1: Who am I?
My name is Brian Lojeck, I'm an Electrical Engineering student at Cal State, Long Beach, and I'm trying to teach myself to play guitar. I don't have a lot of audio knowledge, but I'm good with Google. ;-)
2: WTF, dude?
I recently purchased a Roland Microcube guitar amp used on Ebay. It's a good amp, but had a REALLY crappy speaker installed, which means it makes a farting noise whenever you hit a low note at any reasonable volume. Some research turned up that this is a common problem. In fact, the manual that comes with the Micro Cube says it will sound better through headphones then through the speaker.
Also, some research turned up that a lot of people asked about replacing the speaker, but nobody seemed to answer the question. Here, I discuss and document my trials and tribulations doing just that.
3: The Roland MicroCube, disassembly and investigation:
The Micro-Cube is a tiny (9"x8.25"x6.25") battery powered, 2 watt guitar amp. It's a "modeling" amp, which means it contains audio processing electronics that allows it to simulate the sound of several classic guitar amps.
It's generally regarded as a very good practice amp, especially for apartment dwellers who cannot be too loud. The only true weakness in the design is the fragile, poor quality 5" speaker.
Figure 1: Micro Cube faceplate
Figure 2: Micro Cube top panel
Figure 3: Micro Cube rear panel
I set out to disassemble the amp, to take some measurements. The metal faceplate grill is held on by three wood screws (visible in Figure 1) and a bit of adhesive tape under the Roland logo. Removing the faceplate exposes the speaker, held by 4 screws and connected electrically by friction-fit spade connectors.
Figure 4: Faceplate Removed
Figure 5: Speaker Removed, spade connections visible
Figure 6: The dust cap was crushed by the previous owner. Perhaps he has a young son, kids seem to love poking dust caps.
The plastic pieces over each corner of the chassis were easily removed (just a few small screws on each). The metal plate that holds the controls and forms the rear and top panels of the Cube was tougher, there are several large screws on the back panel, and adhesive tape holding it to the top. You'll need to slip a thin screwdriver or putty knife or something along the top to slowly pry it loose.
Once the back panel and the corner guards were removed, it became clear that my Micro Cube had been dropped at some point. The Cube is VERY well put together, made of MDF, with heavily glued joints. However, most of the glue joints on one side had cracked or given out, showing a pretty solid impact had occured.
This actually worked to my benefit, as one side of my amp was very easy to remove. To remove the faceplate (where the speaker mounts) you need to remove at least one side of the amp, it fits into tounge-and-groove slots all the way around. If your amp is still whole you should be able to drive a thin screwdriver into the glue joints at two corners. Keep in mind the tolex plastic covering will need to be cut to remove the panel completely (it forms a hinge at 3 of the 4 corners), but any damage you cause should be covered by the plastic corner braces on reassembly.
Figure 7: The removed panel, with the faceplate slid out
Figure 8: The joints may need a bit of prying, they are solid
Figure 9: Tounge and Groove construction throughout. All joints are very tight, a sign of good construction
Figure 10: The chassis, with back panel removed
Figure 11: Closeup detail of corner joint
Figure 12: More signs of good construction. Cables are epoxied in place, important when the entire system vibrates. The speaker connections snuk into the photo (upper right)
Figure 13: Oddly enough, with all their care of construction, Roland put the "ugly" side (left) of the faceplate out, and the "pretty" side (right) was hidden inside the amp.
Figure 14: A look inside a Micro Cube, with the speaker removed.
4: The Replacement Speaker
The speakers used in guitar amps distort the sound played through them. This is a good thing, since the wide range of sounds we've come to expect from an electric guitar are really created by distortions in the amp and speaker, not in the guitar itself. If you buy a guitar speaker, you'll find them sold as "British", "American", or other terms to try and describe the flavor of sound they produce.
Modeling amps, however, are different. In a modeling amp the digital circuitry inside handles all the required distortion of the sound. What you want out of the speaker is accurate rendition of the sound created by the amp, more like what you look for in a home or car audio system.
The speaker in the Micro Cube is small, 4-ohm, and has different-sized positive and negative terminals. This tells me it's a speaker meant for car audio use. This, combined with the statement in the instruction manual that the amp sounds better through headphones tells me the replacement speaker I buy should be one from the car audio industry, preferably a 2 or three way speaker with the range to handle all the higher harmonics made by a distorted guitar. In a perfect world I'd use a full subwoofer, midwoofer, midrange, tweeter setup from home audio to replicate ALL of the frequency range, but then I wouldn't be able to fit the package into a 9x8x6 box, and it would take more then two watts to get decent sound.
I had to take some measurements. The "faceplate" is the removable panel that the speaker attaches to.
Full faceplate 8.5"x7.5"x0.5" (actually about 15/32") Visible faceplate 7 1/8" x 6 3/4" Back of faceplate to circuit board 2 7/8"
Note that the thickness of the faceplate is just shy of 1/2". This is going to bite me in the ass in a paragraph or two.
I had three basic choices for mounting the new speaker. I could mount it behind the faceplate, in front of the faceplate, or fill in the hollow area in front of the faceplate, re-tolex the entire amp, and mount a larger speaker on the very front with some kind of decorative cover. I decided to set out to find the largest speaker I could mount while still using the stock steel cover the amp came with (I don't want to change the look substantially), which means it needs to either have a shallow front to top-mount or a shallow rear to bottom-mount.
With some research, I found the Polk Audio DB651s. Car speakers only come in pairs, apparently, so two of these cost me $90 (plus tax and shipping) at Crutchfield.com. I highly suggest Crutchfield, as they have good service and a good return policy. The speakers got decent reviews, and best of all, they fit perfectly within the amp.
Figure 15: The speakers
Figure 16: The measurements shown on the back of the speaker
Figure 17: The speaker, and the decorative cover it comes with
Figure 18: It fits!
Now that I have a speaker, I need to cut a new faceplate, and drill the new hole to fit. This is where I got bit in the ass. I went to Home Depot, picked up a 2'x4'x0.5" sheet of MDF, visited a friend with a table saw, and cut a bunch of replacement faceplates (hoping to sell them here, to be honest). When I tried to insert it, it simply didn't fit. It turns out, the evil bastards at Roland used a faceplate that's about 1/32" less then a half inch, and cut the groove for this plate to VERY close tolerances.
Figure 19: The stack of faceplates I cut
Figure 20: It only fit this far when I hit it with a mallet
A few minutes with some sandpaper, and the issue was fixed. Before you look at these next photos, please keep in mind it's VERY difficult to cut a round hole without a jig of some sort. I think I did pretty good for a guy freehanding it in his garage with a Roto-Zip. Also, note that because I'm mounting a front-mount speaker in a rear-mount position, I won't be able to use most of the stock screw holes (they are too close to the hole the speaker is sounding through). I never claimed to be Norm Abrams.
Figure 21: The gasket was meant for front-mounting, and had to be trimmed.
Figure 22: Hey, stop laughing. It fits, and it doesn't impinge on the speaker's movements.
Figure 23: Note the non-standard screw placement. I'll place another couple like that when I do final assembly.
I did some testing at this point... The audio quality is MUCH improved over the broken speaker that was installed before, and the audio range is greater. Bass still isn't fantastic with a little 6" speaker, but for a little pocket-sized amp it will work.
Now, comes final assembly. I didn't bother with step by step photos, because it goes the same way it came apart, only backwards. The one thing I did different, is I added some speaker grill cloth to the faceplate, under the metal grill, to improve the look, and re-glued the broken joints.
Email email@example.com with questions or comments.
Thanks for reading!
UPDATE: I tried drilling a 1.5" "tuned port" in the bottom of the amp, to replace the one that was in the face plate. I didn't notice any difference in sound. I do notice, though, that the speaker I used is pretty directional; the high tones are stronger when you are in front of the amp.
UPDATE (1/6/09): As I hoped, this article has inspired someone to crack open their own Micro Cube and install a front-mount speaker.
UPDATE (1/7/09): I'm thrilled that another person has taken what I've done here, and used it to complete their own project.
Roger Banks ( firstname.lastname@example.org ) contacted me, and sent me some photos and an MP3 file. I'll cut and paste his own words to describe the project
Anyway in return - here's a photo of what I was doing - I do a lot of live
recording and I wanted to be able to get a line-out from the headphone jack
without cutting off the internal speaker. A wire link would have done, but
instead I fitted a small toggle switch just in case I ever need to use
(I didnt bother labelling the switch because its obvious when it 'on')
I used a RadioShack 275-625A SPDT toggle, but anything would do really.
Ive had some suprisingly cool sounds putting the microcube through the PA
In addition to normal speaker/headphone use, this arrangement allows either:
1) simple direct line-out feed to the PA as well as internal speaker on.
2) mic in front of speaker into PA as well as line-out to recording system
Interestingly, the switching isnt just on the speaker output, Roland seem to
switch it before the power amp stage. It seems a really well designed little
amp to me.
And here's an mp3 of it gigging on New Years Eve off a live-air recorder.
(It was 1:30 am by then and it wasn't only the crowd that were the worse for