Does Your Amplifier Have Class?
  

Courtesy of OhmArt

Over the eons Audiophiles have been subjected to any number of questionable "tweaks" claiming to make dramatic improvements in the sound quality of their systems. Who among us can forget green marker pen to be applied on CD disc edges? A personal favorite of ours was spawned by a major high end audio magazine reviewer who claimed that shining a flashlight on the tweeter in her speakers changed their sound. Ah, Houdini would have been proud.
 
But that's not to say that there aren't sonic differences in component parts of audio systems, with speakers being perhaps the easiest to detect but certainly not the only component to affect the sound of your system. When it comes to amplifiers there's pretty universal agreement that they can and do sound different too. So we thought a broad overview of amplifier types in general might be interesting and even helpful to all you GoldenEar audio aficionados out there.

All Amplifiers Have (a) Class

There are lots of elements in amplifier design and component choices that can have sonic impact. It's generally agreed that a particularly large influence is the "class" of the amplifier's design. There are many amplifier classes but the vast majority of decent audio amplifiers are Class A, AB, G, or D. So what are these classes and why do we care?

Amplifier classes are determined by their circuitry and operating design. The various classes range from very linear operation (typically found in high performance, high fidelity designs) with very low efficiency, to very non-linear designs used for other purposes besides Hi-Fi, but with much higher efficiency. Other classes typically fall somewhere between those two extremes.

We can separate commonly available audio amplifier classes into two basic groups. The first are the more common classes of   A, B and A/B. These are defined by the length of time the devices in their output stage conduct some portion of the output waveform. In other words, how long (or how much of) a certain portion of the output waveform is being amplified by a specific output device in the amplifier's power output stage.

The second set of audio amplifier classes are the more recently developed "switching" types; D, E, F, G, S, T, etc. These use some combination of digital circuits, unique power supply designs and Pulse Width Modulation (PWM) to either constantly switch the signal between "fully-ON" and "fully-OFF" in order to replicate the amplified audio signal or to simply increase efficiency. PWM is explained more fully in the following article. Of these newer designs, Class D, and to some lesser extent Class G, are the ones you're most likely to find currently in today's home audio components. Here's a brief summary of the Classes:

Class A
 
Class A amps are the least efficient design, using lots of power, generating lots of heat and requiring component parts that can deliver this power and withstand and dissipate the heat. Class A amps are large, heavy, power hungry and expensive to buy and operate. But, they are often some of the best sounding amplifiers available for the cost-no-object audiophile.
 
Class B
 
Class B amplifiers offer improved efficiency compared to Class A, but the amp's linearity suffers and there's potential for higher levels of distortion too. In particular, class B amps can generate something called "crossover notch distortion" (see figure below). For this reason, you'll rarely see a Class B amplifier in a high quality audio system.

Class A/B
 
Class A/B amplifiers are a hybrid compromise of classes A and B. Most high quality A/B amplifiers actually perform like Class A amplifiers at low power output, switching to A/B operation as power demands increase. Class A/B designs give up some efficiency to Class B amps but are much more efficient than pure Class A designs. A/B amps are a good compromise, can sound excellent and are the most commonly found non-digital audio amplifier designs in use in the a/v market today.
 
  
A Typical High Performance Class A, A/B Amplifier With It's Large Transformers, Banks of Filter Capacitors and Massive Heat Sinks

Class G
 
Class G amplifiers use a unique power supply that feeds the output section. Under periods of high power demand the amplifier switches to a higher voltage so the amp can deliver higher output levels for brief bursts. Pioneered as "big power/small box" in their first incarnations, high efficiency digital amplifier designs instead have favored the Class D (and similar) approach, outlined below and in the next article..

Class D
 
A class D amplifier, or switching amplifier, is an amplifier in which the amplifying devices (transistors, usually MOSFETs) operate as electronic switches and not as linear gain devices as in other amplifiers. This has become the most common technology found in today's multi-channel A/V receivers due to the reduced size and high efficiency. (Today's 11 channel receivers wouldn't fit through your front door with 11 Class A/B amplifiers inside.) We chose Class D amplifiers in all the GoldenEar powered Triton Towers and subwoofers for very good reasons, including high power, high efficiency, low heat and superb performance in bass frequencies. The following article is a detailed overview of Class D technology.
 
We've tried hard to keep this all as non-technical and non-intimidating as possible. Just imagine, the next time you're at an elegant cocktail party and someone asks if anyone there knows what a Class A amplifier is, you are gonna be a hot ticket!. Oh, and If you have a moment, please let us know what you think about articles like these. Interesting? Informative? Fun? What, are we nuts? Shoot us an email at info@goldenear.com  and let us know .
 
For those interested in further reading on this topic, click here for good a primer from Wikipedia.

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