Signal coaxial wiring, 1 or 2 conductors?

[TheDriver41]

I have both one and two conductor coaxial wiring from Apex Jr. I am building Brian’s kit and I’m isolating the rca inputs from the chassis. Should use a single conductor coaxial wiring and use the shielding for the signal ground? Or use a two conductor wire and not do any thing with the shield? And why? Isn’t the shield useless if not grounded?

I'm also trying to figure out if it would help to ground the shield around the twisted 18ga coaxial speaker cable I got from Apex.

Thanks,
Matt

[digi01]

I think the two conductor coaxial is fine.

[TheDriver41]

Thanks.

Should I do the same thing with the shielding on the speaker wire?

If so would I attach the shielding to the black terminal at both ends?

[troystg]

TheDriver41-

digi01 is correct for one camps thought but like everything there are 2 sides.

Some people prefer to ground the "shield or drain" on the PCB side at the PCB's ground point.

I prefer it the way digi01 has it, but felt obligated to mention the other method.

As for the 18ga shielded twisted pair you have for speaker cables, DON'T hook up the shield/drain to the black terminal on the amp!

Tie both the shields to the amplifier chassis. If you connect them to the negative terminal you will add significant capacitance to the cable.

What I do and suggest is:

1. Take 1/4 of the shield (no need to use all of the shield) and twist it together and make a single wire type conductor.

1a = optional I heat shrink the twisted shield, and the cable where the outer jacket is stripped so there are now 3 conductors sticking out of the heat shrink.

2. Crimp an insulated female 1/4 " quick connect to the shield conductor.

3. Make a small jumper with a male 1/4" quick connect on one side and a fork/spade or ring terminal on the other.

4. Put the 2 spade or ring terminals under a bolt on the amplifier chassis if it is tied to earth ground.

5. Connect the male and female 1/4" terminals together on each speaker wire.

6. Insulate the shield on the speaker side with heat shrink. The sheild should ALWAYS be connected on one side ONLY. That goes for low-level interconnects also.

In coax cable the shield can (and typically is) used as the common conductor but was intended as a drain/shield for a single conductor. Which required running two coax wires thus doubling the costs. Needles to say, that didn't last as a buisness practice.

[TheDriver41]

Thanks guys. I actually understand it now.

rabstg, What all did you shield? It looks like you shield everything but the signal input a speaker output.

How did you connect the shielding to the ground? And it looks like the shielding for the wiring from the refractor broad are touch the chassis. Wouldn't that created a problem?

[troystg]

Hey Driver-

The speaker wires from PCB to binding posts are not shielded.

The input wire is the coax from Steve. Since it is less than a foot long it does not need to be shielded either. If it were longer than a foot I would have used the shielded twisted pair and grounded the shield on ONE side.

On the input AC and rectified DC I just slid the wire through a "space/hole" in the shield on the side so there would be a "tail" to ground.

Chassis is at Earth ground, and the shield is at earth ground. No issues. If you could see under the wire at the central tie down point you would see that all screens are grounded there along with the earth ground from the PCB's.

If you use the coax or slded twisted pair from Steve you will not have any noise issues from the input. Just try not to run them by the transformer or AC input wires and keep the xformer some distance from the audio boards.

Please feel free to ask questions and always ask to clarify... I would rather repeat than see good equipment go up in smoke. Hey wasn't that a Cheech and Chong movie? LOL...

I will be on top of a mountain this weekend (hopfully, IF I don't fall :-)) but I will reply next week to any posts.