While part 1 spoke broadly on the physical laws of sound and how they relate to acoustic guitar amplification, this article will narrow its focus to the types of pickups and preamps available, highlighting their strengths and weaknesses. To get the most from this article it’s important to read part 1 first.
Reproducing the Fundamental
Acoustic pickups that fit under the bridge saddle constitute a large part of the market. Within this genre there are a number of manufacturers, all of which claim to have the best sounding technology.
The strong points are that these pickups have a lot of output and do not visually alter the guitar. If you play a custom or vintage instrument, for which the unaltered appearance is important to you, the latter can be a very attractive feature.
The drawbacks to these pickups are that, historically, they are difficult to balance from bass to treble (although this seems to be improving with better technology) and they tend to be on the bright-and-twangy-sounding side. However, if you like bright-and-twangy (a staple in country music), this is a plus.
If you are into improving the intonation of your instrument and use a wide, compensated saddle (anything significantly wider than 1/8”), it will be more difficult for this type of pickup to ‘sit’ properly. The effectiveness of these pickups is largely dependent upon a snug fit. If the saddle has been widened it makes this more difficult.
Because of the different materials, design, and placement, soundhole pickups have a much mellower sound than under-saddle pickups (read: less twang). These are, in design, similar to electric guitar pickups and, to my ear, sound a lot like a jazz archtop pickup.
The strengths are that they are much easier to install than under-saddle pickups (some you can literally pop-in or take out without removing the strings or using any special tools) and most have movable pole pieces (again, like electric pickups) that can be adjusted to easily compensate for an imbalanced sound.
The drawbacks are that, since they’re magnetic, regular acoustic strings won’t produce balanced volume. This is because bronze (the material that covers the wound portion of an acoustic string) doesn’t generate as strong a reaction in a magnetic field compared to steel (the unwound strings). The two steel strings will always be louder than four bronze strings. A simple way around this is to use a heavy gauge set of electric strings (or create your own custom sets from individual gauges). This way all the strings (wound and plain) are steel and have equal reaction in the magnetic field. Also, magnetic pickups are more susceptible to external noise such as hum produced by a lighting system in a club than their under-saddle counterparts.
Capturing the Overtones
For great sound, the microphone has always reigned supreme in the recording studio. But, one of the things you quickly learn as you gain experience performing and recording is that these situations are worlds apart.
Given the choice, no sane recording engineer would ever place a microphone inside the body of a guitar for a recording session. This is because it’s a small wooden box where standing waves and reflected noise abound.
Using a hypercardioid pattern mic can help block out some of these problems. Finding the exact location for the mic takes some patience and care. A windscreen that covers the mic can help reject some of the reflected frequencies that may not be so pleasant.
It’s also possible to get a mic that clips onto the outside of the instrument. This eliminates some of the above mentioned problems but cuts down the volume level considerably (ie – it’s louder inside the box than outside). Theoretically, this is the best place to put a mic, but it’s not as effective if you are playing with a band. The sound generated by the other performers’ amplifiers will be picked up and bleed into your sound source.
Many companies have produced contact pickups with widely differing results. The better ones rival microphones for their clarity and sonic accuracy while the lesser of the species sound harsh and microphonic.
The strengths are that they are small, have virtually no bleed compared to a microphone, are not susceptible to the reflected sounds created inside the body of the instrument, and have a very dramatic, present sound. When you strum hard or whack the guitar, the audience not only hears it, they feel it.
The downside is that these pickups need to be placed strategically on the guitars top. As with microphone placement, finding the right spot may take some time. Also, contact pickups are among the weakest in signal and have the highest impedance. This can be a deadly combination. To compensate, they must be matched with an extremely high-impedance, low-noise preamp.
Another thing to consider is the material used to affix these pickups to the instrument. My experience has led me to use silicon glue. It can easily be found in any hardware store and is designed to create a solid bond that will last for decades without ever becoming brittle. It begins to bond within a few minutes and is cured inside of 24 hours. The great thing is that it creates a rubbery type bond that, even years later, can be unmounted fairly easily without damaging the pickup or the instrument.
If you are an electric guitarist, a certain amount of noise is accepted, even expected, in the amplification process. In acoustic guitar, however, only the cleanest, most pristine signal is acceptable. If this weren’t a tall enough task, acoustic pickups are usually lower output than their electric counterparts, so the preamp must be of the highest caliber to achieve great results. Check the specs on signal-to-noise ratio. There are certain terminology that can tell you things. Preamps that are discrete-component are usually higher quality and lower noise than integrated preamps (and usually more expensive).
The preamp should be the first thing that the pickup reaches in the signal chain. In theory, the shortest physical space (cable length) between the pickup and the preamp is best. Some pickups have preamps built in as a complete system that can be powered by a battery in the guitar. Another popular setup is to have the preamp built into the endpin jack. If, however, your preamp is outboard, keeping the cable length ten feet or less should be satisfactory.
When the signal leaves a pickup it is high-impedance and unbalanced. In this state it is weak and susceptible to noise (ie – external noise sources such as halogen lights and radio frequencies can degrade your sound). A preamp will convert it to low-impedance. Some, but not all, preamps will take the unbalanced signal and convert it into a balanced one. To have your signal leave the preamp as low-impedance AND balanced is the best scenario. This means that, after leaving the preamp, the signal will be less susceptible to any external noise as it travels through the cable and into the PA system. Many preamps will produce a low-impedance UNBALANCED signal. This is still much better than the original pickup signal, but not as good as a balanced signal. Check the specs on any preamp closely. Just because it has an XLR output does not mean it’s producing a balanced signal when it leaves.
Many of the better preamps available have built-in EQ, phase switching, ground lift, and effects loop. These are all elements that will come in very handy when trying to deal with a live performance situation. The equalizer will allow you to modify the tonality of your sound. It’s very handy to have this at your disposal when performing. If you need to roll off a little bass, you can do it much more effectively (knowing how much you want rolled off) than trying to convey it to the soundperson. Phase reversal can stop low frequency feedback without ever having to adjust the EQ. A ground lift can come in handy in a club with old, noisy wiring. And, even if you think “…I’d never use effects on my acoustic, I’m a purist,” you’d be surprised how much definition a little compression can give to the low end of your sound, without ever sounding like your using an effect. A good, clean effects loop can maximize the use of any pedal.
Live Sound Tips
As I mentioned at the closing of Part 1, live sound is an art. First-hand experience is the best way to learn. By dealing with bad sound situations you learn how to overcome them. If I had to give a few pointers that I thought to be universal in the scheme of acoustic guitar amplification, they would be as follows:
Use as little equalization as possible. The more EQ you use, the thinner (and more ‘electric’) the sound will become. If you have a good instrument with quality pickups and preamps, you don’t want to change that — just refine it. Inevitably, though, every room you play in will emphasize problematic frequencies. You will need to remove these to keep the sound of your instrument clear. My philosophy is to try and solve the problem with as little effort as possible. For instance, if I notice a low rumbling frequency (maybe a low D), I’ll try reversing the phase. Often times that will stop the problem. If it doesn’t, I notch out that specific frequency. By being as conservative as possible with equalization, you will keep the fullness and character of your instrument intact.
When working with two sound sources (pickups), sound check each individually before mixing them together. This way, if there’s a problem frequency (and there may be a different frequency for each pickup, since they’re handling different tasks) you can isolate it first, then move on to putting the finishing touches on your sound.
Stand in different spots on the stage (facing in different directions). You don’t want to find out in the middle of a song that if you move your instrument will begin to feed back. Also, try to get out into the audience area and see if it sounds as good out there as you think it sounds on stage.
When assembling your amplified setup, there are a lot of options. The best thing you can do is assess your needs and see which products best fit them. A lot of companies offer fully integrated systems, from pickups to preamps. These will save you a lot of time (and possibly money) when it comes to creating an entire system, but they usually offer fewer options than a mixed-and-matched system you construct yourself. Assembling a system that leaves room for growth is definitely a smart way to go.