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.
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.
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,
GoatGuy ✓
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,
GoatGuy ✓
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
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,
GoatGuy ✓
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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.
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,
GoatGuy ✓
Are you saying the red and the blue wires connected together at both ends creates a loop?
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?
Ok, that is what he was saying and he was being pedantic, thanks 🙂 Although he did say ground loop?
No, I wasn't being pedantic. I was speaking from the perspective of a retired broadcast engineer with five TV stations, six radio stations and three recording studios under my belt. This site is devoted to the purpose of making audio equipment sound as good as possible.
Introducing that supposedly "benign" little wire loop is simply a bad idea.
1. It serves no useful purpose. It does not lower the resistance of any wire.
2. It does in fact introduce an unwanted and unnessessary noise source into any amplifier it is built into.
Say that innocent little length of wire is 18 inches long. Connecting both ends together creates a resonant RF loop right about in the FM band. Do the math yourself. Wavelength =300/Frequency in MHz. Your noise floor inside the amp just went up. For what reason?
Things are not always as simple as they sound.
Doc
Correction. 18" wires soldered in a loop would actually resonate at 328MHz, not mid FM band. But the concept still applies.
Doc
Doc
It will lower the resistance of the signal connection. This serves no useful purpose.thaumaturge said:
A twisted shielded loop is not going to pick up much RF. My objection to this cable is not that it is harmful but that it is unnecessary when single shielded cables are available. Why not use the cable designed for the job?
Well yup, by golly you got me there! I used a common usage term for a shorted wire loop, whether ground is involved or not.
Excuse me for commenting on this thread at all. Obviously not of interest to the parties involved.
I'm out.
Doc
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No. You used a specific term which applies to a quite different situation, then used the 'years of industry experience' card in a vain attempt to silence those questioning what you said. Now you appear to be going off in a huff because we do not worship.thaumaturge said:
Much simpler to just admit that you made a mistake. We all make mistakes.
Here we go....
No matter your opinion I did use the term "ground loop" in a generic sense, as it is used throughout the broadcasting industry. I fully concede that there is no ground involved. As to silencing those questioning me, you can jabber on for years as far as I care. The years of experience "card" I offered is totally valid, as is the technical arguement I offered over why the practice was and is a bad idea. I do not expect nor desire worship. The simple respect I received from those I mentored along the way is quite sufficient. As I also offered my respect for those who mentored me.
You wanf to argue further... have fun.
Doc
Doc, please... I really appreciate your input and life-learned expertise. No need to duck out...
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