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ADI claims audiophile sound for its Class-D amplifier entry

Posted: 28 Jan 2005 ?? ?Print Version ?Bookmark and Share

Keywords:class d amplification? analog devices? adi? ad199x? amplifier?

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 10W to 50W systems for flat panel TVs, and even 150W devices for home theater set ups.

Analog Devices Inc. (ADI) 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 ADI 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, 5W per channel device), the AD1992 (2W to 10W), the AD1994 (2W to 25W) and AD1996 (2W to 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 5W 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 100W 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 600kHz), 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.

For its part, ADI 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 Bob Adams, ADI's audio product manager. 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 101dB.)

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 (20Hz to 20kHz).

Though it would have been an ideal venue, ADI 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.

- Stephan Ohr


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