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Sound Reinforcement - Forums for Live Sound Professionals - Your Displayed Name Must Be Your Real Full Name To Post In The Live Sound Forums => Installed Sound/Contracting => Topic started by: Jeff Heart on August 09, 2012, 02:17:37 PM

Title: Using RG6 Quad for line level signal to RCA fed subwoofer
Post by: Jeff Heart on August 09, 2012, 02:17:37 PM
This goes against everything I thought I was educated with in reference to proper signal lines.  We are doing a residential install where a high end sub woofer is going to be placed pretty far from the head-end.  It only has RCA inputs.  One of my guys who is a seasoned vet in residential install is telling me to use RG6/u quad shield cable terminated with RCA's for the long run of about 80 feet.  Stating that the shield will help with any EMI.  Can you use RG6 coax cable for this purpose???  I was thinking more on the lines of using an audio balun via cat5e but he doesn't want the need for baluns hanging around behind the sub or rack.  I get one shot at this install because once the walls are sealer up, his particular home will cost a small fortune to re-do the line.   
   Unless I am failing to maintain my knowledge through the years, or something has changed since the last time I read up on cable types, 75 Ohm RG6 CATV cable is entirely incorrect to run a line level audio signal.

thanks for answers in advance,
Jeff
Title: Re: Using RG6 Quad for line level signal to RCA fed subwoofer
Post by: Jeff Heart on August 09, 2012, 02:20:05 PM
Also, he said he has done it a million times and always works for him which is why he is making me second guess myself.  Is he just getting lucky on his installs?
Title: Re: Using RG6 Quad for line level signal to RCA fed subwoofer
Post by: Hal Bissinger/COMSYSTEC on August 09, 2012, 04:56:21 PM
Also, he said he has done it a million times and always works for him which is why he is making me second guess myself.  Is he just getting lucky on his installs?

If you want to convert it to a balanced line go ahead (but I wouldn't use CAT5 and those cheap baluns which really aren't baluns. That I would worry about.) For 80 feet RG-6 should work fine. All it will be is an unbalanced shielded line. Since this is for a sub you wouldn't even have to worry about HF rolloff. Note that they also make compression RCA connectors for RG-6.
 
-Hal
Title: Re: Using RG6 Quad for line level signal to RCA fed subwoofer
Post by: John Livings on August 10, 2012, 12:08:05 AM

Hi Jeff,

Consider running a piece of 1 inch or 1 1/4 inch plastic conduit to the attic/crawl space if there is one.

Also, if the RG-6 has "F" connectors already installed, Just buy 2- female F connector X RCA female connectors (one for each end).

Sometimes a standard F  or RCA connector may not fit the "Quad" cable, Larger ones are made for that cable.

Or just buy 100 feet already made up with F ends already installed.

Regards,  John
Title: Re: Using RG6 Quad for line level signal to RCA fed subwoofer
Post by: Ryan C. Davis on August 10, 2012, 12:56:42 AM
Jeff,

You are just fine with RG6 but I wouldn't waste my time with quad, really hate working with that stuff. It may offer some additional EMI rejection but we've always done fine with regular old RG6. Usually for subs we run RG59-SD which has 95% braiding opposed to the 65% braiding you'll see on standard RG6 or RG59.

To answer your question, your installer isn't misleading you, you're in good shape.
Title: Re: Using RG6 Quad for line level signal to RCA fed subwoofer
Post by: Tim Padrick on August 10, 2012, 02:40:59 AM
I'm thinking that what you may have a problem with is ground loop hum.  This is where running a balanced line with a transformer at one or both ends (consult Jensen Transformer) will save you.
Title: Re: Using RG6 Quad for line level signal to RCA fed subwoofer
Post by: Jeff Heart on August 10, 2012, 11:44:01 AM
Thanks for the input.  I have all the connectors for whatever route we take.  I'm thinking I will end up running the RG6 AND a cat 5.  THat way, if one doesn't do the job, I have the alternate choice .  The client wanted it ran through the baseboard but I will try and get him to use conduit instead, if the architect's design will allow for that.   

I have actually always had great luck with the cat5 and baluns but I also heard someone else mention that "not really a balun" comment.  Exactly what is not really a balun about those devices?  I usually use the Intelix one :  http://www.intelix.com/media/data_sheets/AVO_A2_TechSpecs.pdf
I never got into exactly what it does inside but does it just merely adapt to rj45 without truly balancing the signal?  Is that the difference between these passive cheapy baluns vs the failry expensive "true balun" powered ones?
Title: Re: Using RG6 Quad for line level signal to RCA fed subwoofer
Post by: Hal Bissinger/COMSYSTEC on August 13, 2012, 12:33:51 AM
A "balun" is either a transformer or an active powered device that converts an unbalanced line to two wire balanced or vice versa. Those Intelix units appear to utilize a couple of transformers in each unit. There are other manufacturers (particularly of wall jacks) that do nothing more than connect the unbalanced input to a pair on the CAT5.
 
The term balun actually refers to an RF transformer that serves the same function. The old 300 ohm to 75 ohm coax adapters that used to be used on the back of TVs or antennas is the best example. Personally I don't like that "balun" is now being used as a catch all name to describe anything that allows something to be carried over CAT5. "Converter", "adaptor" or "matching transformer" would be more accurate as the case permits but I guess "balun" is the dumbing down of the term for today's techie types.
 
I would also take issue with using CAT5. I would much rather see a shielded audio cable. Here too, CAT5 seems to have become the standard all-purpose low voltage wire for lazy people. You see it used for anything from doorbells to thermostats to telephone. If you are going to go through the trouble of running audio wire I would use audio wire. While a twisted pair of a CAT5 cable provides decent noise rejection you can't go wrong by having the added insurance of a shield which is standard practice.
 
This does mean that you will have to make another choice for your matching transformers but it should probably work out cheaper because you only need a single transformer on each end. Those Intelix units are 2 channel with two transformers in each housing and with an 8 pin/8 position jack for the CAT5.
 
-Hal   
Title: Baluns
Post by: Mac Kerr on August 13, 2012, 10:49:15 AM
A "balun" is either a transformer or an active powered device that converts an unbalanced line to two wire balanced or vice versa. Those Intelix units appear to utilize a couple of transformers in each unit. There are other manufacturers (particularly of wall jacks) that do nothing more than connect the unbalanced input to a pair on the CAT5.
 
The term balun actually refers to an RF transformer that serves the same function. The old 300 ohm to 75 ohm coax adapters that used to be used on the back of TVs or antennas is the best example. Personally I don't like that "balun" is now being used as a catch all name to describe anything that allows something to be carried over CAT5. "Converter", "adaptor" or "matching transformer" would be more accurate as the case permits but I guess "balun" is the dumbing down of the term for today's techie types.

Thank you.

Mac
Title: Re: Using RG6 Quad for line level signal to RCA fed subwoofer
Post by: duane massey on August 13, 2012, 12:37:51 PM
I'd run one shielded pair, #22. Don't use transformers unless you need to. Try it without first, probably work fine. RG6 would work, but stiffer, harder to work with. Don't see the point of using Cat5 unless you're running multiple signals.
Title: Re: Using RG6 Quad for line level signal to RCA fed subwoofer
Post by: Michael Martin on September 05, 2012, 12:08:07 PM
Jeff,
RG6 is a video cable, and not generally recommended for audio, although that doesn't mean it won't make the connection and work.
A low capacitance 22 gauge (or a 18-20 gauge for the longer run) would be the correct wire for this sub run.

Michael
Title: Re: Using RG6 Quad for line level signal to RCA fed subwoofer
Post by: Marty McCann on September 24, 2012, 04:30:16 PM
We recently had a customer that used the Quad cable in a similar hook up to a powered sub.  The Quad cable professes to offer better EMI rejection, but RFI is an entirely different animal.  The system was picking up RFI and when the interconnection was re-patched using a standard low Z mic cable, the problem went away.
Title: Re: Using RG6 Quad for line level signal to RCA fed subwoofer
Post by: Hal Bissinger/COMSYSTEC on September 24, 2012, 09:21:20 PM
We recently had a customer that used the Quad cable in a similar hook up to a powered sub.  The Quad cable professes to offer better EMI rejection, but RFI is an entirely different animal.  The system was picking up RFI and when the interconnection was re-patched using a standard low Z mic cable, the problem went away.

Interesting. Did you simply replace the RG6 with the mic cable or did you also convert to low impedance balanced?
 
-Hal
Title: Re: Using RG6 Quad for line level signal to RCA fed subwoofer
Post by: Marty McCann on September 26, 2012, 12:05:28 PM

Interesting. Did you simply replace the RG6 with the mic cable or did you also convert to low impedance balanced?
 
-Hal

The power Sub had a balanced input so it was indeed balanced to balanced.  When heavy gauge copper is used in a Quad configuration, it only takes a few millivolts more of induced RF in one of the two balanced inputs to not be adequately rejected by some CMRR circuits.   The cheaper methods using an IC as the differential input may boast a -60 DB CMMR for Hum/Noise or other induced EMI, if they use 0.1 % resistors to null or balance the feedback networks.  However within the substrate of the semi-conductor material itself within the IC, the integration process uses back-biased diodes to set the gain of subsequent stages.  The best tolerances of these substrate diodes is 10%.  Diodes act as detectors for RF.  Thus in cheap products with cheap CMR stages, the best S/N for RFI is more like -20 dB.   -20dB of CMRR may get you by in many environment, but not necessarily in a venue next door to a radio tower.   The beauty of a quality balanced line transformer is that it simply can not pass any RF.
Title: Re: Using RG6 Quad for line level signal to RCA fed subwoofer
Post by: Hal Bissinger/COMSYSTEC on September 26, 2012, 11:26:44 PM
Perhaps we are understanding quad cable differently. What the OP is talking about is a length of quad shielded RG6 coax in an unbalanced configuration. If your inputs were balanced how did you connect the RG6? Center conductor to one side and shield to the other side and ground or two runs? If so I can see why you would have problems. I do agree with your assessment of balanced inputs using op amps.
 
-Hal