Amplifier classes?

[captain_tinker]

Folks,
Here is a question for you, I have noticed that some amplifiers claim to be class A, some class B, some class D, and some class T. I have not yet understood what the differences between them are. Is it a certain amount of wattage per channel that makes them one class or another, or the kind of transformer in them, or the ohm load or the quality of the internal parts, what?

For example, I am looking on Rotel's website, and I see that some amplifiers are labeled as Class A, some as Class D, but then there are others that are not labeled at all one way or another. I am really confused here. How do I know what class an Amp is, and why would I want to pick one class over another? Thanks all!

-capT

 

[DTB300]

Have not read through it as I just did a search. But Wikipedia has something on it:

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

I am sure they are other technical sites that may have more information for you.

Dan

 

[karma]

HI CapT,
You are asking a good but complicated question. The class of an amplifier is a technical distinction mostly having to do with the circuit "topology". Each class biases the output devices (can be either transistors or tubes) differently. This results in a number of characteristics that change from class to class. Each has advantages and disadvantages.

It's too complicated to go into here. I'm not really sure how to summarize the differences. Maybe it would be better for you to google the class and then come back here with specific questions.

One easy thing to talk about is the spec sheets. If no class is called out, you can assume it is a class AB amp. These are by far the most common.

Sparky

 

[Tube60]

It's too complicated to go into here.
Sparky

Actually it's quite simple:
Class A: The output devices are "on" for 100% of the device's duty cycle.

Class B: The output devices are "on" for 50% of the device's duty cycle.

Then there's class A/B, which is very common: up to a certain input voltage, the amp is in class A. Past that, the amp runs in class B to help lower the stresses on the output devices, and allows the manufacturer to use fewer and often cheaper devices, or, in many cases, just to get a ton o' power out of the design!:rocker:
Class D & class T I'm not familiar with, but they're digital based.
HTH!

 

[Robin]

My amplifier is so smart... It doesn't need to take any classes

IMHO, the Anthem Statement P5 is Class A/B and is fantastic sounding, to my ears. Naturally it is a five individual monoblock design, which vertually illiminated the distortion. The P5 is an amazing amplifier.

 

[karma]

Actually it's quite simple:
Class A: The output devices are "on" for 100% of the device's duty cycle.

Class B: The output devices are "on" for 50% of the device's duty cycle.

Then there's class A/B, which is very common: up to a certain input voltage, the amp is in class A. Past that, the amp runs in class B to help lower the stresses on the output devices, and allows the manufacturer to use fewer and often cheaper devices, or, in many cases, just to get a ton o' power out of the design!:rocker:
Class D & class T I'm not familiar with, but they're digital based.
HTH!

HI,
The quote is generally true but that doesn't help much when trying to make a buying decision. You are going to have to research the subject and, if possible, listen to examples of each type. You will also need to understand the advantages and drawbacks of each class.

Sparky

 

[wink]

Actually it's quite simple:
Class A: The output devices are "on" for 100% of the device's duty cycle.

Class B: The output devices are "on" for 50% of the device's duty cycle.

Then there's class A/B, which is very common: up to a certain input voltage, the amp is in class A. Past that, the amp runs in class B to help lower the stresses on the output devices, and allows the manufacturer to use fewer and often cheaper devices, or, in many cases, just to get a ton o' power out of the design!:rocker:
Class D & class T I'm not familiar with, but they're digital based.
HTH!

D does not stand for "digital"
The letter D used to designate this type of amplifier is simply the next letter after C, and does not stand for digital. Class D and Class E amplifiers are sometimes mistakenly described as "digital" because the output waveform superficially resembles a pulse-train of digital symbols, but a Class D amplifier merely converts an input waveform into a continuously pulse-width modulated (square wave) analog signal. (A digital waveform would be pulse-code modulated.)

 

[lugano]

Taken from the audioholics website:

Class A: The transistor amp conducts for the entire cycle of input signal. The conduction angle is 360 deg. These amps run hot, as the transistors in the power amp are on all the time, but the upside is high sound quality.

Class B: In this amp, the positive and negative halves of the signal are dealt with by different parts of the circuit. The output devices continually switch. These amps run cooler, but the sound quality is not as pure.

Class AB: These amps work by biasing the transistor amp at a non-zero DC current much smaller than the peek current of the signal source. The second transistor conducts during negative half cycle of waveform and the currents from the 2 transistors are combined at the load. A compromise between sound quality of Class A and efficiency of Class B. Most modern amp designs employ this method.

Class D: There are really two types of Class D amplifiers.

Analog-Controlled Class D: Switching amplifiers with an analog input signal and an analog control system. Normally some degree of feedback error correction is present

Digitally-Controlled Class D: Amplifiers with a digitally generated control that switches a power stage. No error control is present. Those that do have an error control can be shown to be topologically equivalent to an analog-controlled class D with a DAC in front.

Class G: The power supply voltage in thee amps changes from a lower level to a higher level when larger output swings are required. This is accomplished by utilizing a single class AB output stage connected to two power supply rails by a diode, or a transistor switch. For the majority of program material, the output stage is connected to the lower supply voltage, but it automatically switches to the higher rails for large signal peaks. One other approach uses 2 class AB output stages, each one connected to a different power supply voltage. The strength of the input signal determining the corresponding signal path.

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AFAIK, there is only 1 amplifier called a class-T amp, and it is .. the T-Amp. (The T comes from the Tripath chip used into it)
We're talking about a nice toy (I had 2 of them) which was a Hiroshima A-Bomb some time ago in audiophile circles, mostly because of some booming reviews coming from trusted opinion makers, first of all, Lucio Cadeddu from TNT. HiFi for the masses at $39.95, etc. (review at http://www.tnt-audio.com/ampli/t-amp_e.html ) My score is: one burned out, one sold on eBay. Poor plastic crapola.

 

[captain_tinker]

Have not read through it as I just did a search. But Wikipedia has something on it:

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

I am sure they are other technical sites that may have more information for you.

Dan

Very Interesting read! Thank you! Lugano, you as well, thank you! I still cannot say I am an expert on the subject, but at least that has given me an introduction to the subject and I don't feel like I have no clue anymore. The wikipedia entry also had a mention of damping factor, which was another thing that I was wondering about. Thanks!

Oh and Robin, I am glad your amp doesn't need to take any classes - :haha1: I am sure it already has a PhD in sound reproduction! Oh by the way, I like your little Santa/Jet avatar, got a good chuckle out of that one!

-capT

EDIT: Oh, one other thing, taking into consideration what has been said, if I take the Rotel RB-1070 as an example, it doesn't say it is an A/B amp, but since A/B is the most common, am I to assume then that that is what it is? Just curious...

EDIT2: It is an AB. Detailed here: http://www.rotel.com/support/faqs/faqs-rb1070.htm