A, AB, D or All Three? Pick Your Amp!

Amplifiers, such a range but which is right for you? I’d like to discuss amplifier types and applications.

As a Four Master, Car and Home Stereo are recognised as one of the leading car audio installers in the country. Our knowledge and experience mean that we are ideally placed to choose the right equipment and install it to the highest standard.  We offer a no pressure approach and are happy to offer advice on choosing the right equipment; just recently I had a great conversation over Facebook with an enthusiast in Australia over the benefits of the magnificent Rainbow Profi speakers.

Commonly I am asked “how many Watts is it?” but there is so much more to an amplifier than that. In reality, the Watts figure is often not the most useful specification when choosing between amplifiers. True it gives a simple indication of power but all too often that figure is heavily massaged to look higher. I have tested some so-called 3000W amplifiers to discover that they only made 200W at the terminals into a speaker load. Still an impressive output but certainly not 3000W as claimed.

But I digress; we are looking today at amplifier types and their uses. There are three types of amplifiers in main use today they are classed as A, AB and D. Although they all have the same purpose there are some fundamental differences between them that make them suit different applications.

At the heart of an amplifier are the power transistors, these are responsible for taking a small signal and making it larger, the classification describes how the transistors are used. Of course, there is a massive range of qualities and performance variations related to each type but we will stick to classes.

Class A, a typical sound wave.

As we know, Sound is an oscillating wave. This wave has a positive and a negative component but is symmetrical about the ‘t’ axis. So all we need is one big transistor that can amplify the whole signal to whatever power we want right? Well yes and this is the ideal solution for sonic accuracy; the high output signal is an exact copy of the small input signal. The best amplifiers I have ever heard have all been A class with a sound that is open, transparent and simply a joy to listen to.

The magnificent Audison Thesis Venti

However, all this purity comes at a price; huge power consumption. Those massive transistors are switched on all the time regardless of whether there is a signal there or not or how loud it’s playing. This means heat and lots of it which has to be dissipated. So we need massive power supplies and large heatsinks which make the amplifier large and somewhat inefficient, typically around 20%. That’s why we need to use such large power cable!

So how do we get better efficiency? Class AB.

 

There are many considerations here but in very simplistic terms it is possible to use one transistor to amplify the top half of the wave and another matched transistor to amplify the bottom half. This means that smaller transistors can be used and they are only on for half the time leading to efficiencies of 50 to 70%. This means less heat and smaller power supplies which help to keep the cost down.

The JL Audio JX360, a mainstay of many audio systems

There are some drawbacks to class AB, principally of which is crossover distortion.

As a matched pair of transistors has to amplify the whole wave it is essential that they turn off and on at exactly the right moment (the class B part). Imagine two people writing the letter S where one person only writes one-half and the other person completes it, there is bound to be a little error. The same is true in the amplifier leading to a very slight distortion of the output wave but in truth, modern design can almost eliminate this meaning we can have a really great sounding amp that is pretty efficient. This has meant that AB design has dominated the market for decades.

The highest efficiency is class D.
Let’s get something straight, D does not stand for digital. Class D is just another design type, a really clever one. This design uses pulse width modulation to generate the amplified output which involves switching the output transistors on and off at a very high rate typically around 300 kHz. This High switching frequency means any noise is well above our audible range and the PWM leads to astonishing efficiencies of around 95% or higher. So, at last, we have a very high power, high quality and very efficient amplifier. It is fair to say that most mainstream car amplifiers today are D class as they are smaller, lighter and generate less heat than their A or AB counterparts.

The JL Audio XD400.4, small but powerful

As with almost everything, there has to be a downside and with D class their Achilles heel is radio interference. The switching frequency mentioned might not be audible but it can do a fine job of broadcasting in the FM radio range. This can lead to reduced radio reception at certain frequencies, of course, modern design does mitigate the problem but nevertheless, it can still be an issue.

So to sum up, Class A for the best possible sound (ideal for tweeters), Class AB for improved efficiency (ideal for midrange) and Class D for maximum power from the smallest chassis (ideal for subwoofers). Wouldn’t it be great if there was an amplifier with all three? Well, there is and it is called the 5.1K from Audison.

 

The superb and versatile Audison 5.1K

This is one of my all-time favourite amplifiers and it is the amp that powers my own personal system. The detail of the class A front stage is phenomenal while the AB class mid-stage offers an amazing amount of punch. The D class sub stage has enough power to rattle the windows but offers a fidelity that allows a seamless blend with the rest of the audio stage. This unit never fails to impress and is not too big to make it impossible to fit. It does get hot if driven hard but does not require special cooling if mounted sensibly.

 

If you would like to hear what the 5.1K can do then please feel free to call into our shop on Sunderland Street in Macclesfield and I would be pleased to demonstrate it in our Discovery or on the demonstration stand in store. If you would like any more information on amplifiers in general please call me on 01625 432707.

 

 

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