Tube Amp for Martin Logan Speakers


Hi, I love tube sound through my Martin Logan Aerius-i fronts and Cinema-i center. I currently have a Butler 5150 which is a hybrid, but it busted on me and would cost $700 to fix. I've had china stereo tube amps that were pretty good and gave true tube sound, but not enough drive for higher volumes. I live in condo, so not like I can blast music anyways but still. I got the Butler because I wanted 5 channel tube sound for home theatre (The piercing sound from my Denon 3801 receiver was not pleasant to my ears). It appears there are only three multi-channel tube amps around, from Mcintosh, Butler 5150, and Dared DV-6C. The latter two are hybrids, and the last one was one of the worst tube amps i've ever heard. I have no clue why 6Moons gave the Dared a 2010 award, but maybe it's because it produces only 65W.

So since multichannel tube amps are hard to come by, and they tend to be hybrid, I was thinking maybe it would be best to get three true tube monoblocks to power my fronts. Thing is I wonder if they will be underpowered for my speakers, and not sure which ones are decent for the price. Maybe China made ones would suffice, and they still go for pretty expensive price. I'm wondering if anybody knows of a decent powerful tube monoblock that is affordable, because I can't pay $3000 per block. or maybe best to just repair my Butler. Thing is, I'm not confident that it is reliable. The tubes are soldered in which is weird, and i've taken it to a couple repair guys who both said that the design is not good, because it's very tight inside and more susceptible to being fried from DC voltage areas. it's too sensitive.

Any suggestions for tube monoblocks, even if china made ones? the holy grail for me would be Mcintosh tube amp, but they are hard to come by. Thanks.

smurfmand70
Ralph, as I have come to better appreciate, there are very few if any perfect solutions to audio technical issues. It seems that EE/designer folks like yourself make judgments and compromises to achieve a certain balanced product that has market value within its market niche.

For that reason, I think that it is imprudent to think in absolutes. Having said that, I believe that if one is looking to invest significant sums into their rig, they can avoid obvious pitfalls and clear mismatches, for example, matching a low powered SET amp or a high'ish output impedance tube amp with a low sensitivity rock n' roll impedance curve speaker, like my Paradigm Signature 8s.

(It's only through dumb luck that my Ref 150 can do a decent job driving these beasts. Should have picked another speaker, or kept the speakers and opted for a top drawer SS amp like a Pass or Ayre.)

So, I share your view. We are adults and should be polite, albeit a little tongue-in-cheek humor from time-to-time is ok by me. :)

By the way, I thought the primary focus of your comment about using ZEROs was with respect to matching one of your OTL amps with ESLs like Martin Logans which have extremely low impedance specs at high frequencies. Quite honestly, if I loved ML ESLs, I would have some agita matching that transducer with pretty much any amp, SS or tube, unless the amp was rock solid stable and wouldn't distort driving juice into a .5 ohm load.

Btw, btw, after having read your posts again, I changed the output taps on my Ref 150 from 8 ohms to 4 ohms ... again. Although gain was clearly reduced because I was using the 4 ohm taps (about 2.5 db less than the 8 ohm taps), I think it quite possible that the amp was producing cleaner power (i.e., less distortion).

Bass is clearly more extended and tighter. No surprise there since the output impedance off the 4 ohm taps is less than the 8 ohm taps (therefore higher DF). Plus, speaker impedance in the "power zone" (say 60 Hz to 700 Hz) is for the most part 4 to 5 ohms), ergo a better impedance match for the amp where it is being called upon to deliver most of its juice. Imaging was less forward (probably because midrange/treble was less emphasized as a result of higher impedance), so I turned up the gain and imaging came back. In short the rig plays louder, but not as harsh (i.e., maybe less distortion).

Regards to all,

Bruce

P.S. Back to audio business, I just bought a JVC hi-rez redbook CD of Tango music from Acoustic Sounds. I'll report back with comments. I'm trying to keep track of good hi-rez CD label. So far, +1 for MoFi.
Atmasphere, I included the quotes as per your request.
The first two from you to support my premise that I wasn't putting words in your mouth.
The ones from Duke (AKA: Audiokinesis) with re: to it being easier to steady an impedance by lowering the impedance (and ergo make for a smoother frequency response).
I'm having trouble with my old memory as to whom might have suggested that lowering the impedance could help with bass response (and ergo increase frequency response), so the search for that link might be considerably more difficult. Then again if we can extrapolate the words from a very articulate, very well respected (especially here on Audiogon) amplifier designer; that one can get greater sound pressure into a 4 Ohm load with a ss amp, then we can use that extrapolation to suggest that as frequency response is measured in dBs, one can extend bass response and therefore extend frequency response.:-)
Atmasphere, Ah ha, I found one, I'm confident there is at least one more from a different source. I'll keep scratching my head.
04-05-11: Lrsky
Soundlock makes a good point.
Jim Thiel, starting in 1988, while developing the CS5's discovered that, by using bass drivers that drop precipitously in resistance, that this had a side effect of pulling more current from the amplifier--acting 'almost' as an equalizer on the bottom/bass region.
I have not discussed this, but it seems plausible, AND if Jim said it to be true, I don't need much else to bolster this opinion.
So...with that said--an amp with lots of balls, doubling as the impedence drops more and more, as they do get into the 2 ohm region with the THIEL 3.7--should mate well with a 'Krell-like' bass performer. I don't ascribe to the Magtech amp, as I haven't heard it...yet, he's right on the mark in his assessment. THIEL's need current.
As a side note--one very hard earned lesson by Jim that year was, that virtually everyone exaggerated their current delivery on the amplifiers--very few did what the manufacturers claimed.

Good listening...

Larry
Lrsky (Threads | Answers | This Thread)
Unsound, you state above "that [if] one can get greater sound pressure into a 4 Ohm load with a ss amp, then we can use that extrapolation to suggest that as frequency response is measured in dBs, one can extend bass response and therefore extend frequency response." With respect, IMO, your statement may be a non sequitor.

Here's my thought process. A speaker's SPL is a function of its sensitivity and the amount of watts the amp is driving into the load. While it is true that a 4 ohm load will draw twice as much current as an 8 ohm load, I fail to see why that necessarily means the speaker's SPL is necessarily twice as high at 4 ohms.

Consider my speaker, the Paradigm 8s. These beasts have a impedance curve that looks like a roller coaster, ranging from a low of 4 ohms in the bass/low midrange FR range and a high of 20+ ohms at the midrange/tweeter x-over point of 2K Hz. Yet ... the S8s have a very flat FR.

How can that be, I rhetorically ask? The reason is that these speakers were designed to be driven by a low impedance/high current SS amp. The fact that impedance rises to 20+ ohms at 2K Hz, thereby resulting in the speakers drawing LESS current and power from the amp, only means that the drivers are very sensitive at that frequency. Otherwise, the speaker's measured FR would be grossly skewed.

Consider Maggies. I recall reading that the 1.7s have a flat impedance function of 4 ohms or thereabouts, yet are not sensitive. As a consequence, one needs a beast of a SS amp to drive these pups to decent SPLs. In fact, I also recall reading a review of my Ref 150 that mentions the reviewer hooked up my Ref 150 amp to Maggie 1.7s and found the SPL just adequate.

As far as extended FR is concerned, I surmise that has more to do with the speaker design than it's impedance spec as a function of FR. I suspect what may have a more perceived impact on low end bass response is the amp's DF and the speaker's impedance spec. As Ralph, Al and others have mentioned elsewhere, DF is a factor equal to the quotient of a speaker's impedance at a particular frequency, divided by the amp's output impedance. The output impedance of most SS amps is so low that the amp's DF is not really a relevant stat.

But, if the amp's output impedance is "low'ish," say .5 ohms, and the speaker's impedance is "only" 4 ohms, ironically the DF will only be 8. By contrast, if the speaker's impedance is 8 ohms, then the DF jumps to 16. I surmise that the doubling of the DF may (??) have the effect if making bass sound tighter and perhaps more extended sounding.

I think Ralph was probably on target when he said that if a speaker presented a flat impedance load as a function of FR, say 8 ohms or higher, and zero phase angle over the speaker's full FR bandwidth, I think pretty much any SS or tube amp would be a very happy camper.

But as far as a speaker's FR being flat, extended or whatever, I think that has more to do with the speaker's (i) design objectives and build, (ii) impedance variations, (iii) phase angle stats, and (iv) SPL sensitivity. Of course, also critical is whether the designer intended that the speaker be driven by a SS or tube amp.

Really sorry for the long post. I hope it's cogent and the reasoning linear.

I'm sure I mixed and matched concepts, but hopefully our tech members will correct my meandering musings.

Regards,

Bruce
Bruce, thank you for your thoughtful response. It's late for me now, I'll try to find time in the near future for a more deserving answer. In the mean time, you might want to reread the chronology of the posts above. I think you'll understand why I ended that post to Atmasphere with a :-).