectively driven by transistors.

People are constantly trying to mix the two technologies and in the process are flushing a lot of money down the loo.

The Paradigm paper seeks to expose why, and in the process also expose something that I regard as the Emperor's New Clothes- that aspect wherein products are made to look good on paper and ignore human hearing/perceptual rules; that whole thing that all of us are familiar with where the specs tell you nothing about how it sounds. IMO/IME we as an industry are measuring the wrong things, and there is little or no discussion about that!

I know of a neuro-scientist that has done studies of human hearing- he has found that when the audio playback system violates our perceptual rules, the processing of the sounds in the brain is actually handled in a completely different area!
Atmasphere,
Could you please elaborate a bit about the point in your last post where you say some speakers are more effectively driven by tubes and others by transistors. I do not see much written about this and am curious as to why. Thanks.
I agree with atmosphere that it is the final outcome that matters more than measurements. The same can be said for cables or brands of cassette tapes.
Peterayer, The drivability has to do with whether the driving amplifier is based on Power theory or Voltage Theory, and which approach the speaker designer used.

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.

Voltage Theory (or Voltage Paradigm) is where the amp seeks to make constant voltage into all loads. It will not succeed, but that is the goal. Such an amplifier doubles power as the load is cut in half. Lots of transistor amplifiers do this but only a handful of tube amps do.

The speaker designer may have used an amplifier for reference. If so the speaker obeys the same rules that the amp does. So if the designer favors a tube amp, his speaker will likely follow the Power Theory. An example is Wilson Audio. If the designer used a speaker design program or favors a transistor amplifier, the speaker will be a Voltage Paradigm device. An example is the mbl 101E.

Anytime one kind of amp is used with the other kind of speaker, a tonal anomaly will result due to inappropriate power response. Here are some examples:
*Transistor amp on ESL =>bright high end, weak bass
*transistor amp on horn =>shrill high end
*tube amp with mbl101 => harsh midrange
*tube amp with B&W 802 =>weak bass

I have used the term 'Paradigm' in the past as those who operate within a paradigm seek to describe that which is outside that paradigm as wrong, on account of a restructuring of that individual's world view that the individual is understandably reluctant to do. It is easier for such individuals to see the alternative as wrong, but in so doing does not change the nature of reality.

This reluctance to accept reality has resulted in several debates in the audio world that are on-going and have been so for decades: tube vs transistor, subjectivist vs objectivist and the equipment matching conversation which you have asked about.

To deny the existence of the Power Paradigm denies the validity of ESLs, horns, magnetic planars, full-range drivers and a variety of acoustic suspension and bass reflex designs, as well as many tube amplifiers. The Power Paradigm places a high value on design responsiveness to human hearing/perceptual issues.

To deny the existence of the Voltage Paradigm is nearly impossible- that is what is seen in bench measurements from all magazines. Many acoustic suspension and bass reflex designs use the Voltage Paradigm rule. The Voltage Paradigm places a high value on design responsiveness to bench specification, particularly low distortion.

I side with the idea that audio equipment should obey human hearing/perceptual rules, as I maintain that human hearing is the most important aspect of audio. I get very little argument for the latter, but a lot of resistance to the former!
Ralph,Thank you for that explanation. I agree with your assertion that human hearing is the most important aspect of audio. I have found lately, that the speaker/room relationship or room acoustics may be the second.

I have not thought of or previously read about the "Power Paradigm" vs. the "Voltage Paradigm". It is interesting to view it from the speaker designer's point of view. I used to drive my Eggleston speakers with a Pass XA160 which had constant power to all loads, but I now use the Pass XA100.5 which doubles down as impedance is halved. The speaker's nominal load is 6 ohms, which means that the amps have about the same power. The latter combination sounds much better to me. From your explanation, am I to understand that the XA160 follows the "Power Paradigm" while the XA100.5 follows the "Voltage Paradigm" and that Eggleston probably designed the speaker with the latter in mind? It must sound better to me because the amp is better able to handle the load of the speaker. Or is it also because this paradigm places more value on low distortion, and that sounds better to me? This sounds overly simplified. Pardon me for my ignorance, but I'm interested and would like to better understand the issue. I'm fairly new to the hobby and don't have the technical background. Thanks.
Ralph,
Anytime one kind of amp is used with the other kind of speaker, a tonal anomaly will result due to inappropriate power response. Here are some examples:
*Transistor amp on ESL =>bright high end, weak bass
*transistor amp on horn =>shrill high end
*tube amp with mbl101 => harsh midrange
*tube amp with B&W 802 =>weak bass
Would you define power response? And explain why the 4 cases you listed have said behavior?