TWIST OR SHIELD?

[simonsez]

A simple question....When should i use a twist cable and shield cable? I thought audio signal is best use shield cable? (wrong?) Thank you...

 

[Nora]

I guess when it's inside a metal case, extra cable shielding is not really necessary unless in very critical signals (mic/instrument inputs)
I connect the shields at one end only to the chassis (eg pin 1 on xlr's)

I always twist signal wires if they're long enough, even normals in patchbays

 

[simonsez]

Isn't that simply to use shield cable than twist? so why twist? except for ac tube heater supply of course. I think there is technical reason here.

 

[tmuikku]

By twisting cables you minimize loop area thus minimizing noise current from magnetic fields inside chassis (Or something similar, just reading for EMC course exam). Shielding is effective against electric fields in chassis, you need to terminate it on one end as Arno stated to prevenet noise current flowing in shield which could couple to cable it is supposed to shield. Keep cable ends coming out of the shield as short as possible. Coaxial cable is about as effective as shielded twisted pair against magneticfields, but not for electric fields. (there is more to it, try google or maybe someone more experienced will tell you).

Don't take my word granted, I have no first hand experience on the issue yet  :

 

[simonsez]

Thanks for explain....So..  "what and where"? signal cable use shield? If it near the PT then use twist? What about twist and shield? not necessary?

 

[rotation]

I can't figure  out how to shield wires to/from input transformers. What are you guys using for this? I always wondered why Lundahl only makes pcb transformers, it became clear to me as soon as i wanted to shield Cinemag input transformer with wires..
I know some people use foil for shielding. But how do you do it, do you wrap foil around twisted conductors and solder it at one end. Would normal aluminium foil work, or should i find something else? Also, is it difficult to solder foil?

Miha

 

[Samuel Groner]

I can't figure out how to shield wires to/from input transformers. What are you guys using for this?

Shielded cable? I tend to have large quantities of such available to make XLR cables...

Samuel

 

[rotation]

Hmm, i mean when i have transformer with wires. Do i cut wires very short and connect them with shielded cable?

 

[rotation]

Well, i found this mu-copper foil:
http://www.shieldingsystems.eu/?p=Nieuws&id=125&Lang=2
They say it's special foil for shielding. I might also try "normal" copper foil and see if it makes any difference.
I have problems with low signal wires picking noise quite often, so foil looks to be nice solution for this, i hope it works well.

 

[simonsez]

I think that's only necessary if you use plastic enclosure... ??? What you call the signal interference from cellular phone? electric or magnetic? it's sounds like tat..ta..tat..ta...when a device close to cellular phone.

 

[rotation]

It's called EMI, electro-magnetic interference. I think so.
Twisting wires is against magnetic fields, shielding them is against electric fields. This is what tmuikku told you in his reply to your post, i hope he is correct, i will do some more reading because unshielded wires always bother me. Most of times they are noisy before i move them around a little.

Miha

 

[Nora]

This is how I always understood it:

Twisting cable is necessary for differential or transformer inputs to retain CMRR. You want the interference to be as identical as possible to both signal paths so you make sure the wires are as close as possible (by twisting them)

All unwanted interference is then canceled at the input amplifier/trafo.

Shielding keeps out all the crap (like hum, RFI, etc) before it actually hits the input stage, sending it right to 'ground' (or - preferably - chassis)
This is, most of the time, unnecessary if your wiring is inside a grounded metal chassis, which functions as a shield.

For critical runs, or when there are very disturbing factors inside the case: like mains transformers, shielded cable provides better results.

They're always twisted anyway so it won't get worse than twisted cable

Twisted pairs from multicables (studio wiring leftovers) are very useful, not as thick as regular mic cable. and often freely available as 30cm pieces

 

[tmuikku]

It's called EMI, electro-magnetic interference. I think so.
Twisting wires is against magnetic fields, shielding them is against electric fields. This is what tmuikku told you in his reply to your post, i hope he is correct, i will do some more reading because unshielded wires always bother me. Most of times they are noisy before i move them around a little.

Miha

Yes thats about right. To get your own view on subject you can start reading from here:
http://en.wikipedia.org/wiki/Electromagnetic_compatibility

The subject EMC holds a lot of stuff which relates to all electronics (PCB layouts, cabling, cases, interfaces, grounding...) and is really really great knowledge for analog audio equipment builders and vital for digital system designers. The subject is easiest to get grasp on by reading some book (information in web is scattered, I prefer books..). The math involved is relatively easy too (of course there is lot of complexity, but easy methods exists to calculate worst case scenarios).

You can build stuff without knowing anything about EMC but the book I'm reading (Henry W. Ott: Noice reduction techniques in electronic systems) really opened a lot of stuff and maybe I'll avoid some problems in next builds. Now I can even undestand some of what PRR and others are talking in PP

 

[tv]

TWIST AND SHOUT!

 

[CJ]

no, Twist and Shield.

the best of both worlds, look inside any mic cable if you no believe,  :-\

yes, some transformers, low level ones especially, run  coax right out the ol ying yang :-*

this is the first out of focus shot from my new old camera,

i get the idea this transformer is messin with me, you know what i mean?