Class A Watts


Are class A watts more powerful then class AB, or is a watt just a watt. In other words would a 100 watt class A amp struggle with speakers that a 200 Watt class AB amp can handle just fine? I guess current would matter as well. Anyway, I was just curious.
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Watts are watts, Class A, AB, or D. The advantage Class A has over Class AB is no switching distortion, as the power output of the transistors is always on. This has nothing to do with power output though.
There is a lot more to good sound than watts, except for Reggie Watts.
I tend to agree, Watts are Watts, though I find Class A is the only way for me to listen to SS amps, but that's just me.

Where there does seem a difference is tube and SS watts. The former seems to handle comparitively difficult loads, beter than SS amps with the same rated output, given sufficiently robust output transformers. Whether that is only because of the well known "Benign Clipping", that charecterises tube amps, I am not sure.
My earlier XA100.5 monoblock's from Pass were superior in drive and dynamics compared to my modified Nu-Vista 300 poweramp from Musical fidelity which gave more than 2 times 350 watt by 8 ohm.
I own a Sugden A21SE and at only 30 wpc Class A it does not play louder than other well designed 30 wpc gear, but it is definitely quicker and approaches that oft elusive tube sound. The characteristics of speed, solid bass response and openness of vocals.
If your speakers are low impedance, or have an impedance curve that varies quite a bit, both current delivery and output impedance of the amp (damping factor)will have a greater effect on the sound than the amount of watts that the amp puts out.
However, there are A/B amps with no switching distortion and much quicker than lots of class A amps. Just because it's class A doesn't mean it's better than any other topology. Conceptually, it is ideal with regards to this issue. As far as tubes go, you're not going to get there without mosfets in an ss amp.
Going class A is a method of reducing distortion. In a high end amplifier that is supposed to sound like music, this can really help in a lot of cases.

Class A is not the defining criteria though. There are all kinds of design considerations that affect the way an amplifier can sound.
A watt is a watt unless you are comparing ss watts to tubes watts. Although many people feel that Class A amps sound tubish and it might hold some water but honestly I think they have their own particular sound and would definately give a favorable nod to the pure Class A designs over Class AB. I have heard some digital switching amps that come very close to the tube sound but in the end they still sound more like very good solid state.
06-16-13: Csontos
However, there are A/B amps with no switching distortion and much quicker than lots of class A amps....
Csontos, such an amplifier is, then, not class-ABl it's a class-A amp with some very low off-state bias current. I believe this amp is in the fuzzy region of amplifier classification - what I mean is that, traditionally, pure class-A power amps have their rated output current running thru their output stage whether there is a music signal or not hence they get very (, very) hot just idling. And, traditionally, class-AB amps do not have any bias current running thru the output stage meaning that only one output transistor operates at any given time & the other is off as in totally shut-off. This is what gives the cross-over distortion.
If an amp is running some bias current so that the output transistors are always on but the bias current is (very) low then essentially the amp is biased into class-A. I think that they call these power amps 'sliding class-A power amps' meaning that they have some very low bias current during off-state & the bias increases proportional to the input music signal when it's present.

06-17-13: Phd
A watt is a watt unless you are comparing ss watts to tubes watts.
Phd, a watt is a watt is a watt, no matter what! what say?
that includes tube watts.
From Ralph's many posts on the 'voltage paradigm' vs. the 'power paradigm' I'm made to understand that ss amps output a constant voltage & modulate output current as the speaker impedance varies. Tube power amps, OTOH, output a constant power & modulate both voltage & current as the speaker load varies (while keeping the product of voltage times current = output power constant). So, you "feel" that tube watts go a longer way than ss watts.
But 1 watt is 1 watt & will always be so, whether tube or ss.
Bombaywalla, you know what is, watt for watt tube amps appear to have more power and it could be due to the fact that tube amps clip more softly. Are you buying this?
Bombaywalla... for clarification, are you explaining what some manufacturers such as Plinius refer to as Class A/AB?
thanks.
06-17-13: Phd
Bombaywalla, you know what is, watt for watt tube amps appear to have more power and it could be due to the fact that tube amps clip more softly. Are you buying this?
Phd (Threads | Answers | This Thread)
Phd, I'm struggling to. SS amps can also be designed in such a way that they clip softly just like tube amps. It's all in the implementation details.
Tubes (no matter how old they are) are still (today) a linear amplification device. Tube amps can be designed by using Ohm's Law. OTOH, ss amps are based on semiconductor devices & one needs to have a basic understanding of semiconductor device physics to design with them + ss devices (BJTs, MOSFETs, JFETs, IGBTs, etc) are all exponential/square law devices i.e. not linear amplification devices like tubes. Hence the ss device distortions patterns are quite different from tube devices. I think that is why you feel that tubes clip softly while ss amps do not.


06-17-13: Lowrider57
Bombaywalla... for clarification, are you explaining what some manufacturers such as Plinius refer to as Class A/AB?
thanks.
yes, I believe so altho' Plinius is not a good example. All the Plinius amps I know of actually have a switch that can switch the amp into class-AB & into class-A. In fact, these amps have thermal protection that can be set to 15 min/30 min/60 min where the amp falls into class-AB mode if un-used in class-A mode. So, in Plinius' case, they are actually changing the bias current with deliberate user input (toggle that switch). The sliding class-A amps that I was talking about do the class-A/AB transition automatically with no user input.
For me Pass Labs makes the best overwhole sound for amp's. It is better in timing and drive compared to most tube amp's. But you still have the warm musical sound. Like tube it gives a very deep and wide stage. With many tube amps I Always miss something.
In general, tube amps have output transformers- OTL amps being the exception. So tube amps generally have high impedance output because of the transformers. That means they like high impedance speakers. When the speaker impedance goes low the amperage capability of a tube amp becomes limited. That means the tubes are not able to generate the same voltage levels at lower impedances that they are able to generate with a high impedance speaker. You can short the outputs of a tube amp and not cause any damage. The voltage goes to almost zero. (Don't try that with a SS amp- it will pop because current will try to go to infinity.) Tube amps typically have power output ratings that are the same for 8 ohms and 4 ohms impedances. In contrast, SS amps are low impedance designs. They typically increase their power ratings from 8 ohms to 4 ohms impedance. The big monsters with beefy power supplies can generate gobs of amperage and so their power ratings double from 8 ohms to 4 ohms and then to 2 ohms Some even double at 1 ohm. They probably couldn't run at that power level for very long, however, without overheating. Now a tube amp would have the advantage over SS with a 16 ohm speaker. The tube amp would maintain the same power rating as at 8 ohms- because it could generate higher voltages going into the 16 ohm speaker. The SS amp, however will halve its power rating from 8 ohms to 16 ohms- just as it can double going to 4 ohms, since it's amperage capability is reduced due to the 16 ohm impedance. SS amps and 4 ohm speakers were made for each other. Sure, some tube amps work too- ideally the output transformers are matched or have taps for 4 ohm impedance.
As for Class A power- it's simply a design standard based on the bias or offset voltage/current. I don't see how a class A watt can sound louder than a class A/B watt. Almost all SS amps and tube amps for that matter run class A to some power level and switch automatically over to A/B mode once the bias limit is reached. That power level is based on the bias setting which is related to the amount of heat sinks and idle temperature that the amp has.
So tube amps generally have high impedance output because of the transformers.
Tonywinsc, this is not a correct statement. Tube amps have a high output impedance due to the high output impedance of the tube itself. You need many tubes in parallel to get the overall output impedance low (see some of Atma-sphere amps, CAT JL2, etc). The output transformer is used to transform the high(er) output impedance on the tube side to the lower output impedance on the speaker side. Hence the 8/4/2 Ohm taps on a power amp. So, output transformers save the days for tube amps - without output transformers one would not be able to drive a speaker unless one used special techniques like OTL (Atma-sphere Berning). Thanks.
Yes, of course. I was trying to keep the description more simplified, ie. the output tubes are coupled through the transformers so that the speakers see only the transformers.
If an amp is running some bias current so that the output transistors are always on but the bias current is (very) low then essentially the amp is biased into class-A. I think that they call these power amps 'sliding class-A power amps' meaning that they have some very low bias current during off-state & the bias increases proportional to the input music signal when it's present.

All transistor amps run some bias current on the output devices at idle. This is not sliding class A, this is AB. Sliding A is where the bias is dynamically changed as the input signal changes.

I was trying to keep the description more simplified, ie. the output tubes are coupled through the transformers so that the speakers see only the transformers.

The transformer transforms the speaker impedance to something that the tubes can handle. Conversely, the tubes are part of the source impedance that the speaker 'sees'.

An implication of this is that a class A tube power amplifier may well be operating in the AB region if the transformer has the wrong impedance on a particular tap, like a 4 ohm load on the 8 ohm tap.

BTW it is not correct to say

SS amps and 4 ohm speakers were made for each other.

The why of it is that all amps, tube or transistor (or class D) make more distortion into 4 ohms as opposed to 8 or 16 ohms. This distortion is audible as increased brightness and loss of detail. Put in a nutshell, if **Sound Quality** is your goal, it is best served by a speaker of 8 ohms or more. If **Sound Pressure** is your goal, there is a weak argument for 4 ohms if you have a transistor amp that can double its power (3db). Put another way, there really isn't an argument for 4 ohms in high end audio, but there might be in sound reinforcement.
I disagree.
"The why of it is that all amps, tube or transistor (or class D) make more distortion into 4 ohms as opposed to 8 or 16 ohms. This distortion is audible as increased brightness and loss of detail. Put in a nutshell, if **Sound Quality** is your goal, it is best served by a speaker of 8 ohms or more."

OK, I'm following the theory, but as far as audible distortion, wouldn't this be more evident with a tube amp (let's say a SET amp) using a 4 ohm spkr rather than a SS amp into a 4 ohm speaker.
Wouldn't this distortion with SS amps show up more on test specs, not in a listening room?
I think so.
Ralph (Atmasphere), Class A, AB, etc aside, I think it's also worth mentioning that many tube amps use negative feedback which reduces the output impedance at the taps. I would surmise that if a tube amp's output impedance off the 8 ohm tap is 1 ohm, and say half that off the 4 ohm tap, the tube amp may behave somewhat "SS-like" in the way it drives speakers that were voiced to be driven by a SS amp.

For example, I understand that the output regulation of my amp is approx. 1.2 db off the 8 ohm tap, which I think permits the inference that my amp can drive a speaker with a varying impedance plot so that the speaker's output SPL as a function of FR is pretty flat. Perhaps even better performance off the 4 ohm tap. Of course, there's the issues you have written so much about: odd ordered harmonic distortion and TIM distortion.

I think this post summarizes the numerous on-line and off-line conversation I have had with you and Al.

Regards,

Bruce
Lowrider57, the way the two types of amplifier technologies distort is somewhat different.

With tube amps you might get a bit more of the lower orders (so you would get a richer, fatter sound, that could border on muddy, as those are all terms audiophiles use to describe excessive lower-ordered harmonic distortion), but this is highly dependent on the actual amp design and its output transformer (if it has one).

With SS amps, there seems to be less variation in the distortion types that creep in- it appears to be almost entirely odd orders (the source of brightness/harshness in many amps). It may not be much but because the human ear/brain system uses odd ordered harmonics to sort out how loud a sound is, it is very sensitive to these harmonics.

***IOW, it shows up in the specs and is also audible.***

As an additional note, 4 ohm speakers are very critical of speaker cables so if you want the best results with them the cable can't be skimped. Conversely with 16 ohms the cable is hardly critical at all- so you can run longer lengths of lighter gauge and totally get away with it.

The reason for this is the seemingly very slight DC resistance of the cable is in series with the output impedance of the amplifier, and can affect the damping factor by quite a lot! BTW this particular fact has been known for decades; I first saw it in a nomograph published by RCA in the early 1960s.

Four ohms came in as a means of allowing speaker manufacturers to seem to not loose so much efficiency while at the same time using cheaper drivers that have less precision in the voice coil gaps and the like (this makes the speaker easier/cheaper to build, as much as 10X cheaper, so its a pretty powerful incentive). You get 3 db of Sensitivity back if you have a transistor amp that can double power. The trick is understanding the Sensitivity and Efficiency are not the same thing!!

This is why I say that 4 ohms offers a *sound pressure* advantage, not a sonic advantage.

You can talk to any amplifier manufacturer and they will tell you the same thing- that fact that their amp can double power into 4 ohms is not the same thing as saying that the amp is sounding its best into that impedance.
"You can talk to any amplifier manufacturer and they will tell you the same thing- that fact that their amp can double power into 4 ohms is not the same thing as saying that the amp is sounding its best into that impedance. "

Oh, I totally get that part. Thanks for the thoughtful explanation, Atmasphere.
Watts that?