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In Reply to: Atma-Sphere class A OTLs posted by Ralph on December 27, 2006 at 09:03:27:
"Hi Kurt, All Atma-Sphere amplifiers are Class A. This is easy to demonstrate, since all you have to do is put an ungrounded scope across one of the current limiting resistors in the output section and you will see."Well, if I still had my M60's I would do that measurement right away to see how that looks. I do have an idea of how it actually does look, and my guess is that it's really distorted. Now my impression of an ideal class A amp, A1, A2, it doesn't matter, is that the distortion of each contributing phase would have no appreciable distortion at full power, less than say 5% THD, and mainly even ordered of course. If it's well over this, then I think this is moving out of the realm of a true class A amp in the spirit of what a class A amp is supposed to be - that is, an amp whose opposing phase contributions are always about equal and always there up to the full rated power.
I can only believe that the signal you would see on that current limiting resistor would have huge compression toward cutoff and a pointy end where it is conducting heavily in "class A2", or AB2, whatever. How else can a 6AS7 biased to 60 mA act? It can only swing down to near 0 mA and up to the full 484 mA of current it's required to deliver 60W with the help of its paralleled friends into an 8 ohm load. Its zero power crossing point is a quiescent 60 mA. If this doesn't cut off sharply, but rounds out at the bottom just perfectly like a badly distorting remote cutoff triode could do - like a 6AS7 might do, then the 2nd harmonic distortion alone is:
100 * ( 0.5 * (484mA + 0mA) - 60mA) / (484mA - 60mA) = 43%, BEST CASE.
(That is from equation (6), section 2, chapter 13 of the Radiotron Designer's Handbook for a simple calculation of 2nd harmonic distortion of a sinewave.)
That means that when one side is pulling and the other side is pushing, there is a real extra effort on the side that's pulling than on the side that's pushing. In fact you can calculate how much that is: the pull at max power is 484mA - 60mA = 424mA and the push is 60mA, best case scenario. That means at full power there is 88% contribution from the pull and 12% contribution from the push. An ideal class A amp is 50% - 50%. A class AB amp is ideally 100% - 0%. But also, there is some trickle contribution from any class AB amp in the push direction, so it might be more like 99.99% - 0.01%.
So while one might argue that since this 88% - 12% pull-push contribution is "class A" since there is always that 12% still helping out, it seems to me a very "distorted" class A (not to be confused with the final output signal distortion). I will of course let you have your claim that this is class A, but I still think in my mind that it really resembles class AB in the spirit of its definition more than in some technical definition.
To me a properly designed class A amp for 50%-50% push-pull class A operation needs to have a bias at the quarter-power point of current, in this case 242mA. But you can't have this with a 6AS7 and not burn it out, even at 120VDC.
Now that's only my opinion.
Thanks, and have a happy New Year to you all at Atma-Sphere.
Kurt
Follow Ups:
I still think in my mind that it really resembles class AB in the spirit of its definition more than in some technical definition.I've always been a fan of the technical definition myself :)
If you are the same Kurt that I am guessing you are, you ought to take a listen to what we've been up to in the last 8-9 years. The kit that you built really only hints at what the amps do now days.
I think you might find it interesting also if we had the discussion about Voltage and Power Paradigms. Over the years it seems that you have dealt with a lot of Power Paradigm stuff (in fact we are having this discussion on the SET forum which is clearly Power Paradigm technology), yet you use Voltage Paradigm nomenclature (which is more suited to transistors). If you're interested give me a call 651-690-2246.
"If you are the same Kurt that I am guessing you are, you ought to take a listen to what we've been up to in the last 8-9 years. The kit that you built really only hints at what the amps do now days."Yes, I am probably that same Kurt you knew way back then. I contributed to ASOG and all that when I built the M60 in kit form and modified it ad infinitum and was a rather prolific writer back then. Yes, those are great amps then as I am sure they are better now, especially now that there are teflon caps to replace those polystyrenes. I wish I could afford your products today. But times have changed and I cannot afford any of your products at this time, and I was only able to afford what you had because it was in kit form back then.
And anyway, I am really quite satisfied with what I already have now. I really do believe in the SET sound for me. Even with the OPT's. But, of course, realize that I have direct-coupled passive-equalized amps with cobalt OPTs in a 45 SET package with ultrasonic AC DHT heating. And that just about trumps everything I have heard to date, P-P, SE, OTL, anything. You're not going to make me believe there's much better out there, at any price. Even if it were true, it seems inconsequential to me to try pursuing at this point.
Yes, I think I have hit the end point of this game. If I had a spare $5K to spend, I would improve the drivers of my B-D Design Oris 150's to their top of the line units. Unfortunately, we have a near zero output manufacturing country that causes the Euro to inflate to 1.33 USD from 0.87 USD in less than 5 years. So I no longer look forward to that upgrade.
Kurt
45s don't make a lot of power- I have to imagine that the Oris horns are essential. I've heard very good things about them- we have a few customers running them.I for one am not done with the American economy or production capabilities yet; obviously I think we still have a lot to offer.
It sounds like you have built up a pretty serious custom amp- not too many people use HF to run heaters. Had you thought about building more of them for fun and profit?
"It sounds like you have built up a pretty serious custom amp- not too many people use HF to run heaters. Had you thought about building more of them for fun and profit?"Yes, but then I think about how hard that would be and how much money it would drain me for a very small niche market. I really don't know why manufacturers haven't caught on to the ultrasonic heating for DHT's in SETs because A) it sounds better with a blacker background, and B) nobody wants any hum on even 110 dB sensitive speakers like I have. Also, it only costs about $100 additionally in parts to have this feature. Not bad for high-end SET amps of $5K or more out there. It's just lucky that 45's don't hum all that badly in the first place, but why have any hum?
(You know what's the coolest thing about ultrasonic heating? You use small ferrite toroid cores with 20 or so windings per side and you have an isolation transformer capable of delivering enough power to fully heat a 300B and have it floating properly. Nothing to it! The magic comes from the high frequency for making cheap tiny transformers to do the job. One power supply, one oscillator, one small class AB power amp (a power op amp might do) and two or more small isolation transformers.)
But also, one of the best attributes of my amp is that it has 01A DHT drivers which sound so exceptional there, and there's just no way of putting that tube into production because those delicate thoriated globe DHT's are only found used today. And it's not a good choice for a broad application anyway because the input sensitivity of my amp is now too low for most people. Thankfully, I designed my own phono stage with so much gain it more than makes up for that. But this is an all analog system.
For digital I go to my HT system. That's good enough for digital to me.
Kurt
Its base is a great-sounding 2-channel system that uses ASL SETs and DIY high-sensitivity speakers...
I've added full-range, diffuse-source surround speakers and am still using the Eminent Tech. 12 centerchannel speaker. Preamp is the fabulous-sounding 6-channel, tubed, c-j MET1. Sources are all digital, and they sound plenty good to me.
It's VERY satisfying--BIG, spacious, warm, well-extended and -defined in the bass, and surprisingly so for such inexpensive main drivers, highly transparent--for all the music to which I listen, which is mostly large-scale Classical stuff but also including small-group jazz and even smaller-scaled stuff.
-------------------------------------------------------
Tin-eared audiofool and obsessed landscape fotografer.
http://community.webshots.com/user/jeffreybehr
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