Among the audio components that have become digital in past decades, perhaps the most evasive is the digital amplifier. After all, power is inherently analog, and the loudspeakers connected to an amp's output stage are analog, too. The challenge that lies ahead is in identifying just what makes an amplifier digital. As it stands, the benefits of even the most basic digital architecture are increased efficiency and a higher channel count in a compact footprint. But things get complicated when sonic quality and DSP are added to the variables under consideration. In all likelihood, a variety of digital amplification models will be developed to serve a variety of needs. However it will be difficult to track the progress of this evolution without clarification on a few facts.
Already, there has been very casual use of the term "digital amplifier," observed Gerald Stanley, senior vice president of research at Crown, and holder of a U.S. patent for the Opposed Current Power Converter among many others. "Over the years we've seen people referring to amplifiers that simply had switch-mode power supplies as being digital," he said. "I like to make a joke of it and say that I have been shaving with a digital shaver all of my life. They have switch-mode power supplies in them, that must make them digital."
The actual definition of a digital amplifier Stanley then provided is "a pulse width modulated amplifier, i.e. one whose output stage is switching, using digitally computed width modulation."
Joel Butler, director of electronics engineering for SLS Loudspeakers, and a founder of SLS' recently acquired digital amplifier division, Evenstar, agreed. "You could say that the only digital amp is completely digital in and out, but the fact of the matter is, digital means 'discrete time.' With a digital amp, you're operating totally different types of transistors that aren't really meant for analog linear operation, they're meant to be turned fully on. You operate those at extremely high frequencies, and the way you modulate the pattern in which they turn on and off creates an audio waveform."
Therein lies the distinction. Signal generation in a digital amplifier is dependent on the particular end requirements of a product. So just how 'digital' an amp is depends on the individual developments of manufacturers. More precisely, as Dan Fraser, senior project engineer for Renkus-Heinz explained, "A lot of people mistakenly call Class D 'digital,' where there are actually two distinctly different forms of it. Class D itself means 'pulse width modulated output,' which actually is a form of analog signal. However the digital/analog split depends on how you develop that PWM signal." Certain Class D amplifiers accept audio in a digital format and keep it in the digital domain, whereas others take analog signal and use a completely different process to the PWM.
Some early PWM methods did not impress those who heard them. Distortion was an issue, as ultrasound that came in through the input was turned around and put back into the audio band by the PWM process. Since the early 1990s, when these fledgling attempts were made, various manufacturers have made refinements to their respective PWM processes, and the result is several viable digital amplifier options.
With sonic quality catching up to the efficiency of digital amplifiers, there is a real possibility of further reducing the number of components in the audio chain and streamlining audio system design and monitoring. "A digital amplifier that has the added ability to take a digital input and go through DSP is a great thing," observed Jonathan Laney, senior audio consultant at the Talaske Group. Having recently completed the lengthy design and installation process for the Jay Pritzker Pavilion at Chicago's Millennium Park, Laney was able to maximize the usage of Crown CTs amps with USB3CN modules. "At Millennium Park I'm utilizing DSP solely in the amplifiers to do all my crossover, signal processing and room equalization. All of that is done directly within the amplifier."
Reducing the component count still further while increasing technical capabilities is the evolution of digital amplifier modules within powered loudspeakers. D2Audio, a developer of digital amplifier modules, has already made an impact on the consumer market and began working with loudspeaker manufacturers on the pro side more than a year ago. The result of a year-long collaboration with Renkus-Heinz has produced the DCC-8 speaker, which was designed to be used in groups of two, called the DCC-16. To be shown in its latest iteration at the upcoming NSCA Systems Integration Expo, the DCC-8 can be used in as large a group as four for a 32-speaker element. It uses the D2Audio XA100 8-channel amplifier module with custom firmware and software that enables lobe-free steering.
"The 8-channel module is a natural for the column arrays in the pro space," said Rich Spina, amplifier development manager at D2Audio. "We're glad that Renkus-Heinz noted that and have designed what appears to be a killer product with it. That amplifier module can deliver 125W per channel, and that's more than they need. What makes it all possible is the fact that our PWM Processor IC with embedded DSP is integral to our amplifier, and that it has so many outputs, has a 93-percent efficiency and is small enough to fit inside that column of eight speakers."
The custom DSP inside D2Audio's modules carries several features for the commercial installation market, including crossover, up to eight bands of parametric EQ, driver alignment/time alignment and limiting.
"The other thing that we take advantage of in the DSP that are particular to the commercial and industrial install space is speaker detection," said Skip Taylor, chief technology officer for D2Audio. "It also checks the polarities of the speakers. We do a lot to automate the setup of an installation to alleviate the problems that are associated with having a speaker in a remote location."
That's just the beginning, Spina said, "We're going higher in power."
Also making big plans to get into big power is SLS' Evenstar. Subsequent to its debut of the S8R powered studio monitor, which was developed to meet the standards of SACD developers, SLS has plans to bring these sonic capabilities to powered loudspeakers for the pro market this year.
"The reason why we did a lot of work with this modulator and this technique is because digital amplifier has been plagued with sonic problems," Butler explained. "What we've been trying to get is the extremely high sonic characteristics of a high-end linear amplifier, but without the heat and power loss in the heat sinks. We've been able to do that with the sigma delta modulator."
By taking the one-bit data stream from the sigma delta modulator and not decimating it, SLS has made a high-power amplifier with superb sonic performance.
While D2Audio and SLS are building more power into their amp modules for powered loudspeakers, one rackmount amplifier manufacturer is taking the opposite route. Crown has carried its high-power Class I technology over from the CTs and I-Tech series of amplifiers into the new DrivePack solution for JBL's VerTec DP Series line arrays. DrivePack amplifiers feature Crown's BCA (Balanced Current Amplification) technology, incorporating Class I circuitry with temperature-compensated modulation.
"The DrivePacks run cool enough that there are no fans whatsoever -- they're strictly natural convection," Stanley said.
The diminished heat output of digital amplifiers is a benefit in the equipment room, as well. Cooler amps mean less long-term damage resulting from thermal degradation, and more compact digital amplifiers can be housed in a rack without fear of increasing the air conditioning load. Power consumption can also be reduced. Dean Standing, director of sales at Rane, explained some of the benefits of the new MA 4: "The MA 4 is very efficient as far as its power consumption. You can have up to eight amplifiers on a 15-amp circuit, so that's 32 channels on one normal circuit. That's really efficient power usage."
Rane introduced the MA4 at AES last October, and the company will begin shipping the product in April. Designed for multi-zone applications, the MA4 has built-in dynamics processing for intelligibility, filtered load monitoring, power factor correction and fault detection with remote reporting. Furthermore, when a fault is reported, built-in redundancy switching prevents a total loss of signal. "What we've tried to do with the MA 4 is use the technology to improve performance and to solve problems, rather than being digital for the sake of digital," Standing said. "It was really geared toward integrating features that were dedicated to the install market."
With so many companies developing digital amplifier technology for use in all aspects of audio, the future might be a very different place. "Ten years down the road, the regular analog amplifier will be a specialty item that only a few audiophiles will long for -- the same group that longs for the days of tubes nowadays," Fraser speculated.