Headphone amp swaps modes to cut noise, save power
( 01 Jul 2006 )
—Graham Prophet , EDN Europe
Designers working with portable audio products have a new option in the constant trade-off of performance versus battery life: Wolfson’s WM8985 audio codec. The device has a headphone-output stage that is dynamically switchable from operation in Class A/B to operation in Class D. Class A/B offers a conventional route to a high-quality, low-distortion output but requires a standing current through its output-stage devices and has a higher power demand. Class D has lower losses. A spokesman for Wolfson makes the subtle point that, in Class D, headphone driving is about power consumption rather than power dissipation.
Although manufacturers in recent years have greatly refined the quality of Class D amplifiers, designers may sometimes not want to use them. Class D is by definition a switching-output mode and introduces some level of harmonics and noise. In a device such as a smart phone with an MP3 player, designers may want it to operate in Class D when the RF section of the handset is not active and in Class A/B when it is. In handsets with FM radio, Wolfson notes, the headphone lead is also typically the FM antenna and cannot carry Class D-output waveforms while receiving FM.
Switching on the f ly between modes, the codec can accommodate this requirement. At typical power outputs of 10mW, switching to Class D reduces power consumption by approximately 50%. The 8985 also integrates a DSP for filtering and equalization, differential or line inputs, audio mixing, and a PLL that can accept a wide range of clock inputs. It has a switchable output and one that is in permanent Class A/B mode. SNR figures for the device’s DAC, ADC, and headphone driver are 98, 92.5, and 90dB, respectively. In standby mode, the device uses 360µW.
