Building Your First DIY Speaker - The MB27

[Swerd]

I thought I'd start a thread about building some real speakers, 2-way speakers with a midwoofer and a tweeter. We've had enough subwoofer build .

I hope to interest people who would like to build their own, but have never done it before, and are not sure just how to go about it. There are kits that come with all the necessary parts including assembled and wired crossover boards and cabinets with all the necessary holes. They require only simple assembly, and some even result in decent speakers. An example is the inexpensive BR-1 sold by Parts Express. There are many others sold by Parts Express, Madisound, and Meniscus Audio.

I want to take it one step beyond building a simple kit. I have chosen a proven speaker design which is available on the internet, but no kit for it exists. Building this design does require:

1. All the necessary drivers and crossover parts
2. Providing your own cabinet and cutting holes and recesses in it
3. Building your own crossover from a schematic diagram
4. Assembling and wiring all the components into the cabinet

Because the carpentry for building your own cabinet is a complex subject by itself, I won’t try to describe that in this thread (maybe another time). This thread makes use of premade cabinets that come without any holes cut for drivers or port tube. I will show, in detail, how I cut out the mounting holes for the drivers in the front baffle and the port vent in the rear. And I will show in detail how I made the crossover board, and how I wired it to the drivers and binding posts.

The speaker design is Dennis Murphy's MB27 MurphyBlaster Productions. It is a high quality 2-way design, intended for a small bass reflex cabinet. Many people have built this design successfully – it has stood the test of time. It can be built for a moderate cost, about $273 (as of August 2012) for all parts excluding cabinets. See the parts list. This is far less than what it would cost for a similar quality speaker if you bought it at a retail store. Even with the finished cabinets that I chose to buy (see below),which cost nearly as much as the parts alone for the MB27, the total cost is less than half of what you might pay retail for a pair of quality small 2-ways.

The crossover in the MB27 design was specifically designed for the GR Research G130 woofer and SEAS 27TFFC tweeter. The width of the front baffle, the interior volume of the cabinet, and the mounting positions of the drivers relative to each other on the front baffle, are also critical features of the crossover design. If you change any of these, it may not work as well.

Although the design details are specific for the MB27, I hope my directions and methods will still be useful for anyone who wants to build a different DIY speaker design. It is, however, not my intention to discuss changing major details of the MB27, such as using a different tweeter or woofer. To do that requires a speaker measuring setup and experience in using it correctly. So, for all those who can't resist the temptation to tweak things, I strongly advise you to follow this recipe. If you change it, it will be at your own risk.

Tools & Supplies

• Plunge router with circle cutting jigs (two sizes of Jasper Jigs models 200 & 400) and bits (¼" spiral up-cut bit & 1¼" mortise bit). Other methods do exist for cutting holes for speaker drivers, but I have these tools and will illustrate their use.
• Tools for wiring: wire cutters, small needle-nosed pliers, wire strippers, crimper tool, etc.
• Soldering iron and 63% tin/37% lead (63/37) or 60/40 rosin core solder
• 16 gauge copper wires with red and black insulation (use two colors),cable ties, crimp caps (I like using these instead of screw caps),small & medium sized female quick connects (medium 0.187", and small 0.110"),crimp on terminal rings
• Saw capable of cutting thin plywood or Masonite (hardboard) for crossover boards

For people who need to see the speaker before reading on

 

[Swerd]

Cabinet Modifications

The cabinet as originally designed is 12½" high × 8" wide × 10½" deep. Those are the external dimensions of a cabinet meant to be built with ¾" MDF. See this link: http://murphyblaster.com/files/MB1_Enclosure_Drawings.pdf. You can build your own cabinets by following those plans, or buy pre-made cabinets from a variety of sources for a wide variety in prices.

In this example, the cabinets are finished in gloss black. I have learned the hard way that a smooth glossy paint finish is more difficult for me to do than veneer, so I bought these finished cabinets from Parts Express: Dayton TWC-0.38BK 0.38 ft³ 2-Way Curved Cabinet Gloss Black 302-711. These cabinets have curved sides and have an internal volume of 0.38 ft³.

The front baffles are held in place by 4 bolts, making them removable for cutting the driver mounting holes and installing the crossover boards. These cabinets also come in two other wood finishes, and flat sided cabinets with similar volume, finishes, and construction are also available at lower cost.

Any sturdily built cabinet with a similar internal volume (±10%) and an 8" wide front baffle should work equally well with this design. The curved sides are not required, but they do look cool. These cabinets are not the exact same dimensions as in Dennis Murphy's drawings, but their internal volume is about the same and the front baffle width is exactly the same 8". Those are the critical cabinet dimensions.

Photo 1 shows a cabinet with the front baffle attached. The 4 bolts are loosely attached so they stand out better in the photo.

Photo 2 shows the view from the rear. You can see it came with a pair of ¼" holes for individual binding posts.

Magnets in the grill frame line up with the steel bolts that hold on the front baffle.

 

[Swerd]

Photo 3 shows the cabinets with front baffles removed, providing a view of the interior cross braces.

They extend full height & width, and divide the cabinet floor into two uneven spaces.

As a result, I had to put the crossover on 2 separate boards. Two such boards sit in front of the cabinets in the photo.
The woofer circuit will be on the larger board (left) and will be mounted on the cabinet floor behind the cross brace.
The tweeter circuit will be on smaller board (right) and will sit on the floor in front of the cross brace.

I had to cut out a notch in the lower hole of the cross brace to allow the woofer board to pass through.
Photo 3 shows one notch only on the left cabinet, but both cabinets did get modified.
Later during assembly, I found that two notches were required (Photo 15, later).
Photo 4 shows the Stanley Sharptooth saw I got to do that job. The 10" blade is long enough to reach the cross brace and the teeth cut on both push and pull strokes.

 

[skyline_123]

Anxiously awaiting your next post...

 

[Swerd]

The MBOW1 cabinet drawing shows a tweeter flange diameter of 4⅛" which fits the Seas 27 TFFC. It is horizontally centered 3+3/16" below the top edge of the front baffle. The woofer center is 8" below the top edge. The PE cabinet is 1½" taller but the same width:

MBOW1 Cabinet
8" wide × 12½" high × 10½" deep

PE Dayton 302-711 TWC-0.38BK
8" wide (front) × 14" high x 11.8" deep

To keep the driver vertical locations comparable to those shown in the MBOW1 cabinet drawing, I moved the driver locations lower by ¾".

Tweeter Drill ⅛" hole, centered horizontally, 3 15/16" below the top edge
Woofer Drill ⅛" hole centered horizontally, 8¾" below the top edge

I next marked the layout on the backs of the front baffles (Photo 5).

I drilled ⅛" holes all the way through in the spots marked YES. Ignore the spots marked NO – they are an example of the “measure twice but cut once” rule in action.

These holes will be the pivot points for the circle cutting jig with a plunge router mounted on it.

 

[Swerd]

Photo 6 shows the router with a 1¼" mortise bit (red) mounted on the large Jasper Jig (model 200, grey with many holes).
This jig has an opening large enough for that bit.

I set the plunge depth on my router by putting the flange of the tweeter (plus a bit of gasket tape) directly between the guides.
In my hands, this works better than trying to measure the thickness of the flange and transferring that measurement to the plunge depth gauge.
This is why a plunge router does this job better than a standard router.

Unfortunately, I took the photo showing the back of the router, so you can’t see the plunge depth gauge.

I covered the fronts of the baffles with shelf paper so I wouldn’t scratch their smooth black finish with the router/circle cutting jigs (Photos 7-11).

 

[Swerd]

Photo 7 shows the tweeter flange recess cut. I do this first with the 1¼" bit.
The outer edge of the recess is 4⅛" diameter, just a bit larger than the flange of the tweeter.
It is as deep as the tweeter’s flange (3.5 mm) plus a bit extra for a sealing gasket and the depth of the shelf paper.
Achieving this depth is obviously important, but methods of doing it vary with the plunge router you use.
I attached a shop vacuum cleaner to the router’s dust port to collect the dust as I made the cut.

Photo 8 shows two steps.
First, I replaced the large Jasper Jig with the smaller one (easier to handle) and also switched to a ¼" spiral up-cut bit.
I began to cut out a 2⅞" diameter hole for the tweeter housing.

The cut will eventually go all the way through the baffle board, but I made it in 3 steps of about ¼" on each pass.
My router isn’t powerful enough to cut through ¾" MDF on one pass. Photo 8 shows this cut after the first pass, about ¼" deep.

Before going deeper, I switched to a ½" larger diameter (3⅜") pivot point on the jig, and
made two cuts to allow clearance for the terminals on the rear of the tweeter housing.
These cuts go all the way through (Photo 8). Photo 17 will show the terminals on the back of the tweeter.

 

[Swerd]

Then I returned to the 2⅞" diameter pivot point, and finished cutting the full circle all the way through for the tweeter body (Photo 9).

It’s always a big relief when I see that the tweeter really does fit (Photo 10)!

 

[slipperybidness]

Thanks for taking the time to post this for the DIY newbs.

 

[Swerd]

I cut a 4 13/16" diameter hole for the body of the woofer (Photo 11).
The M130 woofer does not require a recess for the flange.
And finally, I removed the shelf paper and revealed the baffle with the driver holes and an un-marred finish (Photo 12).

I cut a 2" diameter hole on the rear of each cabinet for the port tubes (photo not shown).
I centered them behind the tweeter, 3 15/16" below the top edge.

 

[Swerd]

Crossover Boards

As I mentioned earlier, the interior brace forced me to separate the crossover into 2 separate boards.
I put the tweeter board in the smaller space in front and the woofer board in rear.
I tried my best to measure the available floor space for crossover boards inside the cabinet.

Tweeter Board – a rectangle about 6⅜" × 3⅛" (Photos 3, 4 & 14)

Woofer board – a trapezoid about 5½" (wide side) × 3¾" (narrow side) × 5⅛" tall (Photos 3, 4 & 15)

These spaces inside the 2 cabinets were not exactly the same.
I had to trim each board as needed to fit, so the boards were not identical.
It became important to test fit the boards inside the cabinets as I worked on them.
The tweeter board was easy. Compare the Tweeter Schematic Diagram with Photo 13.

In wiring, I follow the standard convention of Red for Positive (+) and Black for Negative (–). There is one important exception.
The schematic of the crossover for the tweeter shows that it is wired in opposite polarity to that of the woofer.
The tweeter's Negative (–) terminal is wired to the Red (+) side of the circuit and the tweeter's Positive (+) terminal is wired to the Black (–) side
or ground side of the circuit.
This reversed polarity for the tweeter is an intentional part of the crossover.
To avoid confusion, I painted red dots on the tweeter's Negative (–) terminal (Photo 17) so I would remember to attach the red wire from the crossover board to that terminal.

 

[Swerd]

The woofer board, with 1 small and 2 large inductor coils, was more difficult. See the Woofer Schematic Diagram and Photo 14.

 

[Swerd]

Photo 15 shows a test fit of a woofer board after assembly.
It shows the 2 notches I had to cut out of the circular hole in the brace, and
that I also had to trim the black plastic bobbin on the small inductor coil.
Kitchen scissors easily cut these plastic bobbins.
I also cut some off the plastic bobbin edges on the large 1.0 mH white inductor
to keep it from rolling on its edge as I played with different layouts.

 

[Swerd]

And finally, Photo 16 shows both woofer & tweeter boards inside the cabinet.

 

[Swerd]

Final Assembly

• Attach wires to the crossover boards. Label them with tape and a marker so it’s easy to get it right when you later attach them to the woofer & tweeter terminals! Use red nail polish (Photos 17 & 18) to mark the inside tip of the + (red) cabinet binding posts, the positive contact on the woofers, and the negative contact on the tweeters. Remember that this is a 2nd order crossover, the tweeter is meant to be connected with reversed polarity.
  
• If all the crossover boards are fully wired correctly (double check them against the schematic), and you’re certain they fit in the cabinets, go ahead and solder all the connections. Use rosin core solder that is either 60/40 or 63/37 lead/tin.
  
• Drill 1 or 2 holes through the board to fasten it to the cabinet floor with screws. This allows you to remove it if needed. It took some maneuvering to drill these holes in the cabinet floor. Drill the holes through the board just larger than the diameter of the screw threads, and drill holes in the cabinet floor smaller so the screw threads hold. Use stick-on rubber feet under the boards.
  
• Install the cabinet binding posts (Photo 19). I drove them in with a hammer and a block of wood.
  
• Crimp on the rings (Photo 20), or solder on the supplied solder terminals (Photo 19), to the wires from the crossover boards that go on the inside of the binding posts.
  
• Install the crossover boards and screw them to the cabinet. Attach the appropriate wires to the binding posts.
  
• Cut pieces of convoluted foam to fit the interior cabinet walls and install them. I have done this before without using glue, but this time I used hot melt glue. It seemed to work OK.
  
• Install port tubes with 100% silicone sealer inside the lip of the tube. Do this only after installing crossover boards and interior foam. It may be necessary to drive them in with a hammer and a wood block. Allow the silicone sealer enough time to completely dry.
  
• Pre-drill holes for the woofer & tweeter mounting screws. Both accept #8 pan head wood screws. I used ¾" long black screws with heads that accept square drive bits. I genuinely fear slipping while using a Phillips head or normal screw driver and poking a hole in a driver cone, so I use square drive screws and driver for this. They cannot slip. I drilled the holes slightly smaller in diameter than the screw threads and used a drill stop to control how deep I drilled.
  
• Attach the front baffle boards to the cabinets.
  
• Attach wires to woofer and tweeter using either the 0.187" or 0.110" female quick disconnects. I had labeled the wires, so it was easy to know which wire to attach to each terminal. I trimmed the excess wire and crimped-on brass quick disconnects that I found at a local auto parts store (Photo 20). Madisound also sells gold plated brass quick disconnects. The cheap aluminum ones are junk and don’t fit well.
  
• Mount woofer and tweeter to the baffle. Don’t slip while driving the screws. The curved flange of the woofer is easily cracked – don’t over tighten the screws.
  
• Time to listen!

Finally, if the speaker sounds too bright to your tastes, you may experiment with the tweeter L-Pad values. In the tweeter circuit, there is a 7 Ω resistor in series with the tweeter. Try adding 1.0 - 1.5 Ωs extra resistance between it and the tweeter. I find it easier to add extra resistance at the tweeter’s plus terminal rather than on the circuit board. Remember that in this design, the plus side of the tweeter circuit attaches to the negative terminal of the tweeter. I marked the negative terminal with red nail polish so I can’t forget that.

 

[Swerd]

Photos 17-20

Back of Tweeter showing red dots at negative terminal. Both terminals accept 0.110" female quick disconnects.

Back of Woofer showing red dot at positive terminal. The + terminal fits 0.187" female quick disconnect, and the – terminal fits 0.110".

Cabinet Binding Posts

Crimp-On Terminals:
Female Quick Connects 0.187" & 0.110"
Ring Terminals ¼” hole fits Cabinet Binding Posts

 

[Swerd]

When I get smart enough, I'll post a parts list. Right now, I can't figure out how to do it.

 

[Swerd]

µF = micro Farad (a unit of capacitance, 1 µF = 10^-6 Farad)

mH = milli Henry (a unit of inductance, 1 mH = 10^-3 Henry)

Ω = ohm (a unit of resistance)

g = gauge (a unit of wire thickness, 16g is thicker than 19g)

GR Research Speaker kits, loudspeaker design services, sub-woofers and amplifiers.

Madisound Madisound Speaker Components: distributor of loudspeaker drivers and parts for speaker builders.

Parts Express Parts Express: the #1 source for audio, video & speaker building components

In the crossover, you may substitute other parts of nearly identical electronic values if you wish without seriously affecting speaker performance.

 

[Alex2507]

And finally, Photo 16 shows both woofer & tweeter boards inside the cabinet.

The cut spool on the inductor coil says it all.

I still haven't had the time to go through the thread 100% but DM called for 14g inductors. You went with 16g and talked about the small size benefit of yet another drop to 18g? Just checking to see that I got it straight.

BTW, Swerd was good enough to build these for me and as soon as I grow a pair I am telling Cheryl about how I need to go get these and KEW's SongTowers from Maryland. The bumps are there now but I need for at least one testicle to drop before I break the news to her.

Awesome job on the thread. I wish I had time ... but I don't. Wait, it's better this way. Thanks.

 

[Swerd]

The cut spool on the inductor coil says it all.

That's my favorite picture too.

I still haven't had the time to go through the thread 100% but DM called for 14g inductors. You went with 16g and talked about the small size benefit of yet another drop to 18g? Just checking to see that I got it straight.

I wish I had a photo of three 1.0 mH inductors, 14 g, 16 g, and 18 g side by side. The size difference is large. So is the price .

I think the photos imply this, but I'll say it outright. Getting these crossovers to fit inside the curved cabinets was very difficult for me. I only took a picture when I finally got it all in. Notice that nothing was soldered in that photo, as I was still rearranging parts. And none of the photos show the blood from the scraped knuckles I got during the trial & error fit process. Both cabinets got some blood on them, so they are still a matched stereo pair .

I would strongly advise anyone who wants to build one of these speakers to switch to the pre-made rectangular cabinet (it has more useable space inside) or use 18 g wire inductors instead of 16 g.

Yes, the greater DC resistance of 18 g coils will generate more insertion loss, but nothing is worse for sound quality than crossover boards that won't fit in a cabinet !

That's also part of what's so nice about large floorstanding speakers. There is lots of available space for the crossover board.