Class D Audio Amplifiers – the way forward
Class D Audio Amplifiers – another form of class C without the Morse code.
Simply put a class D device is a fast acting switching circuit – either on or off. Think of class C RF equipment – Morse code transmissions were all more or less class C amplified – when there was no RF applied to the power amplifier the output devices were biased off. Morse code transmissions were only on or off – a dot one third the length of a dash. Simple.
Along came first generation audio amplifiers – what was recorded had to be faithfully reproduced, analogously. Class A amplifiers do this remarkably but at a cost. Class A amplifiers are at maximum 50% efficient – the incoming signal goes through at worst various stages of phase inversion but the end result in a properly designed class A circuit is – sine wave in – sine wave out. A photocopy. Class B amplifiers draw current as soon as there is a signal – not very good for distortion so through a simple modification the designer sets a small idle or quiescent current to the output devices – changing the class to AB. AB amplifiers are noted for their maximum 78% efficiency – they idle at a few milli-amperes (bias current of output devices) and as a signal is applied the amplifier draws more and more current depending on input amplitude and load resistance until the onset of distortion. Yes, there are lots of other criteria involved here – but the biggest drawback is the switchover between output devices in push pull configuration causing crossover distortion – most noticeable at lower output levels. Poorly designed amplifiers often sound very harsh at low outputs. Fuzzy. And no, they aren’t all like this – most amplifiers nowadays running in AB class are designed using perfectly matched output devices and idle current is optimized for low input/output signals. Class AB amplifiers are also categorized AB1, AB2 etc. Single Sideband transmission uses class AB power amplification – this is again all to do with the distortion produced and possibly in very poorly designed circuits – loss of intelligence. SSB transmitters are very expensive because they don’t normally get designed by monkeys. Likewise the first class D amplifiers and modern design.
Class D amplifiers and EMI
Class D amplifiers, if not designed and built properly will sound ghastly and more importantly will have the authorities tapping on your door because of the radio interference (EMI). But because they don’t draw current unless it’s really needed as defined by the audio signal are great for the Greenpeace Movement. How exactly do they work?
The building blocks of a class D audio amplifier
Class D amplifiers essentially have only three main circuit blocks – the modulator, switcher and a low pass filter. The audio input signal is chopped into pulses by the modulator at a very high frequency, derived from a high frequency triangular wave. These pulses therefore are very much like our binary 0s and 1s. When we have a ‘1’ the voltage is high and the current is low. When we have a ‘0’ the current is high and voltage is low. As power is defined by current times the voltage we note that the power consumed will always be very low. Theoretically, when we have ‘1’ state the power consumption is zero. What a bargain! Plenty of power for free. Not so fast kiddo!
Class D audio signals
The input signals fed into the pulse width modulator are (i) in the audio spectrum and (ii) a high frequency triangular wave – usually in the order of about 10x maximum audio frequency or about 250kHz.The output of the modulator is a very high frequency (e.g. 250kHz) square wave which has varying pulse lengths determined by the audio components, frequency and amplitude. Because of the very nature of square wave the outputs of a class D amplifier are (i) audio (ii) high frequency square wave and the not so nice bit (iii) harmonics.
The high frequency component plus harmonics are removed from the output to the loudspeaker by applying a LC low pass filter to the output. (sometimes designers use the voice coil of the speaker to serve this purpose – more about this later). Harmonics are a problem – this is where design is critical.
I write this article because I know Class D amplifiers are expensive and there are thousands of DIYers wanting to build their own and they do. Class D amplifier designers and manufacturers pay meticulous attention to board layout. Just as the original switched mode power supplies were very expensive (from IBM) and the pricing has dropped substantially over the years (Nowadays one can be picked up for under 20U$) so will the class D audio amplifier thanks to the interest shown by many chip manufacturers. Good quality power supplies are built using A-grade components and even more importantly use high quality PCBs and surface mount components. Bet’cha that in the next year or two our Asian friends (Taiwan and China) will have amplifier modules on the market producing 200W RMS also for 20U$. I know if I was Asian – I would be pushing this market. Pricing = supply and demand.
Class D Audio and speaker Damage
Class D audio amplifiers are especially susceptible to causing loudspeaker damage on clipping and the manufacturers of such amplifiers specifically concentrate on circuitry to prevent this. Likewise poor filter design – In Texas instruments they remove the filter component entirely by using a different approach to obtaining the output differential signal – in fact creating a third state or 0V state. TI always supply very interesting and useful application notes so their pdf on Overstress testing of loudspeakers using their third state topology I found very interesting. The two links below this are both well written articles and a must read. There are many myths erroneously applied to class D audio amplifiers and in order to respect both the audiophile’s view(s) as well as that of the manufacturer of these class D amplifiers one needs to be very objective. The bottom line is that Class D audio is here to stay – this includes the many other topologies either correctly or incorrectly defined as being class D. With filterless output stages costs have dropped substantially and TI once again is recognised as one of the world leaders. Likewise B&O’s patents on their ICE power packages.
TI Link to Overstress testing of Loudspeakers – a must read article.
Class D amplifiers – What, Why and How – Eric Gaalaas
Speaker Selection for Class D – Wolfson Microelectronics
Class D Audio – notes
Note: The class D amplifier is often called a digital amplifier which is not really an appropriate description of the workings. However, in time it may very well be marketed as a digital amplifier only because there will be integration of the DSP, power supply and high energy output switching circuitry which will hopefully omit the entire audio spectrum from the input but the fact remains there will always be a a linear component and that is the one which we hear – the output signal. Class D circuits until recently were built chiefly to reduce battery drain (cellular phones) and with this technology came the topology swing towards very high energy audio amplifiers used in modern professional amplifiers. It is not debatable that all audio circuits will be class D in the near future – this reduces shipping costs astronomically – which will lead to all home theater and entry level audio to follow suite. With the energy crises on our hands this may be sooner than we think, so team marketing would be using this to their advantage. Class A and Class B (AB) amplifiers will not disappear just like the valve and valve hybrids of today seem to be showing more of an increase in sales than the opposite. Class D amplifiers are therefore not without their own idiosyncrasies and certainly in the minds of the audiophile, not a direct replacement for all audio amplifiers.
Something to think about: The switched mode power supply driven directly from the rectified mains, filtered and operating as a basic switch, modulated by an audio signal optically isolated for safety and driving the loudspeaker through the switching transformer (a safety element as in the SMPS PSU) would cut out many components. Just as ADSL cuts out digital to analogue to digital in the old ‘analogue’ Modems of yesteryear the class D amplifier is only really seeing it’s entrance now. The funny part is that this technology has been available to us for over twenty or more years.
Technical: Just look at the TPA3110D2 from Texas Instruments. The market eagerly awaits.
Here are some links:
Don Elliot ESP
http://sound.westhost.com/projects.htm – superb website
Car Audio from Warmup Electronics
http://szouma.en.madeinchina.com/P6109415/Class-D-amplifier-1000W-1.shtml
Look through the Chinese trade articles and you’ll find out quickly enough who dominates the electronics market.
Be careful of fraudsters – I see pictures of 1800W RMS amplifiers with loudspeaker terminal posts that would barely pass 100mA let alone 100A.
