A week or so ago, I accidentally damaged one of the speakers of my audio system, most probably because of a loud and high-pitched distortion (noise? - it's basically a "pop" when connecting/disconnecting while plugged in) that happened when the system was connected or disconnected from my computer while turned on. I am getting a replacement system, but would want to prevent this problem from occurring once again.

The sound is difficult to describe, but one knowledgeable person I asked replied that it was a full volume DC offset spike. Is this the actual technical term? If so (or not) is there a way to prevent it in the future (besides being really, really, careful?) Electrical problem? The set was directly connected to a wall socket. However, the other is full of interconnected power bars connected to a computer, two routers, three monitors, and an external HDD.

Would a cheater plug or surge/spike protector prevent the problem from occurring once again? The previous system was directly connected to the wall socket.



1 Answer 1


Your friend may indeed be correct.

Here's what happens in audio circuit inputs and outputs that aren't properly designed:

Most audio inputs and outputs are capacitively-coupled. This eliminates DC offset that might be coming from the electronics. In the old days where everything was discrete transistors (or Tubes!), these coupling capacitors were absolutely necessary because the input and output nodes within the circuit were usually nowhere near ground potential.

Modern audio electronic circuits make use of op-amps and such. If the op-amp is supplied with a bipolar power supply (equal value (+) & (-) rails), the input and output nodes within the circuit are usually near ground potential. There is usually offset voltage errors but these are usually quite small - ranging from zero volts up to a few tens of mV.

Because audio electronics with bipolar supplies is usually of high quality, the designers include DC isolation capacitors WITH bleeder resistors to ground so that the net DC voltage at any outside-world connector is zero.

However, most inexpensive audio equipment uses only a single supply voltage, with a virtual-earth reference rail set at half the supply voltage. The input and output nodes within the circuit thus sit at about half the supply voltage.

So: the designers include coupling capacitors to block the DC but pass the AC signal.

Here's where they mess up: they often don't include a bleeder resistor at the outside-world end of the capacitor. The capacitor has some leakage, so any charge that might be on it leaks away when nothing is connected to the in / out connector. That is: the DC voltage on the capacitor is the same at both ends of the capacitor.

Because the circuit-side of the capacitor is sitting at half the supply voltage, the outside-world end of the cap is ALSO sitting at half the supply voltage.

Now you go to plug something into that connector and see a HUGE spike that lasts as long as it takes for the capacitor charge to equalize.

The resulting damage is generally NOT covered by warranty.

  • \$\begingroup\$ Thanks for your reply. However, if I reading you correctly, the system is far from cheap. It's an Edifier S730. Any way to prevent the "popping" sound, in layman terms? \$\endgroup\$
    – FM931
    Commented Mar 28, 2015 at 0:05
  • \$\begingroup\$ Yep. Add resistors from each audio signal (hot) line to the (ground) line at each connector. 10k is reasonable but you can go higher if needed. Note that you need to do this to the equipment at each end of the cable. Also note that the resistors are added to the equipment, NOT the cable. \$\endgroup\$ Commented Mar 28, 2015 at 5:05

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