Wiring 1/4"and XLRs through a wall...

[moiremusic]

I've built an isolation box for amps in my studio in north Texas. It's a clever little framed box on wheels, about 4'x5'x4, with sheetrock walls lined with OC703, uses half a ProCore door, and depending on the results may contain slat resonators across the back wall. The framing, door, and insulation are done. The only thing left is some simple wiring, the outer layers of sheetrock, and some speaker carpet and hardware for the outside.

OK, carpentry is a breeze for me, but when it comes to wiring, I'm an idiot. I've built a simple isolation box for guitar amps, and have purchased the hardware to send 1/4"and XLR cables through the wall of the box. Here are my needs (and you can see the image below for a visualization) :

Two XLR for microphone signals - capable of phantom power, to go from mics in box into mixer or preamp in the room
One 1/4" for mono non-amplified guitar signal - to carry signal from guitar in room to amp in the box
Two 1/4" for mono (each) guitar effects loop - to carry effects loop from amp in box to effects in room and back to amp in box
One 1/4" for mono amplified speaker signal - to carry amplified signal from amp in room to speaker in box

But looking at these jacks, I'm a little lost. Aaronw's post on connections seems pretty helpful, but I'm not sure if I need balanced or unbalanced connections here. So here are two basic questions (edit: refined in hopes of getting a bite):

1) Which of the signals listed above should be balanced and which should be unbalanced?

• a - XLR
  b - non-amplified guitar signal
  c - effects loop signals
  d - amplified speaker signal

2) I was supplied wire by the the manufacturer of the plates, but do I need a different kind of wire for the amplified signal? This will be used when a guitar amp needs to be outside the box and the speaker needs to be inside. My understanding has always been that speaker wire and line/low level wire were different.

[cvanas]

Hi moiremusic.
I'll have a crack at this one. I built one like this as well. It worked great! I have a 50watt Marshall and at a very loud playing volume only a little 'woof woof' could be heard (or rather felt) outside the box.
To answer your posted questions. Only a microphone cable is 'balanced'. It generally contains 3 wires. These correspond to hot, cold and ground wires. Two covered wires and the bare ground wire are inside.
The guitar and speaker cables use two wires only. positive and ground. Guitar cables are shielded, speaker cables are not. Hardware stores have electrical cord, use this for your speaker connection (12 - 16 gauge). You only need a short section.
I hope you don't mind if I include my "head-fakers" guide to sourcing parts. Use what they gave you if it will be easier to solder. Or, if you have any broken mic, guitar or speaker cables lying around, use them. If you don't have any handy but buy alot of gear from your local music store, go there and see if they have any dead cables they can part with. Otherwise, buy it at the cheapest place you can. Using sections of cable it will make it easier to see what your doing.
Might I suggest approaching it like this:

1. Label corresponding jacks on both plates with Sharpie.

2. Cut cables to length and strip ends. Leave enough to work with! You will shove any extra cable length inside box when finished.

3. Start with the panel that will be inside the box. This is the one with the female XLR's (on the left in photo).

4. Solder all cables to connectors on inside panel. Label them as you go with masking tape.
3 wires for each XLR connector, 2 on each guitar, fx send/return, and speaker cable. Look here http://en.wikipedia.org/wiki/XLR
for XLR wiring.
Scroll down page to XLR3 connectors and look at the female/male pin diagram there. This is the only tricky part. Later, when you solder the inside panel cables to the outside panel connectors the ground Pin1 on male must go to pin1 on female, Pin2 to Pin2, etc., or you will have microphone phase problems.
guitar jack connections will be: inside panel A tip to outside panel A tip, inside panel A sleeve (ground wire) to outside panel A sleeve (ground wire), etc.

5. Attach panel to inside of box and feed cables through hole.

6. Solder outside panel connectors up.

7. Try everything (important step).

8. Shove extra cable length into insulation and attach outside panel.

9. Yippee!

Wow, and all those girlfriends called me selfish.
Cam.

[moiremusic]

Thanks so much cvanas! So it sounds like I should just be connecting similar wires like this:



By the way, here are the instructions I was originally given, which is why I found this process so confusing in the first place:

2 XLR - for microphone signals
pin 1 to ground then match the other 2 colors

1 1/4" - for mono amplified speaker signal
negative to sleeve, choose a wire

1 1/4" - for mono non-amplified guitar signal
ground to sleeve

2 1/4" - for mono (each) guitar effects loop
ground to sleeve

I like your instructions a lot better. I'm very open to using old mic/guitar cables for the wiring. I hadn't even thought of that. And for the speaker wire, I can just use regular 14-3 electric wiring like the stuff that I use within the walls of my house?! I've got a ton of that laying around.

[cvanas]

Yeah, you got it. In essence you are just making really short connecting cables inside the wall of your box. The wiring instructions that came with the panels, although correct, are a little on the vague side . Just be sure to connect the correct pins on the XLR's. Don't just solder the pin on the left to the pin on the left of the other panel. It's a little confusing, but if you look at that wikipedia diagram again you can see that if you plugged the male connector into the female the corresponding pins would connect (if that makes sense). For the 1/4" jacks, if you look at where you would solder each wire and follow it with your eye, you can see that one leads to a bent piece of metal that would only touch the tip of the 1/4" cable whereas the other would touch only the sleeve of the cable. Use the electric wire you already have for the speaker connection. Most of the speaker cables I use are lamp cord with switchcraft ends . Don't forget to solder one side first and then feed the wires through the box to do the other, or you will have a lot of extra un-soldering and re-soldering to do. Oh yeah, and test it before you seal it all up.
Cam.
I just reread your first post and noticed that you mentioned XLR's 'capable of phantom power'. All balanced XLR's (three wires) will carry phantom power from a mixer or preamp to power a condensor mic.

[moiremusic]

And did I understand you correctly about the speaker wire? Standard 14-3 electrical wire like I have inside the walls of my house is OK? Or does it have to be braided wire?

[cvanas]

I have never used anything but stranded or braided cables. I didn't think to realize that the electrical wire you were talking about was solid. Duh. I think you would be better off with lamp cord. After a quick websearch it seems that 18 gauge is sufficient for amps up to about 100 watts (and a lot easier to solder to the little pins!).
Cam.

[Ro]

Would a solid-core not be better for speaker wires?

[cvanas]

Yeah, you would think. But in all my time around these things I have never seen anybody use a solid core speaker cable. I have seen plenty of people use guitar cables for speaker cables (which is a big no-no I can understand). Must be a reason for braided only.
Cam.

[Edo Peters]

Solid core is not very smart... there's a possibility of the central core breaking or stretching after some time due to temperature influences or movement strain, and the sole reason why I'd never do that is because of the value of the overall resistance of the wire. Maybe I'm being too strict on myself here, but a braided wire has a number of small cores that form a single wire. The overall resistance is way lower than a single core, thus keeping more of the original signal intact. The braided cores act as parallel resistors and by Ohm's Law, that means less overall resistance.

Check under 'parallel circuits' HERE if you want to read the 'very' technical and theoretical stuff.

Good luck!

[brianrcdd]

I'm afraid I have to disagree with part of what you have said. First, let's make sure we're all talking about the same thing:

Solid= a single solid conductor.

Stranded = many strands of wire twisted together longitudinally to form a single conductor.

Braided = many strands of wire braided together, usually to form a shield around a central conductor.

I agree with you that in most cases, solid core is a bad idea, because of tensile breakage from repeated bending. It also tends to be uncooperative when you try to position it or roll it up, etc. It would also be a mistake to use anything that is for (or even looks like) electrical wire because mistakes can be made in identifying what that wire is used for after the fact. But in theory, a solid wire will certainly carry the signal to a speaker as well as a stranded one.

But your statements on resistance aren't quite right. As the link you included points out right off, resistance to electron flow is determined by (among other things) cross sectional area of the conductor. So all else being equal, if the areas of the individual strands in a stranded wire add up to the area of a solid wire, the resistance will be exactly the same, and it will pass the same current.

But generally speaking, a stranded wire of a certain diameter will actually have slightly higher resistance than a solid wire of the same diameter. This is because there are actually gaps between the strands that can't carry any current.



In the example above, the two different wires probably carry the same current. But also note that the overall diameter of the bundle of strands is larger than the other because of the gaps. In practice, the resistance differences aren't very big. And there are certainly other variables one could consider besides resistance when selecting a wire, but most of those (such as capacitance, inductance, etc.) are fairly small parts of the equation.

It's good that you were thinking it through as you did. The part you missed in your parallel-circuit equation is the resistance of the smaller individual strands, which is much higher than that of the larger conductor. But when you parallel many strands the equation shows that the overall resistance drops back down.

[Edo Peters]

Thanks for clearing that up Brian! I was just thinking along the lines of 1/R = 1/R1 +1/R2 + etc... and you were right that I was using the wrong term 'braided', when I really meant 'stranded'. English is not my native tongue, and even though I trie very hard to keep up with you guys, some words will always be missing from my vocabulary.

[moiremusic]

Well, I'm the one that first incorrectly used the term "braided". I meant stranded.

I'll just get some 14-gauge speaker wire from the local hardware store. I only need 4 feet of it. Thanks everyone for enlightening me in the subject.

[moiremusic]

I stopped off at the hardware store on the way home for that speaker wire, and they were out of 14-gauge. I picked up some 16-gauge. Can I go ahead and use that, or is it important enough that I should try again at another store tomorrow?

[brianrcdd]

16 gauge might work, unless you are running around the world with it. Remember, one of the variables in the equation is length, so the shorter the cable is, the less it is a factor. Just to give you an idea of impact here's some info:

16 gauge = .4018 ohms per 100 ft
14 gauge = .2524 ohms per 100 ft
12 gauge = .1588 ohms per 100 ft

But

16 gauge = .0402 ohms per 10 ft
14 gauge = .0252 ohms per 10 ft
12 gauge = .0158 ohms per 10 ft

You can see how shorter length equals less resistance. So even if you run 100 ft of wire, the resistance of the wire is pretty small compared to the impedance of the speaker, which may be 4 or even 8 ohms. Even if you figure total resistance (coming and going), 100 ft of 14 gauge would only be 11% of the total resistance with a 4 ohm speaker. Another way to look at it: a 4 ohm speaker connected to a 100 watt amplifier would only see about 89 of those watts. If I did my math right, which is always doubtful. The missing power is dissipated as heat along the speaker cable.

On a 10 foot cable, the results are proportionately lower. The same scenario at 10 feet is only about a watt or so of loss. So really, you have to decide if the loss is acceptable, given the length of your cable run. FWIW, I would not run 100 ft of 16 gauge wire to a speaker.

[moiremusic]

Thanks, Brian. I'm only running 4 ft of speaker wire, so it sounds like the difference would be extremely hard to discern.

So does the amount of resistance affect just the level of signal reaching the speaker, or also in the quality of the sound?