Damping Factor - Interesting article
Yep. This is why Stereophile tests with a simulated speaker load. Look at any amp review, and compare tube amps to SS in the measurements. For a counter point though, look at Nelson Pass. He wrote at least one article where he showed that with some speakers, low damping factor, not high, is ideal. Best,E |
kijanki ,... I am generally aware of the relationship between an amplifier's output impedance and a speaker's input impedance, the latter stat which varies as a function of frequency response. What I found surprising is that Benchmark writes that if an amp has an output impedance of 1 ohm, the output level of the speaker could vary by as much as 2 db. I own an ARC Ref 150 SE. Some years back, John Atkinson (JA) reviewed an earlier version of my amp and reported his bench test findings here: https://www.stereophile.com/content/audio-research-reference-150-power-amplifier-measurements JA's bench test findings report that the FR changes of Stereophile's hypothetical test speaker varied by only .8 db off the 8 ohm tap. Curiously, JA measured the Ref 150 as having an output impedance of 1 ohm. FWIW, ARC reports that the Ref 150 has a DF of 14, which permits the inference that the amp's output impedance is .57 ohms (or 8 divided by 14). I know you are a techie. Can you please explain why there appears to be a difference between JA's findings and the Benchmark report. Perhaps Atmasphere (Ralph) or Almarg (Al) can weigh in if they catch this thread. Thanks BIF |
Looking at the Sterephile simulated speaker load FR/Impedance diagram, you can see the lowest impedance is 6 ohm, vs the 2.6 ohm of the Focal. So their simulated speakers isn't as difficult to drive. WIth the Focal, the AR amp would have a higher variation, and you can figure out by how much using the equations from the Benchmark article. |
The Focal is an interesting speaker to chose, since I've analyzed at least one model in depth, and let me tell you, it deliberately dropped the impedance. No doubt in my mind that having a low impedance was a crossover design goal. Since then I've seen they have a remarkably similar profile in many of their speakers, and it does make a difference. It makes a speaker seem more "discerning" of amplifiers. "My Focal must be better because I can hear the difference in amps." is the message. Still, people don't want to chat about that, there are many other high end speakers which brush with low impedance. From B&W to your average ESL. The Stereophile simulated speaker load is more forgiving than some, but you can still see the impedance dependent effects there if you look at a tube amp review. The principle is the same, if not as large. Best, E |
I just looked at the speaker load Stereophile uses: https://www.stereophile.com/reference/60/index.html That is a very easy to drive load compared to a lot of the speakers they review today, or are typical. I wrote to them in the hopes they update it. This speaker is a much more typical load: https://www.stereophile.com/content/bowers-amp-wilkins-804-diamond-loudspeaker I am 100% sure they will ignore me. It's' my impression (with no formal data) that speakers have tended towards harder-to-drive as SS amplifiers got bigger, beefier and more expensive. Even the DIY speakers I make have a tough time staying above 3 Ohms to get decent sensitivity out of them. |
@bifwynne Benchmark selected 8 ohm speaker with 13 - 2.6 ohm impedance variation. One ohm of output impedance would reduce signal to 13/14 and 2.6/3.6 respectively. Ratio of reduced signals would be 1.286 (28.6% difference) = 2.18dB It would be much less for my 6ohm speakers that have minimum impedance of 3.6ohm |
Today I received the Benchmark StarQuad XLR that is referenced in this article. They are replacing 2 Audience Au24 and Au24 SE cables that cost over $1000 combined some years back. The 2 StarQuad’s were $200. It should be an interesting comparison. I expect no difference with my Benchmark gear but audible difference with other gear. |
@douglas_schroeder Yes, the difference between DF=20 and DF=2000 seems to be huge, but not for the purpose of damping. Amplifier with DF=2000 has very low output impedance and will provide steady voltage (ideal voltage source) at all frequencies while speaker impedance vary greatly. It will affect the sound because of that. Damping is different story. When we apply positive voltage speaker membrane moves forward. When membrane moves forward on its own speaker generates also positive voltage, but now current flows from speaker to amplifier. Opposite direction of this current produces opposite action on the membrane and membrane stops. This current depends on back EMF produced by the speaker and impedance in the circuit/loop. This loop contains speaker internal impedance, impedance of speaker wire and the output impedance of an amp. Let's assume for a moment that wire is perfect. Total resistance in the circuit will be resistive portion of the speaker impedance - most likely 6 ohm for 8 ohm speaker and resistance of the amps output. At DF=20 total resistance in the circuit will be 6 ohm + 8/20 ohm = 6.4 ohm while for DF=2000 it will be 6 ohm + 8/2000 ohm = 6.004 ohm. Ratio of braking current in the circuit will be equal to ratio of total resistance - in this case difference of 6.6%. Is it audible? Perhaps for trained ears, but it is not as dramatic as DF=20 vs DF=2000. There will be more sound change from less than perfect driving, than lower damping IMHO. For DF=200 vs DF=2000 difference in damping will be 0.6%. |
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@douglas_schroeder I don’t question what you heard at all. My experience is none compare to yours. I merely stated, that the sound difference you heard could likely be because of less stable output voltage while driving and not because of different damping. As I said, the difference in damping between DF=200 and DF=2000 is only 0.6% but many people believe it is 10x more. The other question is how this high DF was obtained. The easiest way to lower output impedance is to increase feedback. Adding 20dB to feedback lowers output impedance 10 times, but there are consequences of that. One of them is TIM, that produces unpleasant higher order odd harmonics, that make sound bright and unpleasant. Even Benchmark in AHB2, that doesn’t have negative feedback in normal sense (feedback is non-recursive), settled only for DF=350@20Hz. Class D might have inherently low output impedance because output Mosfets always clamp output to low impedance (either GND or VCC), but for the other classes I would even avoid amps with very high DF (like 2000). Something has to give IMHO. |
Kijanki, you are a respectful member of this forum, and I thank you for that politeness. Many would become obnoxious at such an exchange, and I would cease communicating with them at that point. It seemed to me that you questioned the result directly when you said, "Is it audible? Perhaps for trained ears, but it is not as dramatic as DF=20 vs DF=2000. There will be more sound change from less than perfect driving, than lower damping IMHO." My reaction to your comment is that you feel it is quite insignificant. I am telling you that you are wrong; the difference was more on the order of swapping an amp, or as you say, sound change from less than perfect driving (which is a good analogy). Regarding the Benchmark products, I am somewhat familiar with the culture of the Benchmark Audio mindset, and I also go against the grain when it comes to such things as the DAC being influenced by cables, or the amp by power cords. Read my reviews of these components also at Dagogo.com Some audiophiles, perhaps yourself, find it difficult to accept that there are significant sonic changes in situations where they calculate that there should not be. I'm in no position to argue with you about your prodigious specification assessment, but I am in a position to tell you that if you think Benchmark's sensitivity to cables, or Gold Note's PA-1175 Damping Factor function are insignificant, you would benefit from hearing such things. :) |
This is an important discussion. Thank Yous to the the OP and all posters responses. I was unaware of the importance of Damping Factor as a 'factor,' in maximizing synergies between speaker and amplifier, until I started considering Cube Audio's Nenuphar speaker. Both major reviews of the Nenuphar speaker address Damping Factor, and confirm what Erik shared in his post. In the case of the Nenuphar, both (reviewers) Srajan Ebaen and Dawid Grzyb find low / lower damping factor pairings to be ideal / preferred. Specifically, @erik_squires points out: For a counter point though, look at Nelson Pass. He wrote at least one article where he showed that with some speakers, low damping factor, not high, is ideal. Many Nenuphar owners report the same THOUGH I'm sure other factors (in addition to damping factor) are also at play. |
I am copying Doug Schroeder's discussion on "Damping Factor" from his Dagogo Gold Note PA-1175 Mk II amplifier review. Doug's findings with respect to this amp are, I believe, of significant importance and of critical value. For this reason, I am sharing it in it's entirety. The link to the review follows at the bottom of this post. From Page 2 of @douglas_schroeder 's review: "My enthusiasm for the amp was dampened slightly but became a white-hot flame of interest once I began to work with the unusual Damping Factor feature. I can put up with the quirks largely because of two things, the uncanny ability of Italians to get the Midrange perfect, and the “best of both worlds” experience the PA-1175 offers through use of the DF switch. In the current amplifier market Gold Note may have the coolest feature available in the Damping Factor. This is no gimmick or marketing ploy, but a serious uncommon advantage conferred upon the owner. Anyone who has spent time with solid state versus tube amplifiers knows the variance in sound can be dramatic. Speakers react substantially differently when hooked up to a solid-state amp with high power versus a tube amp with lower power. For our objectivist friends reading this, I have no desire to debate my perspective with you. Why am I discussing power when the Damping Factor is all about controlling the excursion of the drivers? For the simple reason that the DF feature alters the sound in such a way that one senses they have switched out the amp from a higher power SS design to a lower power tube design. That is how much the DF control changes the listening event. I have placed enough heavy monoblock class A or A/B amps with my speakers, and also enough heavy tube amps with those same speakers. to know the effect immediately. The DF operates in a fashion that gets similar adjustments to the sound as if had I physically moved two separate amps in and out of the system. As an aside, on this topic we see the proclamations of specifications-driven audiophiles shown to be wrong by the reality of performance. I have seen dismissals of Damping Factor as irrelevant to amplifier operation, or at a minimum much less important than current delivery or wattage. It takes about ten seconds hearing the distinction to know that Damping Factor has a major effect upon a speaker, and while it is not typically treated as seriously by the hobbyist as those other design considerations, it should be. Once I heard the effect of switchable DF, I began paying very careful attention to that specification. Gold note recommends “…use of the low DF (25) setting with high sensitivity speakers and mini-monitors as the amount of energy used to reproduce the lower range of frequencies usually suits these kind of speakers best… Bigger or mid/low sensitivity speakers perform better when driven by a higher power amp with a higher DF.” While I agree in general with that guidance, there are a couple of things to consider, one being the overall quality of the speaker. Some speakers are not terribly precise, regardless of the advertising that claims otherwise. I would tend to run such speakers with the higher DF in order to tighten them up, and I would turn to cabling to tune them in terms of tonality. Conversely, a speaker with such beautiful expression as the just reviewed VanL Speakerworks Silhouette is a treat to hear with either of the DF settings. The Silhouette is a purist’s speaker, a refreshing departure from the crass offerings without the signs of a master craftsman’s touch. It is a minimalist speaker design, as is evidenced in the supreme quality of the 6/5” wooden cone dual motor mid/bass driver and minimal crossover, and as such lets the alternative settings of the PA-1175 be expressed readily with deftness. The Silhouette is not designed to be bone crunching, but soul caressing. See my article for a thorough introduction to this rare gem. The other consideration is that there is no telling what the preference of the listener will be. Some people enjoy a fuller, less tight low end. Some people prefer a sharper, laser guided upper end. Others insist on a predominant midrange that draws attention away from both the upper and lower end, even to the point of suggesting anything lower than 40 Hz is unnecessary for acceptable recreation of the event. For that reason it is worth trying the DF setting in both positions regardless of the speakers you are running. As the PA-1175 is robust, and the power structure is not affected by it, you should not enter into a situation where a speaker will be underpowered by going to the low DF setting." https://www.dagogo.com/gold-note-pa-1175-mkii-solid-state-stereo-amplifier-review/ |
I have paid very close attention to damping factor over the years starting with when I acquired my large Ohm Walsh speakers that use a single 12” driver operating Walsh style with wave bending. I had read the large 12” driver benefitted from high damping and have found that to be very true. I started off with a Carver m4.0t amp that is designed to emulate a tube amp with relatively high output impedance. This did very well with larger Maggie’s and modest tower design B&W P6 but I found I could do better with the Ohms. I landed on the Class D Bel Canto amps which bring a higher level of control to the 12” Walsh style drivers. Everything is clearer and better especially bass which became clearly nuanced and articulate compared to prior. I’ve used other smaller monitor speakers with these amps as well and find the benefits of the higher damping varies more there speaker to speaker. Dynaudio contour monitors benefited but to a lesser degree. Small less extended Triangle Titus monitors even less. I run kef ls50s off the amps now and sound is very detailed and articulate. I heard these prior a few years back when someone brought a pair over to hear and found they sounded as described off my Class D amps but were actually quite bad, very flat and muddy, off a pair of tad hibachi amps I had at the time which were a zero feedback SS design. Huge difference with highly damped Class D versus zero feedback/high output impedance. So the correlation between damping factor and sound quality seems to exist at least based on my experience. Also I should note I tried a different more highly damped Class A SS amp prior to the Class D and yes brightness was an issue there possibly due to the bad harmonics associated with higher damping SS amps. All this is a big reason I am a sold Class D fanboy these days. Especially if you have larger speakers that need a beefy high current SS amp to take vice-like control of the drivers |
What amplifer are you talking about turnbrown? Some amps will have something like DF >400 at 20hz, I have seen some say DF > 300 20hz - 20khz @ 8Ohm. Use the Excel spreadsheet from Benchmark and input the numbers from your speakers. What Benchmark was showing is speakers that range from 2.6 Ohm to 18 Ohm DF of >200 was .1dB difference. Some like Pass use a low DF which produces higher dB in the Bass depending on speakers used. |
djones512,195 posts06-06-2020 8:41am"What amplifer are you talking about turnbrown?...." Marantz Integated Amps are a good example of DF specs with no load or frequency specified. Hegel, who prides themselves on very high DFs, list no load or frequency in their specifications. |
In the Benchmark paper it says most amplifier specs assume 8 Ohm whether that's true for Hegel and marantz I have no idea. What's interesting to me about the article is they are saying low DF of 10 or high as 10,000 is not an issue damping of driver motion but a low DF can affect the sound of speakers because of variations in impedance of the speaker. I notice the Hegel integrateds show a DF of 4000, I can't imagine a speaker bad enough to for the DF to matter at 4000. |
General observation: much of the discussion above focuses on high DFs of 20, 200 or 2000. I surmise that most amps that fall into those DF categories are solid state amps that are constant (or near constant) voltage sources. I was hoping Atmosphere (Ralph) would have chimed in because he manufacturers very fine tube amps that probably have output impedances in the single digits. Anyone have any thoughts about the db spread on tube amps, even those that use NF?? BIF |
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Here is an older paper I found that explains the DF relationship between amp and speaker. It was a little easier for me to understand some of the numbers. https://butleraudio.com/damping2.php |
One last thing: Damping factor theory is basically series network analysis. There’s nothing here very complicated. You just string up your amp’s output impedance and put it in series with the load, then analyze the difference in output vs. F. Amp (+) --> Output Z --> Load Z --> Amp (-) A little work with a spreadsheet and a speaker impedance graph and you too could model this at home! Any EE learns to do this probably in the first semester of electronics. If you want to learn more search for "AC Circuit Analysis" There is one thing I’d like to say though: While network analysis like this is straightforward, the effects I’ve heard from speaker cables has led me to believe that amps are more susceptible to cable and speaker impedance than we would model this way. I don’t think the model as outlined above fully accounts for what I’ve heard in all instances. This does not mean speaker cables are worth $30,000. It just means I think there’s untapped research to be done there. Best, E |
Reaching out to those that know this stuff: does damping factor play a role with (influencing) perceived scale (image size)? If so, how? Thanks. Member @stephendunn from the Cube Audio Nenuphar thread: one area in which the SIT-1 excelled that I believe might be related to damping factor is scale |
According to Pass the SIT-1 damping factor doesn’t change with frequency. The Cube is a single driver speaker, looking at the standard vertical scale at 50dB there’s a lot of axial variation from 20 -300 hz, below 100hz it looks a little strange. It’s probably bumping the bass below 100hz and above 10khz with distortion and then rolling off at about 15khz. Looks like it would sound good paired with the SIT-1 not sure DF would have anything to do with it. The frequency and phase response is flat, distortion |
@david_ten asked: "
Reaching out to those that know this stuff: does damping factor play a
role with (influencing) perceived scale (image size)? If so, how?
Thanks.
" Not directly, to the best of my knowledge, but imo it can indirectly. My understanding is that low damping factor generally calls for high amounts of global negative feedback, which in turn can obscure low-level detail. And soundstage size and depth, along with a sense of immersion or envelopment, benefit from preserving low-level detail. That being said I'm not an amplifier guy, but as a dealer for both types and as a speaker manufacturer and dealer, I've had a fair amount of experience in driving the same speakers with both high and low damping factor amplifiers. Here's another paper which examines the two paradigms of amplifier design, the constant-voltage paradigm (most solid state, generally high damping factor) and constant-power paradigm (mostly tubes, generally low damping factor). This paper was particularly useful to me as a speaker designer: http://www.atma-sphere.com/Resources/Paradigms_in_Amplifier_Design.php Duke |
@audiokinesis Duke, Thank You. I'll reference the paper. 1. Regarding: My understanding is that low damping factor generally calls for high amounts of global negative feedback, which in turn can obscure low-level detail. Is this more of a solid state amp issue? How does it apply to tube amps with zero to very low negative feedback AND corresponding low damping factors? 2. Regarding: I've had a fair amount of experience in driving the same speakers with both high and low damping factor amplifiers. Your generalized findings / results? Thanks. - David. |
I can't really buy into Ralph's paper at least the terminology defined. The example of the constant power amplifier as a tube-amplifier with transformer taps, is in my mind no different from the voltage amplifier paradigm presented, the only difference is the taps on the transformer impedance match the output to what is still essentially a voltage amplifier. The output power of those amplifiers will still change as the load impedance changes, perhaps not as much as if there was more feedback to compensate for the low output impedance of the amplifier, but it will still change as it is inherently a voltage amplifier. |
Let's assume for a moment that wire is perfect.The only problem with that is wire isn't, so the math can't be realized. The example of the constant power amplifier as a tube-amplifier with transformer taps, is in my mind no different from the voltage amplifier paradigm presented, the only difference is the taps on the transformer impedance match the output to what is still essentially a voltage amplifier. The output power of those amplifiers will still change as the load impedance changes, perhaps not as much as if there was more feedback to compensate for the low output impedance of the amplifier, but it will still change as it is inherently a voltage amplifier.@roberttdid One thing you are not getting has to do with the application of feedback. What I have said in that paper is true if the amp has none- what you say above is true if the amp has enough feedback to allow it to behave as a voltage source. Now Duke touched on something of high importance, that relates to @douglas_schroeder 's comments quoted from his review. Loop negative feedback is not a trivial matter in any amplifier, and the amount used can have profound consequence on the sound that derives from the amplifier. I am immediately asking- in an amplifier which has variable damping the easiest way to set that up is by the use of variable feedback- so what is the minimum and what is the maximum feedback? This is a bigger deal that it would seem to appear; if the amplifier has too little feedback (less than about 35dB) the consequence is that the feedback itself will introduce distortion, mostly composed of higher ordered harmonics (and some IM). Somewhere in the area of 35dB and north the amp finally has enough feedback such that is can actually compensate for the distortion introduced by the feedback itself. Now the ear converts all forms of distortion into tonality and can favor that tonality over actual FR errors. The ear is particularly sensitive to the higher ordered harmonics and IMD; the former are used by the ear to calculate sound pressure. If they show up, the amplifier will sound brighter and harsher and louder than real life, even if in 'tiny' amounts that we are used to seeing on spec sheets. This simple fact is at the root of the tubes vs solid state debate! Tubes don't make the higher ordered harmonics in the same way as solid state and so sound 'smoother' as real music does not have these harmonics enhanced either. So its understandable that an amplifier with variable feedback would sound quite different, and not because of damping factor, even though that is being varied. When an amplifier has insufficient feedback it will have colorations and those colorations will overshadow frequency response errors on account of how the brain perceives distortion. This is why two amps can measure flat on the bench but one can sound bright and the other doesn't! Now some of you may have noticed something- that most amplifiers made in the last 70 years don't have enough feedback. This is why solid state amps have been bright and harsh all this time- its only been recently that newer semiconductors have been available to allow amplifiers to be made with enough loop gain. But it appears that you can count those amps on one hand at this point. So the alternative is to simply use no feedback at all- and thus avoid the highly audible distortion caused by the feedback itself. This results in an amplifier with a high output impedance and thus low damping, but many speakers don't need much damping to sound quite realistic. This is why things like SETs exist- put them on the right speaker and the result is excellent. The bottom line is this is all about Gain Bandwidth Product and the resulting loop gain- both of which have been insufficient in the prior art. The Benchmark amplifier is one of the very few non-class D designs that actually gets the feedback into the ballpark. So if you want really natural sound, you either go with an amp like that or go with an amp that uses no feedback at all- and deal with the simple fact that it won't work on all speakers, which is also true of an amplifier that is a perfect voltage source! So you'll have to audition the speaker and amp combination in any event. |
My understanding is that low damping factor generally calls for high amounts of global negative feedbac Doesn’t have to be global. Local feedback can also be used to lower distortion and increase DF. Also, a high output impedance (low DF) can be offset by having more output devices. Still, yes, it is easy and cheap to achieve low damping factors with high amounts of global feedback. |
Another dimension of DF not often discussed is having high DF, and high current through the treble. While conventional speakers tend to have their low points in the mid-bass, where most amps have the highest DF, ESL's are essentially capacitors, and have their lowest impedance in the peak frequency. Having an amp that can do high current and low output impedance at 20 kHz can restore the treble. This is a reason why a lot of tube amps can sound dull with ESLs. Of course, no one cares more about this than Roger Sanders, and his Coda based amps are optimized for low DF and high current across the spectrum. https://www.sanderssoundsystems.com/ |
Another dimension of DF not often discussed is having high DF, and high current through the treble. While conventional speakers tend to have their low points in the mid-bass, where most amps have the highest DF, ESL's are essentially capacitors, and have their lowest impedance in the peak frequency.If an amp has 'high current' (which is a bit of a myth; current can't exist without voltage) then it will at all frequencies. The problem with ESLs is that they typically vary by about 9 or 10:1 in impedance from bass to treble, but their efficiency doesn't vary in lockstep as it is supposed to like you see with box speakers. So an amp that doubles power as impedance is halved is typically way too bright on most ESLs. Martin Logan got around this (sort of) by making their ESLs very low impedance in the bass (4 ohms) so they are only 0.5ohms at 20Khz. Even most solid state amps have troubles into that impedance, thus reducing the brightness that would otherwise manifest. Generally speaking most ESLs don't follow the voltage rules; IOW their impedance curve does not match their sensitivity through their frequency range! |
This is too simplistic a view, and I was thinking specifically just related to the basic output stage which does typically behave much like a voltage source, and is usually configured as a voltage follower, and with a light load (lighter than a speaker), behaves as a voltage source, and with load, as a voltage source with an element of constant and variable impedance. One thing you are not getting has to do with the application of feedback. What I have said in that paper is true if the amp has none- what you say above is true if the amp has enough feedback to allow it to behave as a voltage source. Again, I find this is too simplistic of a view. Simply saying 35db is too little feedback without taking into account the frequency response of the feed-forward and feedback paths, not to mention what the inherent feedback is in the output stage if you are considering that separately makes any hard number in the sand questionable. The statement makes assumptions about the linearity of the feedback network as well. Ditto for Gain-Bandwidth, which is one number, but gain at frequency is far more relevant. Instrumentation op-amps may have very high gain-bandwidth, but are useless at 20KHz. This is a bigger deal that it would seem to appear; if the amplifier has too little feedback (less than about 35dB) the consequence is that the feedback itself will introduce distortion, mostly composed of higher ordered harmonics (and some IM). Somewhere in the area of 35dB and north the amp finally has enough feedback such that is can actually compensate for the distortion introduced by the feedback itself. |
Not sure the justification for this statement. Their response w.r.t. voltage, is fairly flat from mids-highs, with usually a bit of a dip at high frequencies. An amplifier that doubles in power as the impedance is squared will keep the most consistent anechoic output. The brightness is more a factor of their emission shape and how they will interact with most room, and the resultant room response, which will differ from a "point source" dynamic driver. The problem with ESLs is that they typically vary by about 9 or 10:1 in impedance from bass to treble, but their efficiency doesn't vary in lockstep as it is supposed to like you see with box speakers. So an amp that doubles power as impedance is halved is typically way too bright on most ESLs. Martin Logan got around this (sort of) by making their ESLs very low impedance in the bass (4 ohms) so they are only 0.5ohms at 20Khz. Even most solid state amps have troubles into that impedance, thus reducing the brightness that would otherwise manifest. |
An amplifier that doubles in power as the impedance is squared will keep the most consistent anechoic output. Buddy, you went the wrong way. I know of no amp that doubles power as impedance goes from 4 to 16 Ohms. That is certainly not an ideal voltage source anymore. You are also conflating dispersion with relative differences in amp output vs. impedance. The feedback loop of Class D amps looks so different from linear amps I’m not at all sure we should be judging by the same criteria, assuming 35db is even correct. Last I looked there were at least 3 different ways in which Class D amps used feedback. It is not unreasonable to imagine an amp with a 300 DF at 20 Hz but 50 at 20 kHz. At 20 Hz the amp’s output Z is ~ 0.03 Ohms, but at 20 kHz ~ 0.16 Ohms. With a normal dynamic speaker, these are not really significant, but ~ 0.16 Ohms is significant when compared to the 0.5 or less an ESL may present. Some quick math, and you’ll see about 1/4 of the amp’s output voltage is gone. Of course, this is all hypothetical and math-y. Listening alone will tell you if you’d like it. Best, E |
Every amplifier has some feedback. Even emitter resistor is a form of local feedback. The problem with global feedback is, that it corrects with a delay (phase shift from input to output). This delay produces overshoot in time domain (odd harmonics in frequency domain). 40dB feedback means, that amplifier has 100x higher gain without feedback. Since amplifier delays signal from input to output, signal fed back and summed at the input is late. It make very little difference for slow sinewaves, but for fast changing input signals amplifier, for a moment, has 100x higher gain and overshoots. Benchmark is trying to time correct it with separate error amplifier (two sets of output transistors). This overshoot shows in some Stereophile reviews at square wave response. |
This is too simplistic a view, and I was thinking specifically just related to the basic output stage which does typically behave much like a voltage source, and is usually configured as a voltage follower, and with a light load (lighter than a speaker), behaves as a voltage source, and with load, as a voltage source with an element of constant and variable impedance.@roberttdid I've yet to see a tube output section where on its own without feedback, is able to behave as a voltage source. If you can point me to one I would be very interested. You might want to take a look at this image:https://www.radiomuseum.org/r/fisher_80_az80az.html This is a Fisher 80-AZ, typical of a number of amps from this period of the mid-late 1950s, prior to when the voltage rules were adapted by the audio industry. It is equipped with a Damping Control, which is a variable voltage and current feedback system. Note that at 12:Noon the control is marked 'Constant power'. At the extremes the control is allowing the amp to be a voltage source or a current source, as the control operates both forms of feedback. When the two feedbacks means are balanced against each other, that is about the same as zero feedback, hence 'constant power'. Now if you spend time with zero feedback tube amplifiers, and happen to have measured their power response with respect to load, you find that above a certain impedance (depending on the overall output impedance of the circuit) the power decreases quite slowly as impedance is increased, in fact doubling the impedance sees only a small percentage loss of power. Its not perfect of course, but 'constant power' is really not a stretch; a zero feedback tube amp will do pretty well with this as long as the load impedance is high enough. No amplifier is perfect of course and this includes all amps that behave as voltage sources as well. So my description as not too simplistic. It was simply correct. Their response w.r.t. voltage, is fairly flat from mids-highs, with usually a bit of a dip at high frequencies. An amplifier that doubles in power as the impedance is squared will keep the most consistent anechoic output.Huh?? What kind of amp doubles power as impedance is squared? Even a constant current amp only doubles power as impedance is doubled. At any rate this statement is entirely false, as ESLs don't do that. Here's a rather famous ESL impedance curve, the Quad ESL57: http://www.quadesl.com/quad_main.shtml You can see that while it does flatten a bit in part of the midrange, its on the decrease all the way from the peak in the bass. We have a lot of customers with Quads and Sound Labs (80% of our MA-2s built over the last 30 years are running on Sound Labs); these speakers don't seem to behave around voltage rules nor should they, as their impedance curve is not that of a driver in a box with its attendant resonance. This is of course not the only example of a modern high end audio loudspeaker that doesn't use the voltage rules; keep in mind that most SETs are zero feedback and so tend to behave more as power sources than voltage sources, and yet there are speakers on which they do quite well as the designer of the loudspeaker intended that it be that way. |
Every amplifier has some feedback. Even emitter resistor is a form of local feedback. The problem with global feedback is, that it corrects with a delay (phase shift from input to output). This delay produces overshoot in time domain (odd harmonics in frequency domain). 40dB feedback means, that amplifier has 100x higher gain without feedback. Since amplifier delays signal from input to output, signal fed back and summed at the input is late. It make very little difference for slow sinewaves, but for fast changing input signals amplifier, for a moment, has 100x higher gain and overshoots. Benchmark is trying to time correct it with separate error amplifier (two sets of output transistors). This overshoot shows in some Stereophile reviews as square wave response. +1 |
Doing 4 things at once including posting here :-) ... sorry for my error. I meant impedance is halfed, but had V-squared/R on the brain :-) An amplifier that doubles in power as the impedance is squared will keep the most consistent anechoic output. No conflation. It was postulated that ESL sound bright due to higher power output to the speaker as the impedance drops. I claimed that was not true, because though the power goes up, the anechoic response w.r.t. constant voltage over frequency is flat to down at high frequencies. The postulate w.r.t. bright due to amplifier interaction is not the reason, the reason is different dispersion and how that interacts with the room and creates a room response that will be bright (if not done right). You are also conflating dispersion with relative differences in amp output |
atmasphere, I deleted my old post questioning your article as I misinterpreted what your point was and where you were coming from. Put my head into a different mindset and completely agree in principle w.r.t. what you were communicating w.r.t. constant power for a tube output configuration. I don't have a blind attachment to 0 output impedance / infinite damping factor, and I expect most who do don't even know the details of why and why it may not be best from a system standpoint. |
Most modern speakers are simply wrong because they have uneven impedance curves and their impedance is too low in the bass region which then means you need amplifiers with low output impedance/high damping factors/negative feedback/high current supply to try to correct these electrical flaws.Two wrongs trying to make a right.When what we should be using is speakers with benign impedance curves in combination with current drive /high output impedance amplifiers because current drive amplifiers sound so much better than voltage drive.http://education.lenardaudio.com/en/12_amps_8.htmland https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&cad=rja&u... |
It is not an electrical flaw in the speakers. It is primarily a mechanical design choice. The site you linked is good, but it is easy to draw the wrong conclusions if you don’t read in depth. A pure current feedback output is not ideal as it will provide a peaky frequency response, and provides no additional electrical damping for the drivers which can be beneficial. The damping factor the speakers see is never near 0 either due to the crossover impedances. Neither was is really "wrong", just different and the right answer is probably somewhere in the middle ... though I would say most accurately, the correct answer is speaker specific amplification with an amp per speaker, but that is a tough nut for most companies to crack and not easy to market. Good on SGR tackling it at some level. |
@david_ten wrote: "1. Regarding: My understanding is that low damping factor generally calls for high amounts of global negative feedback, which in turn can obscure low-level detail. "Is this more of a solid state amp issue?" In practice probably so. David_ten: "How does it apply to tube amps with zero to very low negative feedback AND corresponding low damping factors?" I would expect it to have all the low-level detail benefits of any other successful low-damping-factor amplifier. David_ten: " Regarding: I’ve had a fair amount of experience in driving the same speakers with both high and low damping factor amplifiers. "Your generalized findings / results?" First a bit of background: I design speakers with fairly high and unusually smooth impedance curves so that they have very similar response with both amplifier types across most of the spectrum, and then include user-adjustable bass tuning to adapt to the effects of amplifier damping factor into the bass region impedance peaks. So frequency response differences are minimal, and generally relegated to the region south of 100 Hz, where there can be a "free lunch" to the tune of an extra 1/4 to 1/3 octave extension with a low-damping-factor amp. The following is my opinion; I can’t "prove" any of this. Consider these to be generalizations; I’m not a writer of audio prose; and [disclaimer] I'm an Atma-Sphere dealer: With low-damping-factor tube amps, I hear more of a sense of ease and liveliness and immersion. I’m hearing more stuff going on, it’s like the voices and instruments have more texture. Music is more engaging at lower sound pressure levels, which I think is related to the low-level details not needing as much SPL in order to be audible. At high sound pressure levels the difference is arguably even more noticeable, as there is an absence of the edginess which often starts to set in as the SPL goes up. I’m hearing more of a difference in soundscape and ambience from one recording to the next, with more of a sense of being "transported into" the recording, especially when it’s a really good one. There is a powerful emotional experience (some might call it spiritual) which some music can convey, but it calls for intensity and density and freedom from distractions. The least expensive amplification I have found which can do this is low-damping-factor tubes... specifically, the Atma-Sphere S-30 and M-60 [dealer disclaimer reminder]. Roberttdid wrote: " The example of the constant power amplifier as a tube-amplifier with transformer taps, is in my mind no different from the voltage amplifier paradigm presented... " As a speaker designer I can tell you that the difference between the two amplifier paradigms is significant, whether or not output transformers are involved. The relationship between the impedance curve and the speaker’s output level differs depending on the amplifier type, such that if the speaker’s impedance curve has significant peaks and dips, it will measure and sound different with the two amplifier types. Duke |
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"With low-damping-factor tube amps, I hear more of a sense of ease and liveliness and immersion. I’m hearing more stuff going on, it’s like the voices and instruments have more texture. Music is more engaging at lower sound pressure levels, which I think is related to the low-level details not needing as much SPL in order to be audible. At high sound pressure levels the difference is arguably even more noticeable, as there is an absence of the edginess which often starts to set in as the SPL goes up. I’m hearing more of a difference in soundscape and ambience from one recording to the next, with more of a sense of being "transported into" the recording, especially when it’s a really good one. There is a powerful emotional experience (some might call it spiritual) which some music can convey, but it calls for intensity and density and freedom from distractions. The least expensive amplification I have found which can do this is low-damping-factor tubes... specifically, the Atma-Sphere S-30 and M-60 [dealer disclaimer reminder]." All that is what I hear with Bakoon amplifiers.Which are class A/B but which are reputed to be zero negative feedback/high output impedance/ current drive/probably low damping factor amplifiers.In simple terms they simply sound incredibly clear and pure.Which I have really only heard elsewhere from expensive SET amps like the Kondo Ongaku. |
