Article : Andy Collinson
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Overview
You may all be familiar with this sound effect. You listen to music through an amplifier, then switch the source to a different input and hear a loud click or "thump" in the speaker system. Not all equipment is affected. Some high end audio systems are not affected, but intercoms,switching systems, and multi-channel systems are often affected.

The Cause
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 charge on the capacitors reach equilibrium. This takes only a fraction of a second and the sound you hear is the annoying "DC" thump. A typical circuit for audio switching is shown below. This is just an example.

dc thump diagram

At input 1, there is a standing dc voltage on the emitter of the transistor in system A. This is isolated by the 47u capacitor. The capacitor will either be discharged or partly charged. When the rotary switch is turned to position 1 a click will be heard as the capacitor charges through the switch and volume control on the following amplifier. A similar condition appears at switch position 3. System A represents part of a preamp while system B represents the output of an amplifier. What can be done to prevent this?

A Solution

One possible solution is shown above. The rotary switch is part of the right hand circuit. The switch contacts now have a dc path to ground. When the switch position is moved there is no charging or discharging of capacitors, as these have already been charged by the inclusion of the new resistors. The potential on the switch contacts is 0 volts, whilst the opposite site of the capacitor plates are at the potential of the preceding stage. I have shown different resistor values as each system has different coupling capacitors. The resistors can be soldered at the remote amplifier switch, alternatively they could be included at the output of the remote audio systems.

Important Note
If you are considering modifying any audio equipment, please be aware that by doing so, you may well invalidate your manufacturers warranty. Please read my disclaimer as I cannot be held responsible for any loss or damage of equipment.
Preventing 'DC' Thump
There is another even simpler way of eliminating 'DC' thump without modifying circuitry..... just turn the volume control down. Many instruction books always recommend this as good practice and I cannot agree more. Switching channels at full volume could, at worst, burn out your loudspeaker cones as well as giving you earache.
Transformer Systems
I use an old fashioned valve (tube) amplifier which is transformer coupled. There are no dc currents flowing in my amplifier, but I do use a remote preamp to listen in another room. The preamp output is biased to 7.5Vdc, but has been eliminated for the next stage by using a capacitor and resistor. Even so, I still make sure that I reduce the volume when switching inputs, and turn the volume to minimum when not in use.

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