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Shielded cable
Shielded cable
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Old 28th May 2019, 05:59 PM   #1
Plimpington2 is offline Plimpington2
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Default Shielded cable

One of the amps I am rebuilding uses shields for the signal lines in two places - from the back plate to the switching, and then again for the run from the front deck to the driver tubes.

The shields from the back deck to the front are THICK, heavy and inflexible wire wound “tubes” with black rubber sheathing. Within each tube are the solid core conductor cables. The tubes are slightly unwoundat the ends and soldered to ground. The shields from the front deck to the driver grids are flexible RCA appearing cables.

I VERY much like the noise results of this arrangement and will use it in my signal lines that run for more than a few inches. My question is, will 3 conductor microphone cable offer good shielding in this application? I think yes, but want to see what other people are using.
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Old 28th May 2019, 08:33 PM   #2
GoatGuy is offline GoatGuy  United States
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Three wire microphone cable has a twisted pair in the center and (typically) an aluminized plastic film — and often a “ground return wire” (naked, no insulation) — in construction. For the shortness of runs inside a chassis, although the mic-cable is “not right”, it definitely would work. Just connect the pair of inner wires together at both ends electrically so they become one. The shield is the ground.

However, at audio frequencies braided-shield, small conductor, flexible, medium-thick insulation wire is cheap, performs well, and gets the job done without all that twisted pair fussing. Buy a hundred feet, and you'll find dozens of uses for the stuff.

Choose stranded core wire though. The solid-core type is better suited for crimp-on BNC/TNC connectors and “plenum” or wiring rack service, generally. Or for HAM radio operators. Well, there are hundreds of uses for it, but the solid core becomes substantially less flexible. Without delivering any discernible acoustic improvement. So… stranded.

Just saying,
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Old 28th May 2019, 09:52 PM   #3
HollowState is offline HollowState  United States
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Quote:
Originally Posted by Plimpington2 View Post
...but want to see what other people are using.
RG-180B/U (silver & Teflon w/very low capacitance/Ft.)(RG-179 is very similar)

AWG Size 30
Conductor Stranding 7/38
No. of Strands 7
Strand Size 38
Dielectric (in) 0.102
Nom. O.D. (in) 0.1410
Cond. Material Silver-Plated Copper Clad Steel
Dielectric Material Polytetrafluoroethylene (PTFE)
Shield Overall Braid Shield
Shield Coverage 91%
Shield Material Silver Coated Copper Braid
Jacket Material Fluorinated Ethylene Propylene (FEP)
Nom. Imp. 95
Vel. of Prop. 70
Nom. Cap. (pF/ft) 15.4
Mil-Spec # MIL-DTL-17/RG-180B/U
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Old 29th May 2019, 04:50 AM   #4
thaumaturge is offline thaumaturge  United States
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Just connect the pair of inner wires together at both ends electrically so they become one. The shield is the ground.
You have just created a ground loop.
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Old 29th May 2019, 06:23 PM   #5
GoatGuy is offline GoatGuy  United States
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Originally Posted by thaumaturge View Post
You have just created a ground loop. Doc
I see how one might think that, but there are 2 ways to 'do it' that don't have a loop per se. One is to connect “red” and “blue” (the pair of inner insulated, twisted conductors) as shown here. No loop.

The other is to connect one (say “blue”) at each end to the braid, leaving just the “red” free to conduct SIG. (sorry, no diagram).

The thing you want to avoid is leaving one of the inner conductors doing “nothing”, electrically floating. And you wouldn't want to connect just one end of the “floating” conductor to ground, either.

BTW… there is a relatively new invention (Mogami brand) that I personally have found to be marvelous… the 4 conductor twisted-braided microphone cable. To keep sanity, its four inner conductors are red, red, blue, blue. The idea is that you treat them like the diagram… twist (at each end) the reds together, likewise for the blues. Treat them as “a single pair” electrically. Magnificent microphone cables. You can use them in the most electrically challenging environments, and basically nothing squeaks thru, interference-wise.

Just saying,
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Last edited by GoatGuy; 29th May 2019 at 06:29 PM.
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Old 29th May 2019, 07:21 PM   #6
scottjoplin is offline scottjoplin  Wales
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Originally Posted by GoatGuy View Post
I see how one might think that
Can you? I can't
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Old 30th May 2019, 09:44 PM   #7
thaumaturge is offline thaumaturge  United States
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Originally Posted by GoatGuy View Post
I see how one might think that, but there are 2 ways to 'do it' that don't have a loop per se. One is to connect “red” and “blue” (the pair of inner insulated, twisted conductors) as shown here. No loop.
Just saying,
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Nope, it is still a loop. It may not induce problems with neither end connected to anything else, but any AC field will induce current to flow in created loop. Think that understanding goes back to Faraday.
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Old 30th May 2019, 11:27 PM   #8
GoatGuy is offline GoatGuy  United States
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Originally Posted by thaumaturge View Post
Nope, it is still a loop. It may not induce problems with neither end connected to anything else, but any AC field will induce current to flow in created loop. Think that understanding goes back to Faraday… Doc
Yes, of course.

The key tho are both of the bold parts. If the topology of the solution doesn't have any measurable interference impact on the signal passing through and has positive impact on the pass-thru by lowering resistance, well … a ('nothing' + 'win' = 'win').

The real problem of ground plane loops both external to, and within the confines of electronic equipment, whether they are amplifiers, recorders, mixers, preamps, signal conditioners, EFX boxes, “chains of things” … is legendary. That too has been known since “Faraday” as you say.

Now… if you could show that there is a negative consequence, well, then I'll keep up my end and demur.

Just saying,
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Old 30th May 2019, 11:40 PM   #9
scottjoplin is offline scottjoplin  Wales
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Originally Posted by thaumaturge View Post
Nope, it is still a loop. It may not induce problems with neither end connected to anything else, but any AC field will induce current to flow in created loop. Think that understanding goes back to Faraday.
Doc
Are you saying the red and the blue wires connected together at both ends creates a loop?
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Old 31st May 2019, 10:35 AM   #10
DF96 is offline DF96  England
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It creates a loop, but a fairly benign loop. Better to buy the right cable, of course. Why do people have this obsession with using twisted pair for unbalanced connections, as though twisted pair was somehow better than the correct cable?
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