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How Class D Digital Amplifiers Work

As Axiom’s chief R&D engineer Tom Cumberland describes it, a digital amplifier is a “power DAC”, and of course a DAC (Digital-to-Analog Converter) is the basis of all digitally recorded media, whether we’re talking about CDs, hi-res audio, Blu-Ray soundtracks, DVD video, and so on. The view of some that “all digital amplifiers are crap” is not true. In fact, the clock rate of a good digital audio amplifier is typically in the range of 350 to 500 kHz (that’s 500,000 Hz). (Axiom’s A1400 digital amplifier uses a 450-kHz clock frequency.) By contrast, even the highest-resolution digital audio system (DVD-Audio and a variant used for Blu-ray soundtracks) runs at 192 kHz, which is far below the clock rate of a good digital amplifier.

Different Forms of Class D Amplification
Though we may think that “digital” means all the circuits in a digital amplifier work in on/off pulses, in fact there are a number of different types, including digital amplifiers that have analog elements.

A digital amplifier will have either analog or digital inputs. Good digital amplifiers with analog inputs can use analog feedback networks to lower the amplifier’s distortion, in much the same way that a Class A/B analog amplifier uses a negative feedback network to lessen the distortion. However, a digital amplifier that accepts only a digital input must rely on the incoming digital signal to lower distortion.

Feedback Networks
Why feedback networks? The reason they are used is that all parts in an amplifier have “tolerances,” which means that any particular part has a range or value in which it operates. Anyone who has examined such basic parts as resistors may have noticed they are specified as being “5%” or “10%” resistors, which means the specified resistor value is accurate within a range of 5% or 10%, respectively. Consequently, because of these variations in parts, a feedback network “looks at” the outgoing signal from the amplifiee — the one that goes to your loudspeakers –and compares it to the incoming audio signal at the amplifier’s inputs. Any deviation in value away from the incoming signal is a distortion, so the negative feedback network applies inverse correction to compensate.

There are even differences in the operation of digital amplifiers. For example, the “ICE” digital amplifiers developed by the Ice Power division of Denmark’s Bang & Olufsen use a very complex negative feedback system due to parts tolerances. B&O holds patents on its “ICE” amplifier, which is basically a Class D switching design (Pulse Width Modulator) with variants that B&O claims reduces distortion to levels associated with Class A amps, while retaining the high efficiency of Class D switching designs.

“IR” (International Rectifier) is the system used by Axiom Audio in its A1400 digital amplifier. Axiom worked with International Rectifier to keep parts tolerances held to the very minimum amount, so that very little negative feedback would be used to correct for anomalies in the output. This approach also made the amplifier more robust in its operation without being subject to oscillations or instability.

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Axiom and IR developed new silicon output devices that drive the MOSFETs in the output stage in such a way as to produce a perfect Pulse Width Modulated square wave at the output before the reconstruction filter.

Pros and Cons of ICE and IR Digital Amplifiers
One of the downsides to using a complex negative feedback network in a digital amplifier of the type used in ICE designs is a potential loss of efficiency. Performance may also suffer because of a slower clock rate.

In an IR type of digital design, which uses very little negative feedback or none at all, the clock rate is higher and efficiency increases. Moreover the high efficiency is combined with high power delivery and higher overall resolution. At full output, Axiom’s A1400 digital amp runs at about 95% efficiency (by comparison, class A/B analog amplifiers run between 50% and 60% efficiency; the remainder is wasted in heat).

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