Integrated Amp Reviews

Like most industries, hi-fi has its share of empty marketing slogans—so many that it makes sense to regard manufacturers' claims about their equipment with a certain degree of skepticism. I kept this in mind as I pressed Greg Stidsen, NAD's director of product development, to explain to me just what the company's PowerDrive technology is.

A lot of what makes high-end monster amps powerful and expensive is the extra balls required to drive a difficult loudspeaker load—one of very low impedance, perhaps in combination with a large phase angle—at frequencies where music has a lot of energy. Such loads require extra power to keep the amplifier from distorting and to keep the character of the sound from changing. In traditional designs, that extra power is used only when the load would stress a lesser amp—when the amplifier isn't working hard, all that extra power is wasted. "So at 8 ohms, all these big parts that you paid extra for" in that monster amplifier you just bought "are just loafing along," wrote Stidsen in an e-mail.

NAD's PowerDrive amplifiers give your speakers what they most crave. When the load is difficult, the circuit maximizes current delivery, but with easier loads—those that don't require so much current—the amplifier switches to "a second, higher voltage winding [that] allows us to employ the full current capability we already have and make it available into 8 ohms," said Stidsen. Simple but clever.

Stidsen claims that what sets PowerDrive apart from other designs—such as the "class-G" approach commonly found in pro-audio equipment—"is the way it determines which of these two voltage supplies should be used at any given moment. The circuit senses the operating conditions of the output stage (voltage, current, heat) and decides which rail to switch in, high current or high voltage. For short-term power output, even at 4 ohms, it will stay on the high-voltage rail." Only when the load starts to stress the amplifier does it switch to the low-voltage, high-current rail. The result is "very low distortion into any loudspeaker load, and we get the maximum power from the parts we use," wrote Stidsen.

How does this affect the sound? "I find that the PowerDrive amplifiers never get that 'strained' sound, the hardening and roughening that occurs with many lesser amplifiers," wrote Stidsen. I don't know if PowerDrive is the reason, but I certainly found the C 372 to have a relaxed sound.—Jim Austin