[Boatanchors] 2nd Newbie Q

[Richard Knoppow]

----- Original Message ----- 
From: "Brian Clarke" <brianclarke01 at optusnet.com.au>
To: "Dennis DuVall" <duvallddennis at gmail.com>; "W2HX" 
<w2hx at w2hx.com>
Cc: "qth.boat" <boatanchors at mailman.qth.net>
Sent: Thursday, September 11, 2014 4:20 AM
Subject: Re: [Boatanchors] 2nd Newbie Q


> For really good quality interconnects at audio frequency, 
> you should run balanced lines, ie, two-core shielded 
> cable. In this way the shield performs only one function - 
> shielding. The two internal cables then carry the 
> differential that is your audio signal. If you try to get 
> the outer shield to perform two functions, ie, shielding 
> and carrying one part of the differential signal, then any 
> mains transformer, motor or electrical noise generator 
> within cooee will induce AC Voltages in the outer shield. 
> This Voltage on the shield will be seen at the 'receiving 
> end' as signal; we will hear it amplified as hum and 
> noise.
>
> Coaxial cable for carrying audio signal is OK for very 
> short lengths inside a chassis, or inside one metal 
> cabinet where there are no enclosed mains transformers. 
> How to use twin-core shielded? At the sensitive 
> (receiving) end, connect the shield and one of the 
> internal pair (eg, black) to the ground and the other of 
> the internal pair (eg, white) to the input 'hot' terminal. 
> At the other (sending) end, connect the black wire to 
> ground and the white wire to the signal output. Leave the 
> shield unconnected.
>
> If after all this, you still have hum and noise, you may 
> need to lift the mains ground lead above the chassis with 
> say a 100 Ohm resistor. This will give you safety and it 
> will significantly attenuate any ground-loop induced 
> Voltage. The next step after this, is to use truly 
> floating differential circuitry, such as transformers. 
> This is the kind of thin,king applied in high quality 
> audio recording studios,
>
> At RF, different rules apply. Here, you should ground the 
> shield at both ends and at somewhere between every 
> one-twentieth and one-fiftieth of a wavelength along the 
> coaxial cable. In this case, you are making the coaxial 
> cable part of the ground plane.
>
> 73 de Brian, VK2GCE.
>
    Using shielded, balanced lines is also what I was taught 
but its interesting that Western Electric promoted the use 
of unbalanced coaxial lines for microphones in the brochures 
for their speech input consoles, for instance the famous 
25B. WE used 50 ohm mic lines but, of course, microphone 
lines are not terminated in the nominal impedance, rather 
its the source (the microphone) that determines the 
impedance.  I have always suspected they did this in the 25B 
because it had switching ahead of the input transformers so 
that providing for balanced lines would have considerably 
complicated the wiring.  Its also true that a lot of early 
broadcast and other installations used unshielded twisted 
pair wiring but I think this is vulnerable to a lot of 
induced noise.
    Shielded three conductor lines are aparently the best 
for audio use with the ground carried through the third wire 
and the shield connected only at one end, usually the signal 
receiving end.  Of course, this will not work for coaxial 
cable of any sort including the RCA type cables commonly 
used for audio and video.


--
Richard Knoppow
Los Angeles
WB6KBL
dickburk at ix.netcom.com