Newsgroups: rec.audio.tech
Subject: Re: Correct way to wire cable to avoid hum?
References: <[email protected]>
Jeremy Taylor  writes:

> Cutting down an example to it's simplest form, how should you make up a
> mono-to-mono cable with phone jack plugs?
> 
> The cable is to link 2 mains powered devices, line out to line in.
> 
> The simple cable has a single core and a conductive shield. Normally at
> each end you would wire the shield to the plug's sleeve terminal and the
> centre conductor to the tip terminal.

This is a correct way to 
 
> What if you specifically wanted to avoid up caused by unequal potentials
> at the common/ground points on the sockets of the 2 devices? I have seen
> it suggested that you should disconnect and insulate the shield from the
> sleeve terminal at one end of the cable. Is this correct? 

Some references recommend this. And this can sometimes work,
although not always. Sometimes disconencting the shield can
make thigns much worse. 

OFr better ideas read:
http://www.epanorama.net/documents/groundloop/index.html

> Is this bad?

If you disconnect the shield and use the cable to connect any ungrounded
device the connection does not work an is very noisy.

A system with ground disconnected is potentially very sentitive
in picking up radio frequency interference.

> If it is ok, will the shield still act adequately as a means of avoiding
> interference?

Disconnecting the shield from one end will very much reduce it's
effectiveness of avoiding interference (especially RFI). 
Sometimes reduces shield

> Or should 2-core cable be used, the 2 wires going to sleeve and tip at
> each end, and the shield present but unconnected? This won't be
> particularly effective at retarting interference, will it?

This approach is not good. If you want to build this kind f
cable with 2-core cable, then connect as you described but
connect the shield to the plug sleeve or other ground.

> Is there a better way of joining these 2 devices up?

Read http://www.epanorama.net/documents/groundloop/index.html

> What if one of the devices was not mains powered but battery powered?
> Any change to the cabling? 

If you use a cable where one end of shield is disconnected
(only the one central wire in cable is connected through) then
this does not work with battery powered device.�The two-core
cable I described above works with battery powered. 
And a cable where the shield is wired to the plug's sleeve terminal 
on both ends and the centre conductor to the tip terminal on both
ends will work perfectly well on battery powered devices
and other devices which are not grounded.

> Or any specific limitations like "the shield
> should be connected to the plug which is inserted to the battery powered
> device"?

Shield should be connected on both ends with battery powered
devices and other "floating" (non-grounded) devices.
 
> Also: Wiring up a mono to stereo cable: Is it bad to wire up the left
> and right tip/ring terminals together?

From one mono source to stereo input this is a correct wiring.

For other direction (stereo output to mono input) I do not
recommend this kind of cable.

> Wiring up 2 mono devices to a stereo device, one to left, one to right,
> taking into account the advice you give me above, where the mono devices
> may be powered by mains or battery (tell me if I need to handle these
> differently) - how should I wire up the grounds in this instance?

A cable where the signal wire and shields are connected on both
ends is the right general purpose cable. It will work on 
battery powered devices nicely. And t will work also with
grounded mains powered devices when the device (input and output)
are powered from same power source (same mains outlet / extension cord).

If you encounter problems like noise, then an audio signal isolation
transformer is a good general purpose cure for inter-chassis ground
noise problems. And for more details read
http://www.epanorama.net/documents/groundloop/index.html

> If I was going to put a 10:1 attenuation circuit in a signal path
> between 2 devices, should I change the way I treat the ground/shield?

No changes needed for this.
 
> Background: I am trying to wire up a radio receiver to a PC's soundcard.
> The radio will at times be a transmitter, so shielding to protect from
> RF interference is an issue. 

You need a cable where the shield is on both ends conneected. DEFINATELY.
Wrapping the cable around a ferrite bar or toroid is a good idea
to reduce interference (place the ferrite near computer input).
On radio applications an isolation transformer on audio line
is also a good idea (you can stop worrying on the ground
potentials and you redice some RF problems).

> The object is to utilise the PC's soundcard
> to decode packet data with amateur radios. The reason for enquiring here
> is the expertise with cabling, grounds, and audio that is here, as the
> amateur radio side of things is to all intents and purposes irrelevant
> except to note that shielding is important. Radios may be mains powered
> or battery powered. Some software can handle a radio on each channel,
> hence the query regarding wiring up 2 mono lines to a stereo plug. Some
> radios put out quite a strong audio line signal, so sometimes an
> attenution circuit is needed - nominally something line this:
> ---signalline-------------/\/100k\/\----------
>                                        |
>                                        >
> radio                                 1k     PC
>                                        <
>                                        |
> ----ground/shield-----------------------------
> 
> I appreciate any help and advice you can give
> Jeremy Taylor

-- 
Tomi Engdahl (http://www.iki.fi/then/)
Take a look at my electronics web links and documents at 
http://www.epanorama.net/