Class D
While the other classes operate in a linear (analog amplifier) mode, Class D amplifiers run the output devices as switches (on or off) at an ultrasonic frequency to create a continuous series of square waves or pulses.

The input signal is made to modulate the pulse width, an operating principle called pulse width modulation. The pulses’ high frequencies are filtered out, and this filtered output (an analog signal) drives the loudspeaker.

• Very efficient (90-95 percent). That’s because the transistors are either on (with high current and no voltage) or off (with high voltage but no current). So there is very low power dissipation compared to linear amplifiers.
• Tends to be small and light because of less cooling and fewer output devices.
• Typically used for high-power car stereos and pro applications where heat, weight and size are considerations.

Class I
• Class I or BCA (Balanced Current Amplifier) design developed by Crown is an efficient system based on Class D switching amplifier technology. The amp needs only a little AC power to operate.
• According to the manufacturer, a BCA amp generates one-tenth the heat of conventional amplifiers so it can work with much less air movement. This reduces fan noise, heatsink size, filter maintenance, and failure due to heat, so the amp can be small and light.
• A BCA amp re-uses the energy returned from the speaker rather than dissipating it as heat or forcing the amp into premature current-limiting. This helps BCA models handle 2-ohm loads without shutting down.

I hope we’ve clarified some of the differences among power amplifiers. They are another component in the chain of audio equipment that we need to understand well.

AES and SynAudCon member Bruce Bartlett is a recording engineer, audio journalist, and microphone engineer. His latest books are “Practical Recording Techniques 5th Ed.” and “Recording Music On Location.”

More articles by Bruce Bartlett on PSW:
Recording Microphone Techniques To Produce Warm, Spacious Stereo
Remastering Jazz Classics: The Dave Brubeck Quartet, Art Pepper, and Sonny Rollins
Deconstructing Hip-Hop To Hear How The Mix Comes Together

This article originally appeared in Live Sound International, 2011.