Add headphones to a Class D amplifier
Connect headphones to the MAX9704 and make a quieter lab or office.
Hiroshi Fukushima, Technical Research Center, D&M Holdings Inc, Kawasaki City, Kanagawa, Japan; Edited by Martin Rowe and Fran Granville -- EDN, July 9, 2009
The MAX9704 from Maxim 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.
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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.
