Electric guitar

Ask the expert: Why does my plugged-in electro-acoustic guitar sound so uneven?

From the March / April 2021 issue of Acoustic guitar | By Martin Keith

Question: “I recently had the action adjusted on my electro-acoustic guitar. The plugged-in sound is now very uneven: the E-high and E-low strings sound hollow and quieter than the others. What happened and what should I do to fix it?Roberta H., by email

A: Ah, the piezo pickups. They have made it much easier to perform live with acoustic guitars, and over the years they have improved dramatically. Originally known for their thin, rough quack, piezo systems have evolved into a large family of acoustic pickups that can be quite musical and rugged, if properly installed. However, that last part is the proverbial Big If – and many (many, many!) Piezo systems suffer from installation issues that result in less than ideal sound.

The most common piezo sensors fall into two categories: under the saddle or top / body mounted. Both have their strengths and weaknesses, and some more advanced systems even combine the two in an attempt to get the best of both. The majority of factory guitars have saddle systems, largely because they are easy to install and offer solid output and less feedback than body pickups. The downside to under saddle pickups is that they tend to have a very strong, spiky transient attack, which can make the guitar harsh or unappealing, especially when played with a heavy right hand. Body sensors tend to have a more airy and natural response, with more instrument character in tone, but they can suffer from weaker output, uneven balance, and problematic feedback. For these reasons, the higher pickups tend to work better for fingerstylists and artists who play at lower volumes in smaller settings.

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Without having your guitar on my workbench, I’d be willing to bet that the culprit is the underside of the nut (the white strip of bone or composite that the strings contact at the bridge). The word piezo comes from the Greek word for pressure, and piezo systems rely on compressing the strings in order to couple with the vibration of the guitar and create a signal. When the mic is sandwiched between the saddle and the bridge, it should be fully and evenly in contact along its entire length. If not, the result will be what you describe: hollow strings and uneven volume.

If your technician adjusted your action by sanding the bottom of the saddle (a common approach), chances are they didn’t sand it perfectly flat and square. Check this by removing the saddle from your guitar (you’ll need to remove the strings, of course!) And placing the bottom edge against a ruler or straight surface. If you can see light or gaps at the outer ends, the saddle should be flattened.

When removing the saddle, note how well it fits into the slot. My rule of thumb for saddle adjustment literally involves my thumb, as well as my index finger: I wish I could remove the saddle using just those two numbers, but it needs to be just snug enough to stay in place when the guitar is playing. came back. If the saddle is too tight, it can get stuck or seized in the slot, preventing the even pressure the pickup needs. However, if it’s too loose, the saddle can tip over in its slot, which also compromises the performance of the mic (and can lead to a cracked bridge!). If the saddle is too tight, lightly sand the faces with 220 grit or larger paper until it fits as described above.

If you’re still having issues with balance but the saddle is perfectly straight and flat, the next possible culprit is the split in the saddle itself. Most flat top guitars aren’t exactly flat – some have slightly domed tops designed during construction, while others were built flat but bulged forward due to string pressure. Some amount of this is normal and is generally taken as a sign of a reactively constructed instrument. However, this top distortion can sometimes result in a flexed bridge, which could mean that the bottom of the slot has taken on a convex shape. If you have a skilled luthier nearby, they can often redirect the saddle slot to create a flat surface if the top has flexed too much. It may be necessary to use jacks or other supports inside the guitar to simulate the pressure of the strings on the top, so that the slot can be flattened with precision.

A slightly less invasive approach to treating curved saddle slits is to create some flexibility in the saddle itself. In cases where flattening was impractical or impossible, I solved string balance issues by drilling five small holes (1/16 of an inch) through the saddle, between each pair of strings. These holes go through the saddle in the same direction as the string paths, and I drill them just below where the saddle is visible at the bridge, so they don’t show on the assembled instrument. . Then I saw a small slit from the bottom of the saddle down to each hole, creating a keyhole shape. This idea follows the same principle as the grooved liner on the inside of your guitar: the slots allow the saddle to flex evenly and help it conform to the curve at the bottom of the slot in your saddle. This quick approach solved many string balance issues for me, in less time than it takes to mount a Dremel tool and cut a slot.

It can be tempting to try and put a little wedge under the weak rope, try to regain some extra pressure and balance the balance. I encountered a lot of them while digging into bridge setups to resolve these pickup issues. In my experience, this approach is generally almost works, but never completely fixes things. Adding a wedge to the weak string will concentrate more pressure on that spot, but it will create a step in the surface that will deprive the adjacent string (s) of good contact. If the strings don’t respond evenly, the problem is that the surfaces aren’t flat and even, so making them even less likely isn’t going to be successful. This is a case where one has to treat the cause, not just the symptom.

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One last point to note: piezo systems are also very sensitive to the signal chain. I have had several clients who brought me instruments to resolve an apparent pickup problem, only to find that the guitar sounded good when plugged into an appropriate input. Any passive piezo mic will need a high impedance input (like an acoustic preamp) to sound good, and even many active on-board systems suffer from mismatch between mic and preamp. If the sound is thin, rough, too compressed, or lacks warmth in the bass, this could easily be the result of a wrong guitar and amp combination. Unfortunately, these same issues can also result from fit and installation issues, so sometimes it takes an experienced technician to tell the difference. I’ve spent what seem like years of my life trying to debug uneven piezo systems, and I know how frustrating it can be. If you’re lucky, a little quick sanding or filing on the bottom of your bridge saddle should make your guitar sound great.

This article originally appeared in the March / April 2021 issue of Acoustic guitar magazine.

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