[PLUG] Feedback insanity

[Larry Brigman]

On Fri, Jan 23, 2009 at 9:27 AM, John Jason Jordan <johnxj at comcast.net> wrote:
> On Fri, 23 Jan 2009 06:48:23 -0800
> Michael Ewan <mhewan1 at comcast.net> dijo:
>
>> Eric Wilhelm wrote:
>> > # from John Jason Jordan
>> > # on Thursday 22 January 2009 23:17:
>> >
>> >
>> >> What does exist in the location where the
>> >> sound comes from is the stereo receiver, a cable modem, an ethernet
>> >> switch and a router. And because of those devices there are quite a
>> >> lot of cables in the area.
>> >>
>> >
>> > Unplug them each and wait perhaps 10 seconds to allow capacitors to
>> > drain?  When the sound stops, take that device in the next room (or
>> > otherwise plug it in by itself) and see if you can replicate it.
>> >
>> > Are any of the wires wrapped around your stereo speaker wires?  But
>> > then, you said the sound isn't coming from the speakers. :-/
>> >
>> > And failing all of that:  how loud is it?  Can you hear it over the
>> > stereo?  Is the stereo loud enough?
>> >
>> > --Eric
>> >
>> Also check for RF interference, I get what sounds like a high pitched
>> whistle from the powered speakers on the PC, but listening closely it's
>> a radio station being picked up by the wiring. Keith I'm sure would have
>> a good explanation and solution to that problem.
>
> The sound is not a radio station. It's exactly that variable wailing
> whine you get when there is feedback in an old PA system. As I move my
> body around the pitch and volume varies. And from past experience with
> old PA systems, the position of the mic, speakers, and the mic gain are
> related to the whine.
>
> I thought Keith's suggestion to mute the mic on the desktop PC was
> excellent. Unfortunately, having muted the mic in alsamixer, the whine
> continues as before. For that matter, there is no microphone plugged in
> on the PC. :(
>
> It's really hard to troubleshoot because the slightest movement of my
> body affects it. When it starts I want to get up and walk over to the
> corner where it is coming from to feel the router, switch and modem
> just to see if I can sense vibration from them. But the minute I move
> the sound stops. Disconnecting any of them would kill my network. And I
> might have to wait for a long time to see if the sound continues
> because it is not always present.
>
> The weird thing is that I sometimes get the sound even when the stereo
> receiver is completely turned off. That means it has to be coming from
> the router, modem or switch. It is also (barely) possible that it is
> coming from the desktop PC, which is sitting on top of the table toward
> that end of the room. For it to be coming from the PC the sound would
> have to be echoing or somehow creating an auditory illusion that it is
> coming from the corner.
>
> The PC is just a $50 box from Fry's with pretty standard stuff in it.
> I'm not even sure the speaker in it is capable of making that much
> sound. I don't even know where the speaker in it is located, except
> that presumably it is toward the front of the case. That means it would
> be facing away from the corner where I heard the sound coming from.
>
> Troubleshooting something like this is maddening.

Combined ground loop w/radio reception?
Is all the equipment plugged in using grounded plugs/receptacles?
Anything using 2 prong cords that don't have one side larger than the other?