Surround Sound Debug

For some reason, this keeps happening to me. I must be doing it wrong. But I’ve either been too stubborn or too distracted to find the better way to do it.

I’ve installed surround sound wiring into my abodes at least five times (twice in my bachelor days at two different apartments {1}, and thrice in my married days in all three of Sarah’s and my mutual houses. So I have a fair amount of experience with fishing wires down walls and into ceiling joists, cutting in low-voltage single-gang electrical boxes, and wiring up multi-channel audio speakers.

But the one step in this many-tiered project that I haven’t mastered is this: the differentiation of the channels during the final wiring.

home-theatre-speakers-diagramHere’s what I mean. You get all the four main channels mounted and wiring run from the four corners of your media room {2}. Add the center channel and you have five sets of wires coming into a single hook-up junction box near your media center cabinet. Next step: terminate those 10 conductors {3} into speaker lugs, or banana jacks, or binding posts.

But which channel pair goes to which speaker location? Doh!

And it’s not just an accident. I have tried various cable labeling in the past, as I did this most recent installation. But inevitably, my installs have included fish-taping cable down finished walls {4}. Therefore, the labels that I’ve used — such as tape or zip ties, etc. — have always come off the cable in the process. Without that identification, it’s hard to know how to hook the cables up.

So what to do? I haven’t found much on the DIY home audio forums on this topic, strangely, which makes me wonder if I’m going about this installation all goofy. Do the pros do it better? Probably.

My first generation surround sound amplifier had a channel test tone utility that was helpful. It would emit a 1KHz tone into each channel for about 2 seconds and automatically increment to the next channel. But this wasn’t ideal for a few reasons: 1) It wasn’t controllable. Once that 2 seconds had expired, you’d have to race to the other speaker to hear the tone. Not very efficient. 2) The tone obviously required that the speaker cable be terminated between the speaker and the amp to be of any use. It’s an audible tone, therefore you need a speaker to hear it. So that’s not too helpful either since I’d rather hook up all the wire after I determine which set of wires goes to which speaker channel.

So here’s how I solved the problem.

Requirements

  • An LED
  • A 1KOhm resistor
  • A 12 battery
  • Some alligator clip leads

Setup

  1. Wire up the LED indicator with the current limiting resistor attached either to the anode or cathode. A more preferred method would be to use a ready-made indicator LED panel lamp that has a built-in resistor. These assemblies come with red (positive) and black (negative) wires. Here’s one from Digi-Key. http://www.digikey.com/product-detail/en/SSI-LXH312ID-150/67-1190-ND/145097
  2. Connect the anode side (red wire) to your unknown speaker wire positive lead.
  3. Connect the cathode side (black wire) to your unknown speaker wire negative lead.
  4. Back at the junction box end with all 5 channels, connect the 12V battery to the positive and negative wires.
  5. If you don’t see the LED light up, first try switching the battery leads on that same channel.
  6. If the LED still doesn’t light, move the battery to the next set of channel wires.
  7. If the LED still doesn’t light, repeat steps 5-6 until you find it.

Theory of operation

The nice thing about this method is that the LED is unidirectional. In other words, it doesn’t light unless (conventional) current is flowing from positive to negative. This is key, because you don’t want to get the positive and negative terminals mixed up to your speakers. Therefore, using a DC incandescent light bulb would be a mistake, since it will light bi-directionally.

Notes

I’m aware that some speaker wire have knurled, striped, or discolored conductor insulators to help you differentiate between the two. This is helpful in most cases. But in two of my installs, I used some Monster brand cable that had little to no helpful indicators. So the above bailed me out.

Project Gallery

Footnotes:

  1. And I might add that I installed in such a non-destructive way as to still earn back my deposit money! []
  2. I’m old-school, and continue to find 5.1 setups sufficient for my tastes, rather than the more cinema-like 7.1 channels []
  3. Each channel has two wires: audio signal and return. []
  4. I’ve never had the good fortune to install media center cable into an unfinished room yet. []

9 Replies to “Surround Sound Debug”

  1. The LED assembly looks like an elegant solution. Wouldn’t a regular electrical light tester have worked?
    You’re wiring to a panel? not directly from the source to speaker?

  2. Common sense rules the day. Thanks for the wisdom from experience! Even though digikey has everything, Sparkfun is run by a CU graduate. That and they make an RTFM sticker, makes me smile.

  3. Try wrapping a sticker about 2′ back on cable. This can alleviate initial damage done to cable and sticker. Yes, it takes more cable, but sticker should be fine. The other thing you can do is tone it out.

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