Audio Amplifier Classes: Which Offer the Best Sound & Performance?

A

admin

Audioholics Robot
Staff member
Confused by the alphabet soup of amplifier classes? Do you wonder what terms like Class A or Class D mean? Long story short, these aren't grading systems with which to judge amplifier quality, but are in fact descriptions of an amplifier's topology. Understandably, each class brings something a bit different to the table, but the end goal is always the same: amplifying the input signal without introducing distortion. Curious to find out the real differences among the different classes? Keep reading to find out!

Read Explaining the Audio Amplifier Classes (A, AB, D, G and H)

Which is your favorite type of amplifier and why do you own it?
 
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G

greatdavide

Audioholic Intern
Really great article, I just have a clarity question, in the thread for the IQ audio mono block amp it was mentioned that efficiency and a lack of dynamic headroom are related. Is that only true for units using SMPS power supplies? In this article you mention that Class D amps with linear power supplies have more dynamic head room than those with SMPS, is the headroom of linear power supply class D amps generally equivalent to the headroom available in Class A or Class A/B designs? And if not is dynamic headroom a product of the amp efficiency or of the power supply design or something else entirely?
 
Steve81

Steve81

Audioholics Five-0
Perhaps add that to the article. :) It would help others that have the same question.
I made a little segue to clarify:
While all the amplifier classes previously mentioned have one or more output devices active all the time, even when the amplifier is effectively idle, Class D amplifiers rapidly switch the output devices between the off and on state; as an example, Class T designs, which are an implementation of Class D designed by Tripath as opposed to a formal class, utilize switching rates on the order of 50MHz.
 
Irvrobinson

Irvrobinson

Audioholic Spartan
I made a little segue to clarify:
"Class T" isn't really a formal class, it's actually a trademark, if I remember correctly, for a proprietary design. I'd just ignore it.
 
Steve81

Steve81

Audioholics Five-0
Can't please everybody :D I don't know that it really hurts anything as is, though if Gene disagrees, it'll be gone soon enough.
 
Gunny

Gunny

Junior Audioholic
Amazingly article. Thanks so much for explaining all of this in clear terms. It's been decades since I studied electronics but this effectively jarred my memory and tied it all together again.
 
JRoss

JRoss

Audiophyte
Hello Steve this probably is a odd question for you guys because it being 12volt. Amplifier class A , AB etc. Have you ever heard of a class A/G ? Can it exists? I have in my collection guessing from mid to late 80s with A/G circuit stenciled on it. Can not find any thing on it. So the question, can it be? Let me know. Thx
 
Steve81

Steve81

Audioholics Five-0
So the question, can it be? Let me know. Thx
It can. Best guess is that it's probably an A/B amp that operates in Class A up to a certain level, and then leverages the additional efficiency of A/B + rail switching (G) for higher output levels.
 
Verdinut

Verdinut

Audioholic Spartan
QSC Audio's most powerful amps operate in Class AB at lower output levels then switch to a higher voltage rail for Class H operation.
 
ski2xblack

ski2xblack

Audioholic Field Marshall
Hello Steve this probably is a odd question for you guys because it being 12volt. Amplifier class A , AB etc. Have you ever heard of a class A/G ? Can it exists? I have in my collection guessing from mid to late 80s with A/G circuit stenciled on it. Can not find any thing on it. So the question, can it be? Let me know. Thx
This could be interpreted as TLS' favorite topology, the Quad current dumpers. Class A amp paired rather cleverly with class B. Not sure if it was ever implemented in mobile audio. Or it could be as Steve stated.

Anywho, I have class a/b and g (h?) amps, as well as a couple SE tube jobs, which don't amplify particularly well but they sure deliver the chocolaty mids.;)
 
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Matthew J Poes

Matthew J Poes

Audioholic Chief
Staff member
I won’t say I have a favorite, I too have heard great examples of all types. However, as both a reviewer and tweaker who constantly is moving amplifiers around, I can confidently say I love Class D amps with SMPS supplies. They are just easier to manage.

I used to DIY amps quite a bit, building from either pre hilt modules, kits, or even of my own designs. I’ve built numerous linear power supplies including capacitance multipliers and CLC supplies with inductors the size of 1kva toroidal transformers. I measured their behavior along with a lot of computer modeling.

There is a quality to SMPS that I think deserves both mention and discussion. They need a little love.

An SMPS supply is inherently regulated. This means the voltage doesn’t drop with load. Instead it regulates the voltage until it hits its limit. This is good for maintaining output under varying load conditions and under highly dynamic conditions. It also ensures the amp distortion and noise doesn’t suddenly rise at the limits. Linear supplies without regulation can’t do this and thus need to either be very large with huge capacitance or use a complex CLC method like i did, which is really expensive and really heavy.

The limitation with SMPS is that you can’t treat their maximum current delivery rating or wattage rating the same as linear. If you consider it like an amplifier, linear soft clips where SMPS hard clips. When a linear supply approaches its limit it just gradually drops the voltage as current increases (which is actually fine for low impedance loads) until it hits a limit. The ripple increases as well, so a lot of capacitance is handy to avoid that. An SMPS won’t reduce voltage, it just hits its limit and a current limiter kicks in. It leads less headroom. I built a 300 watt per channel amplifier (Class A/B) and included a 1200 watt SMPS supply designed for audio use. It couldn’t even reach 300 watts rms before the limiter kicked in. Yet a 1.2kva toroidal transformer and 40,000uf of capacitance would have been plenty with a linear supply to exceed that rating. Instead the amp needed twice the power supply to reach its expected output. In other words, an SMPS doesn’t have to make an amplifier less dynamic, manufacturers just need to start including much larger ones. Like twice as big.

Another nice attribute of the SMPS is that their ripple voltage is on par with the very best linear supplies possible with advanced filtering. No basic RC linear supply can even compare. While many might point to switching noise in SMPS, fact is, in modern well designed Audio grade SMPS, there is no meaningful switching noise on the output. They do have high RFI radiation, but that is easily filtered out. In the end, I think we all need SMPS in all amps.
 
Verdinut

Verdinut

Audioholic Spartan
One example of successful Class A/B and Class A/B/H amplifiers using a SMPS is the DCA series of amps which QSC Audio have been manufacturing since 1998. These have proven that a well designed amplifier can use a switching power supply and do the job as well if not better than the conventional linear supply:

https://www.qsc.com/cinema/products/power-amplifiers/dca-series/

I am using three DCA 1222s and one DCA 1824 in my HT system.
 

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