Al; I think I did misread the statement.
Thanks,
Thanks,
Tube Amps and Friendly Speakers - Back Impedance
The issue of tube amp friendly speakers has been taken apart on the Forum. However, I have another tube amp/speaker compatibility question that I hope will attract the attention of our tech oriented members, especially those members who manufacture speakers or amps. As I will explain below, the question relates to what I call "back impedance." Although this OP is longer than I would have wanted, the subject is complicated. Please accept my apologies. In the end, I'm not sure there is a best answer or practical advice. But here goes.
First a warm-up. I think many tech oriented members, such as Ralph (Atmasphere), Duke (Audiokinesis) and Al (Almarg) to name a few, have written extensively about Voltage and Power Paradigm amps, and how these amps produce current and power when presented with varying speaker impedances that change as a function of frequency.
Very generally, Voltage Paradigm amp (i.e., usually SS amps) usually maintain constant voltage and as a result produce more current, and correlatively more power (i.e., watts), when speaker impedance drops. However, SS amps produce less current and correlatively less power as impedance increases. By contrast, Power Paradigm amps (i.e., usually tube amps) tend to produce constant power as speaker impedance changes. The Atmasphere White Paper on the so-called Voltage and Power Paradigms contains a much more cogent and comprehensive discussion of the highly simplified points in this paragraph.
Second, a tube amp twist. Our tech members have explained at great length that SS amps usually have very low output impedances. This characteristic goes hand-in-hand with high damping factors (DF) and the Voltage Paradigm attribute of SS amps being constant voltage sources.
Third, the tube amp twist is that some tube amps use negative feedback of various types which has the effect of lowering output impedance and raising DF. In short, this design attribute enables a tube amp to perform somewhat solid state-like. That is, this class of tube amps is able to produce output voltages that don't vary very much as speaker impedance changes as a function of frequency.
As a case in point, my tube amp, the ARC Ref 150, has 3 different output taps (4, 8 and 16 ohms), each of which has a different output impedance: 4 ohm tap -- +/- .4 db; 8 ohm tap -- +/- .8 db; and 16 ohm tap -- +/- 1.4 db. Take a look at John Atkinson's 2012 bench test measurements of the Ref 150 to get a better sense for how it performs when presented with a simulated speaker load -- Graph 1 in particular.
Now to the back impedance question. And let me caveat my question by saying that it applies to tube amps that use output transformers -- not OTL amps like Atmasphere amps. In addition, I am thinking about tube amps that have low output impedance.
As I mentioned above, this issue has been discussed before, but I'm not sure sufficiently so. I say this because without knowing more, I would have jumped to the conclusion that a tube amp that has a low output impedance tap like the Ref 150 should perform sufficiently "SS like" so that it could drive speakers that were voiced to be driven by solid state amps. In fact, if the 4 ohm tap produces the lowest output impedance, just use it regardless of the speaker's impedance characteristics (nominal or varying). Not so fast ...
As I also mentioned, the Ref 150, like many tube amps has 3 output taps (4, 8 and 16) that are intended to correspond to the nominal impedance of the speaker. The theory is that the amp and speakers will perform better if there's a good impedance match between the two components. Great! What the heck does that mean and how does it impact performance??
The explanations I read on some of the older Forum posts seems to go like this. One of the key functions of output transformers is to match the impedance load of the speakers to the optimal operating range of a tube amp's output tubes. So, in a crazy way that I still don't understand yet, an output tranny works two ways -- (1) it steps-down the output tubes' plate voltage and high impedance to match the speakers, and at the same time (2) it steps up the impedance loading presented to the output tubes through the interaction of the tranny's primary and secondary windings (or, back impedance). In the latter case, the impedance step-up relates to the speaker's impedance presented to the tranny's secondary windings.
So, if I got this halfway correct, the inference that one should always hook his/her speakers up to the 4 ohm tap just because it generally presents the lowest output impedance to the speakers is flawed. The fallacy is that blindly using the 4 ohm tap may not result in an optimal impedance match for the output tubes.
And I think our tech members mentioned that if the output tubes are presented with a stepped up impedance that is outside the optimal design range of the output tubes, the result could be higher distortion and/or loss of power delivery capability at a given frequency as a function of the speaker's impedance characteristics at that frequency. Perhaps that's why the sage advice of using the tap that sounds best keeps cropping up. There's a lot of variables in play that affect what's comes out of the business end of a speaker, e.g., DF, output voltage regulation, power delivery and distortion, all changing as a function of frequency.
Ok, so using a low impedance tap doesn't solve all the problems with varying speaker impedances. Then, is it practical to know how much variation in a particular speaker's impedance viz-a-viz the amp's output tap impedance can be tolerated to be assured that the back impedance presented to the output tubes is in the tubes' operating sweet spot. Stated differently, if one plugs a speaker having a nominal impedance of 8 ohms into the amp's 8 ohm tap, how much can the speaker's impedance vary, yet still maintain optimal back impedance presented to the output tubes by the output transformer. Plus or minus 2 ohms ??, 4 ohms ??, etc.
If the practical answer is not more than 4 ohms total variation (or -/+ 2 ohms), then that's one heck of a pretty flat speaker by any accounts. So, my hypothetical speaker's impedance should not be greater than 10 ohms or less than 6 ohms or else the amp's output tubes will be operating outside their sweet spot, possibly producing more distortion or less power than predicted, especially if driven hard (e.g., at high gain, especially in the bass region).
So, in summary: is it practical to know how much variation in a particular speaker's impedance viz-a-viz the amp's output tap impedance can be tolerated to be assured that the back impedance presented to the output tube is in the tube's sweet spot? And that is the question!
P.S. I apologize for any typos. Just had eye surgery and my vision is still coming back.
First a warm-up. I think many tech oriented members, such as Ralph (Atmasphere), Duke (Audiokinesis) and Al (Almarg) to name a few, have written extensively about Voltage and Power Paradigm amps, and how these amps produce current and power when presented with varying speaker impedances that change as a function of frequency.
Very generally, Voltage Paradigm amp (i.e., usually SS amps) usually maintain constant voltage and as a result produce more current, and correlatively more power (i.e., watts), when speaker impedance drops. However, SS amps produce less current and correlatively less power as impedance increases. By contrast, Power Paradigm amps (i.e., usually tube amps) tend to produce constant power as speaker impedance changes. The Atmasphere White Paper on the so-called Voltage and Power Paradigms contains a much more cogent and comprehensive discussion of the highly simplified points in this paragraph.
Second, a tube amp twist. Our tech members have explained at great length that SS amps usually have very low output impedances. This characteristic goes hand-in-hand with high damping factors (DF) and the Voltage Paradigm attribute of SS amps being constant voltage sources.
Third, the tube amp twist is that some tube amps use negative feedback of various types which has the effect of lowering output impedance and raising DF. In short, this design attribute enables a tube amp to perform somewhat solid state-like. That is, this class of tube amps is able to produce output voltages that don't vary very much as speaker impedance changes as a function of frequency.
As a case in point, my tube amp, the ARC Ref 150, has 3 different output taps (4, 8 and 16 ohms), each of which has a different output impedance: 4 ohm tap -- +/- .4 db; 8 ohm tap -- +/- .8 db; and 16 ohm tap -- +/- 1.4 db. Take a look at John Atkinson's 2012 bench test measurements of the Ref 150 to get a better sense for how it performs when presented with a simulated speaker load -- Graph 1 in particular.
Now to the back impedance question. And let me caveat my question by saying that it applies to tube amps that use output transformers -- not OTL amps like Atmasphere amps. In addition, I am thinking about tube amps that have low output impedance.
As I mentioned above, this issue has been discussed before, but I'm not sure sufficiently so. I say this because without knowing more, I would have jumped to the conclusion that a tube amp that has a low output impedance tap like the Ref 150 should perform sufficiently "SS like" so that it could drive speakers that were voiced to be driven by solid state amps. In fact, if the 4 ohm tap produces the lowest output impedance, just use it regardless of the speaker's impedance characteristics (nominal or varying). Not so fast ...
As I also mentioned, the Ref 150, like many tube amps has 3 output taps (4, 8 and 16) that are intended to correspond to the nominal impedance of the speaker. The theory is that the amp and speakers will perform better if there's a good impedance match between the two components. Great! What the heck does that mean and how does it impact performance??
The explanations I read on some of the older Forum posts seems to go like this. One of the key functions of output transformers is to match the impedance load of the speakers to the optimal operating range of a tube amp's output tubes. So, in a crazy way that I still don't understand yet, an output tranny works two ways -- (1) it steps-down the output tubes' plate voltage and high impedance to match the speakers, and at the same time (2) it steps up the impedance loading presented to the output tubes through the interaction of the tranny's primary and secondary windings (or, back impedance). In the latter case, the impedance step-up relates to the speaker's impedance presented to the tranny's secondary windings.
So, if I got this halfway correct, the inference that one should always hook his/her speakers up to the 4 ohm tap just because it generally presents the lowest output impedance to the speakers is flawed. The fallacy is that blindly using the 4 ohm tap may not result in an optimal impedance match for the output tubes.
And I think our tech members mentioned that if the output tubes are presented with a stepped up impedance that is outside the optimal design range of the output tubes, the result could be higher distortion and/or loss of power delivery capability at a given frequency as a function of the speaker's impedance characteristics at that frequency. Perhaps that's why the sage advice of using the tap that sounds best keeps cropping up. There's a lot of variables in play that affect what's comes out of the business end of a speaker, e.g., DF, output voltage regulation, power delivery and distortion, all changing as a function of frequency.
Ok, so using a low impedance tap doesn't solve all the problems with varying speaker impedances. Then, is it practical to know how much variation in a particular speaker's impedance viz-a-viz the amp's output tap impedance can be tolerated to be assured that the back impedance presented to the output tubes is in the tubes' operating sweet spot. Stated differently, if one plugs a speaker having a nominal impedance of 8 ohms into the amp's 8 ohm tap, how much can the speaker's impedance vary, yet still maintain optimal back impedance presented to the output tubes by the output transformer. Plus or minus 2 ohms ??, 4 ohms ??, etc.
If the practical answer is not more than 4 ohms total variation (or -/+ 2 ohms), then that's one heck of a pretty flat speaker by any accounts. So, my hypothetical speaker's impedance should not be greater than 10 ohms or less than 6 ohms or else the amp's output tubes will be operating outside their sweet spot, possibly producing more distortion or less power than predicted, especially if driven hard (e.g., at high gain, especially in the bass region).
So, in summary: is it practical to know how much variation in a particular speaker's impedance viz-a-viz the amp's output tap impedance can be tolerated to be assured that the back impedance presented to the output tube is in the tube's sweet spot? And that is the question!
P.S. I apologize for any typos. Just had eye surgery and my vision is still coming back.
67 responses Add your response
"IOW, efficiency is the measure of *power* driving a speaker while sensitivity is the measure of the *voltage* involved and ignores the resulting power. This might be a case of looking at the same equation from a different orientation, as the resulting power of most ss amps is affected during these impedance changes. |
^^ to clarify, its not about orientation. Sensitivity is 2.83 Volts at 1 meter Efficiency is 1 Watt at 1 meter If the speaker is 8 ohms, they are the same. Now imagine a speaker with 90 db sensitivity. If the speaker is 4 ohms, the speaker will have and efficiency rating of 3 db less. This is because to make 2.83 volts, 2 Watts will be required, IOW you need double the power and doubling the power is 3 db. If the speaker is 16 ohms, it will have 3 db more efficiency as you only need 1/2 watt to make 2.83 Volts. Under the Voltage Paradigm, the amp that is a Voltage Source will have variable power output as the speaker load changes. This is useful in a box speaker, where there can be a big impedance peak in the bass due to box resonance with the woofer. The peak throttles back the power and so the bass is not boomy. (The problem here is that this rule mostly applies to some box speakers. For example it does not work so well with ESLs.) Under the Power Paradigm you would design the speaker so that the impedance peak was occurring below the woofer cutoff. This would then take advantage of the amplifier's ability to put out the same or possibly more power into the higher impedance. This can easily win another 1/2 octave of response out of the speaker. The Efficiency rating has been around for well over 60 years. The Sensitivity rating is newer, having been introduced in the 1980s. The Efficiency rating is an artifact of the Power Paradigm, the Sensitivity rating is an artifact of the Voltage Paradigm. Two different approaches, both with strengths and weaknesses, and their own sets of followers. The industry generally does not talk about this, and the result is confusion; hence the equipment matching conversation, of which this thread is an example. |
^^ No. All they do is translate the capacitance to a lower impedance. It is typical as a result that in an ESL the impedance will be high in the bass and low in the extreme highs. The Sound Lab for example is over 30 ohms in the bass. Yet you still need power to move that panel. This is why a 150 watt tube amp can keep up with a 600 watt solid state amp on that speaker- because into 32 ohms a 600 watt solid state amp is only going to make 150 watts. The tube amp might make 145 watts into that impedance. This is why transistors tend to be bass-shy on full range ESLs, and often very bright at high frequencies (where the impedance might be 2 or 3 ohms). |
Mulveling ...., Al (Almarg) and Ralph (Atmasphere) have already answered the question. Ask the company how the speaker was voiced. That is, was it designed to be driven by a Voltage Paradigm amp (solid state - constant voltage source; extremely low output impedance) or a Power Paradigm amp (tube amp) with "high'ish output impedance. This whole thread speaks to the electrical adaptability and compatibility of tube amps with speakers that were voiced to be driven by solid state amps. Although the discussion meandered a bit, that is the topic. If you are curious about your Tannoy speakers, call the company and ask if the speaker was voiced to driven by a solid state amp or a tube amp. I'm sure Al or Ralph will correct me here, but as a general matter, if the Tannoy's impedance and phase angle curves are pretty flat, regardless of whether it is open or closed ported, the speakers may be able to do double duty. As already explained by me and others, some tube amps use negative feedback to achieve certain operating specs, e.g., lower bench distortion, extended bandwidth and lower output impedance. In such cases, if the tube amp has low output impedance, it will perform somewhat solid state like. But that doesn't speak to the issue of back-impedance, which is the subject of this thread. Last point, as regards open port (bass reflex) or closed port (acoustic suspension), again, it really depends on speaker's the impedance curve and at what frequency(ies) it peaks. A speaker with a high impedance peak and highly capacitive (negative) phase angle may be a difficult load for any amp. |
Needed to resurrect this old topic of matching amp output impedence to speaker impedence. I've not read this whole thread, nor do I understand it from a technical point of view. And... I hope I don't have to - we don't do "word problems" in Mississippi! I'm driving a new set of Harbeth 40.2's (which Harbeth states range from 4 ohms to 8 ohms impedence)... with a new Audio Research 150SE tube amplifier. And - am trying to decide which of the 3 impedence taps to connect to on the amp - 4, 8, or 16 ohm? I've tried all three and the 16 ohm taps sound quite a bit better than the other 2 - with the 8 ohm next best and the 4 ohm the "least best." Now, I understand that the amp and speakers are a long way from being "burnt in." So, I realize the sound will improve with burn in. But... the highs are somewhat rolled off if I use anything other than the 16 ohm taps. Though the bass may be a bit less controlled. My questions are - is that to be expected? And... is there any reason I shouldn't use the 16 ohm taps - will it damage the amp / tubes? And I guess - one other question - will the Harbeths sound better with a SS amp driving them, than a tube amp? It seems they sound a bit better all around with my Halo A21 or JC-1's, than the AR150SE at this point in its burn in. |
Try the 8 ohm taps and let the gear bed in there. The sound will improve dramatically. Harbeths aren't tough to drive or terribly demanding although the 40.2's are the toughest of the Harbeths in this regard. The ARC amp you have can drive almost anything. Alan Shaw is a bit amp agnostic and he design with solid state so I've been led to believe. Doesn't mean the ARC won't be the end game but keep an open mind and the one that sounds best is the one you should keep. |
Yes, the tweeter should open up over a bit of time. I agree that you should use the 8 ohm taps during the run-in period. The issue of ss vs. tube and high vs. low power with 40 series Harbeths is, frankly, completely unresolved, with strongly held opinions on all sides. Your ARC actually sounds like a pretty good middle-of-the-road compromise. There are people driving them with 40W of tube power and others with 400W of ss. Some believe certain makes have synergy (but they’re very different makes: Hegel, Luxman, LFD, Quad, Vinnie Rossi among them) and others who contend that they’re agnostic. In other words: no single person has anything like the definitive answer for you, and in this case (as in almost all, but more so), your ears will be the final arbiter. But give them at least 200 hours before you start comparing. |
Thanks for responding to such an old thread... I've also contacted Harbeth and Audio Research for their guidance on this matter. Harbeth responded, but did not answer the impedence question - though they did answer the "tube vs SS" question. They stated specifically that Harbeth designs and voices their speakers using SS amps (didn't say which ones, nor offer any sugestions). They also noted that the Harbeth Users Group posts answer both questions. So, I went there... and... have never seen such a load of BS offered by many individuals - none of whom even came close to answering the questions, or even providing a coherent discussion. They just ramble non-sensically, of which the designer, Alan Shaw, was foremost. It's as if, they struggle to provide incoherent discussion of every topic. But... I do understand... that they use SS amps to design and voice their speakers... so... I guess it would make sense to stick with SS. Also - as many here have noted, the Harbeth speakers are all pretty warm, and rich - making me think the most neutral SS amps would obtain the best sound from them - presuming you prefer the detail and clarity of more neutral speakers. I do, but I love the rich tone of the Harbeths - just want as much detail and clarity as I can get with it (a la ATC's). The sound I'm after is that of the various YouTube links I've posted in other threads - in which they're driven by Hegel integrateds, and MacIntosh integrateds, or other MacIntosh amps, or SS amps. I just wanted to avoid getting an integrated amp, especially with an internal DAC. I reasoned that the "neutral" sound of the Audio Research Ref 150SE would get me there, while providing a bit more resolved highs and airy soundstage of a tube amp. |
To follow-up on my question - ARC responded to me to say that it makes no difference which set of speaker terminals I use (4, 8, or 16 ohm) on my ARC AR150SE tube amp - any will work with my Harbeths - just use the ones that sound the best. Harbeth did not answer that question, though they did "suggest" that I use a SS amp, which is what they use to design and voice their speakers with. In the interim I'm trying my ATI AT6012 - which is designed to be used specifically with Linkwitz Orion speakers - to drive my Harbeths. While it has only 60 watts of power, it sounds fine with the Harbeths - much more transparent, clear and controlled than the AR 150SE at the moment (prior to full burn in). ATI amps have a much more neutral sound than almost all tube amps, and most SS amps - right now it's doing a fine job of driving the Harbeths. The Harbeths definitely need such an amp to "tighten" up their sound and compensate for their warm, rich, bass and mids, and add clarity and definition. |
But... I do understand... that they use SS amps to design and voice their speakers... so... I guess it would make sense to stick with SS.There's a big 'that depends' going on here. You really don't 'voice' a speaker to a particular amp (that is if as a manufacturer you know what you are doing); what Harbeth was probably trying to say is that they voiced the amp to play nice with amps that are voltage sources. And to that end, your ARC qualifies. I suspect that as you allow the system to break-in, you'll find that the lower taps play well on the Harbeth tweeter. Speakers in particular are much more subject to break-in issues- so I would not make any big moves until you've allowed it to do so! |
@bassdukeIL I echo what Ralph just posted. But let me elaborate just a bit. I also own an ARC Ref 150 SE amp. ARC says that it take 600 hours for the amp to break it in. That may be a bit much. In any case, what ARC said is correct, use what ever taps that sound best to you. That said, I surmise you may settle in the end for the 8 ohm taps. The Ref 150 SE has an output impedance of approximately .75 ohms (at low frequencies) off the 8 ohm taps. Less than that off the 4 ohm taps. See John Atkinsons report at https://www.stereophile.com/content/audio-research-reference-150-power-amplifier-measurements JA reports that because the Ref 150 has an output impedance of < 1 ohm off the 8ohm tap, based on Ohm's Law principles, the FR response of your speakers could vary by much s +/- .8 ohms as a function of FR. I surmise that you may find the midrange FR to be a bit bright at the x-over point between the mid and the tweeter because speaker impedance will be high at that FR. But if the speaker impedance is low in the bass region, that may cause the speaker SPL to be a bit attenuated. You may say, ... hey my amp is not supposed to be a tone control. Why should I settle for an amp whose output changes with FR and speaker impedance? Answer: because the Ref 150 SE may (??) just sound better than your SS amp, .... just because. In addition, your speaker, coupled with room affects may also be a "tone control." IOW, if you were to use a speaker mic at the listener's position, you may find that your Harbeths do not produce a ruler flat, straight line FR because the speakers are not flat and because of room affects. Bottom line: the notion of gain on a wire is a dream. So bottom line: first, let your rig break in. Then experiment with the taps that sound best. You may be surprised. BIF |
You may say, ... hey my amp is not supposed to be a tone control. Why should I settle for an amp whose output changes with FR and speaker impedance?One should keep in mind also that the human ear assigns a tonality to all forms of distortion. In many cases it favors this tonality over actual frequency response. IOW the distortion your amp makes is most definitely a tone control! -And one that is pretty hard to adjust. The lower ordered harmonics of tube amps tends to be far less audible to the ear than the higher ordered harmonic distortion of solid state. I've seen people tape socks over their tweeters to try to tone down the harshness of solid state- but because it comes from distortion and not frequency response, it doesn't work. |