Tying conductors together for signal wiring in chassis

[force of 1/2]

Hello Everyone,

I can't seem to find any posts dealing directly with this question:

I have a bunch of shielded 2 conductor console wire (Mogami 2944) that I would like to use to rewire from RCA inputs to the selector switch in a preamp. The preamp is star-grounded, and the selector switch only switches the positive leads.

Mogami 2944 is 2 x 26awg with spiral shield.

With the shield floating (tied to the RCA ground, which is grounded to the star ground) should I:

1- tie the 2 center conductors together and use them as a single positive lead to the selector?

2- just use one of the two conductors?

What would advantages or disadvantages be of either method?
(I am guessing lower resistance, but slightly higher capacitance when tying the conductors together?)

Thanks in advance for your advice!

Best,
John

P.S. The preamp is over 40 years old, and the wire with PVC jacket is visibly oxidized blue-green-- thus why I want to rewire.

[PRR]

Originally Posted by force of 1/2

...should I:
1- tie the 2 center conductors together ...
2- just use one of the two conductors?'...

Whatever is easier. Makes no difference for short lengths inside a box.

I would not fret some green-crap on the wires. It won't likely eat-through in your lifetime. (It did happen to me, but it involved thumbtacks and a leaky ceiling.) I *would* wonder what ELSE in there is turning green, and why.

[DF96]

I assume when the OP says 'positive' he means signal. How is the signal return wired? You talk about a floating shield, but then tell us it is grounded. So is it floating or grounded? The signal return should closely follow the signal, so the loop they enclose is as small as possible.

To wire unbalanced signals you need shield plus one wire. It doesn't matter too much what you do with the spare wire. Having discovered that I had bought the wrong cable, I think I would simply parallel the second wire with the first wire.

[scottjoplin]

I wouldn't leave any conductor unused. I'd suggest the twisted pair as flow and return and the shield connected both ends to chassis, or one end to chassis, or one end to return at the RCA, or both ends to return. What might be interesting is if you tried all the alternatives and posted your findings to see which is the better, it could depend of the rest of your grounding scheme so it would be helpful if you told us that. As PRR says doubtful it would make much difference in an enclosure

[force of 1/2]

Yes, the OP meant ‘signal’ when he wrote ‘positive’.

The original grounding scheme works well, so I don’t need to experiment there.

I called the shield “floating” because it is only attached to ground at one end. I apolgize if I used the wrong term, though I don’t think I did.

The original coax only carries the “signal” (or flow as Scottjoplin called it). The shield does not carry the signal return. The returns are accomplished by short leads from the input jacks to the star ground. The coax cable in question is only carrying “signal” from input jacks to the selector switch.

Also, I did not buy the wrong wire. I bought a bulk of the Mogami 2944 a few years ago at a very cheap price. It is flexible and easy to work with, thus why I am considering using it here.

Back to the actual question:
Does anyone know any reasons (I’d like to hear the science) why tying together a twisted pair inside a shield to make a coaxial wire would present any disadvantages or potential advantages. I know I can use it in any way I choose, but I would like to hear if any one has some solid information to preface the choice.

I am asking out of genuine curiosity. I am not worried about making a “right” or “wrong” choice.

[scottjoplin]

Originally Posted by force of 1/2

The shield does not carry the signal return. The returns are accomplished by short leads from the input jacks to the star ground.

For maximum protection from interference, both signal conductors should be close coupled throughout their length, in a coax this means using the shield as
the return

[PRR]

Originally Posted by scottjoplin

...in a coax this means using the shield as
the return

When you have the luxury (or pre-bought reel) of two conductors under one shield, you take Red as hot, black as return, and shield as shield. Hot/return connected both ends, shield at one end only.

However Force already has a working returns scheme, and I would NOT muck with such a thing if it is working.

As for lower resistance or lower capacitance, you'd need some very complete specifications to say if one is better than the other, and by enough to even be worth the computation. Driving a 4 Ohm speaker you sure do want to parallel all the copper you can get for lowest Ohms loss. Coming out of a 30pFd capacitor mike head, you may want to minimize every last pFd loss. But modern hi-fi interfaces are *intended* to make such parasitics moot, even over longer runs than inside the box. As for shielding,.... well, I can see it either way, but assume that inside-the-box shielding is not the most important thing.

If you need to obsess, copper tubing with ceramic or teflon beads and appropriately sized center conductor "may" be more "pure" than recycled milk-bottle plastic. (OK, good wire is probably still virgin poly.) AM car antennas (a very high impedance at high frequency) use a mostly-hollow construction with very fine center conductor. Radio transmitters use a pipe inside a pipe with ceramic rings at intervals. If you live in these worlds, you gain a feel for what may be best. In Audio, I say jacket color is more important than parallel/not over a foot of run.

[scottjoplin]

Originally Posted by PRR

When you have the luxury (or pre-bought reel) of two conductors under one shield, you take Red as hot, black as return, and shield as shield. Hot/return connected both ends, shield at one end only.

Shield to the chassis? Why connect one end only?

[AndrewT]

Originally Posted by force of 1/2

Hello Everyone,

I can't seem to find any posts dealing directly with this question:

I have a bunch of shielded 2 conductor console wire (Mogami 2944) that I would like to use to rewire from RCA inputs to the selector switch in a preamp. The preamp is star-grounded, and the selector switch only switches the positive leads.

don't allow the "star-ground" to introduce big loops into the two wire signal connections.

Mogami 2944 is 2 x 26awg with spiral shield.

spiral shielding has very poor performance compared to braided shielding

With the shield floating (tied to the RCA ground, which is grounded to the star ground)

the shield/screen should never be floating. If you use the shield/screen as the signal Return then the input end of the shield MUST be connected to the input socket Cold/Return terminal.
the output end of the shield/screen MUST be connected to the signal Return terminal adjacent to the Signal Flow terminal.

If you use the separate shield (stp) as a shield, then connect it to the chassis at the input end by a very low impedance connection.
If possible connect the output end of the shield to the chassis using a low impedance connection. TheIn this case the two internal wires carry the signal. Bothe MUST be connected at BOTH ends. One as signal Flow and the other as Signal Return.

should I:

1- tie the 2 center conductors together and use them as a single positive lead to the selector?

2- just use one of the two conductors?

What would advantages or disadvantages be of either method?
(I am guessing lower resistance, but slightly higher capacitance when tying the conductors together?)

Thanks in advance for your advice!

Best,
John

P.S. The preamp is over 40 years old, and the wire with PVC jacket is visibly oxidized blue-green-- thus why I want to rewire.

Every signal connection is a TWO WIRE connection. The current Flows from the Source and Returns to the Source.


The CURRENT MUST RETURN to the SOURCE.

STP = screened twisted pair.

[AndrewT]

DF said

The signal return should closely follow the signal, so the loop they enclose is as small as possible.

Scot said

both signal conductors should be close coupled throughout their length, in a coax this means using the shield as the return