I doubt you're going to find someone who has compared both in a variety of systems over a lengthy period of time - even top-notch preamps generally have a subtle effect on a system and take months to properly evaluate. I would draw out any person who directly responds to your question about the other components in their system, their listening room, whether they have treated their A/C power supply, the type of music they listen to, etc.
At this level, both of these preamps are going to be superb, and synergy and features should be your focus. If you have long IC runs (2+ meters) to your amp, I would be inclined to consider the top solid-state pre's in order to avoid the impedance issues and problems running long cables that almost all tube preamps (except those using output transformers) suffer from. I would insist upon a preamp that has a polarity switch, and perhaps one that allows you to set input levels independently for each input so as to avoid gross differences in base volume levels among various source components. I don't much like preamps with tube rectification (I think the Rex; not sure about the Atma-Sphere) because I can't keep them turned on 24/7.
I would add to your list of reference-level, truly balanced preamps the Rowland Criterion, ARC's recent-production reference preamps, and Pass's top preamp. The darTZeel preamp is not balanced (Herve Deletraz hates balanced circuits), but it does feature an XLR output on an output transformer, which confers the noise-suppresion benefits of balanced operation for running long IC's. I run a Rowland Coherence II (which is fully differential balanced and battery-powered). |
Output impedance measurements for tube preamps are typically made at 1 khz., and the story will usually be very different at the frequency extremes, especially in the low bass. As for the BAT Rex, Atkinson's measurements in Stereophile revealed, "[I]ts output impedance was a little higher than specified, at 1000 ohms at 20kHz, 415 ohms at 1kHz, and 4.8k ohms at 20Hz. The last is due to the necessarily finite size of the output coupling capacitors. As MF noted, it should not be a significant issue in listening to music as long as the power amplifier has an input impedance greater than about 50k ohms." Atkinson's measurements of the BAT VK-51SE revealed similar performance.
Generally speaking, you need ten times the maximum output impedance of the preamp as compared to the input impedance of the amp to avoid frequency roll-off, i.e., this is why Fremer and Atkinson state that a BAT Rex with a maximum output impedance of 4.8K ohms needs to be used with an amp featuring an input impedance of 50K ohms or higher. There are a lot of amps featuring input impedances of less than 50K ohms, so this is going to be an issue at times.
I know that Atma-Sphere pre's don't use output transformers. Perhaps Ralph can explain how the MP-1 get around this problem? |
Madflloyd:
Some tubes in some tube power supplies can pass a relatively large amount of current and will wear out like output tubes in a power amp will wear out (certain tube preamps actually use output tubes in their power supplies - the ARC Reference 3 uses a 6550, and I believe the big multi-chassis Jadis used to use EL-34's). For example, when Stereophile reviewed the BAT Rex, they went through a couple of 5AR4's in the power supply over the course of several months of leaving the preamp on 24/7 (which they evidently did at BAT's suggestion). The review suggests that the BAT is fused to protect against tube blows in the power supply - you need to consult your owner's manual or ask BAT whether 24/7 operation poses a safety issue.
The other circumstance in which I would tend to think twice about 24/7 operation in a piece of equipment using small-signal tubes (i.e., tube preamps, tube DAC's, tube tuners) is if the tubes are run near their maximum voltage rating. Basically, such a tube will face the same wear issues that an output tube in a power amp will. It did not escape me that the Rex is said to run really hot, but this could be nothing more than the fact that it has a ton of tubes in it - I don't know how its various tubes are run relative to their maximum voltage ratings.
If, however, a tube preamp has solid-state rectification and does not feature a hot-rodded tube circuit (that would be the vast majority of them), I will run it 24/7 in order to maximize sound quality and tube life. As for why 24/7 operation would extend tube life, please see my comments about small-signal tubes and tube performance in the following thread:
http://forum.audiogon.com/cgi-bin/fr.pl?aamps&1281657617&&&/VAC-Signature-MK2a-w-phono-users
I hope this helps. |
Rtnl:
You wrote "Raquel, just for clarification, the BAT should be left on standby when not in use?" I cannot answer this question - you would have to ask BAT.
Again, it is generally much better to leave tube equipment featuring small signal tubes turned on 24/7 in order to maximize tube life, but as explained above, if the piece features tubes in the power supply like the BAT Rex, or operates the tubes in the circuit near their maximum voltage, you may be better off turning the unit on and off.
When you write "[N]onetheless, something must have been not quite right, and I agree to burn-in the unit for a few hundred hours for a review.", I'm afraid I do not understand the idea that you are trying to convey.
My apologies to the author of this thread for the tangent about tube maintenance. As to how this issue applies to the two preamps you have inquired about, the Rex has been discussed. As for the Atma-Sphere, I'm pretty sure it uses solid-state rectification, which, in combination with its use of 12AT7's and especially 6SN7's (they were used in televisions and are extremely robust), would make me run it 24/7. If it were me, I would probably run a Rex 24/7 notwithstanding everything written above unless BAT strongly disagreed, but based upon the Stereophile review, it looks like BAT has no problem with 24/7 operation of this preamp. |
Darkmoebius:
I can think of a few other fully differential balanced phono stages: BAT, Einstein, Rowland. I run a Rowland Cadence. |
Thank you, Mr. Karsten. Incidentally, does your MP-1 use solid-state rectification, and I wonder what the voltages are, relative to maximum recommended voltages, on the 12AT7's and 6SN7's (BTW, are the 6SN7's standard or GTA/GTB's?)? If this information is proprietary or you do not wish to respond for whatever reason, my apologies, I do not mean to pry. My question goes to the issue of tube life and running the unit 24/7 - I would think, assuming moderate voltages and solid-state rectification, that 24/7 operation on this preamp would maximize tube life. My rationale for this is set out in my comments to the following thread:
http://forum.audiogon.com/cgi-bin/fr.pl?aamps&1281657617&&&/VAC-Signature-MK2a-w-phono-users
Best regards. |
Mr. Karsten: Many thanks for your responses. The 6SN7 Sylvania chrome tops in my VAC Renaissance amp, used as driver and splitters, are beginning their eleventh year of use (in on/off service), and for all I know, they may have previously seen service in somebody's T.V. :) |
Mr. Khomenko:
Thanks for the input on the issue of tube life and 24/7 operation. My experience is that, while most people are aware of a debate regarding 24/7 operation and solid-state gear, few people who own or sell tube gear are aware that some manufacturers advocate 24/7 operation of equipment using small-signal tubes in order to extend tube life (my posts on this issue tend to create surprise).
Assuming the conditions necessary for 24/7 use are present that I describe in the links referenced in my posts, the only thing I worry about operating 24/7 with either solid-state equipment or small-signal tube gear (aside from not being home to disconnect equipment when an electrical storm passes through) is what it's doing to the caps, which tend to be rated for only a few thousand hours, but caps will often far outlast their ratings and are relatively cheap and easy to replace, while thermal cycle damage to transistors from on/off operation, particularly where the transistors have gone out of production and the amp thereby becomes worthless, or thermal-cycle damage to irreplaceable, cherished NOS small-signal tubes, is not cheap. If I owned BAT gear (and I may one day, as my system is fully differential balanced from turntable to amps - that's why this thread, about BAT and Atma-Sphere gear, caught my eye), I would follow your instructions and turn my equipment on and off. With all due respect, however, your advice does run contrary to the evidence I have described in my posts referenced above and in numerous other posts I have made over the years here. |
Again with apologies to those who are bothered by this tangent, I think we are actually saying much the same thing, Mr. Khomenko. I have or had mentioned in this thread and/or others that:
(i) operating voltage helps determine tube life, i.e., the more voltage a tube sees, the hotter it runs, and the quicker it will normally die; and
(ii) in order to avoid the sudden, deleterious application of voltage to tubes upon start up, some tube equipment - including your products, I am now learning - features a "soft-start" function that brings up power gradually.
Since we're on the topic and your Rex preamp features tube rectification, I should mention that tube rectification functions like a variac to bring up power gradually.
Not to belabor the point, but based upon the evidence I have seen, GENERALLY speaking, if the small-signal tubes in a given circuit are run at reasonable voltages (i.e., well below their maximum rating and they thus run cool) and the piece uses solid-state rectification without a soft-start or standby feature, which is the case, in my experience, with most tube preamps, 24/7 operation yields better tube life (and better sound). I again quote from the owner's manual of my VAC Renaissance tube amp:
"How long should tubes last? It has long been known in professional circles (and probably now forgotten) that a tube such as the 12AX7 will display BETTER performance characteristics after TWO YEARS of CONTINUAL operation than when it was new. In normal use it is not unusual for a low level [small-signal] tube to last 5 years or longer. Output tubes [i.e., power tubes used in tube power amps] are another story, as they are continuously providing significant amounts of current." (Emphasis original).
Incidentally, if, because of work demands, I know that I'm not going to be listening to my system during a period of a month or more (a common scenario for me at year-end), I will indeed t |
"will indeed t ... urn everything off." Sorry about that.
Messrs. Karsten and Komenkho: Great stuff - thank you. I look forward to the debate about cathode wear and small-signal tubes the next time you sit on a panel together! |
