Removing the DC Thump in = Audio=20 Circuits

Article : Andy=20 Collinson
Email=20 me

Overview
You may = all be=20 familiar with this sound effect. You listen to music through an = amplifier, then=20 switch the source to a different input and hear a loud click or "thump" = in the=20 speaker system. Not all equipment is affected. Some high end audio = systems are not affected, but intercoms,switching systems, and multi-channel = systems are=20 often affected.

The Cause
Most audio outputs = have some=20 standing DC voltage which is separated by a capacitor. The capacitor = will block=20 this DC voltage while allowing the AC or audio to pass. Audio components = using=20 transformer coupling are not affected. The input to an amplifier may = have a=20 capacitive input or just a volume control. When switching inputs, charge = is=20 transferred through the switch as the charge on the capacitors reach=20 equilibrium. This takes only a fraction of a second and the sound you = hear is=20 the annoying "DC" thump. A typical circuit for audio switching is shown = below.=20 This is just an example.=20

At input 1, there is a standing dc voltage on the emitter of the = transistor=20 in system A. This is isolated by the 47u capacitor. The capacitor will = either be=20 discharged or partly charged. When the rotary switch is turned to = position 1 a=20 click will be heard as the capacitor charges through the switch and = volume=20 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=20 output of an amplifier. What can be done to prevent this?

A=20 Solution

One possible solution is shown above. The rotary switch is part of = the right=20 hand circuit. The switch contacts now have a dc path to ground. When the = switch=20 position is moved there is no charging or discharging of capacitors, as = these=20 have already been charged by the inclusion of the new resistors. The = potential=20 on the switch contacts is 0 volts, whilst the opposite site of the = capacitor =20 plates are at the potential of the preceding stage. I have shown = different=20 resistor values as each system has different coupling capacitors. The = resistors =20 can be soldered at the remote amplifier switch, alternatively they could = be=20 included at the output of the remote audio systems.

Important Note
If you are considering = modifying any=20 audio equipment, please be aware that by doing so, you may well = invalidate your=20 manufacturers warranty. Please read my disclaimer=20 as I cannot be held responsible for any loss or damage of = equipment.
Preventing 'DC' Thump
There is another even = simpler=20 way of eliminating 'DC' thump without modifying circuitry..... just turn = the=20 volume control down. Many instruction books always recommend this as = good =20 practice and I cannot agree more. Switching channels at full volume = could, at=20 worst, burn out your loudspeaker cones as well as giving you = earache.
Transformer Systems
I use an old fashioned = valve (tube)=20 amplifier which is transformer coupled. There are no dc currents flowing = in my=20 amplifier, but I do use a remote preamp to listen in another room. The = preamp=20 output is biased to 7.5Vdc, but has been eliminated for the next stage = by using=20 a capacitor and resistor. Even so, I still make sure that I reduce the = volume =20 when switching inputs, and turn the volume to minimum when not in = use.

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