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Author Topic: Question about 3 conductor cable wiring for audio cable  (Read 4927 times)
KU4UV
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Posts: 376




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« on: March 19, 2013, 06:52:30 PM »

OK, I hope this doesn't sound like a stupid question, and perhaps the topic has been covered before, I don't know.  I have a 3 conductor cable that I use for my sound card interface with my radio.  I made my own cable(s) using a 3-conductor cable that I purchased from Radio Shack.  The cable has two wires contained within a braided wire.  The way I currently have the connectors soldered to the phono plugs, I am just using the two inner wires, and not using the shield.  My question is, should I re-do the soldering on the connectors and use the shield as the wire that connects to the shield of the connector, OR am I ok with the way I currently have the plugs wired?  Wouldn't the wires inside the shield be "shielded" from any RFI pickup, since the braid essentially acts as a Faraday cage, even though it not grounded to anything?  I have had no problems so far in the way I have this wired.  Any suggestions?

73,
Mike KU4UV
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AA4PB
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« Reply #1 on: March 19, 2013, 07:05:35 PM »

The shield needs to be grounded in order to exibit any effective shielding of the inner conductors. You can leave the grounded inner conductor in place if desired, but also connect the shield to the ground connection at each end.

The two conductor with shield cable is intended for use in balanced microphone circuits where neither side of the microphone is connected to ground but the shield is grounded.
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KM3K
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« Reply #2 on: March 20, 2013, 06:00:42 AM »

..., but also connect the shield to the ground connection at each end.
I've not had any real experience in this area but I do recall reading in many places that only one side of the shield should be connected to ground so as to avoid ground-loop currents.
If it were my set-up, I'd ground just one side and try the cable; then flip the assembled-cable around the other way and see if things got better or worse.
73 Jerry KM3K
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AA4PB
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« Reply #3 on: March 20, 2013, 06:29:10 AM »

Connecting the shield at only one end (called "telescoping shield") is only effective with a balanced circuit. With an unbalanced circuit you MUST have the common ground return path in order to complete the circuit. Telescoping shields are often used in professional audio installations when there is a problem with AC leakage currents flowing through the shields on long cable runs.
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KM3K
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« Reply #4 on: March 20, 2013, 07:25:30 AM »

With an unbalanced circuit you MUST have the common ground return path in order to complete the circuit.
Seems to me that Mike already has his circuit completed.
His next step is to only ground the shield at one end.
73 Jerry KM3K
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AA4PB
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« Reply #5 on: March 20, 2013, 08:22:15 AM »

I don't see any point in "telescoping" the shield when you have other conductors in the cable carrying ground currents.
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KU4UV
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« Reply #6 on: March 20, 2013, 09:25:44 AM »

Thanks guys.

I have been using the cable with a SCI-6 interface kit from Unified Microsystems and not had any  problems with RF getting into anything.  The interface kit uses isolation on all cables from what I understand, so there shouldn't be any ground loops to begin with, but I never know what else I might need to use this cable for in the future, so I want to get it right.  I too have heard about grounding the shield on one plug, but leaving it ungrounded on the other.  I used to work at a radio station, as well as run the audio board for newscasts and productions at WTVQ-TV, the ABC affiliate in Lexington, KY.  I should KNOW some of this stuff, but never really had to fool with having to build audio cables or anything, just enough knowledge to be dangerous, but far from an expert.  Thanks gang!

73,
Mike KU4UV
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KM3K
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« Reply #7 on: March 20, 2013, 11:23:18 AM »

I don't see any point in "telescoping" the shield when you have other conductors in the cable carrying ground currents.
Don,
The reason for grounding the shield is to drain off to ground any charge induced onto that conductor by RFI.
Mike,
Had I not been interrupted by taking the XYL to lunch, I would have suggested that you really need an isolation-transformer to prevent ground-loops. I just went to the site that explains your kit and I see you will be well isolated with transformers on the audio lines.
Here is why those transformers are needed and why any shield should be grounded only at one end:
1. I assume your computer has a three-wire AC-cord, so its chassis is "grounded".
2. I assume the same for your transceiver.
3. Theoretically, both pieces of gear are at ground potential; but in the real world, this is not often the case, especially so if the two items are not plugged into the same power-strip.
4. Since it is likely there is some difference in potential on those "grounded" chassis, if you connect your shield to each chassis-ground, you'll get a current thru that shield and "that ain't good". That current can couple into the wires inside your shield and "that ain't good" because it will move right along with your desired signal to cause you grief.
5. Lets say that you did not have the shield and had no transformers. That same current would be right on your ground lead and "that ain't good". The transformers break that current.
Hope this makes sense; if not, I can elaborate some more at whatever sticky point there is.
73 Jerry KM3K
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AA4PB
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« Reply #8 on: March 20, 2013, 11:45:41 AM »

Jerry, take a look at the SCI-6 schematic on their web page. The shields don't connect from the radio chassis to the computer chassis. They connect from the radio chassis to one SCI-6 isolation transformer winding and from the other winding to the computer chassis. Even if the radio chassis and the computer chassis are at different potentials, there is no way for current to flow through the shield - even if the shield is connected to the connector shell at both ends. The isolation transformer in the SCI-6 breaks that path.

If he has a two-conductor cable with one conductor connected to the shell at both ends then it probably won't make any difference if he floats one end of the shield. However, if he floats one end of the shield on a single-conductor cable then he'll have no return path and no audio through the interface.

When I've installed balanced pro audio cables with telescoping shields to prevent ground loops I've always left the receive end grounded and floated the other end. Sometimes it is necessary to place a small value ceramic capacitor between ground and the floating end to ensure that it the shield has an RF ground to the equipment chassis to prevent RFI from AM broadcast stations.
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KM3K
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« Reply #9 on: March 20, 2013, 07:32:53 PM »

Jerry, take a look at the SCI-6 schematic on their web page. The shields don't connect from the radio chassis to the computer chassis.
Don, the schematic has nothing there about shields.
When I've installed balanced pro audio cables.....
The schematic of immediate concern has only unbalanced circuits, so I believe it is prudent to stay on topic. Smiley
Sometimes it is necessary to place a small value ceramic capacitor between ground and the floating end to ensure that it the shield has an RF ground to the equipment chassis to prevent RFI from AM broadcast stations.
No comment other than I winced when I read it; I'd have to try out the idea. Actually, if there'd be a RF concern, I'd rather put RF-bypass-caps on the audio-lines to chassis-ground.
Jerry, take a look at the SCI-6 schematic on their web page.
I think note# 2 on the drawing is significant.

Mike, there is one item that I'd do automatically that has not been mentioned.
I'd put a twist in your two-wire cable before putting it into the shield.
I'd go with a twist per inch (therefore 12 twists per foot); others may have different ideas on the amount of twists per foot.

73 Jerry KM3K
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KU4UV
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« Reply #10 on: March 21, 2013, 07:55:24 AM »

Thanks again everyone, lots of good information.

73,
Mike KU4UV
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KD0REQ
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« Reply #11 on: March 26, 2013, 06:48:51 PM »

in classic commercial broadcasting, all the rack cables (shielded back in the day, no outer jacket within racking) are bound together with lacing cord for no ground loops.  there are frame grounds at the distribution equipment, carrying through at the patch panels.  there is copper bonding strapping rack to rack to rack, capable of carrying all the AC feed current in that area.

in today's broadcasting, they are now using single-conductor unbalanced audio now because the equipment is basically uprated consumer stuff... computer video/audio storage, Ether streams, etc.  grounding became a major issue that no longer responded to the old rules carried over by WEco and RCA from the 3-wire telephone days.  this happened about the time I lost all my ties to the industry due to retirements, my change of field, etc.  my understanding is that it's all grounded on both ends, and the big ol' copper strapping tying the rack frames together as of old makes it work.

the "ground one side only" stuff really comes from commercial audio, as in music, concerts, etc., because there is no master bonding between backstage, front stage, the mix board, etc.  if they ran thick bonding straps which are a much more fascinating highway for current, they wouldn't have to worry about ground loops, or failed X/Y capacitors on the amps putting line voltage on the mic windscreen, etc.
« Last Edit: March 26, 2013, 06:55:02 PM by KD0REQ » Logged
G3RZP
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« Reply #12 on: March 27, 2013, 04:37:59 AM »

When using a lead to an external ground, there are circumstances where the ground lead requires shielding. In that case, the shield is connected at the external ground end only.
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AD6KA
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« Reply #13 on: April 01, 2013, 12:24:07 PM »

Quote
"I made my own cable(s) using a 3-conductor cable that I purchased from Radio Shack."

FWIW, when making audio and control cables for the shack,
I've always had better results using this shielded mic cable from
All Electronics. The RS audio cable's "shield" is pretty skint, not
even braided, not to mention the premade cables are expensive.
All Electronics also sells almost any connector you might need.
 
http://www.allelectronics.com/make-a-store/item/MC-2/2-CONDUCTOR-SHIELDED-MICROPHONE-CABLE/1.html

YMMV...73, Ken AD6KA
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