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PA-cab construction tips

28-Mar-2017

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PA Cab Construction Tips


This page is designed to compliment the Cabinet-Plans pages - Click here to open lots of plans!


This page applies to any of the plans that you build from the list, and contains important information to get the construction right...... So take some time to read this, or better still, print it out and refer to it when you build the cabs....

TIM-BE-E-E-E-E-E-E-E-E-R !!!

The best material is Birch plywood, but it's a bit expensive. You can use chip-board, but it's real crap and falls to bits easily..... If you can't afford ply, then go for MDF particleboard....but if you do use that stuff, and there's dust flying about from drilling holes or cutting etc...WEAR A MASK.....MDF dust is not very good for you to inhale !!

Preferably, take the plan to the timber yard, and they will cut the 8'X 4' sheet for you on a wall-saw, so the edges will be nice and straight !!....

Use 3/4" (18mm) board, or 5/8 (12mm) ply for the panels, and 1" square baton for the ......Joints !!.....

JOINTS

ER.........No !!!.......not that kind, more's the pity, cos woodwork construction & spliffs seem to go together well !!....... but the kind that stops your speaker cab falling to bits, rattling and farting etc when the volume gets going.....

here's some pictures of the 'REINFORCED BUTT JOINT'.....(..heh heh huh...he said...butt !!..hu huh uhu etc etc...).... which is the easiest joint to constuct for joining edges together.

Anyhow...... check these Joints out, cos all the joints have to be airtight if possible, so get liberal with the white PVA wood-glue....



That square bit of wood called a 'baton', runs right the length of each joining edge.....as mentioned above, use 1" square baton, and preferably make it 'PLANED' (smooth) timber, as this will create a better seal

So first attach the baton to one panel....Pre-drill the screw holes through the sheet edge, making sure that they are the right size for the screws...(the holes should be tight enuff so that the screws need to be tapped home with a hammer)

next, smear plenty of PVA wood-glue onto the baton, and screw it together, making sure the edges are flush with the sheet edge.

Attach all your batons to one edge of two that meet, before putting together the sheet panels.

SEALING

Obviously in a witty mood today, I'll say this is not about getting a baseball bat and going for a quick weekend in Alaska !!.... Nope... but as mentioned, the cabinet needs to be airtight...YES, even though there may be a port-hole in the front panel, the cabinet edges still need to be air-tight.

Try to get speaker units that drop into the hole from the outside of the cabinet, but if you only have ones that fit from behind/inside, then there will be one panel for every cabinet which is removeable, to facilitate the removal of the driver-unit (speaker)...... This is usually the rear panel, and to help seal this, use insulation strip such as you use to seal doors & windows....It comes on a roll from the hardware shop !!....If you are adding a removable back, put a screw every 6 inches or so... But if possible get front loading speakers that drop in, cos a fully sealed cab is preferable !

As a final touch, run some mastic around all the internal corners and along the edges of the batons where they join the panels.

The speakers should come with a rubber neoprene (wet-suit material) gasket to give a good airtight seal, but if it doesn't, use some more insulation strip.

Check whether the speaker is front or rear mounting....(see image below)....Try to use front mounting if possible..... If it is front mounting, then the final panel can be glued & screwed into place after the holes have been cut and bolt-holes have been drilled.

If the speaker is NOT front mounting, then the final panel will need to be screwed home only, with NO GLUE...... You can seal it with more insulation strip.

LOUDSPEAKER FIXING



The speakers will usually come with nuts & bolts to fix it to the panels. Make sure the holes you drill to take these bolts are TIGHT..... The bolt should have to be firmly hammered home through the hole. (if you do hammer the bolt, place a bit of wood over the bolthead or end, and hit the wood...NOT THE BOLT, otherwise you could shag the thread !!

It's better if the speaker mounts from the front, rather than from behind the panel....If the speaker drops into it's hole from the front, then you wont need to open the cabinet to remove the speaker, and therefore all the panels can be glued & screwed firmly into place.

The cabinet panel that houses the speakers, should be cut to size, and tested to make sure it fits into the assembley. Once it is tested and fits correctly, study the plan to get the center point for the speakers by following the cross marks on the plan...... lay the speaker onto the panel, draw round it, and mark out the holes that need to be drilled to take the bolts...... Cut out the speaker hole, and then pre-drill the bolt holes. Again, these bolt-holes in the panel should be the right size, so that the bolt must be firmly hammered through....NOT a loose fit !!

NEVER USE SCREWS TO HOLD THE SPEAKERS TO THE PANEL....ALWAYS BOLTS & NUTS.

CABINET LINING

It is recommended to line the inside of the cabinet with an acoustically absorbant material. This prevents standing waves inside the cabinet, and improves panel damping. This padding is especially effective in the midrange performance.

Use BAF Wadding, Glassfiber insulation, or Mineral wool insulation, or Wool-Waste (Shoddy)....... These are all good materials, and should be attatched to about 1" - 2" thick around the inside of the cabinet, making sure not to block ant port-holes that have been cut. Fix the absobant material to at least 3 sides of the cabinet inside area with either staples, or glue. You don't need to line tweeter or horn cabinets.



LOUDSPEAKER PROTECTION

Right....You don't want some twat sticking their foot through your precious driver units do you ?!!..... Well then you'll better add some metal grille over the front to protect it huh!!

FINISHING OFF

Ok, you can get accessories from music shops to add extra protection to the corners and you can get cheap angled alloy or plastic/vinyl strip to do the same for the edges.......If you want to add handles, and the type you choose requires that you cut a small hole in the cabinet sides, make sure that the handle unit goes into the hole with plenty of mastic smeared over the facing plate, to make them air-tight !!

After that, you can paint 'em or cover 'em in carpet, or whatever you like.

ELECTRICAL CONNECTIONS

OK, you need a socket on each cabinet to get the signal in. This can be preferably an XLR Cannon type connector for best quality, or cheaper, a standard 1/4" guitar type socket.

Again, make sure the seal is good and tight when adding the socket.

Internal wiring should be either phase -colour coded twin flex or equivilent single-core flex. It should have a resistivity of 10 ohms per 100 ft. ....(er this doesn't mean we NEED 100 feet of the stuff !!)

All the internal wiring should be kept well away from moving speaker parts, so best to fix it to the inside panels or batons with cable clips or whatever to prevent buzzing etc. Crossovers should be fitted to the bottom of cabinets, on to foam sealing strip...

OK.......This is VERY important......When you are wiring a cabinet with multiple speakers inside, such as the Full-Range cabinet below, make sure the speakers are wired IN-PHASE........

Mark one terminal as red, or +, and make sure this goes to all the + speaker terminals, and -(negative) goes to the negative terminals.

You can check the Phasing of multiple speaker cabs' drivers, by connecting a 4.5 volt battery to the positive input terminal and checking out what the cones do.......they should all move in the same direction......If one doesn't, then it's out-of-phase, and it's connections should be reversed.

IMPEDENCE MATCHING

Tres Tres impotant mate !!!.......All amps work into a specific resistive load , usually 4, 8, or 16 OHMS........Optimum performance will be obtained, IF the total resistive load (impedence) of the speaker or multiple speakers is exactly correct for the amp's spec'......if it ain't the same, then you get a mis-matching......

If the total loudspeaker impedence is too HIGH, the power delivered to the speakers from the amp will be REDUCED....= Less volume !!

If the total loudspeaker impedence is too LOW, the power delivered to the speakers will be too HIGH....= Speaker overload & damaged amp !!

Any amount of speakers can be connected to an amp, as long as they are wired up correctly so that the final impedence presented to the amp is an exact match.

You can do this with multiple speakers by wiring them in SERIES, PARALLEL, or a combination of the two, SERIES/PARALLEL......USE THE FIOLLOWING FORMULAE WHERE 'Z' = IMPEDENCE.

FOR SERIES CONNECTION......

Z TOTAL= Z1 + Z2 + Z3 ETC

FOR PARALLEL CONNECTION.....












For combined Series / Parallel, calculate the impedence of each cluster of speakers, and treat that as if it were a single speaker with a single impedence. then calculate the total combined impedence using this formulae:

Z = IMPEDENCE IN OHMS





In multi-way systems with several speakers in a cabinet using a passive cross-over, it's important that each seperate section/bandwidth....Bass, Mid, treble, should present the same impedence.

For example.....In a system of 8 OHMS total impedence, it is important that each section, Bass - Mid - Treble, should individualy have an impedence of 8 OHMS.

So, if each or any individual section has more than one speaker unit, then they must be wired to present 8 OHMS...........so, for example, if you had an 8 OHM per channel amp, delivering to a cabinet with 1 bass speaker, and 2 mid speakers, and a treble speaker with a passive cross-over, then the midrange speakers need to be 4 OHMS each and should be wired in series, creating a total together of 8 OHMS........

The exception to this rule is with Tweeters operating at above 3kHz.......At frequencies above 3 kHz, the tereble units can present a Higher, but NEVER Lower impedence than the other sections......

The effect of a higher Treble impedence, is a Decrease in sound output level, but an Increase in power handling capability, proportional to the mis-match ratio........

So, if a 16 OHM Tweeter is fitted in an 8 OHM system, the power handling capability of the Treble Section would be increased by a factor of 2.....er....got that ?!!.......Good.....


Well, th-th-th-that's all folks !!.....er just one last thing.........

When you put the cab's together, before you seal in the final speaker units, check inside to make sure there is no crap left inside the cabinet, such as screws, bit's of wire, cable clips, beer bottles, your watch etc !!!....