Ground loops in any system are serious problems. Often they manifest
themselves as humm or buzz in audio, which to most, indicates that poor
grounding exists and thus more grounds are needed. Quite the opposite is
typically true! If one has a ground loop problem then it is reasonable to
conclude that more than one ground path exists. Adding some type of
isolation circuit, be it electronic photo-isolators or transformers, then
this action is clear evidence that one has used these means to break a
multiple ground path.
As to shielding, the shield in an audio path is such designed to shield only
and not be a current carrying conductor. In an unbalanced circuit,
correctly implemented, there needs to be 2 conductors and a shield. Oh yes,
I know that we've used the single conductor shielded cable for years in
audio. One conductor for audio HI or + , one conductor for audio LO or -
and shield. In best practice, connector wise, the shield is connected at
one end only and is connected to ground or audio LO or - if the two are the
same. Good practice indicates that the shield should be connected at the
receiving end or input of the device. This gets more complex in newer
technology where there is audio ground and digital ground being two separate
and isolated grounds. In a true balanced audio configuration then a 3rd
conductor is added. Those being used as audio HI or +, audio LO or -,
ground and shield. This is most important where microphones require any
power be it for the preamp or phantom power devices. In a true balanced
circuit there should be no ground current flowing thus the ground can be
eliminated and only the shield connected, but as shield at one end only.
For mikes requiring external power, then a negative return path must be
provided and the ground is mandatory. Keep this in mind, " shield is shield
and must not provide a complete current path" . Once realized, ground loops
are a thing of the past.
This subject is a topic to which I've found even many experienced audio
engineers have little understanding. In installations where audio ground,
digital ground, RF ground and lightning protection ground are required, the
level of technology to achieve such successfully approaches that of the more
complex digital circuits today. It look extremely simple. It isn't!
73
Bob, K4TAX
----- Original Message -----
From: "George Skoubis" <george.skoubis@verizon.net>
To: <tentec@contesting.com>
Sent: Monday, April 19, 2004 9:11 PM
Subject: RE: [TenTec] Orion Jupiter mic connections
I read the addendum to the owners manual and looked at figure 2-5 which
is supposed to follow. I didn't have a cable with 2 wires and a shield
so I went to my local Radio Shack and picked up 50 feet of 278-513 audio
cable (2 wires plus shield 50 ft for 7.99).
I wired the shield to the metal 4 pin connector body (wrapped the wire
around the screw that holds the 2 halves together. One mic lead to pin
1, one mic lead to pin 2. On the mic side I used a shielded connector,
connected the shield to the connector body, and the mic leads to the two
insulated wires.
I use a foot switch and connected it to the PTT connector on the back
of the rig through an interface box I built (connects serial port for
CW, audio in / out, FSK and accessory port on back of Orion).
I have a second floor shack 35 feet straight below my 3 element beam
and I have monitored my signal on CW, FSK, and SSB on a 2nd receiver on
10, 12, 15, 17, and 20 meters at 1500 watts (1000 FSK) and I don't have
problems with RF getting into the audio circuits this way. I've also
keyed SSB from the computer soundcard and listened to the output on the
2nd receiver and that also sounds good with no RF feedback (also tested
with amp 10-20 meters).
I do use audio isolation transformers in all the audio lines between
the rig and computer and some ferrite (all inside the interface box).
It only took a few hours of work to get everything wired and tested
(and a suggestion from a person on this reflector to use serial pin 4,
DTR as PTT and CW key).
I do have horrible feedback if I connect my computer amplifier (I use
a Soundstream car audio amplifier and some Boston Acoustics speakers for
my computer audio) simultaneously through a Y cord with my Audio In cord
into my interconnection interface that leads to the rig. I'm sure this
is a ground loop and I will buy two more isolation transformers when the
local Radio Shack gets some more in stock as the three I purchased were
all they had. I only use the voice keyer in contests so I leave the
audio out from the computer unplugged for now. It is neat to have the
sub receiver sound come out of the left speaker and the main receiver
out of the right.
I did have bad RFI with my Omni VI+ and wish I had taken the time to
"do it right" as I have with the Orion, I would have enjoyed the rig
more.
Take care and good luck whether you decide to keep the Orion or switch
to another rig.
73 de George / KF9YR
-----Original Message-----
From: tentec-bounces@contesting.com
[mailto:tentec-bounces@contesting.com] On Behalf Of Rob Atkinson, K5UJ
Sent: Monday, April 19, 2004 11:40 AM
To: tentec@contesting.com
Cc: k5uj@hotmail.com
Subject: [TenTec] Orion Jupiter mic connections
>The microphone connections on both rigs are unbalanced ( mic + and -
Grnd).
>Now there is nothing wrong with this setup mfrs. have been doing this
forever.
That's exactly the problem.
It would be great if manufacturers built rigs with balanced XLR audio
inputs
and isolation transformers, and a separate PTT jack. A lot of "RF in
the
audio" problems would vanish. Balanced audio lines are much less
susceptable to hum and RF pickup. All decent baseband audio gear is
designed to operate with balanced lines; professional audio and
broadcast
equipment are designed for them, for the aforementioned reasons. But
ham
radio manufacturers seem to have stalled on the idea that everyone
operates
voice with a PTT hand mic, and coiled cord right into the rig, as if
nothing
has changed in the past 15 years or so.
Rob Atkinson
K5UJ
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