Correct Al. Btw, just for fun, I pulled the Stereophile article which listed 2013 recommeded componenents -- speakers in particular. I drilled into the Class A speaker list write-ups and read John Atkinson's bench test reports, focusing on the impedance and phase angle charts. I needed dramamine after looking at some speakers' wacko impedance/phase angle charts. Need I say more?
So the point is, even if one's tube amp has relatively low output impedance and tight output voltage regulation, if the intended speaker has goofy impedance and phase angle dips, peaks and curves, will the impedance values (which change as a function of frequency) presented to the output tubes via the primary windings of the output transformer result in the output tubes operating outside of their sweet spots.
If so, as Al crisply said, "it becomes likely that no tap can be chosen which will allow many tube amplifiers to perform at their best," just at their least worst. |
Mapman, I can't disagree ....., but I'm wondering out loud here if the Thomas Mayer article presents an option or two for those whose speakers permit bi-amping/bi-wiring. In such cases, there may be a way to use the 4 ohm tap for the bass and the 8 or 16 ohm tap for the mid/tweet.
My woofer/mid x-over is about 230 Hz. Might be interesting if I can use the 4 ohm tap for the woofers and then use a higher rated tap for everything north of 230 Hz. Just asking ... that's all.
Hoping Ralph comes back with some thoughts. If so, what looks like an academic OP may have some practical utility. Was hoping I'd hear back from ARC, but so far nada. |
Folks, I'd like to keep this OP open for a while. I've sent some private messages to a few of our fellow tech members. If Al (Almarg), one of our most tech-wise members, is scratching his head a bit, I think clarity and insight would be helpful and much appreciated.
To underscore the point, copied below is the web page that links to Stereophile's list of 2013 recommended loudspeakers. Interested members should drill a click or two into the article to read John Atkinson's bench test measurements for each speaker on the Class A list - Graph 1 in particular. You'll need drammamine when studying the impedance and phase graphs. The B&W 800 D is literally off the chart.
http://www.stereophile.com/content/2013-recommended-components-loudspeakers
Regards,
Bruce |
Thanks Unsound. I assume y
No doubt that having good vision is a blessing. Ditto with good hearing.
And how to better enjoy the blessing of good hearing than to listen to beautiful music, even if certain frequencies may not be amplified within the optimal sweet spot of my amp's output tubes. LOL. Right now I am listening to a superb CD recording of Fandango.
Very nice. Very nice indeed. |
Thanks Unsound. I assume your reference to "insight" is a pun. If so, very clever. :-')
No doubt that having good vision is a blessing. Ditto with good hearing.
And how to better enjoy the blessing of good hearing than to listen to beautiful music, even if certain frequencies may not be amplified within the optimal sweet spot of my amp's output tubes. LOL. Right now I am listening to a superb CD recording of Fandango.
Very nice. Very nice indeed. |
For the technically interested:
Came across this article written by Thomas Mayer, "Speaker Impedance and Amplifier Output Taps: at this web site:
http://vinylsavor.blogspot.com/2011/08/speaker-impedance-and-amplifier-output.html
To head off anyone actually interested in trying the ideas mentioned in the Mayer article, please note Mr. Mayer's DISCLAIMER:
"Vacuum tube circuits work with dangerously high voltages. Do not attempt to build circuits presented on this site if you do not have the required experience and skills to work with such voltages. I assume no responsibility whatsoever for any damage caused by the usage of my circuits."
And that goes to mixing and matching output taps too!
Regards,
BIF |
Thanks Ralph. Al (Almarg) and I were hoping you'd chime in. Did you get a chance to look at the Thomas Mayer article in the URL link above. Mayer presents an interesting option relating to hooking up an amp's output taps. The option really pertains to speakers that have two sets of terminals for bi-wiring or bi-amping like my Paradigm Sig. 8s. Any thoughts??
I'd like to turn around your observation that "if there is a 4-ohm load on the 8 ohm tap, it will cut the load impedance on the tubes by half- and they will make less power and more distortion as a result." What happens if I use the 4 ohm tap and the amp is driving speaker impedance loads in the upper frequencies ranging from 8 to 20 ohms?
Let me share my anecdotal reactions when trying the different taps of my Ref 150:
(1) When using the 16 ohm taps, the music is open and detailed, but bass is clearly thin. I'm not sure about this, but while the music is open and detailed, it seems like the high end is a little too light. Btw, the S8's impedance drops to about 5 or 6 ohms north of 5K Hz.
(2) When using the 4 ohm tap, bass is deeper and a little tighter, but the midrange and treble are darker. Detail and imaging is fair.
(3) The 8 ohm tap seems to be the best compromise. I use a subwoofer so I can supplement SPL below 50 Hz. Bass is not as tight as off the 4 ohm tap, but I get more detail and better imaging.
So Ralph, is picking a particular amplifier tap an exercise best described as choosing the tap which sounds best, or choosing a tap that sounds the least worst.
Last point -- take a look at the Stereophile 2013 list of Class A recommended speakers. As an example, JA measured the B&W 800 D impedance as literally off the chart at certain frequencies. A couple of negative phase angle saddles at -67 degrees too. I suspect, not a very tube friendly speaker.
The URL link is here:
http://www.stereophile.com/content/bampw-800-diamond-loudspeaker-measurements
Thanks for the insights.
Bruce |
Swampwalker, there may be a practical answer here. I'm hoping to hear back from Ralph.
Take a look at the Thomas Mayer link in my earlier post. It may be possible to creatively use all the taps on my amp to better match impedance because my speakers can be bi-wired. If Ralph thinks so, please award me a +1.
If not, then I deserve a demerit of -1. I promise to accept my lashing with grace and dignity. :0( |
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. |
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 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. |
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. |
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. ;-) |
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. ;>') |
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. ;-) |
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 |
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. |
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 |
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 |
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, 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 |
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. |
@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-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 |
