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The CLASSiC-D, Part 1

World's first DIY high-power high-performance Class-D amplifier: 250Ω into 4Ω; 150W into 8Ω

By John Clarke

Class-D or switching amplifiers are made by the squillions and used in countless TV sets, home audio systems and a host of other applications ranging from iPod players and phones to large amplifiers in commercial applications. So they are obviously reliable when they are mass produced.

However, in the past when we have taken a typical Class-D chipset and tried to adapt it to a do-it-yourself design for publication in SILICON CHIP, we have been lamentably unsuccessful. Inevitably the chipsets were surface-mount devices and some employed quite critical heatsinking for the main amplifier itself. And inevitably again, we consistently blew devices as we tried to devise a reliable DIY design.

So much so, that Leo Simpson, the publisher of this magazine, had sworn off attempting another Class-D
amplifier. Time heals all wounds though and eventually he relented when he saw the details and specs of this proposed design. Yes, it does use a surface-mount driver chip but the pin spacing is quite reasonable for hand-soldering. More particularly, the main switching Mosfets are conventional TO-220 devices that are easy to solder and heatsink. All the other components are conventional leaded devices and the result is that this Class-D amplifier is easy to assemble.

That’s its first big advantage. Its second big advantage is ruggedness and reliability. It delivers heaps of power and has all sorts of protection built in so we have not blown up a succession of devices during development. Well, back up a minute, we did blow some in the early stages but those problems have all been sorted out.

Efficiency is the third big advantage, in common with all Class-D switching amplifiers. Typical efficiency is around 90% and that means that this amplifier will deliver considerably more power from a given power supply than would be possible with a typical linear amplifier such as our Ultra-LD design.

High-quality sound is the final advantage of this design and this is its outstanding feature. Most Class-D amplifiers are only average in this respect and this applies to the vast majority of sound equipment used in homes today.

We’ve christened the new module the CLASSiC-D. Why the CLASSiC-D moniker? Well, “CLASS” stands for class (what else?), “SiC” is for SILICON CHIP and “D” describes the class of operation.

What is Class-D?

So what is a Class-D amplifier and how does it differ from a conventional amplifier? Put simply, conventional audio amplifiers are either Class-A, Class-B or Class-AB (a combination of the first two). These amplifiers have their output driver transistors (or Mosfets) operating linearly and if you trace the signal through them, you will find that its shape is unchanged but increased in amplitude as it passes through successive stages to the output.

Class-D amplifiers operate in an entirely different mode whereby the output Mosfet or bipolar transistors operate as switches rather than in their linear region and are either fully switched on or fully switched off. When switched on (or off), the power losses within the Mosfets (or output transistors) are almost zero. Thus a Class-D amplifier is far more efficient and generates much less heat than linear Class-A, Class-B and Class-AB designs.

In a Class-D amplifier, the output devices are switched at a very high frequency and the duty cycle is varied by the input audio signal. This is called pulse width modulation (PWM). After filtering to remove the high-frequency switching from the output, the result is an amplified version of the input signal.

With Class-D it is often (mistakenly) assumed that “D” stands for digital. Not true. It was called Class-D because the previous amplifier classes were A, B, AB and C. So when switching amplifiers were first devised many decades ago, it was natural to call them Class-D. 


• High efficiency

• High power

• Low distortion and noise

• Bridging option for driving 8Ω loads with two modules

• Over-current protection

• Over-temperature protection

• Under-voltage switch-off

• Over-voltage switch-off

• DC offset protection

• Fault indicator

• Amplifier running indicator

• Optional speaker protector module

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