News and New Products

FROM EDN EUROPE: Digital amplifier topology aims high in audio

By Graham Prophet -- EDN Europe, 3/3/2005

Audio amplifiers that use switching (pulse-width-modulated) waveforms have progressed very rapidly. Usually designated Class D, they have been used chiefly for their high efficiency, allowing audio to be produced at reasonable levels from portable or battery-powered equipment. They have also allowed high audio power levels to be generated from compact amplifiers with reduced thermal losses (and, therefore, heatsinking) (Picture). However, although audio quality has steadily improved, they have not been regarded as being capable of competing with high-quality "conventional" audio amplifier designs for the "hi-fi" market.

Zetex has developed a configuration that it believes is sufficiently different to warrant a new classification, and has created the name "Class Z". In demonstrating the amplifier in prototype form at the recent Consumer Electronics Show in Las Vegas, Audio Marketing Manager Craig Bell says the company had a positive response. Listeners used to identifying the failings of audio amplifiers acknowledged that, as Bell puts it, "if they had not been told it was a digital amplifier, they would not have known."

The design sets out to identify the specific shortcomings of a digital amplifier signal chain, and to correct them. Although Class D handles a nominally perfect square wave through a signal chain that comprises PWM generator, output driver and output power switches, the square wave in practice becomes less and less perfect at each stage. Distortion can take the form of ringing, changes to edge speeds and other degradations. At the amplifier output there is a reconstruction (low-pass) L-C filter followed by the variable load of the loudspeaker. The speaker load is not precisely known to the amplifier designer and can vary dramatically, both between loads and across the audio frequency range for any given speaker. In Class Z, one of the techniques Zetex applies is to measure the error present in the output waveform and use that data to generate a feedback signal to correct the error. The error measurement is made in the analogue domain, and the result is digitised and fed back into a noise-shaping stage at the output of the PWM controller. Pulse-width errors are compensated by interpolating on a time-average-basis; and other errors are compensated by measuring and adjusting the total energy in each pulse. Conventional negative feedback around the complete amplifier loop cannot be used in a Class-D design, Bell says, because of the (typically 2 msec) group delay through the signal path. Hence, most digital amplifiers are open-loop designs. This direct-digital-feedback technique overcomes that problem, and allows very low figures for total harmonic distortion (THD) to be achieved. The technique allows the output stage to run with low shoot-through current, for high efficiency; and gives a low effective output impedance for a high damping factor, (in the thousands at low frequencies, Bell says) to handle "awkward" speaker loads. It also removes the need for a very high-quality power supply, as psu imperfections no longer couple directly into the output. THD is not, Bell asserts, the most effective measure of performance. Zetex uses an intemodulation test with 20Hz and 1 kHz tones; the Class-Z design produces some sidebands around the 1-kHz frequency in the output, but at very low level, and almost no other modulation or noise products.

Zetex will produce an integrated amplifier control IC incorporating the digital feedback mechanisms, scheduled for production late in 2005.

Zetex, +44 161 622 4444, www.zetex.com.