Amplifiers deliver a digital gain
-- EDN, January 4, 2001
One of the last bastions of the all-analog world, the audio amplifier, is about to get a much-needed digital upgrade, claim companies such as Apogee Technology, Cirrus Logic (www.cirrus.com), Jam Technologies (www.jamtech.com), and Texas Instruments. Digital amplifiers replace the DAC and linear amplifier of conventional designs with a DSP-controlled PWM driving switching power transistors, or rectifiers. Historically, Class D analog amplifiers, with their "binary," discrete load connection to the power supply, achieved efficiency improvements over always-active first-generation Class AB designs. All-digital amplifiers, which aren't continuously switching, take these improvements one step further. Their advocates claim that these amplifiers offer efficiency, SNR, EMI, CMOS integration, power-supply tolerance, output-impedance variability, scalability, and other strengths that analog or hybrid amplifiers can't match.
Both Apogee Technology and Texas Instruments unveiled their first-generation product offerings at Comdex. Apogee's $6.98 (1000) DDX-20xx chip set, scheduled for production in this quarter, consists of the 3.3V DDX-2000 controller in 44-lead QFPs and the surface-mount, dual-channel, H-bridge DDX-2060 power device. Apogee uses a damped-ternary modulation scheme that connects the speaker to ground, disconnecting it from the power supply, when no output power is required.
The DDX-20xx chip set delivers a claimed maximum 89% efficiency and 93-dB, A-weighted SNR. Driving a 1-kHz, 1W output signal into an 8Ω load, the chip set has a THD (total harmonic distortion) plus noise of 0.08%. At 1% THD plus noise, per-channel output power is 42W, and both mono "bridged" mode and higher power "staged" mode options are also available. EB-2060 evaluation boards for stereo or mono cost $198. Apogee also offers the DDX-S200 two-channel controller core, which the company has synthesized to 0.5- and 0.35-µm processes. The core includes test vectors; accepts 44.1- and 48-kHz input-sampling rates; and supports auto-mute, volume control, and dynamic-gain compression (anticlipping).
TI's $8.95 (50,000), surface-mount-packaged TAS5xxx, now in production, derives from technology acquired when the company bought digital-amplifier pioneer Toccata Technologies in March 2000. The dual-channel chip set comprises the TAS5000 controller and two TAS5100 H-bridges. The TAS5000 accepts incoming I2S, 44.1- to 96-kHz-sampled, 16- to 24-bit data streams. TI specs the chip set at 30W (continuous) per channel but declines to provide distortion and noise specifications other than nebulously worded "clean and crisp" verbiage. Cirrus Logic's first chip set, targeting low-output-power, portable-audio-headphone-amplifier applications, should arrive in the first half of this year, and higher power variants will follow. Jam Technologies' Web site is heavy on marketing claims but, at least for now, devoid of product details.
Apogee Technology, 1-781-551-9450, www.apogeeddx.com.
Texas Instruments, 1-972-995-2011, www.ti.com.
-by Brian Dipert
