Digital Amplifiers

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Digital technology has the ability to make good products better. It gives us the opportunity to develop previously unthinkable devices and reinvent analog products as digital ones of smaller size that have offer increased functionality and consume less power. The home theater market is no exception when it comes to reaping the benefits of digital technology. Thanks to the digital revolution the music tape became the CD, the VHS tape became the DVD, as camera film became flash memory. The next product on the list appears to be the analog amplifier. After all, the analog amplifier is long overdue for a digital replacement.

We’ve been amplifying analog audio signals since the invention of the first electrical phonograph in 1925. The first breed of amplifiers employed vacuum tubes to boost the analog signal to an amplitude (voltage) that could move a speaker cone. 1947 saw the invention of the transistor which became a foundation for transistor amplifiers that eventually replaced most tube amplifiers. Today, three major types (or topologies) of analog amplifiers exist: class A, class B, and class A/B. There is also a class T amplifier produced by Tripath Technology that combines analog and digital components. Of course tube amplifiers have been perfected over the years and are still the choice of many golden-eared audiophiles. However, both vacuum tube and transistor technologies are relatively inefficient and produce a lot of heat as a by-product of their designs. In order to mass produce analog amplifiers at a reasonable cost the quality of many parts inside them is compromised. This results in loss of quality through signal distortion and degradation.

The newest addition to the amplifier world is the class D amplifier or the digital amplifier. But beware, not all class D amplifiers are actually digital. Some class D amplifiers accept an analog signal and use an analog control system. Other class D amplifiers use a digitally generated control.

Thought the technology has been around for longer than a decade, it is only now that digital amplifiers are becoming accepted by audio enthusiasts.

Early digital amplifiers had problems meeting analog level performance specifications and competitive costs. They also depended on analog interfaces and controls making them not entirely digital. But today, sophisticated class D amplifiers use digital controls eliminating performance degradation due to dependence on analog component variations.

Manufacturers are taking different approaches to producing digital amplifier chips that offer varying levels of performance. Conventional class D amplifiers utilize binary, or bi-state, operation. Binary consists of two switching states, positive and negative power. In binary operation, the cancellation of two full level signals is required for small outputs. Thus the amplifier has to continuously provide full power which translates into wasted power. Apogee Technology Inc has tackled this issue by introducing a third state – the zero power state – in their digital amplifier. During the time when neither supply connection is required, both speaker terminals are connected to each other providing damping to the loudspeaker. Hence the ternary approach only supplies power to produce a signal. Apogee’s digital amplification chips are used in a wide variety of today products including home theaters in a box and flat panel televisions. Texas Instruments is also a big player in the digital amplifier chip market. Their PurePath Digital amplifiers meet stringent audio quality demands and are used in many of today’s digitally amplified A/V and DVD receivers.

So what advantages do digital amplifiers present over analog ones? Digital amplifiers are highly efficient with power conversion at over 90% (only 10% or less is lost to heat), they are virtually immune to noise, the digital signal path does not rely on analog component tolerances, and they offer greater flexibility and configurability of the signal path. A digitally amplified system allows the audio signal to stay in the digital domain for practically the entire signal chain, from the source to the speakers. The conversion to the analog domain occurs in a passive filter or the speaker itself, so there is virtually no loss of quality. Analog amplifiers on the other hand are very inefficient at power conversion (nearly 50% of the power is lost to heat). This means that to achieve 100 watts of output on a speaker the amplifier is losing 100 watts as heat that must be dissipated into a heat sink. On top of that the power supply must be designed to supply up to twice the output power. Analog amplifiers are also susceptible to noise, and the signal path relies on other component tolerances.

Additionally, digital amplifiers do not require large heat sinks found inside analog amplifiers, they are substantially smaller in size and are much lighter in weight. This allows digital amplifiers to be incorporated into products such as small electronic devices and flat-panel televisions. Digital amplification is enabling new possibilities for designers of consumer audio products.

The signal chain of a digital amplifier goes through the following steps (see Figure 1):

• A Digital Sound Processor (DSP) receives digital source material from a DVD or CD, decodes and/or processes it, then outputs it as PCM data
• The PWM processor receives the PCM data, over samples it, applies noise shaping, and then converts the data to a PWM format
• The PWM processor then passes this PWM data to the power stage, which converts the 3.3 volt PWM signal input to a higher output (usually from 20 to 40 volts), and then applies the signal to a speaker or second-order low-pass filter where it is now in the analog domain

The signal chain of an analog amplifier is shown for comparison in Figure 2. In the analog signal chain the PCM data from the DSP would get converted into an analog signal by a Digital to Analog Converter (DAC), and then be amplified. This low level analog signal is very susceptible to noise so care must be taken in the design of this type of amplifier. Digital amplifiers do not have DACs at all.

What about sound quality? The audio quality of first class D amplifiers was inferior to good quality analog amplifiers due to distortion. As a result, these amplifiers were used mostly in low-frequency subwoofer applications where the audio fidelity of class D amplifiers was acceptable. This is because the switching speed of the transistors, and lower sound quality are masked by the lower frequencies being produced by the subs, since distortion is harder to discern at low frequencies. Today, improvements in digital amplifier technology are placing the digital amplifiers on a level playing field with their analog counterparts. Refined PWM stages are available from several different component manufacturers with varying levels of performance allowing digital amplifiers to deliver similar performance to analog amplifiers. In November 2004, D2Audio’s XRT became the world’s first amplifier to meet THX Ultra2 amplifier performance, proving that a well designed digital amplifier can meet even stingent home theater demands. However, digital amplifiers are still a hard sell to the golden-eared home theater enthusiasts. Most of today’s class D amplifiers can deliver appropriate mids and highs but lack a little in the bottom end.

Digital amplifers first appeared in computer speaker systems, but thanks to companies like Texas Instruments and Apogee Technology Inc. (among many others), digital amplifiers are quickly making headway into the entry and mid-level home theater products. For example, Jamo’s DVR50 receiver and Kenwood’s VRS-N8100 receiver feature digital amplifiers (see Kenwood review in this issue). The Spherex Xbox 5.1 system reviewed in our last issue was also powered by a digital amplifier. Now, it is only a matter of time before digital amplifiers make their way into high-end audio components. Yamaha has already jumped on the bandwagon with the introduction of their audiophile quality MX-D1 digital amplifiers at the 2004 Consumer Electronics Show (CES) in Las Vegas. Audio Design Associates also offers an audiophile quality home and theater A/V receiver that uses Apogee’s digital amplifier chips for additional zones. After all, digital seems to be the way of the future.

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