2. amps-preamps
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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

Showing 8 responses by jandidden

jandidden
8 posts
 
Output impedance does go down with nfb - that's a basic tenet. Easy to check: take a non-nfb amp and change the load d
from 8 to 4 ohms. You will see that the output voltage drops a bit. Now apply nfb, do the same thing and you will see that the drop is less, showing that the output impedance has dropped. In fact, it drops exactly with the feedback ratio. Apply 20dB nfb and it drops by a factoe of 10.
Of course we all know that Zout is not physically a resistor or sumptin'....

Jan
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.
jandidden
8 posts
 
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
8 posts
 
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?
jandidden
8 posts
 
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.
jandidden
8 posts
 
My 2nd post above, where I say: 'The Iload is therefore 0.5 amps' should be of course 1 amp (from 8 V across 8 ohms). Duhhh. :-)
jandidden
8 posts
 
So, to recap, what did we learn:

- output impedance of a 'box' is *defined* as the change in output voltage divided by the current change that caused the voltage variation;
- nfb makes the output voltage change smaller for the same current change, so *by definition* lowers the output impedance.

We also saw that nfb does not change the 'internal' output impedance of an amp - nfb operates strictly outside the amp proper. Also, we saw that (to a first order) this change in output impedance due to nfb does not change the amps output capability. Lowering the output impedance towards zero does not all of a sudden produce an amp that can deliver gobs of power in very small loads. The as-designed amp capability does not change.

A corollary we did not discuss but should be quite obvious: since damping factor is *defined* as load impedance divided by output impedance, output impedance drop due to nfb consequently increases damping factor in the same measure.

This was a good exchange!
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