Sizing a Headphone Output Coupling Capacitor for the Actual Load

A coupling capacitor that passes bass into a line input may strongly attenuate the same signal into headphones. The load resistance can differ by orders of magnitude. Identify whether the output requires DC blocking, then calculate the high pass response with the actual supported load.

Confirm the output architecture

Determine whether the amplifier output sits at a DC bias, operates around ground using another supply arrangement, or drives both load terminals. Do not add or remove a series capacitor merely because another headphone product uses one.

The TI headphone coupling capacitor application note shows the low frequency relationship between output capacitance and load impedance. Its circuit options also illustrate why the return connection must be considered as part of the complete stereo output.

Calculate the initial corner

For a simple series capacitor feeding a resistive load with negligible source resistance, the corner is one divided by two pi, load resistance, and capacitance. Use that model as a starting point and include additional output resistance when it is significant.

A hypothetical 220 microfarad capacitor into 32 ohms gives a corner near 22.6 Hz. At the corner, the amplitude is already about three decibels below the high frequency response. Therefore, placing the corner at the lowest wanted frequency does not imply a flat response down to that frequency.

Include variation and stereo interaction

Check the minimum effective capacitance and the supported headphone impedance range. A smaller load lowers the time constant and raises the corner. A real headphone also has frequency dependent impedance, so verify its measured response rather than relying exclusively on a resistor model.

If the channels share any return impedance or coupling element, drive one channel while observing the other. A change that saves a capacitor can introduce interaction between channels and requires a topology specific assessment.

Verify voltage and transient behavior

Check capacitor polarity and voltage across startup, normal playback, shutdown, and connection changes. Use a capacitor technology and rating appropriate to the expected DC bias and AC signal; do not infer polarity only from the connector label.

Measure low frequency gain with the intended load at moderate level, then repeat near the required maximum output to check current delivery and distortion. A small signal response measurement cannot establish the amplifier’s large signal capability.

Save the output architecture, load range, capacitance assumptions, response curves, and startup behavior. Choose the value from an explicit allowed attenuation at the lowest useful frequency. The goal is a verified loaded output response, rather than a capacitor that appears large enough by comparison with unrelated line level circuitry.