What are the most powerful Pure Class A Amplifiers ever made?

caesar

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May 30, 2010
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Anyone familiar with Symphonic Line Kraft 400's?

Reading the subjective description, makes me salivate:

"... hat could be so entirely special about a pair of amplifiers?

Everything. These are magnificent beasts.

As Peter Forsell has pointed out, what we hear from the speaker is, in essence, the electricity itself. In this sense, the Kraft 400s stand alone as the ultimate expression of purity in design when it comes to the power supply and short, simple signal paths. This paradoxical fusion of gargantuan and minimalist concepts would prove a futile gesture indeed if it didn't actually serve the music in some mighty special way. But it does—magically.

The foundation of their sound was the richly complex and textured, powerful, transparent, and huge soundstage the 400s hung about the speakers and the listening room. The ambient soundfield had an integrity that I had never before experienced. They developed a spread and layering of soundstage of perfect control—all musical elements absolutely stable. The classic descriptions of wide, deep, layered, and so on lose their meaning in this context. Put a CAT/Forsell combo or a JP-80MC and a JS1/J1 in front of the 400s, and the original acoustic is simply there before you. Cue up an Insider or the Symphonic Line RG-8 phono cartridge on The Forsell, and you are transported. There are no room boundaries to overcome, no sense of displacement of your listening room's acoustic. You're simply there...."

http://www.stereophile.com/content/symphonic-line-kraft-400-monoblock-power-amplifier
 

JackD201

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Apr 20, 2010
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Hmmm. 1995 review. I wonder how the current Kraft amps hold up against the competition today.
 

caesar

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Hmmm. 1995 review. I wonder how the current Kraft amps hold up against the competition today.

My VTL is from that era. And other than a bit less openness on the highs, I like it better than their newer designs. Also, the Boulder 2000 is from that era as well, and considered one of the best to this day. If I could find one for dirt cheap, it would be very tempting. But the problem is each of those boys is 260 lbs.
 

trueblue

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Jan 23, 2013
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Theoretical pure Class A efficiency is 25% ( not 50 % ) When degraded with 20% that would require 1500 watts from the AC supply.
Class A is one of the most misused and minsunderstod topologies in high end. Pure Class A is no guarantee of great sound, it depends on how it is made but many manufactures claims Class A because real class A is expensive to manufacture. All pure class a amplifiers must have huge powersupplies and huge heatsinks,. these are the most costly parts in any amplifier, so when looking at a claimed class a amplifier, the weight and powerconsumption will tell most of the story. Class a amplifiers predicting what comes next and can adjust the bias to match the music ? sure, and pigs fly - but it can still be a nice sounding amplifier.
Why not? Theoretical class A efficiency at nominal impedance is 50%. Even if you degrade it by 20% almost any standard AC installations can supply 750 watts. But it will heat a lot.
 

JackD201

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My VTL is from that era. And other than a bit less openness on the highs, I like it better than their newer designs. Also, the Boulder 2000 is from that era as well, and considered one of the best to this day. If I could find one for dirt cheap, it would be very tempting. But the problem is each of those boys is 260 lbs.

True, true. Newer doesn't automatically mean better. I was just wondering what they might have done in the past 18 or 19 years since they still make the Kraft line albeit the strongest is 350 and no longer 400.
 

microstrip

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Theoretical pure Class A efficiency is 25% ( not 50 % ) When degraded with 20% that would require 1500 watts from the AC supply.
Class A is one of the most misused and minsunderstod topologies in high end. Pure Class A is no guarantee of great sound, it depends on how it is made but many manufactures claims Class A because real class A is expensive to manufacture. All pure class a amplifiers must have huge powersupplies and huge heatsinks,. these are the most costly parts in any amplifier, so when looking at a claimed class a amplifier, the weight and powerconsumption will tell most of the story. Class a amplifiers predicting what comes next and can adjust the bias to match the music ? sure, and pigs fly - but it can still be a nice sounding amplifier.

Theoretical maximum power efficiency of a pure class A transformer coupled amplifier or pushpull amplifier is 50% - I was surely assuming the best possibility. The 25% value refers to single ended operation with capacitive coupling or similar. I surely agree with you that Class A is one of the most misused and misunderstood topologies in high end.

Surely these are maximum values, impossible to reach in tube power amplifiers. A Jadis JA200 is rated at 160W in class A and has an idle power of 440W.
 

trueblue

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Jan 23, 2013
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Theoretical maximum power efficiency of a pure class A transformer coupled amplifier or pushpull amplifier is 50% - I was surely assuming the best possibility. The 25% value refers to single ended operation with capacitive coupling or similar. I surely agree with you that Class A is one of the most misused and misunderstood topologies in high end.

Surely these are maximum values, impossible to reach in tube power amplifiers. A Jadis JA200 is rated at 160W in class A and has an idle power of 440W.

I was referring to Solid State amplifiers
 

microstrip

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I was referring to Solid State amplifiers

Push pull Solid State amplifiers also have a maximum 50% theoretical efficiency. See http://sound.westhost.com/efficiency.htm or Operational Amplifiers: Theory and Design by Johan Huijsing . Chapter 5.1 Power Efficiency of Output Stages - page 109 fig 5.1.3 . It was an usual question in electronics problem classes.
 

MylesBAstor

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MylesBAstor

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Brian Walsh

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Atma-Sphere MA-3 output transformerless monoblocks are conservatively rated at 500 watts Class A.

1073616781.jpg
 

Brian Walsh

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We could have really used these in January.

;)

I've been told by Ralph Karsten of Atma-Sphere that effectively the MA-3 doesn't produce that much more heat than the MA-2. Each amplifier has 3 banks of 14 output tubes each which can be turned on or off to suit listening requirements.
 

mep

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To be responsible for the care and feeding of that many output tubes is simply masochistic.
 

microstrip

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I've been told by Ralph Karsten of Atma-Sphere that effectively the MA-3 doesn't produce that much more heat than the MA-2. Each amplifier has 3 banks of 14 output tubes each which can be turned on or off to suit listening requirements.

Interesting.You get a MA-1.4, a MA-2.8 and a MA-4.2 amplifier when you get a MA-3 - normalizing the model reference to the number of power tubes (the MA-2 has 20 tubes). Do you know what is the total capacitance of the MA-3?
 

microstrip

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To be responsible for the care and feeding of that many output tubes is simply masochistic.

No Mark. The 6AS7 tubes last long, are rugged and intrinsically self-fused. It even includes a tube tester. Caring of the MA-3 should be much easier than of your old Defy-7!
 

mep

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No Mark. The 6AS7 tubes last long, are rugged and intrinsically self-fused. It even includes a tube tester. Caring of the MA-3 should be much easier than of your old Defy-7!

Francisco-I'm simply talking about the cost of retubing them and keeping them running assuming all of the output tubes live a nice long life.
 

MylesBAstor

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yes, really, class a is a bias scheme, not a distortion scheme. I can bias about any device class a and produce any distortion I want, but distortion seems a bad word for many philes here, despite its applicability, aah well. sigh.

Oh boy.
 

ack

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Distortion can be good. It's the only hope you have to re-shape BS you read on the internet into something that may even be remotely perceived as sarcastic.
 

microstrip

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yes, really, class a is a bias scheme, not a distortion scheme. I can bias about any device class a and produce any distortion I want, but distortion seems a bad word for many philes here, despite its applicability, aah well. sigh.

Congratulations. Are you going to educate us on how to do it? I am sure every electronics audio designer will come to WBF!
 

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