Posted on Thu, 25 May, 2017
Posted by Bob


Arcam G Class Technology is a whole new way of fusing the audio purity of Class A designs with a 21st nod to efficiency, compact size & energy efficiency.

Today on The Audio Affair blog, we get technical; today’s topic of conversation revolves around amplifiers – specifically power amplifiers, an essential component in our passion for audio.

Power amplifiers are an essential part of our passion; no matter what the source material, no matter whether one works in the digital or analogue domain, at some point, power amplifiers are what push the speakers and create the sounds we love so much.

Highly respected British audio stalwarts, Arcam, debut their G Class technology in 2009’s award-winning AVR600 amplifier. But what *is* Class G, and for that matter, what does Class-A, Class-AB and Class-D mean anyway?

It’s time for a technology lesson here at The Audio Affair Blog; open your copybooks, sharpen a pencil, and PAY ATTENTION AT THE BACK! It’s time to learn all about the history of amplification, and Arcam G Class technology.

Arcam FMJ A49

Arcam’s A49 G Class Amplifier

Class-A – Audio Purity…At a Cost!

Before we start talking about amplification ‘classes’ it’s important to realise one thing – the letters were initially assigned in order of evolution: Class-A being the first amplifier topology invented, followed by Class-B, Class-C etc.

In the beginning, there was only one kind of audio amplifier ‘topology’, Class-A amplification.  Class-A amplification is based upon just one single amplification device (this could be a valve or a transistor) which handles both the ‘push’ and the ‘pull’ of an oscillating signal. Class-A amplification wrings the most from just one single amplification device, and it makes the amplifier work very hard as a result.

The significant advantage to a Class-A design is it produces a very pure reproduction of the input signal with very little distortion. This makes it very attractive to Audiophiles, and Class-A designs are often prized for their sonic character.

The big disadvantage of Class-A amplification is that it’s very inefficient; the output device is, in effect, always ‘turned on’, leading to a lot of power being wasted in the form of heat.

Class-B – Efficient but Flawed

Very early on in the history of amplification, designers sought to wring ever more power and efficiency from their designs and components. One major breakthrough was the concept of ‘push-pull’. Rather than asking one transistor (or valve) to do all the work, push-pull splits the load between pairs of devices.

Think of it like a see-saw with two people: As one person goes up, the other goes down and so on. It’s a much more efficient use of power and components and leads to a more powerful, cooler-running amplifier. There is, however, a major problem with Class-B amplifiers, and that’s an effect known as crossover distortion.

Crossover distortion is a nasty effect which occurs when the phase of one cycle, switches to the next. Let’s go back to our see-saw analogy to explain this; as one person goes up, there’s a small period before the other person at the bottom can ‘push off’. The see-saw lurches a little bit, it’s not a smooth transition.

The same thing happens in a Class-B amplifier; as one device hands over to the other device, there’s a momentary ‘pause’ in the signal which causes crossover distortion. An audible and very undesirable effect, especially noticeable at low volumes.

Crossover Distortion

Class-AB – The Compromise

The design goal of virtually any amplifier engineer for 70+ years now has been how to harness the audio purity and sonic ‘Nirvana’ of a Class-A topology, with the efficiency of a Class-B, push-pull amplifier. One significant development was that of Class-AB.

In a Class-AB amplifier, the amplifier works very closely to that of a Class-A amplifier, at low volumes; this is the typical area where crossover distortion occurs. Once the amplifier is above that critical area of crossover distortion, it transitions into Class-B operation, for higher efficiency and power.

Class-AB amplification is very much ‘The Norm’ for audio amplifiers; it’s fair to say that the vast majority of Hi-Fi amplifiers will be operating in Class-AB, as it offers a good compromise between efficiency and low distortion.

Inevitably though, engineers don’t sit well with the concept of a ‘compromise’ and various designs have appeared over the years aiming for nothing short of perfection

Discovering the ‘Holy Grail’ – Sonic Purity with High Efficiency

There have been various attempts over the years to ‘better’ the compromise of Class-AB amplification: Quad pioneered their ‘current dumping’ technology in the 405 Amplifier of the late 1970s – its prodigy can be seen in the modern Quad Artera Power Amplifier. French Mavericks, Devialet, were founded on the ADH Intelligence design, where a Class A amplifier was coupled with a Class D amplifier.

It was perhaps inevitable then, that we should expect to see highly respected British audio stalwarts, Arcam, have an attempt to crack this ‘engineering holy grail’. How to create an amplifier design which fuses the sonic purity of Class-A amplification, with greater efficiency, light weight and compact dimensions.

In 2009, Arcam released their AVR 600 AV amplifier; the very first amplifier to utilise the then brand new Arcam G Class technology. G Class was to go on to form the backbone of Arcam’s High-End Amplifier line to date.
AVR600

Arcam G Class – Audio Purity with Efficiency

Arcam G Class works in a markedly different way to other amplifier topologies such as Class-A and Class-B, yet inevitably there are some parallels. One of the problems with Class-A amplification, if you remember, was that huge amounts of energy were lost at low volume levels, as the output device was effectively turned fully on.

Arcam’s approach is to dynamically monitor the power requirements of the output stage, and ‘throttle back’ the input power to increase efficiency. Think if you will of a sports car with a huge V8 engine. At slow speeds, all that capacity is being wasted, but if we were to switch half of the cylinders off, it’d be much more efficient, whilst still being able to deliver huge power when required.

This, in effect, is how Arcam G Class technology works. At low volumes, the G Class amplifier works in pure Class-A, running from a low-power voltage rail. Power is saved, and crossover distortion is eliminated. If however there is demand for more power (a loud music transient for example) then the Arcam G Class circuitry will switch to a higher power rail and ‘rev up’ the amplifier.

Arcam G Class circuitry is the cornerstone of Arcam’s current High-End amplifier line-up, such is Arcam’s faith in the technology. Let’s take a brief overview of the models in the current line up which feature this new development in amplification.

The G Class Range

Arcam’s current G Class range comprises a range of Hi-Fi Amplifiers: Power Amplifiers, Integrated Amplifiers and AV Amplifiers which all benefit from Arcam G Class technology.

Arcam G Class Integrated Amplifiers

Arcam’s range of G Class enabled Integrated amplifiers starts with the A29 (80w per channel), moves up to the A39 (120w per channel – 20watts in Class A) and then reaches the range-topping A49 with a massive 200w per channel, up to 50w in Pure Class A.

Arcam G Class Power Amplifiers

Arcam G Class Technology is also available in stand-alone, power amplifier format; Ideal for those who may wish to add it to a pre-existing, favourite pre-amplifier, or couple it with Arcam’s own excellent range of Pre-Amplifiers. The Arcam Power Amplifier range is made up entirely of equipment featuring G Class technology.

Starting with the P349 which delivers 180w per-channel (50w class A), then moving up to the P49 which offers a whopping 200w per channel (50w Class A).

For those requiring multi-channel operation, there’s the four-channel P249; a great way to expand an AV amplifier to full Dolby Atmos, or any situation where four channels of Arcam G Class amplification is required in one package.

Arcam G Class AV Receivers

Arcam offer two, G Class equipped AV Receivers: Firstly, the SR250, an Audiophile focussed, 2-channel AV Receiver with a raft of high-end features, including network audio support. The AVR850 is the multichannel version of the SR250, offering all the functionality of the SR250 but enhanced with additional channels for full 5.1 Surround capability.