Removing the "DC Thump" from Audio Circuits

Posted on May 25, 2012

You may be familiar with this effect. You switch audio equipment such as an amplifier to a different input and there is a loud click or `thump` in the speaker system. Not all equipment is affected. Some high end audio systems never suffer, but intercoms switching systems are often affected. Most audio outputs have some standing DC voltage which is separated by a capacitor. The capacitor will block this DC voltage while allowing the AC or audio to pass. Audio components using transformer coupling are not affected. The input to an amplifier may have a capacitive input or just a volume control. When switching inputs, charge is transferred through the switch as the input or output capacitors or both reach a state of equilibrium.

Removing the
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This takes only a fraction of a second and what you hear is the annoying dc thump. A typical circuit for audio switching is shown below. This is just an example. At input 1, there is a standing dc voltage on the emitter of the transistor in system A. Thi

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