AudioCircle
Audio/Video Gear and Systems => All Solid State => Topic started by: grsimmon on 11 Jun 2017, 07:10 pm
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I'm aware that certain Parasound amps operate in Class A for perhaps the first 7-9 watts of operation, then transition to Class A/B thereafter.
Are there other amps on the market that operate in this manner?
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http://www.sst.audio/son-of-ampzilla/
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Many. The Pass Labs X series all play to some threshold in class A then switch to AB. My X-350 runs to 40 Watts in class A then switches. There are more. I also have a vintage Harman Kardon Citation 16A and it is biased in class A for the first 1.5 Watts then switcheroo.
Making a list will just require a bit of Googling. :thumb:
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Emotiva
https://emotiva.com/products/amplifiers/xpa-1
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ALL audiophile amps have a slight bias to class A to stop crossover distortion.
but so called all class A amps typically really are just highly biased to class A, but still do switch to AB at high output.
So to answer the op question: ALL modern production solid state amps are biased to Class A to some small (or large) degree.
A very few amplifiers are pure class A
The real problem with 'pure' class A in an amp is heat. With pure class A the entire power is dissipated as heat when the amp is idling. The blowtorch of true 100% class A means that amp is HOT, and huge heatsinks to dissipate the heat. (so that pure class A amp with 50 watts per weighs 100 lbs.)
So in general if you can name it, and it is not a class D amp, it IS a class A for some percentage (from small to large)
It is a curious thing that the new 'cubed Bryston amp series is LESS biased to class A than the squared, yet does sound better.
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Here is Bryston's statement on why they do not go into Class-A... http://www.audiocircle.com/index.php?topic=96377.msg964084#msg964084
The last paragraph from that post:
A not-so-obvious disadvantage with class A designs is that this high idling current has consequences to the distortion levels far beyond the theoretical elimination of crossover artifacts, (which even in itself is debatable). Transistors have numerous types of distortion mechanisms, among which are deviations from linearity under conditions of simultaneous high voltage and high current. These are, of course, the exact parameters necessary to class A operation, and a typical Class A amplifier runs distortion levels at least 10 times, and often over 100 times, as high as a Class AB amplifier of similar power, or around 0.1%. A careful inspection pf the distortion spectrum also reveals that all the'harmonics are increased, including those represented by the crossover distortion at which the class A operation was aimed in the first place!
It is a curious thing that the new 'cubed Bryston amp series is LESS biased to class A than the squared, yet does sound better.
The improved sound is owed to the new input circuit (output circuitry remains the same from the SST2 models). Plus they brought down the bias to meet tighter power regulations, but still kept it at the required level to reduce distortion.
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Odyssey amps, start class A then switch to A/B. They sound relax but high in detail.
Lots of info here.
http://www.audiocircle.com/index.php?board=10
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All of Pass Labs amps run in class A and switch to AB. Some run in Class A for the first 20 watts and others for the first 160 watts.
Vincent Audio, AVA are others to name a few.
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Pretty sure all First Watt amps are pure Class A. They aren't space heaters either....because they have less generous output, and simpler circuits, than their Pass Labs brethren that switch to A/B. Just saying that a class A amp (or A - A/B switching amp) doesn't necessarily need to be a giant space heater. It fully depends on output in class A and how deeply biased into class A the amp is. Sorry if a little off topic.
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Pretty sure all First Watt amps are pure Class A. They aren't space heaters either....because they have less generous output, and simpler circuits, than their Pass Labs brethren that switch to A/B. Just saying that a class A amp (or A - A/B switching amp) doesn't necessarily need to be a giant space heater. It fully depends on output in class A and how deeply biased into class A the amp is. Sorry if a little off topic.
Yes, but living here in Minnesota, my Pass X250 comes in handy during the winter. My BAT preamp with its 8 tubes may be even hotter. :D
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I'm next to you in Wisconsin, and was on the opposite side of you in South Dakota before that. The space heater effect can be useful at times. :thumb:
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For Nostalgia, the Bedini 25/25 and the mighty Sumo Gold and Nine Plus.
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http://www.sst.audio/son-of-ampzilla/
Interesting, I'm looking at their trinaural processor down the road
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Pretty sure all First Watt amps are pure Class A. They aren't space heaters either....because they have less generous output, and simpler circuits, than their Pass Labs brethren that switch to A/B. Just saying that a class A amp (or A - A/B switching amp) doesn't necessarily need to be a giant space heater. It fully depends on output in class A and how deeply biased into class A the amp is. Sorry if a little off topic.
No problem, that's good to know and thank you
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Minimizing crossover notch distortion isn't the only advantage to real class A operation - it also means buckets of current available and traditionally with a doubling of wattage as you drop in load impedance. Even my little Threshold S-200 that's only class A to about 20 watts per channel runs HOT. Something like the monster Krell monblocs that were for sale locally a few weeks back -are tempting until you try to explain to SWMBO'd why you always turn on the air conditioner when you listen to music !
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I have a Threshold s300 150x2, class A up to 10 watts but it's never been hot, nor have I noticed the Coda CSX being hot. Ventilation matters, I wonder if anything else does like cable gauge etc. I'm sure the load and listening habits. My X250 got warm to the touch but not what I'd consider hot.
I had a set of Conrad Johnson tube monoblocks, now that threw some noticeable heat.
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My Aragon 2004 mk2 says goes to 50w in a then a/b after that it says
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I'm aware that certain Parasound amps operate in Class A for perhaps the first 7-9 watts of operation, then transition to Class A/B thereafter.
Are there other amps on the market that operate in this manner?
There are different circuit configurations to consider when thinking Class A. Most amps are push-pull designs using an output device for each side of the signal phase. In a Class B there is theoretically a hard on-off point for each device as it crosses 0 phase. Class AB introduces current to allow each device to conduct some amount around the 0 phase point. So the output devices are "always on" for some amount of the opposite phase cycle and for some amount of power output and this is what is called being in Class A mode. It is not so easy to say that a higher bias current level is better for any given push-pull amp design. There are tradeoffs.
A single ended Class A uses one output device for the full signal phase and inserts bias at a midpoint in conductively. So, this device will run hot as it is never off. With this configuration you truly have no crossover distortion (good) but potential for higher distortion outside the range of the device to conduct (low power overhead).
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I'm not a tech-y guy, so I don't really understand what class my Krell FPB 400cx works in. But it gets hot as Hades, that's for sure.
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My 30 watt Cairn 4808A amp does the first 10 watts in class A. It runs very hot when idling.
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My 30 watt Cairn 4808A amp does the first 10 watts in class A.
My amp also does the first 10 watts in Class A. But I'm only using about 4 watts at normal listening levels, so am I nearly always in class A mode? And whenever I cross into AB, how would I even know? If I can't tell the difference between A and AB, then is the supposed superiority of Class A overrated? I've been wondering about this for many years.
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When I had a Pass XA30.5, whenever it would shift to class A/B, the sound became very slightly hard or mechanical. It was barely noticeable, but over sustained periods I could tell that something just didn't sound right. It's difficult to pinpoint, but I'd describe it as a slight lessening of naturalness and flow. Of course, this is just my experience with one amp and there are likely other things at play (like distortion, both with the amp and speakers). I think it would be very difficult to prove either way whether class A or class A/B is better or worse than the other, to our ears. This is very subjective with a lot of variables.
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Early B., in most cases the class A rated power also includes the power used by the front end input stage which is operating in class A. You may be transitioning to class B well before you have dissipated 10 watts in the output stage.
Scotty