Special Report: Class D Amplifiers

Class D Primer: Designing a Class D

Designing a Class D amplifier is a very different exercise than creating a traditional linear Class AB design. The transistors must be turned on and off with incredible precision. This balance is so delicate that any stray capacitance or inductance could cause distortion on one hand or the amplifier going up in smoke on the other. That’s why switching amplifiers are almost exclusively built with surface-mount components. (Conventional circuit boards and “through-hole” construction introduce capacitance and inductance variations that can cause the Class D amplifier’s output stage not to function correctly.)

There’s also quite a bit of mathematics behind a switching output stage; the popular ICEpower module was the result of four years of research at the Technical University of
Denmark. For these reasons, most amplifier manufacturers buy an “off-the-shelf ” output-stage module and driver chip from one of several OEM suppliers. The amplifier manufacturer then designs the surrounding circuitry, power supply, and chassis, and perhaps even modifies the module with higher-quality parts. It’s worth noting that the purity of the DC supply to the switching transistors is of paramount importance. The output transistors effectively connect the DC supply to the loudspeaker terminals; any noise or ripple in the DC will be fed to the loudspeaker. Ripple (a 120Hz modulation of the DC supply caused by imperfect filtering of the incoming 60Hz AC power) on the power supply will cause amplitude modulation of the audio signal.

The ICEpower module, shown above, was developed for Bang & Olufsen, and is used in the Rowland line as well as in the Cary A 306 (among others). Another popular module is the Tripath, used in the Audio Research 300.2. (Tripath calls its technology Class T rather than Class D.) The Kharma MP150 employs a technique developed by Bruno Putzeys when he was at Philips Applied Technologies. (Bruno also designed the entire Kharma MP150. Hear from him in the Class D Designers Roundtable).

There’s no question that Class D technology will dominate mass market audio products, from the amplifiers in flat-panel televisions to car audio to home-theaters-in-a-box. The size, weight, heat, and cost advantages are just too compelling. Whether switching technology is good enough to displace linear amplifiers in the quest for the absolute sound is still an open question.

Top | Previous | Next