How to Build a USB Guitar

Bill Machrone, Contributing Editor, PC Magazine

The first question is why you'd want a USB port in an electric guitar, and the answer is simple: convenience. By putting a small USB codec in a guitar, you're putting an extra sound board in there. So when you record, your guitar is digitized locally and the signal transmitted to your computer over a USB cable. You plug your headphones into the guitar, so you have more mobility and fewer cords to get tangled. The guitar still has all its standard analog capabilities, but USB output as well.

This is a PC Magazine Build-it project, one of many ongoing examples of cool things you can do with computer technology.

For our project, I've selected a simple, well-made guitar and the Micro USB interface that's bundled with M-Audio Session, a flexible home recording package that has many of the features of GarageBand, but records one track at a time. M-Audio provided several Micro USB interfaces for experimentation and agreed to provide a discount on Session for PC Magazine readers.

Here's the victim guitar on the operating table. It's an SX STL Ash, a decent, inexpensive ($120) Telecaster-inspired guitar from Rondo Music. This is Rondo's budget line; I did a bit of setup work on it to make it a player. Rondo's higher-end Agile line is considered a "secret weapon" by many pro musicians--high quality, durable, but considerably less expensive than name brands.

A Tele-style guitar is ideal for our purposes--bolt-on neck, lots of room in the control panel and under the pickguard if we need it. The full-thickness lower edge gives us a nice place to mount jacks without gunking up the face of the guitar.

The heavy ash body and jumbo frets give it great sustain. The belly cut is Strat-like, deeper than those found on Telecasters, and the armrest bevel is Strat-like too, so it's a very comfortable guitar to play.

The Micro USB interface has two 1/8 inch jacks, one for guitar in, the other for stereo headphone out. It has a short cord with a USB plug. It opens easily with just your fingernails and a little pressure.

It has Micronas USB codec onboard, preconfigured to record a mono input at a 16-bit, 44.1KHz sampling rate--CD quality. It outputs the signal in both the left and right channels.

Here's an approximation of the jack and module layout. The Micro USB module fits easily in the control cavity.

The topmost jack is the normal guitar analog output. I'm changing the stock jack (very inexpensive) to a rugged Switchcraft jack. The second jack was intended to be an auxiliary input jack. I misread the documentation on the Micronas chip and thought I could run a second guitar or a microphone into the Micro board and have the guitar on one channel and voice or second guitar on another. Unfortunately, I didn't discover my mistake until the project was mostly finished, so my guitar has an unnecessary, useless input jack.

the third jack is a stereo headphone jack. Most small headphones have 1/8 inch plugs, but I find 1/8 inch jacks to be weak and poorly constructed. For durability, I went with a 1/4 inch jack. I prefer to use headphones with a 1/4 inch  jack or I use a 1/4 to 1/8 inch adapter on the headphones.

The USB female jack is from L-Com, part number ECF504-12AAS. It's the smallest panel jack I could find, and I liked the chromed plastic shell. I figured that a male-male 10 foot cable would provide good mobility. I decided that the mini USB connector would not be rugged enough, so went with a full-sized A-style connector. A B-style connector would also work, and A-to-B cables are easier to find than A-to-A cables.

Shop around when you look for long or unusual USB cables. My male-male cable was only $2.50 from an Amazon affiliate; I could have spent much more.

 

I struck a centerline with an adjustable square, laid out the holes, and then drilled 7/8 inch holes with a spade-spur bit. I'd have preferred to do this on the drill press, but it required too much reconfiguration and jigging/clamping. Freehanding with an electric drill worked fine.
I drilled a smaller hole for the USB cable and marked out the size of the connector ears on the guitar body.

I cut the edges with a chisel and then used a small router to clear out the inner portion. You have to be careful when chiseling into end grain not to hit it too hard or go for too much--it's easy to split the wood and do serious damage.

I opened the USB jack shell with a jeweler's screwdriver. The jack and circuit board slide out of the shell. I cut the excess length off the circuit board and also shortened the shell with a hobby saw.
I made a note of which colors went where and removed the wires from the jack and shortened the cable on the Micro USB module. I then stripped and soldered the wires from the module into the female connector board in their proper places.
Here's the shortened shell in the mortised recess I made for it, plus an aluminum cover plate for the rest of the jacks. I made the rectangular opening on a benchtop milling machine, but you can do as good a job by drilling the corners, cutting the straights with a coping saw, and filing the edges.

I decided to recess the cover plate, which is unnecessary, but just felt like the right thing to do. I held it in position and traced around it with a hobby knife to cut through the finish and into the wood for subsequent routing and chiseling.

Here's the routing in progress. As with the deeper mortise, I used a sharp chisel to define the edge and then routed up to it.

This router base for the Dremel rotary tool is sold by Stewart-MacDonald, a supplier of musical instrument tools and materials. It's far superior to the plastic Dremel router base. I used a flat-bottomed 1/8 inch router bit.

On the aforementioned benchtop milling machine, I drilled the 3/8 inch holes for the jacks with a step drill. Step drills are good for thin metal because there is no tendency to catch in the metal and no spiral flutes to pull or distort the work.
I had originally intended to remove the jacks and solder directly to the Micro USB board, but decided that the jacks wouldn't get any wear and tear inside the guitar. One connects to the guitar output, the other to the stereo headphone jack.
Here's the completed side panel--with the useless Aux In jack. I did the labels with a Dymo LabelWriter, using black on clear tape. Use a second nut or a spacer washer inside so the jack fits flush with the outside nut. I have a spacer and a lockwasher behind the panel.
Now that the electronics are squared away, it's time to turn our attention to the guitar's playability. The feel of a guitar mostly comes from the way the neck is set up. From the factory, you can expect that the nut is not cut deeply enough, so the strings are too high off the fretboard.

I used special nut-slot files, available from Stewart-MacDonald and other lutherie suppliers.

You can also expect that the fretboard has too much relief, or belly in the middle. While this is good for preventing string buzz when you strum hard, I prefer a more gentle approach. So I take most of the relief out and set the board nearly flat, using the truss rod adjustment at the left.

I suggest that you not start hacking away at your guitar if you've never done a setup before. I highly recommend Dan Erlewine's Guitar Player Repair Guide as a clear, authoritative guide to repairs and setups.

If you really want to learn about guitarmaking, setups, electronics, and the like, visit the Musical Instrument Makers Forum. I've been a regular there for ten years, and you'll find a wealth of information in the Forum's library, plus knowledgeable advice from experienced  members. The BS factor is lower than just about anywhere else on the Web.

On an inexpensive factory guitar like this one, the frets are seldom as level as they should be, which causes buzzing. With high, jumbo frets like these, I could just sand all of the frets with a flat straightedge, like the side of a level, and then re-round the fret tops, but why take off more material than you have to? Instead, I use a variety of small straightedges to "rock" the frets three at a time, left, middle, and right, so I can see and feel high spots. I scrape the high spots with the straightedge or use a hollow-curved fret file to remove the high spots.

You can see how big those frets are--considerably larger than those you'd find on a Fender Telecaster, but in keeping with current style. They make it easy to bend notes or add vibrato.

 

I covered the fretboard with masking tape to prevent damage during the smoothing and polishing phase.

Many factory-fresh guitars also typically have sharp fret edges hanging out a bit beyond the edge of the fretboard. They can actually cut your hand if you're not careful, and they're uncomfortable to play. I ran a file down both sides to trim them back. One nice thing about this SX guitar is the bound, or plastic-edged fretboard: I didn't have to worry about cutting into wood or refinishing after I smoothed the fret ends. The plastic doesn't need finishing/refinishing.

The fret file has concave edges, filled with diamond grit. A couple of strokes at each end of the fret made it smooth and round, but left visible scratches on the frets. I also filed any of the frets that I had scraped level, to make them rounded again.

Then I went back over the entire fretboard with 600 grit sandpaper held over the file, so the concave would guide the paper and polish out the scratches. I followed up with 1200 grit paper, which left a smooth, matte finish on the frets.

I then buffed the frets with an emery-charged wheel. This left bright, smooth frets that are a pleasure to play on.
This guitar is also getting inlaid PC Magazine and M-Audio logos in the fretboard. I came up with a simple, "cheater" way to do this, instead of the usual abalone, mother-of-pearl, or other exotic materials. I routed logo-sized recesses in the neck and printed the logos on clear transparency film. I laid aluminum foil in the recesses, then the logos, then covered them with clear epoxy.

Note that the router base is riding on the vise jaws, not on the frets, to give a consistent platform from which to create the recess.

I then used a small tool steel scraper to remove the excess epoxy and shape it to the curve of the fretboard. I wet-sanded it with 600, 800, 1000, and 1200 grit paper to restore the shine, and buffed it with successively finer buffing compounds.
Here's the finished product. Action is light and smooth; the guitar is a pleasure to play.
The finished guitar, all back together, ready to rock--and record.

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