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-   -   Wire runs within the chassis. Shielding properly. (http://www.diyaudio.com/forums/chip-amps/228956-wire-runs-within-chassis-shielding-properly.html)

FenderBender11 29th January 2013 09:31 PM

Wire runs within the chassis. Shielding properly.
 
1 Attachment(s)
Hello everyone,

My chipamp (LM3886 x 2 + LM4780 (sub)) has been going strong for about a year now. I've always had a few quirks with it though. I ran out of wire when I did the initial build so I used some really old cheap and nasty unshielded twisted pair wire for the signal. Probably not the best of ideas. I've also been getting a little buzzing on high frequencies on one of the channels.

So I bought some wire from Markertek. I got some good quality Mogami stuff and I am going to rewire the parts I skimped on.

My question is: For each wire run between different components do you attach the wires shield to ground on just the "input" side. For example, my signal comes from 2 RCA jacks to a 4PDT switch (to headphone amp and to power amp), then to subwoofer crossover board. I run wire from the crossover's INPUT terminals(where I just inserted the wire, not on the crossover's output) to the Left and Right channel amp.

So the main wire runs are RCA --> Switch --> Subwoofer crossover --> Left and Right channel.

If you didn't get what I meant with the input terminals and subwoofer thing, I'm essentially just using the input terminals to splice the wire so that I can have the signal go to the subwoofer board AND to the left and right channel.

Here's a partial drawing. Where do the cable shield grounds get attached?

FenderBender11 30th January 2013 07:52 PM

I hate asking again because I know everyone here is a "volunteer", but any ideas?

Frank Berry 30th January 2013 07:58 PM

You don't want to connect it at more than one point. I have always connected the shield at the output end but I don't think that it matters greatly.

FenderBender11 30th January 2013 08:58 PM

Quote:

Originally Posted by Frank Berry (Post 3349110)
You don't want to connect it at more than one point. I have always connected the shield at the output end but I don't think that it matters greatly.

So the shield itself will only ever get soldered once?

Frank Berry 31st January 2013 04:27 PM

If you have two inner conductors, the shield should be connected at one end only.
Connecting the shield at both ends can result in a nasty ground loop.

FenderBender11 31st January 2013 09:17 PM

Quote:

Originally Posted by Frank Berry (Post 3350263)
If you have two inner conductors, the shield should be connected at one end only.
Connecting the shield at both ends can result in a nasty ground loop.

Right I understand that if there was one single run. So if I had multiple runs, as in the signal was hooked up to the switch as I said....The shield still should only be at one...

Keith Cary 1st February 2013 04:57 PM

shielding ground
 
If the input jack is grounded on the chassis and the amp is also grounded on the chassis then right, you shouldn't also have the shielding making a second path. So if you have a switch mid-way, I'd ground the shield at the input jack, but not at the switch and for the leg after the switch I'd ground it at the amp but not at the switch. I'm assuming (I know, I shouldn't assume) the switch will also have its body grounded at the chassis. But if not, then you should ground the switch body to one side or the other. So, everything grounded somewhere but no complete second path, no ground loops.

gootee 2nd February 2013 11:38 PM

You don't want the signal ground connected to the chassis, at the jacks. It should only connect to the input resistor just before the chipamp (from which it should have its own separate conductor to star ground). Otherwise, you've created a large "enclosed loop area" between the signal and signal ground conductors, which would make a very good antenna, for anything from AC hum to RF.

All RCA jacks should be isolated from the chassis, and from any other ground.

Anyway, I would use shielded twisted pair, at least internally, with the shield connected to chassis at one end only (and NOT connected to the signal ground, or any part of the RCA jack).

I don't understand your diagram. If you use isolated jacks (insulated from chassis) as you should, and proper internal grounding, and your cables have only a center conductor plus a shield that doubles as the signal ground, then you would want to connect the shield at both ends, everywhere.

FenderBender11 4th February 2013 07:19 PM

Quote:

Originally Posted by gootee (Post 3353557)
You don't want the signal ground connected to the chassis, at the jacks. It should only connect to the input resistor just before the chipamp (from which it should have its own separate conductor to star ground). Otherwise, you've created a large "enclosed loop area" between the signal and signal ground conductors, which would make a very good antenna, for anything from AC hum to RF.

All RCA jacks should be isolated from the chassis, and from any other ground.

Anyway, I would use shielded twisted pair, at least internally, with the shield connected to chassis at one end only (and NOT connected to the signal ground, or any part of the RCA jack).

I don't understand your diagram. If you use isolated jacks (insulated from chassis) as you should, and proper internal grounding, and your cables have only a center conductor plus a shield that doubles as the signal ground, then you would want to connect the shield at both ends, everywhere.

Sorry for the diagram.

I have a wooden chassis...I know terrible for RF suppression. Right now, I have the shield connected to the signal ground only at the RCA inputs. No metal chassis so yes it is isolated.

Should I do as Keith says but not grounding to earth?

Thanks.

gootee 5th February 2013 04:56 AM

The main thing is that the signal and signal ground should never get separated. The signal ground needs to stay ONLY right next to the signal, all the way from the source output to the input shunt resistor at the first active device in whatever amplifier it goes to.

If the ground connects to anything else along the way, and can also take a different path, that doesn't stay right next to the signal, then the enclosed loop area between the signal and ground will be a big antenna. (Similarly, two untwisted wires, with any gap or space between them, would not be good.)

So your switch should be switching both the signal and the ground, and should connect both to only one source and only one destination at a time.

If the switch connects one or more of the grounds from different amps together, that would not be good.

I would much prefer to use shielded twisted pair. Then the shield is just a shield and should NOT be connected to the signal ground. Then you would connect the shield to chassis ground at one end only, always.

With a shielded cable that has only one single inner conductor and an outer "shield" that is also the signal ground, you should always need to connect the shield at both ends, if everything else is set up correctly. Otherwise you would force the signal ground to find some other route, which would make a large enclosed loop area between signal and ground, which makes a much better hum (and RF) antenna.


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