Bombaywalla, I just caught your post re the Soundstage measurements of the SF speaker. I surmise that you would agree with the general view of many that when deciding on a tube amp's output taps to use for a particular speaker, that one should use the taps that sound best to that person's ears.
That said, if I was asked to guess what would be the best output tap selection for the SF speakers based JUST on graphs and NOT ears, I might have guessed the 4 ohm taps would be optimal. Here's why.
Btw, I assume the SFs were voiced to be driven by a SS amp. I also tracked down a J Atkinson bench test report of the Prima Luna HP amp. JA's output impedance measurements are pretty typical: 4 ohm taps (between 1.1 and 1.3 ohms); 8 ohm taps (about double the 4 ohm taps).
First off, the SFs are moderately sensitive. Soundstage reports 88.3 db. That bodes well when worrying about driving speakers with rough impedance and phase angle plots in the bass frequencies.
Second point. As JA reports, the amp's output voltage will vary with speaker load because of the amp's "high'ish" output impedance. Presumably, the amp's output voltage should vary less off the 4 ohm taps because output impedance is half that of the 8 ohm taps.
Third point. The Soundstage report shows that SF's FR drops off pretty sharply below 70 or 80 Hz. I suspect that the owner may be using a self powered subwoofer. If so, my reactions are even more on point.
Fourth point. The Soundstage report also shows that the SF's impedance ranges between 3 and 5 ohms between 50 and 100 Hz. In addition, the SFs are mildly capacitive (neg. 23 degrees or so) in that frequency range. Could be worse. Further, speaker impedance stays at 5 ohms or below up to 1000 Hz.
Conclusion. All in, my guess ... based JUST on the numbers ... is that the 4 ohm taps would yield a cleaner sound with less frequency response variation because of the amp's output impedance off the 4 ohm taps. In addition, the back impedance off the primary taps of the output trannies would probably be a better impedance match for the tubes because the amp is asked to deliver most of its power output in the 3 to 5 ohm range.
All the "techno" BS aside, I recommend that the owner of the SFs and the Prima Luna gear should use the taps that sound best to his ears. |
Thanks Bombaywalla ... but what is the purpose of partial cathode coupling. Is it a form of local negative feedback?? Does it ameliorate some of the adverse affects associated with NFB that Ralph has written about, e.g., TIM distortion that raises the level of odd ordered harmonics??
ARC has used this type of topology for many years. |
Czarivey ... I think your post slaps quite a bit of paint with a very wide brush.
My ARC Ref 150 SE has a relatively low output impedance, resulting in a damping factor of 14 (assumes an 8 ohm load; would be half if the load is 4 ohms). That ain't bad for the bass. In addition, it has a 1040 joule power supply, which helps the amp to muscle its way through demanding loads where serious current (and correlatively power) is needed.
And based on my own ears, I do not concur that my tube amp "has no bass" or that its power is "mostly applied toward mids and highs." I surmise that Ralph (Atmasphere) would take similar exception about his highly regarded amps. |
@Kijanki ... thanks for your informative response. You wrote:
"Damping factor of 14 is OK. 8ohm speaker's impedance is mostly resistive. Assuming, that it is approx 6 ohm it limits effective DF to 1.33 . Amps DF of 14 will make it worse only by 9.5% - irrelevant."
In my case, I drive my speakers, which have roller coaster impedance and phase angle plots, off the 4 ohm taps. To my ears, those taps sound "overall" the best. While it may result in non-optimal impedance matching at higher frequencies, I surmise that the power demands place on the amp are not that significant at such frequencies. Further, as mentioned above, the amp has a very robust power supply. Hence, I doubt that the mismatch is resulting in considerable distortion.
Kinjank, you also write that:
"There is a lot of local NFB in almost every amp. Any resistance in cathode is a form of NFB. Global NFB doesn't have to create TIM if it is applied within certain limits. It improves pretty much everything - bandwidth, output impedance, THD & IMD. Great sounding amp with small amount of NFB requires great design and quality components. Unfortunately it is cheaper to achieve the same using cheaper design and excessive amount of NFB hence creating overshoots (odd harmonics in frequency domain) and unpleasant bright sound."
I surmised the same as posted above. I wrote that ARC said the use of NFB is part of an overall engineering solution that balances many variables.
Thanks again.
BIF |
Beavis ... there are ... many other factors. As Kijanki posted, music is more than listening to sine waves. |
Thanks Bombaywalla. I really enjoyed reading the 1983 Atkinson and Messenger interview of Mr. Johnson. It was like taking a trip back in time.
IMO, since that time, ARC has made many significant refinements to their technology that have taken its product line many level beyond where ARC was holding back in 1983.
Wish I could say I understood the patent app stuff ... but I didn't. I'm not a EE.
I think Mr. Johnson wisely commented that trying to judge the quality of an amp by throwing around cold stats like slew rate, phase angle shift, bandwidth, and so forth is a fool's errand. It made perfect sense to me when he said many technical factors are taken into consideration when designing an amp that sounds good. To even think that a perfect amp can be viewed as "gain on a wire" is an absurdity.
Thanks again.
|
Al and Ralph ... help me with the terminology. When I think of headroom, I think of an amp's ability to handle short term dynamic transients, which is part of real music ... especially classical music.
To be more specific, my tube amp has a 1040 joule power supply. In my "un-technical" way, I interpret that as "head room" because my amp presumably can handle short term power demands that exceed its rated power output of 150 wpc, subject to FR, speaker impedance and tap output impedance. Am I mixing and matching terms and concepts here?? |
Thanks Al ... the problem is my misuse of terminology.
Perhaps a better way for me to think about my amp's performance window is that it does not appear to choke when asked to deliver power. Perhaps that is because I am not really tasking the amp all that much during "normal" operations.
In my layman's way of thinking, what I think of as "headroom" is my perception that if the amp is making say 25 to 50 watts of power during "normal" operations, a transient peak that pushes the amp out to 100+ watts is well within its rated power capability. Not headroom in the technical sense, but headroom insofar as the amp can produce a lot of power at reasonably low distortion numbers and remain stable.
Another fine point from the Atkinson report is that my amp will produce rated power in those cases where the load impedance matches the nominal tap value, i.e., an 8 ohm load plugged into the 8 ohm tap; or a 4 ohm load plugged into the 4 ohm tap. However, the amp will not produce rated power where there is an impedance mismatch between load and nominal tap value.
That said, in my case, my speakers have a 4 to 6 ohm saddle in a good part of the low frequency range, say 70 to 500 Hz. Impedance goes vertical past 700 Hz.
I surmise that most of the power demands placed on my amp fall within those goal posts. Given the foregoing, the 4 ohm taps make the most sense from a impedance and power matching perspective. And as an aside ... my rig sounds the best to my ears off the 4 ohm taps too. |
Bombaywalla, as you probably know, my amp is the ARC Ref 150 SE. I mention this because ARC designed my amp, and indeed, most of its tube amps, using NFB. In the case of the Ref 150 SE, I believe the NFB stat is 14 db.
Ralph has written quite a bit about the negative trade-offs of using NFB. Do you consider 14db to be "a lot of global negative feedback"??
FWIW, ARC describes its Ref 150 circuit topology as follows:
"Output stage coupling is a combination of “ultralinear” and Audio Research’s patented “partially cathode-coupled” topology, which is superior to conventional pentode or triode operation."
Whatever that means. :) LOL |
@Bombaywalla ... "I'm not even treading into the realm of "is 14dB too much global negative feedback?"!! ;-)" Scardy-cat! :) LOL
Re NFB: FWIW, many years ago, I recall speaking with either Lenard or Kal (ARC customer service tech rep; Len has since retired) ... I can't remember which one now ... and I asked about ARC's use of NFB in their amps. The answer was that many electrical variables are taken into consideration when designing an amp. So ... ARC's end product is the result of many engineering compromises that are made to achieve an optimal result. No perfect answers or solutions to an engineering problem that entails making trade-offs.
@Bombaywalla ... do you understand the term "[o]utput stage coupling is a combination of “ultralinear” and Audio Research’s patented “partially cathode-coupled” topology ..."?? I have a rough understanding of ultralinear .... No clue about "partially cathode-coupled” topology. Any idea?? |
