When I shut down my computer and the room is silent, I notice the speakers humming. Doesn't seem to affect the sound otherwise. Lots of crossed wires on the floor behind the computer, but I've tried different ways to isolate the speaker cables and power cord, to no avail. Logitech Z2300 speaker system. Any ideas on why I'm getting this hum, and how to stop it? Thanks.
Is your computer near to one of those fluorescent lamps?
Hey Ken, actually "Hum" is not a grounding issue, that's called a "Hiss", "Hum" is an oscillation of the AC at the frequency of mains electricity (60 or 50Hz)
I'm definitely sure it's not a grounding issue because that would sound like a high pitch hiss that could go away with touching the "non (or poor) grounded" spot...
Zigzag3143 and Punkster: Thanks. No fluorescents near the computer or in the room, in fact. I had the speakers power cord plugged into a grounded strip at first--still humming. Then I plugged into a wall socket--still humming.
Hum can be caused by a problem in the power supply allowing noise to pass through and be amplified. This is a filtering problem, and the common way to eliminate it is to add a coil (choke) and capacitor as follows (the use of ferrite beads on the power leads may also prove benficial):
A standard PSU as found in speakers consists of a transformer, a rectifier and some form of power conditioning/smoothing (filter). This filter is often just a single capacitor across the rectifier output. The job of this capacitor is to store energy and to supply it to the rest of the circuitry. With reference to the above image, the dotted waveform represents the output from the rectifier. As you can see, this is far from smooth. It is the job of the capacitor to store some of the energy that is supplied and to release it to the rest of the circuitry when voltage level of the rectifier output drops (as it does on a cyclic basis). Even the PSU found in computers suffers from this, although the effects are very much minimised because of the higher frequencies involved (the higher the frequency of the voltage pulses on the input, the smoother the filtered output will be).
Again with reference to the above image, the closer to horizontal that we can get the filtered output, the less noise will be imparted to the rest of the circuitry. This is done by selecting appropriate value components for L and C. In addition, the use of ferrite beads on the input/output leads of the filter will prove beneficial.
Amplifier/speaker hiss is usually caused by having a floating input with the gain turned right up. The effect can be exacerbated if excess hum is also present.
One way to reduce this is to ensure that the amplifier/speakers is plugged into the same power strip as the rest of the system and to ensure that the gain is not set too high.
Dwarf nailed it..... most inexpensive computer speakers have shoddy power supplys I corrected 90% of the hum in a cheap set just by beefing up the filter capacitors.
Ap
While no expert the last 5 articles I have read including the IEEE say
A one-volt difference, while seemingly insignificant, can cause large loop currents to flow through the ground conductor of the signal cable. Depending on the circuit configuration of the audio hardware, the audio signal impedance and the impedance of the signal cable ground, the currents may cause induced voltages in the audio circuitry. This is a source of the familiar 'hum' problem.
OK, zigzag3143. And how do I solve the problem? Thanks. I get the feeling that curing this problem could be more complicated than I suspected.
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