OT audio cable question

epirb wrote on 1/21/2004, 8:04 PM
Spot maybe you know, or anyone else out there.How do you feel about using quadshield RG6 for audio signals? Not as the norm but I have a 12 channel board that unfortunatly I dont have the room for by my computer. but i have 6 RG6's run to computer area and I can place the board in a spot where I could use two of those for L/R channel inputs. I usually use canare ends when I'm using the RG6 for vid signals, but I know that its more important to balance the input/output on audio signals. is the 75 ohm impeadence of Rg6 acceptable?

Comments

musicvid10 wrote on 1/21/2004, 8:29 PM
Since you're using unbalanced sources and inputs (as indicated by your choice of coax as a conduit) it doesn't make a darned bit of difference. Unbalanced inputs run the gamut from 10K- to 1M- Ohm impedance, so the effect of your cable impedance is negligible. Of course, if you are running a 1-2 volt nominal line level through more than about 25 feet of coax, you may want to use direct boxes or lift the ground to eliminate ground loops, thereby reducing AC hum. I strongly recommend direct boxes at both ends, and good XLR cable in between if you are over 30-50 feet.

Hope this helps.
Spot|DSE wrote on 1/21/2004, 8:42 PM
Well...the impedance is acceptable, but noise is a whole nother issue. If it's not balanced, it's an antenna for noise. Balanced, with good output transformers, will keep noise from entering the system, and computers and video are terrific noise makers. That would be my bigger issue. You can wire the Quadshield to be balanced, but I don't know how well the shield would respond. Should be OK. I've never used vid cable for audio, so can't intelligently comment. Use DI's for best scenario.
{edit} oops, musicvid was posting as I was. The only additional comment I'd make is while he recommends boxes after 25 feet, I'll recommend the same thing but at 12 feet or more. Look at various charts, you'll see significant noise rise at about 15 feet on most cables.
johnmeyer wrote on 1/21/2004, 11:11 PM
How do you feel about using quadshield RG6 for audio signals?

It's done all the time. Balanced, as noted above, is the professional way to go, but the RG-59 or RG-6 "trick" is used quite a bit in home theater setup to get longer runs of cable without losing too much signal or introducting too much noise.

Another alternative for longer runs -- one I have used here -- is to use a gadget that converts your unbalanced audio and video to balanced signals, and then lets you run them over Cat 5 cable, the same stuff that is used for 100baseT local area networks. You plug your unbalanced RCA audio and video connectors into one gadget, and then plug a Cat 5 cable, terminated with a standard RJ-45 connector, into the other side. You do the same thing at the other end of the run. I've got one running several hundred fit through a minefield of noise sources, and the audio is perfectly quiet. I did have a problem with the video (hum bars) that was caused by standard ground loop power problems, and I solved that by using a humbucker. It turns out that these devices provide isolation for the audio, but not for the video.

I highly recommend these. Mine is a Unicom Video/Audio Adapter P/N VAA-U501-A. See it here:

Video/Audio Adapter
farss wrote on 1/21/2004, 11:41 PM
These are always difficult calls. Balanced has obviously got the better noise rejection but even the very best transformers are still another thing in the signal path.
In the field, no question, you really need balanced, you never know how much RF or other nasties you are going to run into. In the studio may be another matter. I read a report from one guy who ripped all the transformers out of everything and got a significant improvement in signal quality. I'd add though he put a Faraday cage around the room as well!
epirb wrote on 1/22/2004, 6:13 AM
thanks for all the info guys, I think maybe the DI box deal might be the way to go.I think i have a one somewhere , My run is around 30 ft. I made sure that when I ran all the RG6 along with a couple of cat5E's that they do not run anywhere near any a/c lines, except one place where they are still 1ft apart and intersect @ 90 deg.(these are all in wall cables that I ran when I remodeled)
Plus the board is an old Fostex with only unbalanced monitor and stereo outputs. Prety much using the board to run my mike for narr. and stuff and maybe my synth's for some little music projects or tracks.
I was already using a gnd loop iso. when I have it set up temp. near the computer w/ shorter cable runs.
musicvid10 wrote on 1/22/2004, 11:49 AM
Another option to transformer direct boxes are active direct boxes such as the Behringer.
Although they introduce a teeny bit of hiss into the line, they completely wipe out AC hum and can amplify the signal so that very long runs are possible without signal degradation. The Behringer has the advantage that batteries are not needed if you have phantom power available at the board inputs. They can also handle speaker level inputs if necessary.

A lower-priced solution with the same intent is the Audio Buddy by M-Audio ($99). It has two XLR and two 1/4" inputs, and two 1/4" balanced/unbalanced outputs, separate gain controls, and 2 Ch. phantom power supply. Just the thing for long stereo runs and location shoots. It runs off a standard 9V adapter. I find the Audio Buddy works extremely well if you keep the gains on the conservative side.