Press Release

500W Class D subwoofer amplifier boosts audio performance

April 2005

Achieving THD+N figures of better than 0.05% from 1W up to 90% full power, a 500W RMS into 4? analog input Class D reference design from Zetex is setting new standards of audio performance in active subwoofers. Backed by a single channel evaluation board - the ZXCD500MOEVAL" the design also demonstrates how amplifier cost and size can be significantly reduced.

Based on the Acoustar™ ZXCD1000 modulator IC, this 500W switching amplifier owes much of its outstanding audio performance to a unique feedback architecture designed to reduce distortion. By taking its feedback signal from the filtered amplifier output, this high gain circuit design compensates for bridge mismatches, power supply fluctuations and filter non-linearities, to achieve a reduction in THD+N across the entire power band.

An additional benefit of the circuit™s feedback architecture is a significant reduction in amplifier output impedance. The resulting damping factor, of greater than 285, is a characteristic more often associated with Class AB linear amplifiers.

The reference design includes a mute/soft start circuit that provides an output mute facility via a momentary switch and pop suppression on start-up. In addition, an undervoltage detect circuit provides anti-pop at power down. The Zetex solution exhibits a noise floor of -130dB, a DNR of 98.5dB, and a flat frequency response from 10Hz through to 250Hz.

In order to generate 500W into 4?, a bridge voltage of 70V is required. The evaluation board simply uses a split rail input voltage of +/- 35V to power the output stage, with subsidiary bias voltages provided by an integral regulator daughter board.

Comprehensive circuit protection is another key aspect of the ZXCD500MOEVAL reference design. High-side and low-side sense circuits provide short circuit protection, DC offset detection and protection against thermal overload are also included.

The MOSFETs used in the Class D amplifier™s H-bridge output stage are all N-channel types. In bridge tied load configuration, the TO220 packaged devices are chosen for their current handling capacity, low on-resistance and high switching speed.