[Architect7]

I know that Parasound Halo amps provide the first ~20wpc in Class A power, what other companies/models do this?  Thanks!

[Elizabeth]

Modern amps almost all have some class "A" bias for the first few watts.
Some more, some less.
Generally class "A" is hugely inefficient. And so the hotter the amp runs at idle, the more it is running class "A" (accounting for size. really it is the btu's and bigger amps with a lot more cooling area may seem cooler than a small amp with fewer fins)

[FullRangeMan]

The deal is add more some good sound features to a amp to grant good sound, as monoclock, no feedback, SE, etc
This FirstWatt SIT1 are single ended, monobloc, and zero feedback and moderated priced:
http://www.firstwatt.com/sit1.html

[RDavidson]

Maybe I'm being too obvious here but all newer Pass Labs amps do this, including the XA series. In the specifications of the XA series, they only list the high bias class A output, but actually go into class A/B after their specified Class A watts are used up. See the Stereophile review of the XA30.5 for example. The X series are a little more conventional and have more massive A/B output than the XA series. Basically, the X series is still biased into Class A, but not as highly as the XA, so you'll run out of Class A watts sooner with an X vs XA series (in the same price range). Very similar circuitry between the two series. Class A bias range is the main difference. Both series of amps are VERY robust. I have yet to see my XA 30.5 (via the meter) leave it's Class A range, and supposedly it only has 30 watts of class A power into 8 ohms.

First Watt amps don't go into class A/B at all. They're Class A only, low power, and are pretty application specific, comparatively speaking.

[WGH]

Krell amps are another obvious choice if you need a little more power.

[ssglx]

Monarchy amps do!

[davidflas]

Emotiva does.

https://emotiva.com/products/amplifiers/xpa-1l

[dflee]

It doesn't get mentioned much around here but Coda makes a series of class A-A/B amps.

Don

[srclose]

Clayton Audio

[Davey]

In my opinion, the definition of Class A has gotten a little squishy through the years.  In my book only devices that are constantly biased exactly halfway between cutoff and saturation (or the equivalent in valves) can be considered Class "A".

Anything that runs...."Class A for the first few watts and then switches to A/B" (or similar quasi-biasing techniques) are not true Class A.
That definition cuts down the selection of power amps considerably. 

I'm a dinosaur. 

Dave.

[RDavidson]

In my opinion, the definition of Class A has gotten a little squishy through the years.  In my book only devices that are constantly biased exactly halfway between cutoff and saturation (or the equivalent in valves) can be considered Class "A".

Anything that runs...."Class A for the first few watts and then switches to A/B" (or similar quasi-biasing techniques) are not true Class A.
That definition cuts down the selection of power amps considerably. 

I'm a dinosaur. 

Dave.

Your book?
I see what you're saying, BUT it is obviously possible to design true Class A amps that switch to A/B. I mean, what you're saying kind of flies in the face of the guy who practically wrote THE actual book on Class A solid state design and continues to pioneer the art (Nelson Pass), if you don't believe his Pass Labs amps are true Class A as spec'd, before switching to A/B. I guess what I'm saying is just because an amp doesn't stay in high bias Class A only, doesn't mean it is wrong or is a less "true" amp. However, and I agree with what I think you're getting at, that not all Class A  amps are built the same or meet the same "true" Class A spec.....ie the "squishy" Class A amps. I'm not going to toss any names out there.

[rif]

And what confuses me more is when Nelson Pass, and others, talk about single ended class A and push-pull class A. 

Does it really matter if the amp meets your requirements and sounds good?

[Davey]

Your book?
I see what you're saying, BUT it is obviously possible to design true Class A amps that switch to A/B. I mean, what you're saying kind of flies in the face of the guy who practically wrote THE actual book on Class A solid state design and continues to pioneer the art (Nelson Pass), if you don't believe his Pass Labs amps are true Class A as spec'd, before switching to A/B. I guess what I'm saying is just because an amp doesn't stay in high bias Class A only, doesn't mean it is wrong or is a less "true" amp. However, and I agree with what I think you're getting at, that not all Class A  amps are built the same or meet the same "true" Class A spec.....ie the "squishy" Class A amps. I'm not going to toss any names out there.

No book, sorry. 

Not to take anything away from Nelson as he's certainly done some terrific contemporary work on Class A amp design in the consumer audio field.  However, Class A solid state design has been around for a bit longer than him.    It dates all the way back into the late 40's with the development of the first semiconductor transistors.

I appreciate your comments and don't disagree.  I think maybe it's more of a semantical (is that a word?) definition between "True Class A" and "Class A"  ??

Anyways, these "quasi" biasing techniques probably do fit in the definition of the OP's original query and satisfy the "spirit" of his question/requirement.

I don't know why you don't want to toss any names out there. 

Dave.

[Quiet Earth]

I would love to hear these Valvet E1-R monoblocks someday : http://www.highend-electronics.com/img/Productinfo_VALVET-E1r.pdf

They are Class A, single ended, and use only one power transistor in the output stage. This seems like the cleanest/clearest way to go if you have efficient speakers.

I wonder why more companies don't offer this kind of product in the solid state market. Why should low power single ended amps be limited to triodes and pentodes? 


 

Not to take anything away from Nelson as he's certainly done some terrific contemporary work on Class A amp design in the consumer audio field.  However, Class A solid state design has been around for a bit longer than him.    It dates all the way back into the late 40's with the development of the first semiconductor transistors.

+1 on that.

[rodge827]

ATC does a good job with it, I enjoyed my time with the SIA2-150.

http://www.atcloudspeakers.co.uk/hi-fi/electronics/power-amplifiers/s1a2-150/

http://www.atcloudspeakers.co.uk/hi-fi/electronics/power-amplifiers/p1/

[Freo-1]

First Watt M2 is 25 WPC Class A with no (negative) feedback.  It also has a passive transformer gain stage, and a killer bias setup for the output.  Very nice.

[Roger A. Modjeski]

Virtually any class AB amp can be made class A very easily. Put on bigger heat sinks, crank up the bias to the rail voltage/ twice your speaker impedance, make sure the power transformer can take the extra load without getting too hot (many can) and now you are class A. Enjoy

I think class A amps are fine, but don't buy more watts than you need. Nelson makes many of his First Watt class A amps 10 watts which is enough for most listeners. At 10 watts the numbers are much more reasonable. May I digress here and say that Nelson is one of the most reasonable and talented designers in this industry and I always enjoy speaking with him. Lately were discussing amp camp, Burning Amp (which is sadly cancelled for this year) and my proposition to open an audio school. http://www.audiocircle.com/index.php?topic=125987.0. He and I are all for educating the audio enthusiast and cutting through the bullshit in this industry. I recall Paul Klipsch distributed yellow buttons at the shows that simply said, in old English lettering BULLSHIT Sadly there seems to be more and more of it out there.

It is pure foolishness to own a 100 watt amp and use it at 1 % of its power. it is downright absurd to own a 1000 watt amplifier unless you are in the live sound concert business or own a terribly inefficient speaker that you want to play at 120 dB. Emotiva has a new 1000 watt monoblock advertised in Stereophile.  Their motto is "rethinking high-end" It appears they are not thinking about audio but perhaps cashing in on the more power the better fad. Someone, I don't know who, has started a tread that says "a bigger amp controls a speaker better and the bigger the better" This is simply not true. High available current and damping control a speaker.

If you don't know how much power you are using find out! Don't guess.  Most digital votmeters have a peak reading fucntion, some even beep each time you hit a new peak. How about spending $50 on a volt meter before you spend thousands on an amp that is much bigger than you need. Just hook that meter across your speaker terminals and be ready for a big surprise. If you have speakers that are 90 dB and you listen at 90 dB peak you will see a peak of about 4 volts. A few volts is what I usually see. Of course if you listen at 110 dB you will need a 100 watt amp and start saving for a hearing aid. Recently I visited someone who was playing 100 dB peaks. I had to leave the room. I have a SPL meter application on my iPhone that shows peak and average levels. If you have any smart phone SPL meters are free and you have no excuse not knowing your listening level.

One other thing about the class A region. Higher power amps have a smaller class A region than low power amps for a given amount of heat sinking (at constant temperature). Anyone want to comment why?

[RDavidson]

No book, sorry. 

Not to take anything away from Nelson as he's certainly done some terrific contemporary work on Class A amp design in the consumer audio field.  However, Class A solid state design has been around for a bit longer than him.    It dates all the way back into the late 40's with the development of the first semiconductor transistors.

I appreciate your comments and don't disagree.  I think maybe it's more of a semantical (is that a word?) definition between "True Class A" and "Class A"  ??

Anyways, these "quasi" biasing techniques probably do fit in the definition of the OP's original query and satisfy the "spirit" of his question/requirement.

I don't know why you don't want to toss any names out there. 

Dave.

Now hold on, I didn't say Nelson Pass invented class A amplifiers, but I don't think there's any disputing that he is the foremost expert and pioneer of modern class A solid state design. I don't know of anyone else present or past who has done more work or continues to experiment and develop the art. If there's a book on class A solid state amp design with 10 chapters in it, Nelson would've written 8 or 9 of them. That's all I meant by saying that he pretty much wrote THE book.

Regardless, I agree with everything you're saying.

Ha! You're evil. Sorry, I'm not in the mood for starting fires (by calling out the "quasi-Class A" amp makers or those being "semantical" with their Class A spec). As I'm sure there are many out there enjoying these amps, I see no reason to cause mass ownership panic, resulting in Audiogon making more money than Apple this year. Those who seek enlightenment will do so on their own. Regardless of spec or marketing or whatever, all that matters is what sounds good to you.

So, maybe we should just get back to the topic of class A and supposed class A amps that switch to class A/B.

[Roger A. Modjeski]

In my opinion, the definition of Class A has gotten a little squishy through the years.  In my book only devices that are constantly biased exactly halfway between cutoff and saturation (or the equivalent in valves) can be considered Class "A".

Anything that runs...."Class A for the first few watts and then switches to A/B" (or similar quasi-biasing techniques) are not true Class A.
That definition cuts down the selection of power amps considerably. 

I'm a dinosaur. 

Dave.

Your definition of class A applies only to single ended amplifiers. These were the first amplifier ever made and with tubes of course, starting with the De Forest Audion in 1906. Virtually every tube table radio has a class A single ended output stage. Car radios of the 1950's, 60's and into the 70's had a single ended transistor, class A output stage consisting of one power transistor. The car radio makers were right on top of power transistors because it simplified the power amp of the radio enormously. If is quite common to find the name DELCO or Motorola on early power transistors in and out of car radios. The output transistor was prevalent for many years while the rest of the ratio was still tubed.

When we get to push-pull the definition changes. In a push-pull amplifier class A is defined by having the transistor that is going toward cutoff never completely cutting off. Whether made with tubes or transistors the definition is the same.

I believe Nelson Pass is the inventor, or first made popular, the notion of sliding class A where the bias level follows the signal and attempts to keep the transistor going toward cutoff from cutting off. This was in the early 1970s at Threshold. I recall the amp, it was big with a lovely large meter on the front.

Finding out where a class AB amp leaves class A is very easy. Take the idle current (typically 50-100 mA), double it, square that and divide by the load impedance. That is the peak power, the RMS is half that. For a generous example lets use 100 mA which is typical of a cool running AB 100 watt amplifier. That works out to 0.32 watts peak, 0.16 watts RMS for the class A region. The power dissipated is 0.1 amps x the total rail of 100 volts turns out to be 10 watts per channel. Add in driver power of another 10 watts per channel some other losses and you have about 50 watts of idle draw. That is a fairly cool running amp typical of SAE and many early SS amps.

[RDavidson]

I think class A amps are fine, but don't buy more watts than you need. Nelson makes many of his class A amps 10 watts which is enough for most listeners. At 10 watts the numbers are much more reasonable. May I digress here and say that Nelson is one of the most reasonable and talented designers in this industry and I always enjoy speaking with him. Lately were discussing amp camp, Burning Amp (which is sadly cancelled for this year) and my proposition to open an audio school. http://www.audiocircle.com/index.php?topic=125987.0. He and I are all for educating the audio enthusiast and cutting through the bullshit in this industry. I recall Paul Klipsch distributed yellow buttons at the shows that simply said, in old English lettering BULLSHIT Sadly there seems to be more and more of it out there.

Now back to the topic: From the above example we see that we simply multiply the output power by 5 to get the idle dissipation. With Nelsons 10 watts that is only 50 watts which is, you might say is rather green.

It is pure foolishness to own a 100 watt amp and use it at 1 % of its power. it is downright absurd to own a 1000 watt amplifier unless you are in the live sound concert business or own a terribly inefficient speaker that you want to play at 120 dB. Emotiva has a new 1000 watt monoblock advertised in Stereophile.  Their motto is "rethinking high-end" It appears they are not thinking about audio but perhaps cashing in on the more power the better fad. Someone, I don't know who, has started a tread that says "a bigger amp controls a speaker better and the bigger the better" This is simply not true. High available current and damping control a speaker.

If you don't know how much power you are using find out! Don't guess.  Most digital votmeters have a peak reading fucntion, some even beep each time you hit a new peak. How about spending $50 on a volt meter before you spend thousands on an amp that is much bigger than you need. Just hook that meter across your speaker terminals and be ready for a big surprise. If you have speakers that are 90 dB and you listen at 90 dB peak you will see a peak of about 4 volts. A few volts is what I usually see. Of course if you listen at 110 dB you will need a 100 watt amp and start saving for a hearing aid. Recently I visited someone who was playing 100 dB peaks. I had to leave the room. I have a SPL meter application on my iPhone that shows peak and average levels. If you have any smart phone SPL meters are free and you have no excuse not knowing your listening level.

Preach Roger! I'm really glad you chimed-in here.