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M1BYT > TECH 11.05.07 09:42l 56 Lines 2763 Bytes #999 (0) @ WW
BID : F84052M1BYT
Read: GUEST OE7FMI
Subj: Re: their is a pair of phone sockets o
Path: DB0FHN<DB0NOE<DB0GAP<DB0GPP<DB0OFI<DB0PRT<DB0LHR<F6KFT<LX0PAC<LX0NET<
DB0LJ<DB0RES<DK0WUE<GB7FCR
Sent: 070507/1013Z @:GB7FCR.#16.GBR.EU #:30009 [Blackpool] FBB-7.03a $:F84052M1
From: M1BYT@GB7FCR.#16.GBR.EU
To : TECH@WW
M1EVN Opined with considerable skill:-
> there is a great hum when I connect these to the cd in on my home stereo
> Question - how do get rid o' hum
Sorry, but it is not clear what you are connecting which causes the hum.
Usually this type of hum is caused by one or other of two possibilities.
First is a lack of earth screening, where the cores can pick up the mains hum
from the surrounding mains wiring in your home. Putting your hands near the
cables would help confirm whether this is the cause, as the intensity of the
hum will change. The fix is to find the defect and repair the screening.
Second it is what is known as an earth loop. If both appliances are supplied
with an earth from the mains and the earth screening is continuous [1], it is
possible for a tiny 50Hz mains current to flow along the screen and induce hum
current onto the input of an amplifier. Imagine a triangle with one unit
connected to one mains socket, the other unit connected to another socket -
with the earths of both units linked by the screening of the audio wire. 50Hz
current can flow around that triangle, thus generating the hum in the
amplifiers input.
[1] The screen is the outer braiding in small audio and coaxial cables,
intended to prevent external signals getting into (or out of) or being picked
up by the inner cores.
This is easy to confirm, just unplug from the mains socket the thing you are
connecting to the home stereo which is causing the hum - with the home stereo
turned on. If the hum stops, it is due to an earth loop.
There are two possible solutions to an earth loop. One way would be to
disconnect the mains earth from the thing you are connecting to the home
stereo, but this is certainly NOT recommended and there is also the
possibility of mains input filters causing there own problems. The correct
method is to cut through the screening around the audio wire, close to the
item which is causing the problem - thus breaking the loop.
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º 73 from Harry, M1BYT @ GB7FCR.#16.GBR.EU º
ÌÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ͹
º International Museums Weekend 16th+17th June 2007 º
º www.ukradioamateur.co.uk/imw/index.html º
ÌÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ͹
º The ARC's FL/RAE courses + Newcomers Introduction º
º to amateur radio:- http://www.ukradioamateur.co.uk/ º
ÌÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍËÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ͹
º QTH: Garforth, W. Yorks.. º Timed 10:10 º
º harry.m1byt@tiscali.co.uk º 07 May 07 º
ÈÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÊÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍͼ
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