View Full Version : Loudspeaker issue.
8th Apr 2012, 02:17
We have a desktop computer with a couple of loudspeakers attached, and under the desk is a 'gang bar' into which half a dozen peripherals are plugged, printer, modem, telephone transformer for cordless phone, etc. etc. including a small desk light which is switched on by a rotary switch that is also a dimmer, this is currently fitted with a standard bayonet cap 60 watt bulb, not one of the new eco-fluro things, or halogen.
When we turn on this light the adjacent loudspeaker 'booms' for a few seconds, not a sizzling sound as if there might be a loose plug, or wire, but a repetitive 'boom' at the rate of approx 1/2 second interval repeated for about 10 seconds.
WTF ? Is this some interaction through the computer connection to the loudspeaker, in which case might it be injurious to the computer, or is it some totally different cause ? Is a first for me, and 'tis a puzzlement.
8th Apr 2012, 04:58
Count the exact number of booms and then we'll know more.
8th Apr 2012, 08:30
Having changed the lamp for one with a simple ON/OFF switch, all we have so far experienced is a very loud bass 'click' - which does suggest some sort of electrical interference, tho' whether the loudspeaker is finding that out for itself, or if the computer is transmitting it, is still to be ascertained.
Although a reason, and solution, would be better, we know we have an issue.
8th Apr 2012, 11:03
are switch dimmers like that resistance or chopper / thyristor controlled? If chopper I can just about see how it could create feedback through the mains though surely it would be a lot lot faster?
8th Apr 2012, 11:42
If you switch off the speakers and listen via headphones you might determine if it's via the PC or just the speakers.
I've often noticed a sound through the speakers when an adjacent switch is toggled - I've always assumed it's via the PC, but then I often hear mobile phone interference, so it's possible that EMR is affecting the speaker directly.
8th Apr 2012, 12:27
First things first, plug the lamp into somewhere else and see whether the booms disappear.
Also, try moving the lamp nearer/further to/from the speakers, and see whether you can work out which component is giving the interference. My money's on the variable resistor.
8th Apr 2012, 13:05
I don't have a specific diagnosis or a solution, but the Altec Lansing speakers (which contain an audio amplifier) and serve my desktop machine are annoyingly vulnerable to interference from nearby radio frequency sources. My own HF and VHF amateur radio transmissions are the most obvious - these have been minimized by a dedicated grounding system for the radio equipment. However even parking the cordless telephone in its charging cradle in the wrong place (admittedly close to the 'control' speaker) will produce noises in the speakers obviously coming from that equipment.
Your problems suggests a similar set of circumstances. I'm reluctant to suggest a possible specific solution because the easiest one involves the fitting of a suppression capacitor to the switch of the offending lamp and this can be tricky and downright dangerous if not done correctly - mains voltages are probably involved.
The other possibility is to suppress interfering signal sources at the input to the amplifier within the speaker system - assuming it's that kind of arrangement. This again requires specific components and identification of where they are needed - difficult to direct without knowing the specifics of the equipment. In my observation it can also be difficult to get to the internals of some of the speaker systems around, my own being a good example.
As for the succession of booms noted in the original report; that is a real puzzle but seems to be linked somehow to the circuitry of that particular lamp, however I'm at a loss to explain it beyond that observation.
It's annoying that in this day and age, when problems like this can be predicted and usually prevented by the installation of quite low cost components in the production line of both 'transmitting' and especially 'receiving' equipment, that this isn't done. :\
8th Apr 2012, 14:01
I would suggest the sound does not go via the computer, I can't explain it technically, but in my experience it is quite common for speakers to get interference when various lights/electrical appliances are turned on/off.
PS: Where's West Lakes? He may know the answer!:ok:
8th Apr 2012, 15:09
Peripheral to this, my desk speakers at work, powered up but computer not necessarily on, would pick up RT from my controllers in the adjacent tower. It unnerved them to realise I could listen in from my office :}
....pick up RT from my controllers in the adjacent tower
I'll pick up the occasional half of a cell-phone conversation on my desktop's speakers. Thought I was hearing voices the first time it happened. Turned out I was.
8th Apr 2012, 21:56
Thanks, chaps. Will work through the various suggestions.