Class D Amplifiers Challenge "Golden Ears"

Conventional wisdom has it that class D amplifiers occupy an applications niche in which efficiency is paramount, and that several factors compromise sound quality. The latter includes difficulty in implementing feedback, mediocre power-supply rejection ratio (PSRR), and the drawbacks of constant switching-frequency pulse-width modulators (PWMs).

Countering the conventional wisdom, Analog Devices (ADI) integrated a delta-sigma modulator with its AD1991 class D driver. It also incorporated a feedback loop to deliver a family of class D amplifiers with "audiophile performance" for the high-end automotive and flat-panel TV markets (Fig. 1).

First, ADI replaced the usual class D constant-frequency PWM with a seventh-order feed-forward delta-sigma modulator, which spreads the switching-induced interference (Fig. 2). This is particularly important in the automotive OEM market, where electromagnetic-interference requirements are quite rigorous. (In fact, according ot the company, no current-generation product from any manufacturer totally meets U.S. automakers' standards for interference with AM-radio reception.) Next, ADI had to create a delta-sigma modulator that could accept feedback from the chip's analog outputs. This was the key to minimizing PSRR.

Of course, many audiophiles are the kind of people who argue with Electronic Design's Bob Pease about the "warmth" of vacuum-tube amplifiers and the virtues of silver-plated copper plumbing for speaker leads. Consequently, pleasing them with a switching amplifier is a tall order.

Before designers could do anything about subjective criteria (which ADI is pursuing with focus studies), the company had to minimize total harmonic distortion plus noise (THD+N). In this respect, the company succeeded in producing typical THD+N figures below 0.005%, which is a good order of magnitude better than the general run of class D amps.

ADI's fleet of class D chips consists of the AD1990, 92, 94, and 96, differentiated by their maximum output power: 4 to 40 W in stereo mode, 8 to 80 W in monophonic mode—with better than 85% efficiency. (Using external FETs, the chips can drive in excess of 100 W.) Dynamic range is greater than 101 dB, while PSRR from dc to 2 kHz is better than 60 dB.

The chips require three supplies: separate 5-V supplies for the analog and digital portions, and 8- to 20-V supplies for the output power sections.

Meanwhile, at January's Consumer Electronics Show, Zetex Semiconductors from the U.K. demonstrated eight-channel audio modules based on class D chips. (Their introduction is expected later this year.) Unlike ADI's AD1900-series, Zetex's chips don't integrate the power FETs. Also unlike ADI, Zetex's target market is audio-video receivers and DVD players, but these chips use delta-sigma modulation with closed-loop feedback as well.

Zetex claims 0.005% THD+N. But at CES, the company also discussed two other specifications: two-tone intermodulation distortion (IMD) and damping factor (Fig. 3). Interaction between a bass tone and a treble tone in the IMD spectrum is an indicator of PSRR. The other figure of merit, damping factor, reveals the effectiveness of feedback on bass response. It can be measured by switching load impedance while applying a constant signal. Zetex says its demo modules exhibit damping factors on the order of 200. Previously, damping factors in the 20 to 30 range were considered adequate for class D amplifiers.

Analog Devices' AD1900 series is available now. In 1000-unit quantities, the 4-W AD1990 costs $2.90, the 10-W AD1992 goes for $3.18, the 25-W AD1994 costs $4.45, and the 40-W AD1996 is $6.72. When they become available later this year, Zetex's eight-channel chips will cost around $3.75 in 10,000-unit quantities.