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Author Topic: Breakout boxes (again)...  (Read 7863 times)

William Nexsen

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Breakout boxes (again)...
« on: November 22, 2004, 02:08:13 am »

I'm curious to hear how you handle wiring multiple input lines in new construction.  I've done it various ways: snakes, snakes with fan ends, snakes into boxes with terminal strips or punchdown blocks, etc.  Each seems to have its own merits and drawbacks.  I'd like to be able to settle on one good way to do it, and price future installs accordingly.

One of my goals would be that the breakout point should be:
   a.  concealed (as in a closet or engineering space)
   b.  easily accessible for service
   c.  can be installed during prewiring or roughing-in
   d.  ideally would provide some sort of access to the locations for the jacks in final installation.

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Tom Young

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Re: Breakout boxes (again)...
« Reply #1 on: November 22, 2004, 09:12:35 pm »

I'm not sure if I'm on track (understand your question), but in professional sound system designs for performance spaces and HOW the most common way is for the mic and line level cables to be individual shielded pairs (easier to pull than snake cable), the ldspkr cables are twisted pairs of THHN (common building wire in 10-12 gauge), etc AND for no splices to be done at anything other than ldspkr levels (all runs are home-runs from point to point) and for direct termination to multipins and XLR's at plug panels.  The only place where punch blocks may occur is at rear of patchbays.  Ldspkr lines have screw terminals within racks or at terminal boxes near racks.  Every splice, terminal and punch down point is a source for oxidation and loss of signal integrity over time.

As important as the above is that the wires/cables be installed in metal conduit (EMT) and separated from one another PLUS from the power lines.  This is based on the data from Giddings' "Audio System Design and Installation" - which is THE bible for installed systems but is out of print ..... can be found used.

Hope that helps.  Get the book.
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Tom Young
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William Nexsen

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Re: Breakout boxes (again)...
« Reply #2 on: November 24, 2004, 07:35:35 am »

Tom,

Thanks for the response.  You're correct, I am referring to primarily to HOW installs.  It seems impractical to me on these jobs to pull the input lines as you describe.  

What I usually encounter is that the construction is dried-in and interior walls sealed before the platform/dias/pulpit area is framed.  I generally pull a multipair from FOH to a closet or engineering space behind the platform at rough-in, then run the individual lines out to their final locations when the platform is framed.  In most cases, some aspect of the platform changes between the drawing and actual construction.  If I have a breakout box, I can accomodate the client's changes easily and provide for future requirements by having additional lines already pulled back to FOH.

OTOH, it would probably easier to just pull individual lines as you describe and leave plenty of excess for changes in the design.

BTW, I do have the Giddings book, but it isn't bedtime reading.  I wish there was a much more practical "do it like this" with pictures book.  

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Karl P(eterson)

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Re: Breakout boxes (again)...
« Reply #3 on: November 24, 2004, 04:12:03 pm »

Well (Sadly?) Sound system design and discipline is beyond "do it like this" with pictures book. The complexity of everything from acoustical considerations to using sane wiring techniques to proper signal isolation etc really makes such a (simplified) book almost impossible (and still be of any general use).

In reality, the process goes more like "It can be done this way.... unless this is like that, but thats only assuming this. ... .... We will have to test!!". The only saving grace here is that (at least I believe) the thought process, the rules and the disciplines just start getting embedded into your mind, and eventually its just second nature to think that way.

But at this stage in the game, I am sure you know all that already <g>.

In any case, On to what I meant to comment on.

As for snake/breakout boxes and such in a HOW, the churches I have been involved in have been largely contemporary churches, and we have done different things on most of them. One thing that I do try to avoid in my designs is stage pockets. The major reasons I do this are that unless you double or even triple up most everything, there is never enough where you want them when you need them. Also, a lot of stages are redone every week, and the boxes will never be in quite the right place. Add that to there magnetic like nature to having things dumped and poured down them, and well, its a small laundry list.

So, instead of stage boxes, most of the time I will recommend a whirlwind (or comparable) system that is mounted into a pull out rack (or with access to the back) with quick disconnects in 8 channel blocks for drop snakes, full patching on the front of the stage box, and a MASS punch  on the back to tie into "pulled-lines" to FOH or otherwise. This can obviously change depending on the situations at play. Sometimes thats out of the price of the church, and we have made our own stage patch panel and then bought cheapish box to fanout 8 channel snakes. Another job i helped on briefly was a fair more complex with everything being 3-way transformer isolated with two of the sends being done via mass-punch and a on-stage mon board off of a MASS disconnect. There is a lot of flexibility here to make the system as simple or complex as it needs to be.

This situation also facilitates easier work for rough-in. All that is needed is the correct conduit from the place where the "patch panel" will be to where your board is. You don't care what they do with the stage. After the fact you mount the in-wall rack or what have you, lay out your cable and get your puller or even run multicore if you like (although I normally would just stick to individual pairs again).

Please tell me you run conduit if at all possible..........

Karl "Its great when I see a romex wire-tied to mic cable" P
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Tim Padrick

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Re: Breakout boxes (again)...
« Reply #4 on: December 23, 2004, 07:05:36 pm »

One thing I must disagree with: unless mandated by code, do not run speaker cable in metal conduit.  The resultant change in the impedance of the cable can noticibly degrade the sound of the system.  If there's no way around it, put the amps at the speaker location(s).

Tom Young

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Re: Breakout boxes (again)...
« Reply #5 on: December 23, 2004, 09:58:09 pm »

"One thing I must disagree with: unless mandated by code, do not run speaker cable in metal conduit. The resultant change in the impedance of the cable can noticibly degrade the sound of the system. If there's no way around it, put the amps at the speaker location(s)."

This is completely incorrect.  

There is definite benefit from running each level of audio system signal in separate metal conduit.  You should also use twisted conductors. Giddings' book "Audio System Design and Installation" details this and it is common practice in professionally designed installed sound systems.

There is NO effect on impedance from this.
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Tom Young
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Tim Padrick

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Re: Breakout boxes (again)...
« Reply #6 on: December 26, 2004, 10:12:21 pm »

Tom Young wrote on Thu, 23 December 2004 20:58

"One thing I must disagree with: unless mandated by code, do not run speaker cable in metal conduit. The resultant change in the impedance of the cable can noticibly degrade the sound of the system. If there's no way around it, put the amps at the speaker location(s)."

This is completely incorrect.  

There is definite benefit from running each level of audio system signal in separate metal conduit.  You should also use twisted conductors. Giddings' book "Audio System Design and Installation" details this and it is common practice in professionally designed installed sound systems.

There is NO effect on impedance from this.


Just having the two conductors in proximity to each other affects the impedance that the amp sees.  This is why the old 300 Ohm antenna wire had a space in between the conductors instead of having them together like lamp cord - spacing the conductors reduces signal bandwidth loss by increasing the inductance and/or reducing the capacitance of the cable.  Twisting the conductors has a similar effect.  Listen to a system with the SPEAKER cables running in or on metal for their entire length, and then without there being any metal in close proximity to the speaker cables.  Try it both at low levels and really hammering the system.  If you do,  I think you will reconsider.  

Deric Craig

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Re: Breakout boxes (again)...
« Reply #7 on: December 27, 2004, 04:55:42 pm »

Hello- I found it interesting about the 300 ohm twin lead and speaker cable comparison.
  The 300 ohm antenna cable was used to connect a appropriately designed TV antenna to a 300 ohm balanced input on older TV's. This line is a unshielded/balanced type design. The RF in the VHF and UHF cable like this, basically "follows" this cable to the TV and not much really flows within the conductors. It is more in a field around the 300 ohm line and by using the stand off insulators, it would reduce absorbsion of signal into very close or touching metal like TV tower, alum siding, HVAC ducts , structural steel, etc. The signal level is in the order of microvolts and the cable must be kept at a distance from metal.
  Coax, with the shield being at basically ground potential, keeps the RF basically within the cable and is much more forgiving when run near metal. It is of course shielded and is a unbalanced interface.
   I feel that with speaker lines flowing many volts and along with current, that together being at levels commensurate with the power delivered to the load, and being a low impedance load, that any little bit of capacitance added by being in conduit or against metal will have hardly any effect to the audio and not any change in impedance. Yes , there is "skin effect" that would cause the higher frequencies to tend to travel the speaker cable strands outside diameter and lends a element of truth to higher strand count permitting better frequency response, but after a certain number of strands, it would be a point of diminishing return.  Just my 2 cents        Deric
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John Roberts {JR}

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Re: Breakout boxes (again)...
« Reply #8 on: December 27, 2004, 05:29:07 pm »

Agreed, and skin effects below 20kHz will be minimal too.

JR
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Andy Peters

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Re: Breakout boxes (again)...
« Reply #9 on: December 29, 2004, 02:17:46 pm »

TimmyP wrote on Sun, 26 December 2004 20:12

Just having the two conductors in proximity to each other affects the impedance that the amp sees.  This is why the old 300 Ohm antenna wire had a space in between the conductors instead of having them together like lamp cord - spacing the conductors reduces signal bandwidth loss by increasing the inductance and/or reducing the capacitance of the cable.


Tim, both the separation between the conductors and their radii set the cable impedance.  Of course RF systems require matched impedances; otherwise, your VSWR goes to hell and you can't listen to the radio or watch your TV.  But this is a transmission-line effect which is not relevant at audio frequencies.

Quote:

Twisting the conductors has a similar effect.


Twisting the wires in a pair keeps reduces EMI and also ensures that any noise present is induced equally on both wires in the pair.  These effects reduce crosstalk between different wire pairs in your snake.  Yes, the twisting helps to set the cable impedance.  But cable impedance is not relevant at audio frequencies.

Quote:

Listen to a system with the SPEAKER cables running in or on metal for their entire length, and then without there being any metal in close proximity to the speaker cables.  Try it both at low levels and really hammering the system.  If you do,  I think you will reconsider.


Yes, the metal conduit affects the cable impedance, but we all know (and I'll repeat it) that cable impedance is irrelevant at audio frequencies.  That's not to say that not there's a lumped capacitance between the cabling and the conduit.  I'd imagine that you'd need a fairly long conduit, or an amp with lousy drive capability, to cause the effects you're describing.

--a
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Deric Craig

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Re: Breakout boxes (again)...
« Reply #10 on: December 29, 2004, 03:30:13 pm »

Andy - I am glad you also commented, I had typed out a more descriptive note and lost the dang thing by accident and didn't want to go back thru all of it again. So wrote a shorter version to send. Right on. I enjoy the topics, contribute when I can and appreciate the knowledge that all of you share. Wishing a good 2005 for all.         Deric
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Tim Padrick

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Re: Breakout boxes (again)...
« Reply #11 on: December 29, 2004, 10:54:33 pm »

Again, the proof is in the listening.  I've done hi-fi installs where the speaker cable had to traverse heating ducts, and the cable had to be suspended from the floor because there was a noticible degredation when it was left lying on the ductwork.

John Roberts {JR}

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Re: Breakout boxes (again)...
« Reply #12 on: December 29, 2004, 11:22:18 pm »

Could you offer specifically how the sound degraded? Perhaps some general details of the before and after that you are comparing.


JR
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Andy Peters

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Re: Breakout boxes (again)...
« Reply #13 on: December 29, 2004, 11:49:04 pm »

TimmyP wrote on Wed, 29 December 2004 20:54

Again, the proof is in the listening.  I've done hi-fi installs where the speaker cable had to traverse heating ducts, and the cable had to be suspended from the floor because there was a noticible degredation when it was left lying on the ductwork.


Sounds like some kinda weird microphonic action.

-a
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"This isn't some upside down inverted Socratic method where you throw out your best guess answers and I correct your work." -- JR


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Tom Young

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Re: Breakout boxes (again)...
« Reply #14 on: December 30, 2004, 02:17:42 pm »

"Again, the proof is in the listening."

Hardly.  If that was the case then directional AC and audio cables, magical dots and green sharpie on the edges of CD's *would* all be worthwhile.

"I've done hi-fi installs where the speaker cable had to traverse heating ducts, and the cable had to be suspended from the floor because there was a noticible degredation when it was left lying on the ductwork."

With all due respect; why don't you at least attempt to quantify this or otherwise back it up ?  If you were right, the entire installed audio industry would be beholding to you for enligtening us all.
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Tom Young
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Hal Bissinger/COMSYSTEC

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Re: Breakout boxes (again)...
« Reply #15 on: December 30, 2004, 07:13:53 pm »

Again, the proof is in the listening.

Yeah, right and that's why shyster companies have a market for $50/foot speaker cable, $120 "audio grade" receptacles and all the other useless crap. They mark it up about 10,000% then suck people into thinking they have to have it.

PT Barnum said it best, "there's a sucker born every day".

I once heard of a guy who wanted his aluminum underground electrical service cable replaced with copper because it would make his (ahem) high end AV system sound better and the video clearer. At least he wanted it replaced until he found out how much it would cost.

Guess he'll just have to suffer along with the rest of us.

-Hal

John Roberts {JR}

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Re: Breakout boxes (again)...
« Reply #16 on: December 30, 2004, 08:57:27 pm »

It is usually not productive to argue with somebody about "what" they heard. However if some audible phenomenon is reliably repeatable a thoughtful investigation will often lead to some valid scientific explanation for the "why".

The given explanation doesn't agree with the experience of others so let's wait for more detail which may reveal the actual problem.

JR
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