Connect headphones to Class D amplifier

Maxim's MAX9704 is a small and efficient Class D audio power amplifier. Its fully balanced inputs and Class D outputs. make it a convenient chip to directly drive speakers. Sometimes, though, you want to have a headphone output to keep the office environment. Class D power amplifiers usually have fully balanced, bridged outputs on each channel. If the amplifier drives separate speakers, you can use an attenuator circuit (figure 1). A problem arises, however, with grounded headphones: Stereo headphones use three-pole plugs with which the negative side of each speaker connects to a common ground. Thus, you may think that you can't directly connect headphones to a Class D amplifier without using a transformer.

To solve the problem, look at the output waveform of the MAX9704 as it swings (figure 2). Each channel output alternates between high and low. You can take advantage of the fact that the channels aren't on at the same time by configuring your circuit like the one in figure 3.

Figure 4 shows the circuit details. Because the MAX9704 alternates the outputs of each channel, the R₃/R₆ combination doesn't affect the channel's drivers. Resistors R₃ and R₂ connect to the left output terminal. Resistors R₄ and R₁ connect to the right output terminal. The inactive channel's output voltage must be the same voltage, which means that R₄, R₁, and R₆ connect to the same voltage when the left-channel output is active. R₃, R₁, and R₅ connect to the same voltage when the right-channel output is active. The values of R₁ and R₂ affect how much crosstalk you get between channels. The values in figure 4 provide sufficient channel separation.

About the author
Hiroshi Fukushima contributed this article.

This article is a Design Idea selected for re-publication by the editors. It was first published on July 9, 2009 in EDN.com.

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