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. |
@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 Â
https://www.stereophile.com/content/audio-research-reference-150-power-amplifier-measurementsJA 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  |
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. Â 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! |
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. Â
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. Â
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). Â
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.  |
Thanks for responding to such an old thread...
I've also contacted Harbeth and Audio Research for their guidance on this matter. Â
Harbeth responded, but did not answer the impedence question - though they did answer theÂ
"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).Â
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. Â
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. Â
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.    |
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. |
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. Â 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. |
Needed to resurrect this old topic of matching amp output impedence to speaker impedence. Â
I've not read this whole thread, nor do I understand it from a technical point of view.Â
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."Â Â
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. Â
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. Â
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.  |
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. |
Fascinating discussion! Now I have to wonder what the heck happens to the impedance curve of my Tannoy Canterbury with its adjustable ports, and whether it might be the case that one type of amp is optimal for open ports while another is optimal for closed. |
^^ 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). |
^Would the stepup transformers of the ESL's compensate for this? |
^^ The reason is their impedance curve is not based on resonance in a box, it is instead is based on a capacitance. |
^Though the generalized efficiency rating may have been around longer, the more specific sensitivity rating would appear to be actually more informative/helpful.
Please explain why ESL speakers would not work so well with this rule. |
^^ 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. |
"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. |
Al; I think I did misread the statement.
Thanks, |
What you post is true, however: impedance affects sensitivity. Correct- but its important to understand that impedance has no effect on *efficiency* whatsoever. IOW, efficiency is the measure of *power* driving a speaker while sensitivity is the measure of the *voltage* involved and ignores the resulting power. Audiophiles and speaker designers are both guilty of conflating the two terms as the same; the result can be confusion and people talking past each other. |
Unsound, you may be right. But here's a little exercise that might be of interest to you and/or others that responds to your last quip. I make several references to the 2013 Stereophile list of recommended components, which includes recommended tube amps.
Drill into the Stereophile reports for the Class A recommended tube amps, and in particular John Atkinson's bench test reports. He runs all the tubes amps through the pretty much the same bench tests. I think Graph 1 reports the results of the amp's voltage output across the full frequency spectrum when driving a simulated speaker load that presents varying impedances and phase angles. JA also measures the amp's output impedances off the various output taps.
What you will learn from the exercise is whether the tube amp in question will perform SS like with respect to varying speaker impedances - that is operate like a constant voltage source. As has been said numerous times, this is largely a function of the amp's output impedance. What you will NOT learn is anything about back impedance, other than the reviewer's subjective opinion about how well the amp sounds on the reference speakers used.
Here's an important caveat -- just because the tube amp being reviewed sounds great (or not) on the reviewer's reference speakers could be misinformation on how well the amp will sound on YOUR speakers. This is because the electrical characteristics of YOUR speaker may be very different that the reviewer's reference speakers. And I'm not even talking about the speakers' acoustic performance (i.e., design and build).
My advice to members who are interested in buying a tube amp is to try and grab a bench test report about the amp that will show the types of information disclosed in the JA reports. Look for a tube amp with low output impedance. Presumably, that type of amp will be able to adapt best to changing speaker impedances -- that is act like a constant voltage. Correlatively, the amp will have a DF that will not be in the sub-basement. This is not my subjective opinion -- it's Ohm's Law.
At the same time, try not to go overboard with a tube amp that uses tons of NF for the all reasons that Ralph Karsten, Al (Almarg) and other have already explained. Namely, TIM distortion and odd ordered harmonic distortion.
In addition, the amp should have robust power supply and wpc specs. That may help mitigate some of the problems associated with non-optimal amp and speaker impedance matching. AT the same time, I would try to match the amp with speakers having the flattest possible impedance and phase angle curves.
Finally, and most important -- listen carefully to what looks like might be a good match on paper. Actual results may not line up with projected results. And, as stated many times in this OP, one stat that could affect sound quality is how well the tube amp and speaker can match back impedance.
I sincerely hope this OP has been helpful to the members. I've learned a lot from our techie members and my amateur experiments. I realize that there are technical gaps that I simply cannot bridge because of the multitude of electrical variables in play. Nevertheless, I think one can avoid making highly probable bad matches if certain key electrical attributes that simply don't mesh well are avoided.
Best,
Bruce |
Clockmeister ... not sure exactly what Ken Zelin had in mind. If he noticed that the sound and timber of the system changed as a function of the amp's output tap used, and therefore suggested that one compensate by adjusting room acoustic characteristics, that kinda looked passed the underlying reason for the acoustic difference.
But a fix is a fix be it a Band-Aid of moving the furniture around or me trying to creatively use all the taps on my amp. Whatever works I suppose. |
Unsound .... I'm not aware of any tube amp that functions "SS-like" and that can adapt to a speaker's impedance variations.
Not to say there isn't such a beast out there. Just don't know what the model is.
The closest analogy I can think of is the ARC HD 220, a hybrid tube/SS amp, which matches a tube based front end with a SS output stage.
Notwithstanding the foregoing variation, I wonder if there's a tube amp configuration that can do double as your inquiry posits??
But here's a EE question for the techies. New Sensor is coming out with a new audio power tube, the KT-150 which may be a drop in replacement for the KT-120 -- TBD??? ARC has advised me NOT to drop in the KT-150 yet because its engineering department is still studying it.
I wonder out loud if the "sweet spot impedance range" of an output tube changes if a tube model is changed, i.e., the KT-120 viz the KT-150? Or, is an output tube's sweet spot impedance range more dependent on the circuit topology???
Bruce |
No argument here with that, other than perhaps that most tube amps don't function like ss amps with regard to adapting to impedance variations. |
Unsound .... I'm not sure if we are saying the same thing, but just speaking past each other. Let me try it another way re sensitivity.
I surmise Ralph and Al might say that matching (1) a speaker with low sensitivity and that was designed to be driven by a high power SS amp, with (2) a low powered tube amp that functions "solid state like" (i.e., because it has low output impedance and tight output voltage regulation) might stress the tube amp. The result might be increased distortion, possible clipping and overall acoustic degradation.
So, if I had my druthers and wanted to match a tube amp that functioned solid state like with speakers that were voiced to be driven by a SS amp, I would try to match highly sensitive speakers (e.g., 90+ db) with a tube amp with a lots of wpc and a robust power supply. That's why I think my Paradigm S8 (v3) speakers (92 db sensitivity) work pretty ok with my ARC Ref 150 (150 wpc and 1000 joule power supply).
Optimally, I would really love it if my S8s had a flat 8 ohm impedance curve and zero phase angle function across the entire frequency range. Or, if the output section of my amp was indifferent to changes in back impedance off the primary windings of the output tranny.
Just my opinion.
Bruce |
When Ken Zelin worked for McIntosh, he prepared a graphic slide suggesting customers use the different taps on their Mac amps to adjust for room anomalies. You can find the graphic by googling Ken Zelin and output taps. It's available on the klipsch forums. |
^What you post is true, however: impedance affects sensitivity. The lower the numerical impedance the lower the sensitivity, and conversely the higher the numerical impedance the higher the sensitivity. So the issue of diminishing power output (so long as there is enough required power left, and dollar for dollar extra power is typically more readably available, and over a greater frequency range too) at higher impedances for ss amps, becomes moot. On the other hand as speakers' impedance changes so does it's sensitivity or demand for power change, something that typical tube amps can't adapt to, and ergo the linearity of the frequency response is compromised. Of course if the impedance is ruler flat it won't be an issue for either technology. But flatter impedance speakers seem to be easier to design with lower numerical impedances than higher numerical impedances, and ss amps tend to work better into lower impedance loads than tube amps. |
F1a & Bruce, I concur with everything in your latest posts.
Best regards,
-- Al |
Which all goes back to the OP with the very insightful follow up link.
|
11-01-13: Unsound
Ss amps will more often than not provide a more linear frequency response whether the impedance increases or decreases. True, fundamentally because the majority of speakers are designed with the expectation that they will be used with solid state amplifiers. Meaning that they conform to what Ralph refers to as voltage paradigm principles. Best regards, -- Al |
Unsound, I don't think that's quite the way it works.
As Al and Ralph Karsten have explained, SS amps are constant voltage sources -- Voltage Paradigm amps. As impedance increases, a SS amp - being a constant voltage source and typically ultra low output impedance -- will decrease current output and concurrently power delivered to the speaker. If the speaker was voiced (i.e., designed) to be driven by a SS amp, and as designed has a flat FR, that's just the way it was designed to function.
By contrast, tube amps with high output impedances are not considered constant voltage sources. Power output is less susceptible to changes in speaker impedance -- so called Power Paradigm amps. Therefore, if a Power Paradigm amp drives a speaker that was designed to be driven by a SS amp, it will deliver more power at high impedance peaks and therefore more SPL than the designer intended. Conversely, the amp will deliver less power than was intended at low impedances and therefore less SPL.
That's why if you read my posts about using NF in a tube amp, you will see such amps have lower output impedance and tighter output voltage regulation. Hence, the tube amp operates SS-like, similar to a Voltage Paradigm amp.
But that does not speak to the issue of this thread which is back-impedance. I've killed the issue as explained above. Too tired to write more. I'll let Al or Ralph fill in. |
That the impedance increases and consequently that the power output decreases with a ss amp becomes moot, as the speakers sensitivity typically increases with impedance as well. Ss amps will more often than not provide a more linear frequency response whether the impedance increases or decreases. |
11-01-13: Minorl
... really good amps double their power output when the load impedance is reduced in half. And conversely, those amps halve their power output when the load impedance is doubled. Please re-read Bruce's statement, which refers to load impedance INCREASING. Also, it is important to distinguish between how an amplifier's MAXIMUM power capability varies as a function of load impedance, and how its power delivery varies as a function of load impedance when it is operated WITHIN its maximum capabilities, whatever they may be. It is the latter that is being discussed, NOT variation of MAXIMUM power capability as a function of load impedance. For a given input signal level, ANY amplifier having near zero output impedance, such as most solid state amplifiers, WILL double power delivery into a halved load impedance, as long as it is operated within the limits of its voltage, current, power, and thermal capabilities. And conversely, its power delivery into a doubled load impedance will be cut in half. Those are simply consequences of Ohm's Law. That is all consistent with Bruce's original statement, as well as with Ohm's Law. I'm surprised that you disagree. I suspect that you've misread what has been said. Regards, -- Al |
An amp will deliver specified rated output if the output drivers/pre drivers and power supply is robust enough to support this. An over simplification but really good amps double their power output when the load impedance is reduced in half. I agree that many don't, but some well designed amps do. There are many compromises in amp design and this is a major one. This is why the biggest and "best" had huge regulated power supplies. Which is also the major factor in the cost of those amps. As I said, I'm not trying to divert this most interesting discussion, but for well designed amps with well designed capable power supplies, I simply don't agree.
enjoy |
Thanks Al. I got lazy and wrote too quickly. Of course you're 100% correct. The exercise was an attempt to match the amp's 4 ohm tap configuration with the speaker's low end terminals which are dedicated to the woofers and midrange. And to match the amp's 8 ohm tap configuration with the tweeters, which cross over from the midrange at 2.2K Hz and where there is a 21 ohms impedance peak which drops to about 5 or 6 ohms at 4 or 5K Hz.
As you say, the amp's output impedance off the 4 ohm tap configuration is .55 or .6 ohms, which ARC determined was an optimal match for a purely resistive 4 ohm speaker load. Output voltage regulation is +/- .4 db, which is pretty tight.
The amp's output impedance off the 8 ohm tap configuration is .8 ohms, which again ARC determined was an optimal match for a purely resistive 8 ohm speaker load. Output voltage regulation is +/- .8 db, still respectable.
I suspect I didn't like the sound because the amp's taps only did rough justice in matching amp and speaker impedance, plus the 8 ohm tap configuration is about 2.5 db greater than the 4 ohm tap configuration.
Last point, even though my speakers have some rough phase angles, the power supply of my amp is pretty robust -- 150 wpc, plus a 1000 joule power supply. Presumably, that's muscle enough to do a good job with moderately difficult speaker loads.
Regards,
Bruce |
Yes you're right Al. Often pro reviewers will claim that the 16 ohm tap sounds more transparent because "the secondary windings are minimal thus there's less wire corrupting the signal" when in fact the opposite is true. The lower the tap, the less of the secondary is involved... correct? |
Bruce, enjoy! I have several minor corrections and clarifications to most of the recent posts, though (excluding Ralph's, of course!). 10-30-13: Bifwynne
LT: The zero tap goes to the neg. terminal and the 4 ohm tap feeds the hot. If I've got that right, the amp's output impedance with the LT configuration is 4 ohms, which matches the LT woofer and lower midrange impedance ... As I believe you realize, the output impedance of the 4 ohm tap of your amp (and that of most other tube amps, aside mainly from some OTL's and SETs) is much less than 4 ohms. It would be more correct to say that the load impedance that the tap is designed to drive or work into is 4 ohms. UT: The 4 ohm tap is split wire, so I'm using the other head of the 4 ohm bi-wire to feed the negative terminal. Then, I'm running Tom's single cable off the 16 ohm tap to the hot. If I've got this right, the amp's output impedance on the UT configuration is 8 ohms.... An important point is that the amp's voltage gain off the 4 ohm taps is about 2.5 db less than off the 8 ohm taps. As a result, the speakers may be a tad bright.
10-31-13: F1a
Is the UT impedance really 8 ohms? Would it not be 12 - the difference between 16 and 4 ohm taps?? If so, the gain may even be higher than 2.5 dB. Both statements are not correct. The load impedance that from the perspective of the amplifier is optimal for connection between the 4 ohm and 16 ohm taps is 4 ohms. As indicated in the paper Bruce referenced earlier, when 4, 8, and 16 ohm taps are provided the 4 ohm tap will normally be the center tap of the secondary winding of the transformer, with an equal number of turns "above" and "below" that point. That follows from the fact that transformers transform impedances in proportion to the square of the turns ratio. Presumably the transformer is designed such that a 4 ohm load connected between the common (negative or black) terminal and the 4 ohm tap will present the same impedance to the output tubes as when a 16 ohm load is connected between the common terminal and the 16 ohm tap. Therefore the number of secondary windings between the common terminal and the 16 ohm tap will be twice the number of windings between the common terminal and the 4 ohm tap. Therefore the number of secondary windings between the 4 and 16 ohm taps will be the same as the number of windings between the common terminal and the 4 ohm tap. 11-01-13: Minorl
"However, SS amps produce less current and correlatively less power as impedance increases."
Very interesting post. I'm enjoying the interactions. However one clarification. Your statement in quotes above is really not correct. For well designed solid state amplifiers with well designed power supplies, this statement is not accurate. Not starting a tube vs solid state conversation, just a clarification. I don't see anything wrong with Bruce's statement, Minorl. As I'm sure you realize, almost all solid state amps have output impedances that are close to zero (i.e., a very small fraction of 1 ohm). Therefore, as a consequence of Ohm's Law and assuming that the amp is operated within the limits of its voltage, current, power, and thermal capabilities, the higher the load impedance the less current and power it will deliver, while delivering essentially constant voltage as load impedance varies. Power supply robustness will increase the CAPABILITIES of the amp under demanding conditions (especially into LOW impedances, and difficult impedance phase angles), and hopefully improve its sonics under most conditions. Best regards, -- Al |
"However, SS amps produce less current and correlatively less power as impedance increases."
Very interesting post. I'm enjoying the interactions. However one clarification. Your statement in quotes above is really not correct. For well designed solid state amplifiers with well designed power supplies, this statement is not accurate. Not starting a tube vs solid state conversation, just a clarification.
enjoy |
...most of the speakers on the 2013 Stereophile Class A list have some pretty wicked impedance and phase angle graphs, suggesting non-optimal tube amp matches. Stereophile does not recognize anything other than the Voltage Paradigm. So loudspeakers that might work a lot better with tubes probably won't get the same rating as they likely won't measure as well. One take-away from this is that if you have sorted out that you like what tubes do, then the thing to do is find a loudspeaker that works best with tubes. Overall you will get better sound for your investment dollar in the tube amp. I think this is true regardless of whether the tube amp is capable of true voltage-source response or not! There is a good reason why some loudspeakers have a reputation for being 'hard to drive'. You want to pay attention to that, because a hard to drive speaker is less likely to perform as well **because** it makes the amp work harder: that is true regardless of the amplifier technology. |
Final report: I played around with pretty much every variation and permutation of the amp taps and speaker terminals I could think up. I just couldn't find an overall better combo that bested using the amp's 8 ohm taps on both the LT and UT terminals. I lose some bass tightness and extension, but not enough to justify using the amp's 4 ohm taps or all of the taps in the various configurations mentioned above.
Well my back-impedance experiment is over. I've got great electronics and very good speakers. Gonna just enjoy the music and turn up the juice until my wife screams. ;-) |
Fla, I get your question, but that's what ARC told me. I don't know why.
In any case, last night, I switched everything around. I'm using the zero ohm tap for both UT and LT (negative) terminals and separately wiring the 4 ohm hot lead to the LT (hot) and the the 8 ohm tap to the UT (hot). As I suspected, the speakers are a tad bright, but the bass is better. I'll play around with it for a while and see what sounds best.
Valuable lessons here for those interested in tube amps. Lesson One -- be mindful of the impedances off the amp's output taps. The lower, the tighter the voltage regulatons and the "SS-like" the amp will operate (i.e., constant voltage source).
Lesson Two -- notwithstanding Lesson One -- keep in mind the the output trannies work both ways to the match voltage and the IMPEDANCE of the amp's output section to the speakers.
The trannies step down voltage and impedance to match the speakers via the secondary windings. At the same time, the trannies step up the speaker impedance via the primary windings to match the sweet spot of the output tubes.
My humble non-techie advice: if your set on using a tube amp that uses output trannies, try to match it with speakers that have the flattest possible impedance curve. My non-techie reaction is that most of the speakers on the 2013 Stereophile Class A list have some pretty wicked impedance and phase angle graphs, suggesting non-optimal tube amp matches. Just me musing. ;>') |
Is the UT impedance really 8 ohms? Would it not be 12 - the difference between 16 and 4 ohm taps?? If so, the gain may even be higher than 2.5 dB |
Back again. Tom Tutay rigged up a set of temporary speakers cables to let me use all 3 taps of my amp. As a reminder, my speakers can be bi-wired/bi-amped. I believe the lower terminals (LT) feed the woofers/mid range. The upper terminals (UT) feed the tweeter.
Here's what I'm trying. LT: The zero tap goes to the neg. terminal and the 4 ohm tap feeds the hot. If I've got that right, the amp's output impedance with the LT configuration is 4 ohms, which matches the LT woofer and lower midrange impedance, which is about 4 ohms from 50 Hz to 700 Hz; impedance then rises to about 21 ohms at the 2.2K Hz tweeter/midrange cross-over point.
UT: The 4 ohm tap is split wire, so I'm using the other head of the 4 ohm bi-wire to feed the negative terminal. Then, I'm running Tom's single cable off the 16 ohm tap to the hot. If I've got this right, the amp's output impedance on the UT configuration is 8 ohms. Not a perfect impedance match with the speakers, but better. The UT tweeter impedance ranges from 21 ohms at the 2.2K cross-over point and then falls to about 5 or 6 ohms at 4K or 5K Hz.
An important point is that the amp's voltage gain off the 4 ohm taps is about 2.5 db less than off the 8 ohm taps. As a result, the speakers may be a tad bright.
I'll try this set up for a while and see how I like it. Be back. ;-) |
Right questions Mapman, but wrong focus. I surmise that most speakers are voiced to be driven by SS amps. If my surmise is valid, the bias is that one should buy speakers first and then match the best SS amp, unless the speaker can do double duty.
I suggest you look at the specs on Stereophile's 2013 Class A speakers. Most are not very tube amp friendly. Don't misunderstand me ... I am very pleased with my ARC Ref 150. What's frustrating is that I have no idea how good my ARC Ref 150 really sounds because, as Al said above, the impedance varies so much as a function of frequency, it's difficult to know when it's functioning within the sweet spot range of its output section.
Oh well, another expensive lesson learned. |
One review I read indicated Daedulus were champs in the "tonality" dept. at RMAF. Probably not a coincidence...
Speakers are kind of like wind socks and amps like the wind. The wind has to deliver enough air to keep the thing properly inflated. Really big balloons may take a lot of air, and some amps may not be able to deliver enough juice with some speakers to do the job properly. Amp technology is becoming more efficient than ever these days (green in a sense) so there are possibilities now with high efficiency amps that did not exist before, so the utility of high efficiency easy to drive speakers is reduced somewhat overall, but as long as the shoe fits, you should be good. |
Well said, Bruce. And it seems to me to be safe to assume that the reason a lot of audiophiles find themselves on an amplifier merry-go-round at times is not that there is anything inferior about the amplifiers they've tried, it's just that the impedance characteristics of the speakers make amplifier selection excessively critical.
But hey, there are exceptions. My Daedalus Ulysses, for example, would be suitable matches for just about any amplifier known to mankind, except for the really flea-powered types. They are something like 6 +/- 1 ohm at all frequencies above 100 Hz, with a gradual rise to around 10 ohms at 20 Hz. And with benign phase angles, high efficiency, and high power handling capability. The 6 ohm nominal impedance, also, happens to be a perfect match for one of my amp's three output taps, which is indicated as being for 4 to 8 ohm loads.
And as I indicated in your thread about auditioning speakers, Daedalus provides 30-day return privileges, less two-way shipping and $300.
Best,
-- Al |
Which comes first, the speakers or the amp? :^)
Guess which way amp makers tend to lean?
And those who make speakers?
The rest of us are more "moderate" and less
extreme in our leanings I suppose.
I am more in the "speakers" first camp. Just make
sure the speakers you hear and like are being driven to
their max, and the specific amp to do it is easier to choose
from tehre.
But it'll work the other way also if done right, choose your
amp and then your speakers. But the speakers you'd really
like to have in that room may not be the right ones for you
then.
Just don't do either in a vacuum (no pun intended). |
^Without touching upon the specific technical issues you brought up in this particular thread, I've been saying things along these lines for some time now here, much to the disagreement of quite a few. And furthermore, that it might be more likely to find speakers with a steadier low impedance than a steadier high impedance, which might provide ss amps with some advantages in that regard too. |
Thanks Al. I hope to hear back from the Paradigm tech folks re the creative hook up idea. If they respond, I'll post it here if it's useful.
On reflection, I find this issue quite interesting, but also a bit troubling. Folks tout the benefits of tube amps over SS amps. Yet, at the same time, I am not reading about application limitations, e.g., Voltage vs Power Paradigm characteristics and back impedance matching issues.
Al ... in the end, it seems like the issue of speaker/amp compatibility has gone full circle. Even if an amp can be a de facto constant voltage source and thereby behave "SS-like," the issue of back impedance remains.
So, in the context of matching tube friendly speakers with tube amps having low output impedance and tight output voltage regulation characteristics, one should STILL try to pick a speaker with as flat an impedance curve as possible.
Problem is, as illustrated by drilling into the bench measurements of Stereophile's 2013 list of Class A speakers, most of the listed speakers have flaky impedance and phase angle characteristics that are not very tube amp friendly.
I guess it's back to the drawing board for me. Either I find a speaker that is a top performer and has tube friendly impedance and phase angle characteristics, or I try out high-end SS amps.
I have another thread running that speaks to how one can meaningfully audition speakers in the current environment. Unfortunately, with the demise of B&M hi-end audio stores, it's very hit and miss. |
I couldn't send along ARC's drawing, but it illustrates hooking the speaker's low end terminal up to the 0 (-) and 4 (+) ohm taps; and the high end terminal to the 4 (-) and 16 (+) Ohm taps. Hi Bruce, That suggestion is actually the same as in the first figure of the Vinylsavor paper you had linked to earlier. See that figure and the two paragraphs of the paper which are just above it. That arrangement should function well, and will provide proper phasing and level matching. In terms of impedance matching and output levels, from the perspective of the output tubes and from the perspective of the speaker it would be similar to connecting both sections of the speaker to the 4 ohm tap. I don't know whether or not it would be likely to provide any benefit in terms of the performance of the transformer itself. Best regards, -- Al |
Here is the response from ARC:
Hello Bruce,
Yes, this is possible if you can bi-wire your speakers.
You can hook one part of the speaker to the zero to 4 ohm tap and the other part of the speaker to the zero to 8 ohm tap. Check with speaker manufacturer for help to determine which part of the speaker works best at 4 and which part works best at 8 ohms. Only problem with this solution is you may not have the ability to balance the sound level between taps.
If you want to get creative, you could also try bi wiring using the set up in the attached file. It may give you better results. Have fun!
Best Regards,
Kalvin Dahl
Customer Support
P.S. -- I couldn't send along ARC's drawing, but it illustrates hooking the speaker's low end terminal up to the 0 (-) and 4 (+) ohm taps; and the high end terminal to the 4 (-) and 16 (+) Ohm taps. |
Thanks Ralph. What's that old adage, "curiosity killed the cat." I hope that's not me. I e-mailed ARC with my inquiry and asked whether they would be willing to advise on how I might try this little experiment. I hope the ARC audio-cops don't confiscate my gear because, like Richard Fader of Fort Lee, NJ (SNL fame, circa 1975, courtesy of Gilda Radner/Rosanna Rosannadanna), I ask too many questions. :-(
Thanks again,
Bruce
P.S. If the idea has legs, it's kinda' a cool tweak that others may want to look into.
P.S.S. - Duke LeJeune (Audiokinesis) called me about the Q. He said he was quite interested in the issue and wanted to do more reading on the subject. He promised to get back to me after he digs himself out from under the pile of paperwork that accumulated while he was at RMAF. |
