Path: EDN Asia >> Design Centre >> Consumer Electronics >> Connect headphones to Class D amplifier
Consumer Electronics Share print

Connect headphones to Class D amplifier

17 Nov 2015  | Hiroshi Fukushima

Share this page with your friends

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.

image name

Figure 1: A Class D amplifier has separate drivers for each speaker.

image name

Figure 2: The MAX9704 applies power to one channel at a time.

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.

image name

Figure 3: This speaker configuration lets you connect headphones with a common ground to a Class D amplifier.

image name

Figure 4: With the resistors in place, you can connect headphones to the MAX9704 amplifier.

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

Want to more of this to be delivered to you for FREE?

Subscribe to EDN Asia alerts and receive the latest design ideas and product news in your inbox.

Got to make sure you're not a robot. Please enter the code displayed on the right.

Time to activate your subscription - it's easy!

We have sent an activate request to your registerd e-email. Simply click on the link to activate your subscription.

We're doing this to protect your privacy and ensure you successfully receive your e-mail alerts.

Add New Comment
Visitor (To avoid code verification, simply login or register with us. It is fast and free!)
*Verify code:
Tech Impact

Regional Roundup
Control this smart glass with the blink of an eye
K-Glass 2 detects users' eye movements to point the cursor to recognise computer icons or objects in the Internet, and uses winks for commands. The researchers call this interface the "i-Mouse."

GlobalFoundries extends grants to Singapore students
ARM, Tencent Games team up to improve mobile gaming

News | Products | Design Features | Regional Roundup | Tech Impact