STP for unbalanced Stereo

P

Peter Kowalczyk

Freshman
Apr 12, 2013
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Truckee, CA
I was talking to an electrician today about running an unbalanced stereo signal about 50,' and we debated the difference between shielded twisted pair and dual coaxial cables for this application.

A typical stereo RCA cable is composed of a pair of coaxial cables, typically molded together in a physically parallel configuration. In the past, I've used shielded-twisted-pair mic cable (which I have) instead of dual coax (which I don't) for unbalanced stereo signals. While dual RCA cable has separate shields for the left and right signals, a 3.5mm or 1/4" stereo connector uses the same common for them both.

Is there any reason why a shielded-twisted-pair microphone cable isn't as good for an unbalanced stereo run as a pair of individually-shielded coaxial cables?
 
Re: STP for unbalanced Stereo

Hello

While it is quite true, that there is more crosstalk between L&R in microphone cable vs "diode"-cable ( = normal stereo rca cable ) with a distance of 50 feet / 15 meters I would say in MOST cases there are possibly no problems with either one - I used to feed two monitpr/FB-lines with 100/30 snake that had just one too few lines on it - no problem - output impedance was typical 50-200 ohms...

Now - if we are landing on HiFi-land ( hopefully not - I am out ) we can talk about the difference with having one vs two shields between those two places - obviously with diode-cable one can leave the shield unconnected in one connector of four...

Conclusion - whichever cable you have, will work - just make sure connectors are well soldered.


Nuuska
 
Re: STP for unbalanced Stereo

I wouldn't worry about $11 worth of cable I would be concerned with noise pickup. Test the setup before committing to a permanent install. If its in the wall have the electrician quote you flex conduit and hope he doesn't take you to the cleaners. That way if things change down the road you have an option. Personally I'd go with balanced and be done with it. Have him provide you with a conduit ground lead that you can fasten to the chassis of the two kit you're connecting.
 
Re: STP for unbalanced Stereo

For a long unbalanced analog interconnects, the one most important thing is low end-to-end resistance of the return/shield wire.
Bill Whitlock, Jim Brown, Henry W. Ott and Bruno Putzeys all write about the importance of this low resistance.

High frequency roll-off is not a problem with reasonable components.
 
Re: STP for unbalanced Stereo

Yeah, I wasn't thrilled with the idea of having to run 50' unbalanced to begin with. But it's moot now, as the source (TV cable box) is being moved adjacent to the receiver (Zone mixer). 12" - 18" of unbalanced I can live with. Thanks for your thoughts, everyone.
 
Re: STP for unbalanced Stereo

The big thing to understand is what makes a regular twisted pair balanced line preferred over unbalanced.

It is the common mode rejection ratio of the input stage it is plugged into. This CMMR allows balanced lines to be run long distances (think telephone lines) with minimal issue.

Basically since the signal travels on the conductors-a shield is not really needed to keep out the noise. The whole basic concept of twisting the wires is to expose AS MUCH AS POSSIBLE of each conductor TO THE NOISE! I know that sound weird-but if the input stage has a lot of common mode rejection-then anything that is common to both conductors will be cancelled-to the extent of the CMMR.

An unbalanced signal does not have this type of rejection. Since the shield is actually one of the conductors, and it "shields" the center conductor from the noise-there is a much greater possibility of noise getting into the system

If it HAS to be unbalanced-try this little trick.

Use a regular balanced mic line. Hook the hot to the hot (input and output). Now hook the neg/ground to the neg/ground on the input and output.

NOW hook the shield to the OUTPUT or SENDING device and DO NOT hook the shield to the input side.

This way any noise that gets on the shield will find a lower impedance path to ground (outputs are always lower impedance than inputs-these days anyway-it did not used to be that way in years past).

The signal can still travel on the wires-but it will be traveling with less noise.
 
Re: STP for unbalanced Stereo

Ivan Beaver said:
If it HAS to be unbalanced-try this little trick.

Use a regular balanced mic line. Hook the hot to the hot (input and output). Now hook the neg/ground to the neg/ground on the input and output.

NOW hook the shield to the OUTPUT or SENDING device and DO NOT hook the shield to the input side.

This way any noise that gets on the shield will find a lower impedance path to ground (outputs are always lower impedance than inputs-these days anyway-it did not used to be that way in years past).

The signal can still travel on the wires-but it will be traveling with less noise.
Click to expand...

Good tip, that makes sense - the twisted pair in an STP is impedance balanced (unlike either conductor to shield), so noise will couple equally, allowing the CMRR of a differential input (mic pre) to reject it.
 
Re: STP for unbalanced Stereo

Timo Ulkuniemi said:
Hello

This is known to work great on guitars, too - but for obvious reasons the amp-end has to be, where shield connects.
Click to expand...
It works best when the amp is actually grounded 9ie not a 2 prong AC cable).

Of course it also depends on how well the input jack is actually attached to the ground wire. Does the ground connection go through the chassis or to the circuit board as the path of least resistance?

You mileage may vary-but it can help.
 
Re: STP for unbalanced Stereo

Kevin Graf said:
For a long unbalanced analog interconnects, the one most important thing is low end-to-end resistance of the return/shield wire.
Bill Whitlock, Jim Brown, Henry W. Ott and Bruno Putzeys all write about the importance of this low resistance.
Click to expand...

Thank you. This is exactly what I was thinking when I read the OP.

If it were me, I'd invest a little extra into making it a balanced connection, even if it's just a pair of input transformers, to achieve that 99.99% certainty that it work. No way would I pull 50 feet of unbalanced stereo cabling through a wall and hope that it works without excessive noise.
 
Re: STP for unbalanced Stereo

Ivan Beaver said:
If it HAS to be unbalanced-try this little trick.

Use a regular balanced mic line. Hook the hot to the hot (input and output). Now hook the neg/ground to the neg/ground on the input and output.

NOW hook the shield to the OUTPUT or SENDING device and DO NOT hook the shield to the input side.
Click to expand...

Excellent suggestion!

quasi-balanced.jpg
 
Re: STP for unbalanced Stereo

That paper is in my library somewhere. As a voracious reader, I am not always sure where I first encountered ideas, and sometimes the info just pops back to the top of my head.

Since so much of the internet is just pointers to bookmarks of links to catalog pages...it is nice to be reminded where the original info is.

Sent from my DROID RAZR HD
 
Re: STP for unbalanced Stereo

Jay Barracato said:
That paper is in my library somewhere. As a voracious reader, I am not always sure where I first encountered ideas, and sometimes the info just pops back to the top of my head.

Since so much of the internet is just pointers to bookmarks of links to catalog pages...it is nice to be reminded where the original info is.
Click to expand...

I spend a lot of time reading too. I have most of those papers from Jensen stashed away in my active and backup drives. I started saving a lot of it on my computer because I would read it and then I couldn't remember where I saw it later.
 
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