In transformer-coupled tube amps, the physical mechanisms that cause crossover distortion are very differnt from a direct-coupled bipolar transistor amplifier. In a push-pull amplifier (like both the Marantz and Mac), the concern is hysteris distortion in the output transformer as the current is transferred from one side of the primary to the other. The specific design subtleties in making a quality output transformer are way over my head . . . but suffice it to say that there's a great number of details that can be tweaked to make it work it's best in the particular circuit.
There's also the differnce that the final stage of most solid-state amplifiers is a current amplifier, and the output stage of a tube amp is a power amplifier (both a current and a voltage amp) . . . and tetrodes and pentodes have extra grids . . . so the number of various feedback configurations available increases quite a bit, all of which affect how the amplifier behaves around the crossover point.
True Class B tube amps do seem to be, as a group, less linear than their counterparts running class AB, the McIntosh amps being a bit of an exception to that (they're allmost class B), but the McIntosh "Unithy Coupled" circuit has much more local feedback than most designs, and very high-quality transformers. The Marantz transformers are also very high quality, but designed very differently for the "Ultralinear" circuit. The Model 2 is pretty straightforward Class AB1.
I'm a huge fan of the MC275 in any vintage . . . and the re-issues are excellent, probably better than the original . . . and they're reliable, and available new with a warranty. Also IMO one that's very fairly priced - these were all reasons that I chose it to pair with the existing Marantzes in the system you mention. I just feel that the Model 2 is a big step better than even the MC275, and would have preferred to four Model 2s . . . or even eight of them with a fully active crossover.
But that would be like opening a whole case of 1940s-era Inglenook cabernet . . . which would be awesome, and I tip my hat to those who can afford to do such things. But we didn't have that kind of budget, so I put a 1999 Caymus next to the vintage stuff we already had. |
Kijanki, we're definately running into some terminological inconsistencies in this field - but if we're talking about very low signal levels, I'd say that the amp in question must certainly be a Class B amp that for marketing reasons was being labelled as AB. If it occurred at higher power levels (say above a few watts), then I could see how a Class AB amp could exhibit this behavior.
100mA is probably about right for a single pair of bipolar transistors biased Class B across some pretty low emitter resistors, maybe 0.15 ohm? But I think of 0.22, 0.33, and 0.47 ohm as being the common values, so I'd say that most amps are biased proportonately lower for Class B.
It does seem quaint to use an all-NPN output stage these days . . . maybe for an inexpensive paging-system amplifier it might make sense, but for hi-fi, your're right. Crazy. |
Kirkus - I don't do much with discrete stuff so I remember wrong definition of class B (as a one without bias current). Now I realized that it has to have bias current in order to break Vbe voltage of output transistors. |
Plinius considers all-NPN to be a good way to go for their high-end models, even today. I personally find it a bit clumsy, and that alone might be worse off than slightly different transistor characteristics. Nature seems to value simplicity more highly than complexity.
I agree that many amps marketed as "class AB" are really Class B. The threshold between the two is whatever one wants it to be these days.
I disagree with the "amplifier phase delay" problems with horizontal biamping. There are plenty more phase-inducing circumstances in the system that are worse than the phase of a linear amp, like the speaker drivers' phases. Whether or not a particular horizontal biamp will work well is more dependent on luck than technicalities. However, in vertical biamping, you need the same amps because they are each reproducing the same frequencies and, as most of us know, not all amps sound the same.
Gp phan - Don't worry about what Class the amp is. Just listen and enjoy. If you are really curious about a Class A, just get one and try it for yourself. Besides, there is a lot of sonic performance overlap between Classes. Potential merits can be discussed all day but in the end, they really don't matter. Afterall, it is the sound that should count most.
Arthur |
And you're correct in the assertion that increasing bias doesn't improve the problem, it just increases the signal level at which it occurs. But it does reduce the audibility of the problem. You now have distortion only when the output signal is very high and you have no crossover distortion (GM doubling or whatever you like to call it - lets say transition distortion) when output signal is very low and it runs in Class A (both sides conducting). A small absolute amount of distortion on a large signal is better than the same absolute distortion on a small signal. In one case the listener may notice the transition distortion (large part of the overall signal) while in the other case it will be much less audible due to it being a smaller proportion of a much larger signal. |
You now have distortion only when the output signal is very high and you have no crossover distortion (GM doubling or whatever you like to call it - lets say transition distortion) when output signal is very low and it runs in Class A (both sides conducting). This is quite correct, but the gm-doubling transition distortion is much worse than the crossover distortion. So as to audibility . . . it all depends on the application - each road has its burdens to bear. A small absolute amount of distortion on a large signal is better than the same absolute distortion on a small signal. I've heard it asserted that crossover distortion manifests itself (it drives THD upward) more as the signal level is reduced . . . and honestly, I'm not sure whether or not it's true. It seems to make intuitive sense, but I've measured lots of amplifiers, and I'm doubtful as to whether or not the measured data supports it. Complicating the issue is that THD+N of course rises in a linear manner with a reduction of signal level . . . but when you measure just the noise, you get the same results. I think it may be that as the signal levels are reduced, the proportion of the total signal that's in the crossover region increases, but the crossover non-linearities are at the same time being spread out across a larger proportion of the waveform, making them less severe. Whether or not these opposing factors cancel each out is the question, and I certainly haven't the skill to investigate it with pure mathematics, and my current measurement equipment isn't sensitive enough to find the answer emperically. |
Shadorne - tests show that underbias and overbias causes increase in distortion (especially higher order odd harmonics) for all signals (in wide range of output power). Increasing bias above optimal level increases non-linear area (of non-constant gm-doubling) and makes it worse.
Proper way to do it is to bias it into class A so nonlinear area would never be reached and gm doubling would appear constant for all signal all the time. Unfortunately this requires (typical) bias of 150% of anticipated max output current. At least that's how I understand it. |
tests show that underbias and overbias causes increase in distortion Agreed but I don't understand the point? Of course you have to match the bias precisely between both sides as otherwise you'll get loads of higher harmonic distortion. However you can design circuits that help achieve this and help maintain this under various conditions - it all boils down to careful topology. |
Shadorne - careful topology is always important but I think that as long as transconductance in biased area is non linear extending this area doesn't alleviate the problem but makes it worse (extends nonlinear area).
This problem is different from crossover distortions or switching distortions. Of course huge amounts of negative feedback will make everything nice and linear but for the price (TIM) and even then still not as good as class A. The only strength of class AB is that it costs less and therefore better quality amp (power, parts etc) can be made. |
Class A amps have finesse, ambience, subtlety, blossom and decay of sound that gives it a more realistic quality. However, they are the most expensive amps per watt, the hottest running, uses the most electricity per watt output, most tempermental, requires the most attention and upkeep of all the amp types. I say that, because most pure class A amps use tubes...until recently. Check out this month's issue of Stereophile Magazine and you will see a review on one of the most recent amp models from Pass Labs. It's a pure class A SOLID STATE amp which got rave reviews. I am referring to the XA- .5 models. It is very hard to have a high powered pure class A amp which is why the high powered ones cost many tens of thousands of dollars. However, the 30 watt stereo amp, and the 60 watt mono amp are each only five thousand dollars. This is very reasonable for the quality of these amps. |
The wine analogy that Kirkus used might be the way to go here, but with wine the you can try many types easily and see what you like and have a collection of Cabernet, Pinot, Chardonnay, etc.
But with Hi-Fi, you can't collect to the same degree, and you cannot sample to the same degree. So I think for many people we rely on the audio equipment equivalent of the master sommelier, like reviews from Stereophile as stated above or sound engineers with a lot of experience and people who have been into it for a long time. The expense of audio, and the rate of audio collection, generate a reliance on expert advice in this kind of situation.
So what is the advice for the guy who started this thread, he likes his McIntosh gear. What would be an interesting flavor change, would a Plinius SA100 mkIII be an interesting taste choice? Better is pretty subjective, but how would a class A amp with his McIntosh pre be different? |
My days of sampling different amplifier classes are over. I don't want to knock anybody thus won't mention various equipment brands. Yet I can tell you that through the years I listen to equipment from a lot of major brands. None of them remotely come close to what the real Class A amplifier can do. As to those who have other priorities to spend money at this moment, it does not cost anything to just go listen to these equipment. When you see that much a difference, perspective and priority can develop different meanings. Because of the type of discussions and reports people have about the XA.5 series and that I own and know how Pass Lab XA60.5 sounds, it's obvious as to my recommendation. I am just too in awe of the difference to sit still on this.
Now from a different angle, the more expensive a purchase, the longer it better stays with me. Even $100 is a good amount of money, needless to say $1000 or $2000 that Gp_phan is thinking about. I am not about to spend $1000 when realizing that a year from now I would loose so much just because I really want something else instead. But that's just me. |
Spatine, I share your respect for Nelson Pass and his products, even though they're not my cup of tea. But since his amps are generally single-ended designs with a high output impedance, I would say that the "class-A-ness" isn't necessarily the main source of their sonic characteristics, and the advantages/disadvantages of these aspects of his designs require separate consideration.
That's the main reason why I chose the Levinson ML-2/ML-3 for comparison . . . where the circuit designs are much more similar, whereby the Class A/Class AB distinction is THE big difference. |
If anyone is interested there is a master class lecture by Douglas Self on audio amplifier design. His talk is interesting for its lengthy discussion of distortion mechanisms and the new Cambridge 840 XD design which he mentions briefly (a kind of offset bias to bring the crossover region to where it is less obtrusive). He did not mention active speaker designs with limited bandwidth as a solution to the many necessary compromises required to achieve wide bandwidth in conventional approach- but he talks about nearly everything else. |
I'll second the recommendation on Self's work . . . he also has a couple of excellent books on the subject. I don't necessarily agree with all of his approaches and conclusions, but everything he presents he does with wonderful concise logic, and thoroughly cites all his sources - which seems to be rare in the audio field these days. He's definately one of the modern masters of audio amplifier design, and a delight to read as well. |
There's also Rod Elliot's explanation , if it hasn't been mentioned already.
http://sound.westhost.com/class-a.htm
Lot's of other articles/projects too. |
I also have a Mark Levinson amp that has that AB class sound. To me the push-pull method has it's advantages such as clarity, efficiency, and low distortion. I've had this amp for ten years and I have enjoyed every minute of it... until now. Reacently I noticed that while the sound has the above advantages, it seems to lack depth and realism. A tuning fork has the same advantages as a class AB amplifier, but would you rather hear a musical piece played with multiple tuning forks, or musical instruments with their own distinct overtones. Class A amps have more distortion than class AB, but perhaps it's that distortion that gives class A amps their flavor, and realism. Anyway, that's my two cents worth. |
Kirkus, perhaps Nelson Pass refines Class A amplifier design to a higher level, but the fundamental Class A characteristics that I currently experience, wrote about in this thread and elsewhere is the same thing that Redwoodgarden and numerous audiogoners have described. These characteristics set this particular design so far apart from others that I can have the realism of music at home rather having to severely limit myself to just live concerts here and there. Secondly I did somewhat follow the engineering discussion here and on the internet. But there is only so much the oscilloscope and math can help you assess reality. Ultimately it's the human ears that decide one way or the other. Thirdly Levinson ML2/3 were made late 1970's and early 1980's. Don't you think it's too risky to recommend people buying equipment that old? Finally, the newer Levinson amplifiers to me have their strength in precision and clarity, and that was the basis for my entry into the audiophile world a few decades ago. It really was a fine start, considering what's offered at the national chain stores. |
