combining two different wire gauges....

Discussion in 'Home Theater Projects' started by Ted Lee, Feb 15, 2005.

  1. Ted Lee

    Ted Lee Lead Actor

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    let's say i have a long length of wire which is 18g. then i splice it to a short length of wire which is 14g. has my speaker wire's resistance (or whatever) changed in any meaningful way?

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    i had my new home pre-wired for 7.1. what they ended up doing was running some pretty thin wire, then looping it from one location to the other. i looked at the wire and it's definitely pretty thin...probably around 18g.

    however, i know i'll have to splice the wire coming out of the niche since the amount they left was pretty short. i just wonder if it's worth splicing with thicker wire or if (at this point) it really matters?
     
  2. Chu Gai

    Chu Gai Lead Actor

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    Ahhhh, now comes the question. How much will you agonize over this in the days, weeks, and years to come and have you finished kicking yourself in your "Sugartastic" arse for not specing the wire?

    Any idea on the lengths you're dealing with Ted? Also, what about the possibility of using the existing wire as a 'pull' to snake some 12 gauge through the walls?
     
  3. Ted Lee

    Ted Lee Lead Actor

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    [​IMG]

    yeah, i should have known better. i did ask the guy if it was single-runs to each location, but whatever.

    my estimate is approximately 25' runs (from the bottom of the media niche, up the wall, then along the ceiling to each rear), and about 15' runs to the front speakers.

    as far as pulling the wire, i'm not so sure. i believe they stapled the wire to the studs, so i don't think i can pull them loose, then up. in a worse-case scenario, i can get into the attic, so i may be able to do some creative mickey-mousing...but considering i paid a decent chunk of change to have them wire it, i'd rather not.

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    so whaddya think chu? 20g wire on a 25' run???
     
  4. Chu Gai

    Chu Gai Lead Actor

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    Well I'll be having dinner shortly and get back to you a bit later (unless someone else does). This is in-wall wire Ted? The CL2/3 stuff?
     
  5. Chu Gai

    Chu Gai Lead Actor

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    Consider the following table abstracted from some work done by Fred Davis. The table estimates the length of wire needed to give you a 0.1 dB drop at 20 kHz as a function of what your speaker's impedance is at 20 kHz.

    Min Speaker...Max Cable Length (ft) for 0.1 dBV loss
    Impedance...........Cable Gauge.....................
    .................18.......16........14........12

    8 ohms.....7.1 .... 11.0 .... 16.3 .... 22.5

    6 ohms.....5.3 .... 8.2 .... 12.2 .... 16.9

    4 ohms.....3.6 .... 5.5 .... 8.1 .... 11.2

    2 ohms.....1.8 .... 2.7 .... 4.1 .... 5.6

    Therefore Ted, if your speakers are 4 ohms at 20 kHz, it'll take 3.6 feet of 18 gauge wire to get a 0.1 dB loss. So a 20 foot run will cause your signal to be down about 0.5 dB at the top end. Is that audible under controlled conditions? I'd say doubtful to maybe, just maybe, if you've got real good young ears with 20-20 hearing. Maybe. I wouldn't count on perfect scores though.

    I don't see this as all that big a thing for the rears. Yes the distance is greater so ideally you'd want a nice fat 12 or 10 gauge, but these side and rear speakers are really there to flesh out the cinematic experience for effects. Going 7.1 or 5.1 I'd say the more important things for you to work on would be speaker placement, getting your sub tuned in for a decent room response with the BFD, and probable room treatments. I'd place the wire way below all of those. Your call though buddy.
     
  6. Ted Lee

    Ted Lee Lead Actor

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    great info as usual chu.

    yep, it's inwall (not sure if it's cl2 or cl3).

    i seriously doubt i'll be able to hear a .5db down from any end (top or bottom), so i'm not too worried about it - especially after seeing those specs.

    thx bud!

    ps, when you say signal down .5db, does that mean i can boost the channel by .5db (from the receiver) and "make up" for it....or are those two different concepts?
     
  7. Adil M

    Adil M Supporting Actor

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    Chu,
    Out of curiousity, did you calculate that table or reference it. It seems a bit severe by almost a factor of 2.

    Ted,
    Don't sweat it if they stapled it already (Not a fan of evenly placed metal staples and speaker wire).
    Focus on placement and your speakers.

    Have you thought of just scrapping the previous plan?
    Maybe trim the wire and put a panel on the wall w/ binding posts. Then connect your speakers to the panels. It will probably look cleaner.
     
  8. Chu Gai

    Chu Gai Lead Actor

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    I referenced it from something Fred Davis wrote. For the sake of thoroughness, he included the loss due to skin effect at 20 kHz.
    If you're interested Adil, I've got a paper by him that was in the AES titled Loudspeaker and Cable Interactions. It's a little over 1 meg in size. If you're interested, drop me a PM with your email address and I'll send it off to you.

    Sort of two different concepts. So tell me, did they put up that drywall with screws or nails?
     
  9. Allan Jayne

    Allan Jayne Cinematographer

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    >>> Consider the following table abstracted...
    Is this for one way or round trip using a cable with two identical conductors of the gauge specified?

    Thicker wire for the remaining (outside the wall) portions of the runs is better. It won't, however, undo the degradation suffered during the 18 gauge portions of the runs.

    Video hints:
    http://members.aol.com/ajaynejr/video.htm
     

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