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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
bifwynne
2,206 posts
 
Jan, please re-read my non-tech take 2 or 3 posts north of this post. What you describe is exactly the effect of using NF -- the amp compensates for the output voltage drop based on the amount of NF applied to increase the output voltage to compensate for the interaction of the amp's internal impedance load and the external load of the speakers.

The effect is more precisely described as the amp is behaving as though it has low output impedance. So ... the NF circuit does not really change the amp's internal impedance. Instead, it acts like a servo to cause the amp to compensate for the Ohm's Law effects.

Of course, as Al, Ralph and others have said, the amp can only do this if it operating within its power delivery limits -- i.e., SOA (safe operating area). Also, there are rarely any free rides. Hence, NF has its own problems, e.g., increased odd ordered harmonic distortion ... and its detractors, e.g., Ralph. IMO, amp designers make trade-offs in order to design and build amps that perform at a certain level at a certain price point.

Probably still not saying this correctly, but it goes to show how the terminology can get in the way even though we are all really saying the same thing.
unsound
7,327 posts
 
Every once in a while I remember to use my memory :-).
One of the speakers used here was the earlier ML Aerius and not the OPs ML Aerius i, nevertheless it might be germane:

http://www.stereophile.com/reference/810/index.html
jandidden
8 posts
 
Byfwinne, yes I agree. The internal amp Zout is not changed of course.
Re: increased 3rd with nfb: that has been laid to rest many years ago. Most people know the (in)famous Baxandall graph showing increased harmonics (even harmonics that were not there without nfb) with increasing nfb. Then, when you continue to increase nfb, the harmonics eventually became invisible again - all of them.
There are two sides to this. One, not everybody realised that the worst you could do in this situation, was to use moderate nfb. For lowest harmonics, in this situation (see below), use none, or use a lot!
Second, this was a single, crummy FET stage. It has been shown that if you look at this with a whole amplifier rather than with a single crummy stage, the effect is not seen. If you start with a reasonable amplifier, applying nfb decreases ALL harmonics.
jandidden
8 posts
 
Bifwynne, if I may, not trying to split any hairs, but the term SOA has a very specific meaning for ss devices. The Safe Operating Area defines the combinations of current and voltage across the device, that it can withstand for a certain time before being destroyed. For instance, a power transistor might be rated for 100W dissipation, but that's not the full story. 10 amps at 10V, being 100W, would be OK indefinitely. BUT, 1 amp at 100V would NOT be OK indefinitely, although it is also 100W. That 1A/100V combination can only be withstood for, say, 100 milliseconds, and then it will let out all the magic smoke*.
Now over to amplifiers: they also have an area where they will operate as designed. As you noted, drive the level too high, they will clip. Drive too much level in too low a load and they may eventually overheat and break. One common accident on the test bench is full power at 20kHz - not many amps can hold out on that, and of course during music reproduction, they don't have to. But all this is not referred to as 'the SOA' of the amp.

* The reason is that at higher voltages, the current tends to hog a small part of the transistor die, so that small spot will heat up much more that the rest of the die. That small spot cannot handle the 100W the whole transistor could handle. It blows.
georgehifi's avatar
georgehifi
10,312 posts
 
I would like to thank Jan Didden for laying this furfie
"NF feedback DOES NOT effect Z=out or damping factor" out in the open, in his usual calm and palpable way.
His is a bottomless pit of correct information over at the much more techincal forums, along with the usual others like Nelson Pass, John Curl, and many others, there should be more of them to police bad info on forums.

http://www.wordsense.eu/furfie/

Cheers George
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