Introduction: An Open Back Cabinet for Guitar and Bass

About: PhD chemist, who works very hard at totally worthless but amusing hobbies!

On a whim I purchased a used Electroharmonx 22 Caliber—a neat little pedal-sized amplifier that puts out 22W. (unfortunately this pedal is no longer in production, but it's big brother the 44 Magnum is) It arrived, and I realized that I didn't have a cabinet to run it through. After considering buying a cabinet and a speaker, I decided to saw up some low-grade 1/2” plywood and build my own. My pedal specifies 8ohms, so I selected a 10” Celestian rated at 30W and 8ohms. The project was finished in a Saturday, and a couple of minutes after work for finishing. It's holding up great—very solid, no rattling and good “open” sound. The total cost was approximately $40--including a new Celestian speaker. Many parts were salvaged from my parts supply. Good heavy speaker wire was used, along with left over 1/2" plywood. It was a fun project, and very satisfying. A great introduction to building audio equipment.

Step 1: Speaker Cabinets

There are basically 2 types of cabinets. (well, 3 if you include ported cabinets). Sealed cabinets are literally gas tight—the joints are caulked, openings are gasketed, etc. The air contained in the box acts as a sort of “spring” that helps the cone recover after it moves. The sound is sometimes described as “tight”. This Instructable, with minor modifications, would work equally well for a sealed cabinet. The design is more complicated because the volume of the box is an important factor. Normally the speaker manufacturer will provide information about the volume of air that should be used.

The cabinet described here is an open-back design. It is a design found very commonly on guitar and bass amplifiers. Old Fender, Marshall, Ampeg, and Vox amps used this extensively. Normally, you think about the sound coming out the front of the speaker cone. The reality is that sound also comes out the back of the cone—and out of phase with the front. This leads to some sound cancellation, and a slightly diminished volume. I have never found this to be a problem—you can still crank the volume from the amplifier to get the required volume. The sound of open cabinets is sometimes described as “loose”. Placed near a wall, or especially a corner, they just fill the room.

Whether open or sealed, the cabinet needs to be very strong. Even at 22W the speaker jerks violently. Poor corner joints will eventually rattle loose—and buzzing components will NOT enhance your sound. In fact with this cabinet, the first version did not use a gasket or caulk behind the flange on the speaker. It buzzed so bad, I seriously thought the amplifier was broken or the speaker was blown! Build it right, build it solid.

Step 2: Design Considerations

This Instructable is meant as a starting point for your project. I needed a 1-speaker cabinet that could handle 20W. There was no real need to use more than one speaker. If you are running something hotter, or maybe you would like a cabinet with two distinctly different speakers (kind of a neat idea) there is no reason the box couldn't be expanded for 2 or 4 speakers. The issue to keep in mind is the output impedance of your amplifier head. Amp heads will always clearly state what the impedance is. 4, 8, and 16 ohm are very common. Speakers can be purchased in the same values. I chose 1 8ohm speaker to keep it simple. However, I could have built a cabinet with 2 4ohms (in series), or 2 16ohms (in parallel). If you're not familiar with the calculations, there are dozens of examples available online. The size of the speaker can really influence the sound. I have some toy 9V amps that run off of small 4” or 6x9” speakers. At 1/2W, I guess it really doesn't matter. It makes noise, and it's amusing. If you want to produce actual top notch sound—for actual recording or live playing, I would say 8” is the absolute minimum for a guitar. 10” (like this one) is noticeably better, and I regret being too cheap to buy a 12” right from the start. Splurge and get a good speaker. This one was $35 new. A 12” would be slightly more. There are several online suppliers, and some notable brands include Celestion, Jensen, Peavey, and Eminence. (this one was purchased through Antique Electronic Supply--great folks to deal with). Speakers are also sometimes available "used" from people who bought an amp and upgraded the factory drivers. Check your local music shop.

The cabinet needs to be built strong. While not the most attractive material, plywood is very strong, and very dimensionally stable. In this cabinet, I used 1/2” grade C plywood. It can be finished any way you like. This one is in clear polyurethane which can take a lot of abuse. Dimensional lumber could also be used, but construction would require planing, probably more complicated jointery, etc. It could really produce something beautiful. But my mission here was something abuse-resistant, and cheap. I would say 1/2" is the minimum thickness. If I was going to build a 2 or 4 speaker unit, I would step it up to 3/4". In the roughly 1'x1' size here, 1/2" plywood is plenty strong.

The shape and volume of an open backed cabinet is not terribly critical. I built mine for small size, stability so it wouldn't get knocked over, and materials available. If the cabinet was going to be used as a monitor, it would be possible to make the sides into the shape of a trapazoid, so the speaker was pointed slightly upwards. It would complicated the build a little bit, but should not be beyond any “handyman's” skills. For a 10" speaker, the sides were cut to 12 and 13", to form a box with 12" inside lengths. This left a mounting board with 1" of space minimum between the flange and the sides.

Step 3: The Build

As stated, I used grade C plywood because it was available (free) in a thickness of 1/2”. To go with my home-built cigar box guitars, I decided to finish it in clear poly. The plywood has a heavy grain and knots—it goes with the low-tech look of the instruments. If you want something really sharp, use good veneer plywood, or at least a smoother grade of plywood to accept stain or paint. I started my build by ripping the plywood in to 6 1/2” pieces, and cut 4 of them. 2 of them to 12”, and 2 of them to 13”. These were assembled into a square (with the 13” pieces but jointed to the 12” pieces). Thick pine glue blocks were installed on the inside corners using Titebond II glue, and square drive drywall screws. Make sure to use a square on the corners, and to double check, measure the diagonals. If the box is square, the diagonals should be the same distance.

To ensure a tight fit for the front (the speaker board) the square box was placed on a sheet of 1/2” plywood and carefully traced. The wood was then cut on a table saw to just a hair oversize. The sound board was then carefully shaved down on the table saw (removing a fraction of a saw blade) until it fit, but required a bit of pounding with my fist. Before the front was fit into place, glue blocks were installed 1 1/4” inside the box.To ensure that the front was never going to rattle loose, some cheap super glue was applied from the inside in several passes. If there are significant gaps, the gaps can be filled with baking soda prior to the super glue.

The front requires a hole to accept the speaker. Most speakers have a template that can be traced. My speaker actually did not have a template. I prepared one by tracing the speaker (face down) on a sheet of paper, including the mounting holes. I then measured the width of the flange, and from this determined the radius of the required hole. The hole was cut with a jigsaw. It turned out to be a hair too small for the speaker cone. To make the cone fit, a round rasp was used to file the inside edge of the hole a bit into a bevel.

The back is not completely open—there is a length of pine covering about 1/3 of the opening. This helps the sound, and also the structural integrity of the box. The 4” x 3/4” pine was cut to exactly the correct length. It was then installed in the box with glue blocks and drywall screws. The front was installed again with glue, and with small finishing nails driven in to the edge of the board. To ensure that the front was absolutely rigid, and old model making trick was employed. The small gap between the sound board and the sides were filled in with baking soda, and then the entire thing was glued in place with cheap super glue (from the inside—totally invisible from the front). I have been playing this thing full blast for months, and the box is absolutely solid.

The box can be finished however you want. I did mine in spray polyurethane. Paint would be fine, or even fabric covered for a professional look

Step 4: Finishing

After the cabinet is built and the wood is finished, the cabinet can be completed. This cabinet has a galvanized handle that appears to be for use on a screen door. The hardware store can provide many options, or you could go really low-tech, and drill holes in the top and install a piece of rope to serve as a handle. The cabinet was also fitted with rubber feet. It sits now on carpet, so the feet are not required, but I thought if it ever sits on hardwood, I am asking to damage the floor. Also, there is a potential for rattling and buzzing.

My amplifier pedal has standard 1/4" phono jacks. I decided to run a cable from the cabinet to the pedal. Normal audio cables really are not designed for use after the amp, so in this case I used some good heavy speaker cable. It was soldered to a 1/4" male phono jack (radio shack). Mind the polarity of the plugs. The standard is for the center to be hot or (+), and the shaft to be negative or (-). My wire was color coded. I put gold on the center, and soldered it to the (+) on the speaker. I also tied a knot in the wire inside the cabinet to serve as stress relief. You really don't want the wire pulling on the speaker .

Finally, the speaker can be installed. Years back, while installing a speaker with regular pan-head wood screws, the screwdriver slipped and went directly through the cone! I will never mount a speaker like that again. This speaker has a flange with 4 holes. I placed the speaker in the hole, marked the mounting holes, and removed the speaker. I drilled holes for 1/4" machine screws, and installed the speaker with bolts. This speaker did not come with a gasket. To ensure the speaker would not rattle, A thin bead of black silicone was laid down, and the speaker was placed in the hole. Careful that caulk does not get on the speaker cone! Then the bolts were tightened. Get them snug, but not so tight that it distorts the flange. Really the silicone should hold the speaker in place.

Step 5: Parting Thoughts

The cabinet would benefit from some type of screen or grill. I'm browsing sources of these, but it looks like they are commonly available from car audio dealers. I need to get that finished before it's time to move. The speakers will not tolerate getting stabbed with something.

Speaking of driver selection, I bought a 30W because it's a good bit higher than my 22W amp. Make sure your speaker cabinet can handle your amplifier played at full blast. If the amp is hitting the speaker too hard, the speaker can feel too much "excursion" and blow out. For example, I mentioned that the 22W Electroharmonix is no longer in production, but the 44W is. As far as I know, there is not a single speaker that could take that much power. The amp pedal is also rated at 8ohms, so a cabinet similar to this could be built--but with 2 4ohm speakers in series (impedance adds in series). Alternatively, it could be built with 2 16ohm speakers in parallel. At some point I intend to get the 44W, and build some kind of micro-Stack speaker cab.

This was a fun and satisfying project, and shockingly loud out of the little Electroharmonix pedal.

Finally--please observe all safety considerations while performing this work. I used a table saw--which obviously can do a lot of damage to your fingers. A circular saw could also be used--again, extreme care needs to be used. Watch the fingers, and wear eye protection. Have fun--if you build a similar project, or have already built one, I would love to see it! Always looking for new and interesting music projects!!

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