I purchased 15ft of ZEROpoint QTRX by Phoenix Gold . Its 4 conductor cable (4 channels). ZeroPoint to make RCA cables for my install. I have not worked with this non-standard type of cable before. Each channel has a twisted pair for signal and ground, and then all the pairs are wrapped by an outer foil shield along with two more unjacketed shield wires. The question is as follows: I know how to solder each twisted pair to the RCA ends, but I am not clear as to where to attach the foil shield and unjacketed shield wires, or do these remain unconnected on either end. The ends of each twisted pair are supposed to to be passed through 1 of 4 tubes in the end of the rubber jacket cap (its easier to understand if you look at the pic in the link above) and then one RCA each is soldered at the end of each rubber jacket tube. there is no additional hole in the end jacket for the shield to exit so to connect it to chassis ground it would have to be passed through one of rubber jacket tubes for the RCA ends, which will essentially cause it to come in contact with the RCA. I need some info here from someone who has used it pleez...

 

It sounds like you meant to say "EIGHT" conductor cable???? You've got to be REALLY specific with your wording when asking for help about a very specific question... (4 channels, each with a balanced twisted pair equals 8 conductors, plus the foil shield "drain" wire(s).)

If you are using these wires connected to a speaker level amplifier output, DO NOT CONNECT THE SHIELD WIRES... LEAVE THEM OPEN! Amplifier outputs do NOT like to see the extra capacitance provided by the shielded cable. Twisted pairs, by themselves, however, are the perfect way to feed speakers. I assume, though, that since you are talking about making up RCA cables, that you are connecting this multichannel cable between your LOCs and new power amp? Is that correct? If so, LEAVE THE FOIL SHIELD CUT BACK AND UNATTACHED TO ANYTHING... Use some shrink tubing or electrical tape around the entire stripped bundle to insulate this foil. THEN CONNECT THE DRAIN WIRES TO CHASSIS GROUND AT ONE END ONLY... Since there should be no current through this chassis ground wire, because the signal is being carried on the floating balanced pairs, you could probably tie the drain wires to chassis at both ends, but it would serve no purpose... So connect the shield drain wires to the easiest chassis ground that you can find. That may be under the dash near the headunit/LOCs or it may be back in the trunk near your new amp?? I'm not sure as to why there would be more than ONE drain wire???? Is each twisted pair shielded by itself with a foil and drain wire wrapped around each pair, with an additional overall foil shield wrapped around the entire bundle with its own drain wire on top of that? In this case, I would expect to see 5 drain wires. IN ANY CASE, CHASSIS GROUND EACH DRAIN WIRE AT ONE END ONLY JUST TO BE SAFE WITH REGARD TO GROUND LOOPS... This will serve to provide a "STATIC" ground to the overall bundle of wires. It will NOT cause any ground loops by being connected to ground at only one end, but will still shield the bundle from picking up RFI, EMI, Engine ignition noise, etc. Since the drain wires are unjacketed... you will need to sleeve them yourself. Strip some insulation from a spare piece of wire and thread the drain wire through it. Or you can buy some teflon sleeving which works really well, or you could put 1/16th inch shrink tubing around the drain wire. Just sleeve it so that it DOES NOT come into contact with the floating balanced RCA connectors. If that happens , you COULD SMOKE THE AMP. If the drain wire is too short to reach a decent chassis ground... solder an extention lead to make it longer. Or strip the entire bundle back further to expose a longer drain wire and then cut back the twisted pairs to their proper length. It all depends upon how much spare wire you have in order to do that. REMEMBER, that since this chassis ground carries no current... it's really not that important as to the gauge of drain wire used. The purpose is just to have it at ground potential so that interference cannot enter the shielded bundle.

ALSO REMEMBER, if you are using this cable to connect from the head unit speaker wire outputs to your LOCs... DO NOT ATTACH THE SHIELD DRAIN WIRE AT ALL!!! Again, a speaker level amp output signal does NOT want to be fed down a shielded cable. It does like a twisted pair of wires as opposed to parallel zipcord type of wires, however. So if you're using it in this application, Just cut back the foil AND drain wires and insulate them so that they touch nothing...

Hope this helps...
Southbound

 

Southbound: you know its funny that you responded to my question because after i shut off the computer and went into the garage to start to pull my car apart i thought to myself "gee you know, i hope Southbound reads my post and responds because he's usually got the answer (and you sure pack alot of detail into your answer! ). Now as for my use of the cables, you are right that there are 8 wires, plus a foil shield, plus 2 drains. I dont know why there are 2 drains, but there are. The cable is being used to install a new head unit (Premier DEH940MP) to a 4 channel Soundstream amp and a PPI sub amp. I was going to wire the outer shield (the "drain" as you call it) to chassis ground, separately from the RCA connectors, just as you have suggested. There remains one puzzle here however. On "typical" RCA cables, the ground side doubles as a shield to the cable, running the whole length of the cable, shielding the signal wire all the way. In the case of this new kind of cable, given the way it is to be hooked up to the RCA ends, there is no shield running the last few inches up into the RCA end. I am wondering what will shield this last 2 inches or so of the wire from noise in this case. While I will surely keep the pair twisted all the way to the RCA end, there is no provision to have a foil shield follow around the twisted pair all the way into the RCA end. What is going to happen is anyone's guess because i am at least a good day away from getting it hooked up. Ahead of me for tommorrow is to run all the wires and then to fabricate a custom faceplate for the head unit (i have a sheet of burl wood ABS plastic that i ordered from Select Products which will do the trick nicely. (I have the tan interior with the fake burl wood). Then i will just finish off the rest of the dash to match the headunit trimplate with one of those (fake) wood trim kits.) DO you have any thoughts as to why Phoenix Gold would make RCA cable this way, without a shield surrounding the signal all the way from RCA end to RCA end?? The shop i picked the cable up from said this new kind of cable has twice the shielding resistance to engine noise, etc. I didnt really look at the cable until i got it home, and by the then shop was closed, as is Phoenix Gold for the 4th of July, so I was left a little baffled.

 

Having an inch or two at each end unshielded shouldn't cause you too much grief. THAT IS IF THE INPUTS AND OUTPUTS ARE TRULY BALANCED...

As you say, in a "typical UNBALANCED" RCA connector, one side is ground and one side is the hot signal wire. The ground side is shield all the way along the cable. Sometimes it is a foil shield and sometimes it is a braided shield. A lot of the braided shields do NOT cover the entire center wire. Strip back some CHEAP video cable and you will see that it is only 50% shielded. Therefore, if you have a foil shield, you have 100% coverage and are better shielded throughout the entire run. This may be what the sales people were talking about. There are some cables that do provide 100% braided shield, and there are also some that provide DOUBLE shielding in the form of an inner foil shield covered by a braid. This is really nice shielding. They must be providing two drain wires to assure that contact is made to the foil over the complete run.

Back to unbalanced vs. balanced lines. The benefit of a balanced line is that it is ISOLATED from ground and has two signal wires, one positive and one negative. Because of this, the signals in each of the two wires are 180 degrees out of phase with each other. Therefore, they have a property known as CMRR. (common mode rejection ratio) This means that both wires are susceptible to the same amount of RFI, EMI, interference, etc. However, since they are both floating the same amount above ground (balanced) they will both pick up the same amount of interference on EACH wire. Since each wire is 180 degrees out of phase, the interference will be CANCELLED out at the input of the balanced differential amp. This technique is used in ALL professional audio gear, especially whenever LONG wire runs are required. Balanced cables usually have three wires and use an XLR connector instead of an RCA connector. In the XLR connector there is a ground, a negative and a positive signal connection. IN a TYPICAL UNBALANCED line... the ONE signal wire is prone to pick up hum and noise and interference that will NOT be cancelled out at the input of the amp... rather it will be further amplified by the single ended (unbalanced) amp. This is usually OK for short runs with shielded cables, where the power supply is clean. Also, balanced lines are typically immune from ground loop problems, since there is no actual signal current being drawn through the ground wire.

A further benefit of the twisted pair/shielded cable is the fact that electromagnetic induced crosstalk is minimized when two cables are crossed at 90 degree angles. If you run two cables side by side in parallel, the signal from one cable will crosstalk into the adjacent cable due to parallel inductance. That is why it's good practice to have separate cable runs cross each other at a physical 90 degree angle. THIS IS CRUCIAL with high impedance circuits. It's not as big of an issue with speaker level low impedance wiring. SOOOOOOO, the reason for twisting wires in cables is to have them cancel out any electromagnetic crosstalk. If you take a close look at a twisted cable, you will see that the wires actually come near to crossing over each other back and forth in a pattern of 90 degree angles. This is much better than just having the wires running in parallel next to each other. I have seen cables that have individual twisted pairs that have individual foil and drain wires around each pair of wires for added isolation.

SO with all of the considerations that I just listed, Your overall immunity to interference and crosstalk should be much improved as compared to a typical two wire unbalanced RCA connection. What you need to make sure of is that the output of the headunit and the input of the amp are INDEED balanced and floating from ground. You can do this with an ohm meter by checking the two wires to the metal case of the head unit and amp. If they are balanced they will be floating by some amount from the actual chassis ground and you will want to hook things up as described in my previous post. Use the two twisted wires for the RCA and ground the shield wire at one end while making sure to keep it from touching either of the two signal wires. IF however, you find that one of the signal wires ohms out directly to the case of the head unit or amp... The circuit is unbalanced and is referenced to ground. In this case you may want to hook things up differently to avoid a possible ground loop condition. If one of the signal wires ohms out to ground and then you connect the shield wire to a different ground location, you could end up with a ground loop... This is always a fun game to figure out the proper grounding technique. I've had to tie the drain wire to BOTH ends of the negative signal wire in a twisted pair cable that is used in an UNBALANCED circuit just to avoid ground loops. AND Sometimes, I've just tied the shield and the negative signal wire together at one end to avoid ground loops. Using this type of cable in an UNBALANCED application could provide you with the type of problems that have prompted your questions.

If you find that neither of the two signal wires from both inputs and outputs do not have direct conductivity to ground... THEN YOU SHOULD BE GOLDEN, AND HAVE NO PROBLEMS USING THIS KIND OF CABLE, AND IT WOULD BE PREFERRED TO THE TYPICAL COAX TYPE OF CABLE USED FOR UNBALANCED LINES.

Again, I wouldn't worry about the last couple of inches of unshielded cable. It is still twisted at that point, and the rest of the cable run is far less prone to pick anything up as compared to the unbalanced coax cable that you refer to as TYPICAL... Too bad they don't use a three pin connector and individually shielded pairs for you to carry the drain wire through to ground on each run... THAT IS HOW PROFESSIONAL SOUND EQUIPMENT IS WIRED...

Hope this cleared things up, rather than confusing you. You may be right in that you will not know for sure how things turn out until you actually fire it up and listen.

GOOD LUCK! Southbound

 

Great info and thanks! Actually, if there is any problem with the last few inches without a shield, i can always tape over the metal RCA ends with black tape, and then wrap some foil shielding over the last few inches and run the drain over it also. I found that a good foil to use is the hi-temp all metallic duct tape with the adhesive on the back (NOTE, this is NOT the same typical gray duct tape that you find at the hardware store with the cloth fibers embedded in it)

 

Well Southbound I am soldering the cables up right now. I called the shop this morning to ask about why there is no shield at the last inch or so where you solder on the RCA ends. He told me (and i have known him for many years as a knowledgable dealer so i trust what he tells me) that you dont need any shield because the number of wraps of the two wires in each line is designed to eliminate any noise even without a shield. He said that normally you dont even need to hook up the drain to anything, but that if there is any noise present, to ground it at the amplifer side only. I will let you know how it goes.