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Class AB vs Class D?

post #1 of 33
Thread Starter 
Hi all,

In my research for subwoofers I've seen class AB amps offered as sort of feature or selling point on certain subwoofers over class D amps. From what I understand, class D is the most common choice, due to it's overall efficiency, but I'm wondering, should I show any preference to subwoofers that use class AB amps? I realize that the whole is greater than the sum of the parts and there's a lot more to a good sub than its amplifier class; however, would there be any sonic benefits to class AB? What if I said my intended application leaned more towards music in a smaller room?

From the googling/searching I've done, opinions seemed to be all the over the map. I guess I'm hoping for an alternate, fresh perspective on it. Any thoughts would be appreciated!
post #2 of 33
'There are so many different "classes" of amplifiers being used of sub amps these days, I am serious if anyone can really tell a difference in audio output. (not in heat or energy consumption but good old fashion sound!"

*subscribed
post #3 of 33
Quote:
Originally Posted by Tom4221 View Post

I'm wondering, should I show any preference to subwoofers that use class AB amps?

No

Quote:
Originally Posted by Tom4221 View Post

would there be any sonic benefits to class AB?

No

Quote:
Originally Posted by Tom4221 View Post

What if I said my intended application leaned more towards music in a smaller room?

Doesn't matter.

Quote:
Originally Posted by Tom4221 View Post

...... opinions seemed to be all the over the map

Opinions, yes, all over the map. However factually, there's no inherent advantage with regard to amp class. There's typically superb examples of each individual type.

I'm unclear if you're referring to stand-alone, outboard amps, or typical powered subwoofers. However, either way a subwoofer/amp combo purchase should be determined by budget, room size, main loudspeakers, and listening preferences. Care to elaborate?
post #4 of 33
Some sub manufacturers like Velodyne are slowly changing over to class D amps. I first experienced class D's when I was in the 12 volt business marketing the Memphis Car Audio line. The main difference is less heat, size and improved efficiency. The draw back is that in some cases, due to its more critical design, distortion values can be higher. However, in today's world any electronic device that uses less power is less taxing on your wallet and therefore considered a better buy, especially if it earns a Energy Star rating.



Ian
Edited by mailiang - 11/14/12 at 3:32pm
post #5 of 33
Thread Starter 
Thanks for the replies all.
post #6 of 33
There is NO reason in my mind to pick anything but a class D amplifier for subwoofers. DO NOT BOTHER with class AB for a vast majority of the applications you would consider for subwoofers.

1. MUCH Cheaper price per watt.
2. no discernible difference in audio quality (especially for bass)
3. extreme amounts of total wattage available (seemingly limitless power)
4. Cooler, so MUCH easier to stack them in a rack without special cooling fans and dust management
5. Much more efficient from a power (waste) perspective
6. Due to high efficiency they could be used always-on if needed as well and not waste much power in idle mode.
7. Due to the cheaper price, you can get much more powerful amplifiers for the money which allows you to drive more complex loads or 2-4 ohm speakers without problems.

I now have 7 Crown XLS series amplifiers in my rack powering audio channels and bass shakers.

Additional advantage to PRO amps: standard XLR balanced designs avoid ground loops inherently. The Neutrik connectors are VASTLY easier to work with over the long run for amp-speaker wiring (2 channels / 4 wires per cable).


I will simply NEVER go back to class AB for surround channels and subwoofers EVER.


Having said all this, I am still using a Pass Labs Class A amplifier (hot and inefficient) for my main channels which are Avant-garde Horns. Class A is also amazing while producing such a pure signal with totally silent background with NO audible hiss.
post #7 of 33
Another way to look at the class A/B versus D is audition the sound of the Pioneer Elites (SC-67/68) they seem to have a handle on class D sounding very good IMO.
post #8 of 33
So this applies to powered subs as well? I'm looking at two subs. ML Dynamo 700W (class AB) and SVS SB-1000 (class D). ML for $470 (used, excellent condition) and SVS for $500 (new). Thoughts?
post #9 of 33
Quote:
Originally Posted by dude74 View Post

So this applies to powered subs as well? I'm looking at two subs. ML Dynamo 700W (class AB) and SVS SB-1000 (class D). ML for $470 (used, excellent condition) and SVS for $500 (new). Thoughts?
The amp class doesn't matter. The advantage to D is higher efficiency, so it doesn't require as much power supply capacity for equal output. Until recently D cost more, but as its usage has become more widespread prices have come down. Chances are it will soon own the market if for no other reason than the lesser power supply requirement results in lower cost. At that point Luddites will universally proclaim the superiority of AB simply based on price, much as they now proclaim that tubes are better than SS.
post #10 of 33
Quote:
Originally Posted by Bill Fitzmaurice View Post

...much as they now proclaim that tubes are better than SS."

Better, or for their personal sonic tastes, preferable?
post #11 of 33
I've heard a TON of amps from Class A, AB, B, D, etc. Granted my experience has been in the car audio field with amps like Audison, Image Dynamics, old PPI art, JL, Zed, etc etc, but nowadays I can hardly (if even at all) tell the difference between class D and A/AB. I don't think there is any reason to go with class A if you're looking at a competently designed class D or higher efficiency amp
post #12 of 33
Is that the order of efficiency, A being the lowest and D being higher efficiency? I've seen somebody mention that the class D amps actually switch on and off many times per second or something like that, to increase efficiency. Does this sound correct, or am I making this up? :/
post #13 of 33
So, I just read a review on a certain sub, saying that class AB is more efficient than "the class D switching amps". What actually is the switching?
post #14 of 33
Quote:
Originally Posted by dude74 View Post

What actually is the switching?
The power supply. And AB is less efficient than D, AB being on average less than 70%, D on average 90%.
post #15 of 33
Actually, the output stage switches in a class D amplifier, although the power supply may also utilize class-D circuitry. One the design issues with class D is that the power supply modulates the output.

Bill, where did you find efficiency numbers? For audio power amps I would have guessed lower for both AB and D real-world amps but it is not something I have researched nor read in reviews (that I recall). IIRC efficiency for push-pull class A is 50 % and class B 80 % max theoretical; I would have guessed AB at 60 - 65 % and D ~80 % for practical audio amps (due to driver circuits and output stage losses) but I do not know. Love to find some references.

Curious, tnx - Don
post #16 of 33
Here is a nice post on the differences between the aforementioned Class amps, additionally, BASH amps are discussed as well. Incidentally, my latest sub purchase runs off the latter.

http://www.hifivision.com/speakers/9598-diff-between-bash-class-d-amps-subs.html


nx211
post #17 of 33
That's good info. So for the two subs I'm looking at, SVS is more efficient, but the ML may put out better all around sound? Or should I be looking for BASH options? Obviously, different manufacturers put out units of different quality. I know SVS (D) is highly regarded around here, but how do all the pros here think the ML (AB) matches up in sound quality/efficiency? Sorry if I'm going a little off topic...
post #18 of 33
I'm surprised no-one has mentioned the class G and H amplifiers. From what I can gather, they kind of sound like AB hybrids of sorts.

http://en.wikipedia.org/wiki/Electronic_amplifier#Classes_G_and_H

I believe Rythmik is using class H amplifiers on some of their subs.
post #19 of 33
Some of Emotiva's amps are class H, as well as those from many other manufacturers. We could discuss A, B, C, AB, D (using PWM, PFM, and various combinations), E, F, G, H, and various permutations, but that is probably beyond the scope of this thread -- and most readers. Wikipedia has several articles that walk through the various amplifier classes.

For a subwoofer class D would seem to be a good choice as you can get lots of power with not much heat, and low output impedance and low distortion is not too hard to realize at subwoofer frequencies. Since most power in audio systems generally goes to the sub, that is the place to go for efficiency. We are also less sensitive to distortion at sub frequencies, which is not to say that class D amps have high distortion -- they don't. Getting the same power in a class AB, H, whatever amp is going to cost more and dissipate (waste) a lot more power than using a class D amp.

I do not recall the various Rythmik amp flavors, but their design uses a servo circuit to provice low distortion and low output impedance so is more advanced than just providing a sub amp in (or on) the box. Again, plenty of systems without servos sound just fine (though I prefer my Rythmiks).
post #20 of 33
Quote:
Originally Posted by DonH50 View Post

Some of Emotiva's amps are class H, as well as those from many other manufacturers. We could discuss A, B, C, AB, D (using PWM, PFM, and various combinations), E, F, G, H, and various permutations, but that is probably beyond the scope of this thread -- and most readers. Wikipedia has several articles that walk through the various amplifier classes.

For a subwoofer class D would seem to be a good choice as you can get lots of power with not much heat, and low output impedance and low distortion is not too hard to realize at subwoofer frequencies. Since most power in audio systems generally goes to the sub, that is the place to go for efficiency. We are also less sensitive to distortion at sub frequencies, which is not to say that class D amps have high distortion -- they don't. Getting the same power in a class AB, H, whatever amp is going to cost more and dissipate (waste) a lot more power than using a class D amp.

I do not recall the various Rythmik amp flavors, but their design uses a servo circuit to provice low distortion and low output impedance so is more advanced than just providing a sub amp in (or on) the box. Again, plenty of systems without servos sound just fine (though I prefer my Rythmiks).
I agree with everything you presented here. I just figured since the OP was inquiring about different flavors of amplification, he might want to expand his horizons and look at what is available beyond the usual AB, D or BASH amplifiers mentioned.

I probably did muddy the water a bit and I'm sure the OP is probably muttering to himself......."decisions, decisions". And that's a good thing! smile.gif
post #21 of 33
Alrighty, my off-the-cuff definitions, take with grain of salt. I did not read the Wiki, this is based off my own memories/experience and may be wrong and/or out of date. Hopefully only the latter...

  • Class A outputs conduct all the current all the time. No crossover or switching distortion, but they run hot and waste a lot of power. At some point (low-impedance loads, high power) they will slip out of class A assuming finite rails and power supply capacity.
  • Class AB runs in class A (fully on) for low power levels, which means no low-level crossover distortion as devices switch on and off (you are already well above the zero point when the biasing changes). Much less wasted power than pure class A and thus less heat, energy draw.
  • Classes G and H are A or AB amplifiers but the voltage rails track the signal, reducing with small signals and raising for large signals. This saves power since the power supply tracks the output signal rather than staying at a fixed (high) value. There can be issues with the designs as bias and output stage modulation happens as the power supply varies (assuming non-infinite PSRR). Class G uses switches to select from pre-determined discrete steps; class H uses continuously variable circuits to constantly monitor and adjust the power supply voltages.
  • Class D uses switches that short the output to the plus or minus power rail (some include switches to ground, 0 V) at a high rate with duty cycle and/or frequency dependent upon the desired output signal level, sort of like a 1-bit DAC. An output filter converts the pulse stream to regular old audio and suppresses the high-frequency switching energy. Mostly. Class E and F are variations on class D, and some amps include a small A or AB amp in the design to reduce distortion right around 0 V (where a "dead zone" can occur in these amplifiers). Because they just switch on and off as needed, efficiency can be very high.

I did not list C, BASH, T, or some other more esoteric classes because I have not seen them often (well, C is a popular choice in RF circuits) and do not know them as well. I have designed high-speed analog ICs, not a lot of audio power amps (except as a hobby when I was younger, and hopefully in a few years if and when I retire).

Distortion depends upon many factors other than just the bias class. Many of the class A amps I have had/heard had higher distortion ratings than most class AB amps due to lower feedback ratios.

For the vast majority of applications amplifier class would not be a factor in my choice of amplifier. If you want the most Watts for the money, especially in a sub amp, and without heating the neighborhood, class D is a good choice.

IMO - Don
post #22 of 33
Not to bring back a dead thread here but this is a question that is related and I didnt want to start another thread to clutter the forums.

When you are looking at your mains what are the major differences between Class A and Class D? I understand that Class A is more linear but obviously much less efficient. However, I also have heard that Class D can handle the dynamics better.

I am really curious because I am between building something with the nCore modules or FirstWatt F5.
post #23 of 33
Quote:
Originally Posted by Kimeran View Post

Not to bring back a dead thread here but this is a question that is related and I didnt want to start another thread to clutter the forums.

When you are looking at your mains what are the major differences between Class A and Class D?

Wrong question. Class A and AB are so different from class D that the easier question to answer is: "How are "Class A and Class D the same?"

The answer is: Other than that they accept an analog signal, output a hopefully more powerful analog signal, and accept DC power to do it, there are almost no similarities.

After that, figure out which one interests you more, and look it up with google. There are tons of articles about each on the web. Come back and ask any questions that you may have after thoroughly studying the existing documentation.
Quote:
I understand that Class A is more linear but obviously much less efficient.

Audiophile myth. It turns out that Class AB with optimally chosen bias is the most linear. Class A is just Class AB with suboptimal bias causing more distortion and poorer efficiency than would be necessary.
Quote:
However, I also have heard that Class D can handle the dynamics better.

Audiophile myth. Class D is more efficient. But, handling dynamics is more a matter of linearity. We live in a world which by hifi standards has no lack of power, so efficienct use of power is secondary. Class AB is the most linear and offers the best frequency response. That's true today. Once Class D is more fully developed, it will probably be superior on all grounds.

I read my first articles about Class D in the 1960s, and since then it has been limited by the slow rate of development of solid state technology. Class D mostly progresses based on the power and speed of switching transistors which is subject to slow improvement.

Quote:
I am really curious because I am between building something with the nCore modules or FirstWatt F5.

You can always tell pioneers by the arrows sticking out of their backs. ;-)
post #24 of 33
Not sure about the theoretical aspects and maybe I bought into the class A hype a bit. I also have class D amps that sound quite decent too btw.

Pass labs class A amps sound SO dead quiet at baseline... No hiss, no noise whatsoever even putting your ear up to my horn speakers. I have yet to find a class AB amp that I have used personally with such a deep background of silence.

If only my preamp worked as well as my pass labs amp, i would be in bliss.

I wish my pre-pro was made by OPPO and it had a bunch of ESS reference dacs and rigged for "ultra quiet".
post #25 of 33
Noise floor is only loosely, if at all, coupled to amplifier class.
post #26 of 33
post #27 of 33
Quote:
Originally Posted by blazar View Post

Not sure about the theoretical aspects and maybe I bought into the class A hype a bit. I also have class D amps that sound quite decent too btw.

Pass labs class A amps sound SO dead quiet at baseline... No hiss, no noise whatsoever even putting your ear up to my horn speakers. I have yet to find a class AB amp that I have used personally with such a deep background of silence.

If only my preamp worked as well as my pass labs amp, i would be in bliss.

I wish my pre-pro was made by OPPO and it had a bunch of ESS reference dacs and rigged for "ultra quiet".

There are high SNR A/B Amps. ATI amps for example with a SNR of 120db.
post #28 of 33
Quote:
Originally Posted by aboroth00 View Post

There are high SNR A/B Amps. ATI amps for example with a SNR of 120db.
These have S/N >110dB, and no one I know of is yearning to trade them in for Class A. cool.gif
http://www.sesystems.com/Assets/file/Powersoft_k10-Data.pdf
post #29 of 33
Quote:
Originally Posted by Bill Fitzmaurice View Post

These have S/N >110dB, and no one I know of is yearning to trade them in for Class A. cool.gif
http://www.sesystems.com/Assets/file/Powersoft_k10-Data.pdf

I guess they don't want a space heater tongue.gif.
post #30 of 33
Quote:
Originally Posted by Bill Fitzmaurice View Post

These have S/N >110dB, and no one I know of is yearning to trade them in for Class A. cool.gif
http://www.sesystems.com/Assets/file/Powersoft_k10-Data.pdf

Damn that looks nice...
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