[HiFi1972]

As in skimp on the quality of the shield cable, for example, I got a small roll of teflon-coated SPC but have a ton of cheap generic XLR cables around, so I was thinking of using one of the cheaper cables as my shield; what do you guys think about this?

 

[FallenAngel]

Can't skip the shield for IC, only for headphones.

 

[HiFi1972]

Not "Skip", "Skimp" - as in use a lesser-quality cable for the shield.

 

[FallenAngel]

Oh, whatever, it'll be used to connect a chassis.

 

[Fitz]

Quote:

Originally Posted by FallenAngel /img/forum/go_quote.gif Can't skip the shield for IC, only for headphones.

The shield/ground certainly can be skipped on balanced interconnects; it's not a signal conductor like unbalanced interconnects. Especially with a DIY cable of the sort where it isn't actually a shield anyways.

 

[apatN]

Thanks I was interested in this as well.
So basically with any balanced cable all you need is two pins/wires?

 

[qusp]

yes, IMO the shield is superfluous in balanced IC's whether HP or other cable. For instance I built a cable for my active studio monitors that goes from my interface/dac to the monitors. I only used a twisted pair for each channel. these cables pass behind my computer passed a couple of PSU's, 2 CRT monitors (large) and still there is no detectable noise. as has been said it doesnt carry any type of signal, just a shield, but the nature of balanced means that a shield is largely irrelevant IMO. maybe if your cables are super long and you live in an electrically noisy environment you might be better to cover yourself. for instance if it was for s studio setup with wires that went from the control room to the floor of the studio. also maybe for a microphone cable. but not for this purpose IMO

 

[FallenAngel]

Well, the shield generally connects the PSU ground and likely the chassis. Unless you think your cables are worse quality as a conductor than the chassis of the amp/dac, sure try to make it good, but overall, use whatever. Of course if you want to go "audiophile", you can always do something fancy with it, but it's more for show than anything else.

 

[qusp]

Quote:

Originally Posted by FallenAngel /img/forum/go_quote.gif Well, the shield generally connects the PSU ground and likely the chassis. Unless you think your cables are worse quality as a conductor than the chassis of the amp/dac, sure try to make it good, but overall, use whatever. Of course if you want to go "audiophile", you can always do something fancy with it, but it's more for show than anything else.

true that

 

[amb]

Between certain equipment the ground lead in an XLR cable cannot be omitted. For example, if you're connecting between a source and a balanced amp that is comprised of 4-channels (a separate + and - amp per channel, where each of these amps are ground-referenced), then the ground wire is absolutely essential. In this scenario the ground wire isn't "just a shield" for tying the chassis together. As such, if you're going to use boutique wires for the "hot" and "cold" signals, it makes sense to use the same for the ground.

Only a true "floating" fully-differential amp (such as β24) could operate properly without the XLR ground. But even then, a ground shield is a good idea.

 

[qusp]

OK well; so you are saying only with a balanced amp that is ground referenced yes?? because i'm connecting a balanced dac with a balanced active monitor so a balanced amp in a set of speakers; 2 in each channel. no problem here. but yes I see your point. there is exceptions; not terribly common though. I agree that in a single ended set-up that a shield is a good idea if there is noise about; but with a differential drive system I havent found a need for it. no harm done though if its there obviously

 

[amb]

qusp, without knowing the topology of your speaker's internal amp, you can't assume anything.

 

[qusp]

Quote:

Originally Posted by amb /img/forum/go_quote.gif qusp, without knowing the topology of your speaker's internal amp, you can't assume anything.

fair enough; i'll have a look into it; not the only time i've done it though. but worth looking into.

 

[Fitz]

Quote:

Originally Posted by amb /img/forum/go_quote.gif Between certain equipment the ground lead in an XLR cable cannot be omitted. For example, if you're connecting between a source and a balanced amp that is comprised of 4-channels (a separate + and - amp per channel, where each of these amps are ground-referenced), then the ground wire is absolutely essential. In this scenario the ground wire isn't "just a shield" for tying the chassis together. As such, if you're going to use boutique wires for the "hot" and "cold" signals, it makes sense to use the same for the ground. Only a true "floating" fully-differential amp (such as β24) could operate properly without the XLR ground. But even then, a ground shield is a good idea.

Can you elaborate on this? Because I've done just that in the past, using a balanced amp (that uses 4 separate amp channels, not 2 differential ones) with only pins 2 and 3 connected and pin 1 floating on the cables. I've yet to run into a situation where the ground connection was necessary except as a shield, not counting things like phantom powered microphones.

 

[amb]

On a 4-channel amp, each of the + and - amps per channel are independently ground-referenced. It doesn't "know" what the other half is doing or what it should be doing relatively to the other.

When you connect a truly differential signal source (no ground reference) to the inputs of an amp like this, each of the + and - amps "see" a signal that isn't really referenced to a ground that it "knows" about, unless the ground from the source is also connected. At best it sort of "works", but the + and - halves may not output an equal but opposite phase signal. Now, if the source and the amp both have 3-prong AC cords and their grounds happen to be connected via the wall wiring, then it would "work" by happenstance. But relying on wall AC wiring to carry a ground reference is not ideal.

The possible solutions are:
1. Connect the source XLR ground to the amp ground. This establishes the "zero reference" between the two.
2. Use a input coupling transformer at the amp input (with a single primary and a center-tapped secondary, ground the secondary center tap). This performs the proper "Phase split" without requiring a ground connection from the source.
3. Design a truly differential amp that could support a floating ground balanced input without a connected ground reference. The β24, for example, has a common-mode feedback (CMFB) mechanism that establishes the reference. The OPA1632, THAT1646 and other fully-differential opamps also have a similar mechanism.