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
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
George don't make me blush :-).
Anyway. Atmaspere's statement that the INTERNAL Zout doesn't change has another side to it. It suggests that you can open up an amp and point to a component and say see, there's the output impedance. This is not the case. Apart from a few minor things like the resistance of the cabling from PC board to speaker connector, the Zout is sort of virtual. One cause for instance is the fact that the gain of the output transistors drops when you request more output current, so the output voltage drops a bit and that is seen as 'Zout'. As Bitwynne noted, what nfb does is counteract the drop in output voltage so it appears that Zout is now less. But the 'internal Zout' and the Zout with nfb are equally virtual.
Jandidden, keep in mind this thread is not about transistor amps. But more importantly you have to be careful here not not violate something called Kirchoff's Law.

Actually you will find that such is impossible as it is a law of physics, and not part of legal code :)

This law is known also as the law of energy conservation and simply states that the amount of energy in an electrical network is equal to the amount of energy going into that network.

What feedback does not affect output impedance:

Now if we take two output circuits, one with a high impedance, for example the single ended output of a tube preamp, and that of a transistor power amplifier we will see that due to the lower output impedance of the amplifier that it will drive a lower impedance.

Now if what you and George say is true, that adding negative feedback lowers output impedance, it then follows that if we add feedback to the preamp circuit its output impedance will become so low that at some point we can drive a 4 ohm load effortlessly.

But what we find is that is not the case. Now you may argue that the preamp output is not relevant, so let's take the case of an SET with a 10 ohm output impedance on the 8 ohm tap. If what you say is correct, its output power will increase if feedback is added when asked to drive a 4 ohm load off of that tap. But again, what we find is that the 4 ohm output power is unchanged.

The reason for this is if negative feedback really did decrease the output impedance, the resulting circuit would have the increased current to drive a lower impedance. That of course would violate Kirchoff's Law.

Of course, the real way to provide for greater current ability is more output devices, larger heatsinks, output transformers, power transformers and the like.

IOW, what is happening is that the term 'output impedance' as used with the Voltage Paradigm is a charged term that actually refers to servo gain in the output circuit and not the actual impedance of the devices involved (all types of which have an impedance greater than zero ohms).

One might state that the issue here is semantic- I point it out simply because its use in the context of teh Voltage Paradigm leads to a lot of confusion- but that is how the audio industry is set up.

If you are having trouble following this, it is because you are operating within the Voltage Paradigm. The word 'paradigm' has to do with a platform of thought; quite often viewpoints outside of that platform are hard to think about or might be considered blasphemous.

A further note- people have accused me of making up the two Paradigms (voltage and power) that I mention in the article at this link:

http://www.atma-sphere.com/Resources/Paradigms_in_Amplifier_Design.php

In case I run into that I refer them to this google search on the Fisher A-80 amplifier

https://www.google.com/search?client=ubuntu&channel=fs&q=fisher+a-80+amplifier&ie=utf-8&oe=utf-8

The very first hit you get at this link is a YouTube image of the damping control of the Fisher amplifier. It is marked 'Constant Voltage' at fully counter clockwise, 'Constant Power' at noon and 'Constant Current' at fully clockwise.

This is because there was a time when not all loudspeakers conformed to the voltage model, despite George's and Bruce Rosenblit's remonstrations. Such speakers are still made today, and in increasing numbers. Any speaker than can be driven successfully by an SET will be an example.

ESLs are also an example as it will be found that their impedance curve is not based on the resonant impedance of a driver in a box. If you could show that the impedance curve of the speaker (which in most ESLs varies by about 10:1) is also an efficiency vs frequency curve then you would have an argument that the speaker is a Voltage Paradigm device. IOW the ML ESLs are a low impedance Power Paradigm loudspeaker.

Atmasphere, your conclusion that someone (certainly not me!) would think if you lower the Zout to zero in a preamp, it can effortlessly drive 4 ohms, is rather far-fetched. As I'm sure you know, Zout has NOTHING to do with what you can or cannot drive, and I would expect that most people here would be well aware of that. A D-type battery cell has a Zout of less than an ohm but nobody would expect it to be able to start their car. :-) . I really have no idea what gave you that particular idea.

BTW I gave the example of ss amps' Zout being caused by for instance the transistor gain droop with increased load current. In a tube amp, there are causes for 'Zout' like the internal output tube(s) resistance and transformer losses that cause a drop in output voltage if you increase load current (for instance by connecting a lower impedance speaker). My point was that this Zout in the amp is largely 'virtual' in the sense that you cannot point to a resistor in the amp that would be the Zout. Now what nfb does is making the drop in output voltage with more load less so, thus causing the Zout to appear lower.

Actually it is quite easy to measure Zout on your amp. Just connect an 8 ohms resistor as load and set Vout to say 8 volts. The Iload is therefore 0.5 amps. Now connect a 4 ohms resistor and let's say the Vout drops to 7 volts. The Iload is now of course 7 V/4 ohms = 1.75 amps. So we now know that Vout drops 1 V if the current increases 0.75 amps. Therefore Zout is 1 V / 0.75 amps = 1.3334 ohms. Easy, no?
Atmasphere, I read your last post again - are you trying to say that even when nfb decreases the voltage drop due to increased current output (the Zout), that doesn't mean that the amp can all of a sudden deliver more power? If so, I fully agree. The amp has it's intrinsic limits in output current and output voltage and nfb cannot change that.
Even with nfb, when the output voltage goes up, it eventually gets up to the supply voltage and cannot go higher of course. Same with the output current - the current capability of the amp cannot increase due to nfb. What nfb will do is to keep 'trying' (if you will excuse this antropomorphism) to keep up the output voltage right up to the supply voltage. This is, of course, the reason why an nfb amp generally clips much sharper than a non-nfb amp.