Analog Devices claims audiophile sound for its Class-D amplifier entry

Norwood, Mass. ï Raising the bar for high performance in consumer audio systems, Analog Devices Inc. today (January 24th) introduced a family of audio amplifiers that combine the space- and heat-saving advantages of Class-D amplification, with dramatic improvements in sound quality and reduced EMI (electro magnetic interference).

Featuring less than 0.005 percent THD+N (total harmonic distortion plus noise), and up to 2 – 40 W output power, the devices deliver an exceptional level of audio performance that no other single-chip Class-D device has been able to achieve. This unique combination of features resolves design challenges in consumer applications, such as HDTV and home theater systems, that demand high performance audio in the smallest possible space. EMI reduction prevents interference with other electronic equipment, a critical requirement in the design of car audio systems.

The AD199x Class-D amplifiers outperform competing PWM (pulse width modulation) based Class-D schemes, through the combination of a highly-integrated single-chip design featuring an internal sigma-delta modulator and a closed-loop sigma-delta architecture. Ultimately, this enables higher audio fidelity in half the board space, satisfying the demands of advanced electronics such as thin flat panel TVs and multichannel surround sound car stereos.

“New consumer electronics applications, especially advanced TVs, are designed as much for their aesthetic appeal as their audio/video performance. Class-D, with its inherent space and heat advantages, is a critical enabling technology in this revolution,” said Dick Meaney, vice president of the precision converter group at Analog Devices, Inc. “Analog Devices' new audio amplifiers take Class-D technology to the next level without compromising audiophile sound quality. Leading the industry in high performance and EMI reduction, the devices are the best choice for audio amplification in the demanding car audio market.”

Best-in-class Audio Fidelity and EMI Reduction

Analog Devices' AD199x Class-D audio amplifiers outperform competing devices on two key performance benchmarks: audio fidelity and EMI. These requirements were previously the most difficult to attain using Class-D architecture, but the AD199x amplifiers deliver audiophile sound quality (THD<0.005 percent; SNR>101 dB; PSRR>65 dB), while producing 30 percent less heat. The device's breakthrough performance is achieved through Analog Devices' closed-loop, mixed signal integration of 7th order sigma-delta modulator technology with high power output drive and bridge circuitry.

Radiated and conducted out-of-band RF emissions are minimized with Analog Devices' advanced modulation technique and closed-loop, sigma-delta architecture, enabling a significant reduction in EMI.

Designers have embraced Class-D integrated circuits as a solution to the challenge of building multi-channel high performance audio systems that fit into compact spaces with minimal heat dissipation. As a result, worldwide sales of Class-D products have grown 200 percent to almost $84 million in 2004. According to Forward Concepts, an industry analyst firm, the Class-D market will continue to increase—by up to 68 percent this year alone.

More About the AD199x Class-D Amplifier Family

The AD199x amplifiers, including the AD1990, AD1992, AD1994 and AD1996, are single-chip devices containing an integrated stereo modulator and stereo “bridge-tied” load (BTL) power stage. This compact design reduces board space and component requirements. Power levels range from stereo 4 W (8 W) to stereo 40 W (80 W). A modulator-only version of the device is also available. This, coupled with external high power FETs, enables very high power amplification (only limited by the power stage design).

The AD199x amplifiers can be integrated with other Analog Devices audio products, including ADCs (analog-to-digital converters), DACs (digital-to-analog converters) and CODECs, as well as SigmaDSP and SHARC audio processors.

Availability and Pricing

The AD199x Class-D amplifiers are now sampling, with full production quantities available in Q2 2005.

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Look here for product information on the AD1990.

The image of Class D amplification has certainly changed over the years we've been tracking it. There was a time, under the sway of audiophiles and “Golden Ears” equipment reviewers, when semiconductor manufacturers wouldn't touch Class D with a barge pole. Now, it seems, everyone wants to get into the act. We're witnessing a proliferation of Class D products, from a variety of manufacturers, and covering a wide range of power outputs from low-power headphone amps, to 10- to 50-watt systems for flat panel TVs, and even 150-watt devices for home theater set ups.

Analog Devices is targeting a “sweet spot” in the Class D market, the 5 to 40 watts-per-channel amplifiers used by bookshelf stereo systems, integrated DVD players and wide-screen flat panel TVs. You always want to pay attention to Analog Devices in the audio market. The company has previously made its mark with data converters and DSPs (rather than with Class AB amplifiers), but has claimed slots for its Sharc DSP among a number of high-end A/V receiver makers. One of its power-output parts (the AD1991) is already designed into a version of Sharp's Aquos LCD TV. But it is clear that ADI wants a larger share of the audio pie.

The AD199x amplifiers, including the AD1990 (a 2-channel, 5-watt per channel device), the AD1992 (2 – 10W), the AD1994 (2 – 25W) and AD1996 (2 – 40W), are single-chip ICs containing integrated stereo sigma-delta modulators and a power output stage using a stereo “bridge-tied” load (BTL). The amplifiers are built with a BCD process (a BiCMOS process with integrated DMOS power FET). With an RDS-ON of less than 0.3 ohms, ADI claims its amplifier is better than 80 percent efficient (driving 5 watts into a 6-ohm load). This means minimal heat generation (and smaller heat sinks) in space-constrained applications like flat panel televisions, PC audio systems, and automotive consoles.

The use of outboard FETs, ADI maintains, would increase the output power capability to 100 watts or more.

With the majority of Class D amplifiers, a pulse width modulator delivers a series of variable width pulses to a pair output FETs (wired typically in a push-pull configuration). While the output of the switch pair approximates an analog signal, further smoothing is applied by an outboard inductor-capacitor filter. As I've written before, the success of Class D very often depends on good filter design. Neglect is this area can result in an amplifier which, to quote the Golden Ears set, could bite your ears off.

Thus, a large number of Class D participant are working on “filterless” or “filter-free” designs, which attempt to minimize the capacitor and inductor requirement on the tail end of the amplifier.

ADI's contribution to this task involves the use of a sigma-delta modulator in place of the traditional pulse width modulator. I'm not entirely sure how the sigma-delta modulator functions in this application (I thought I heard “spread spectrum,” centered at 600 kHz), but I suspect it behaves similarly to the sigma-delta modulators in audio data converters. In sigma-delta data converters, the modulator affects a form of oversampling, which in turn provides two benefits. First, it pushes digital quantization noise upward in frequency, where it is essentially out of the range of human hearing and (in terms of capacitor values) much easier to filter. Second, it puts amplitude variations on a broader slope, enabling audio signals to be replicated with a switched capacitor (charging up on one switch cycle, discharging on the next). This would certainly smooth and eliminate some of the harmonics associated with Class D amplifiers.

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For its part, Analog Devices claims simpler filter design. The low distortion specs (less than 0.005 percent THD) are a consequence of the amplifier functioning “closed loop,” through a two-pole, 2nd-order filter. “This does not require design teams tweaking every resistor and inductor,” said Bill Slattery, ADI's product-line manager, digital-audio-product group. Much of the control is proved by the sigma-delta modulator which acts like a 7th-order feed-forward filter. (Claimed SNR is better than 101 dB.)

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While ADI feels its distortion specs are superior to its competitors, it does show the predictable tendency to increase at higher power levels. We'll have to check the datasheets to verify Bob Adam's assertion that the 0.005 percent distortion spec maintains itself across the audio frequency band (20 Hz to 20 kHz).

Though it would have been an ideal venue, Analog Devices did not demonstrate amplifiers with these devices at the Consumer Electronics Show (CES) earlier this month. So, claims for audiophile performance will still need to be tested. But, in principle, ADI is off to a good start.

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