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Acoustic Instruments and Amplification, Chapter 7

Pre-amps and Equalization
Before we get into a discussion of preamps, perhaps an overview of exactly what amplification does is in order. While we may think of an amplifier as a device that just makes our instrument louder, it is really more of a copy machine and overhead projector combined, ie, it "takes a picture" of our basic signal, then makes a larger copy of it, and sends that to the next amplification stage. For any given amplification system, there may be as many as a dozen gain stages.first, the preamp, then on to an amp that splits or sends signals to the respective efx loops and auxiliary sends, there is amplification for the channel equalizer, then an amp to combine all of these disparate signals back together. After that(in a typical mixing board), there are the output mixing busses which combine signals from the various channels, then finally, an output amp to send signal through an outboard eq (typically a graphic), and then onto the power amp(which in itself has at least three gain stages).

Most of these amplification stages are used to combine and refine the signal, and are not necessarily used for gain. An active(preamped) signal is usually much easier for the amp designer to use as it provides improved impedance matching linearity, which in turn, reduces phase shift problems. This is especially helpful in equalization design, where phase inversion is used to boost or reduce a specific bandwidth.

Modern amplifier stages have almost no discernable harmonic distortion or phase shifthowever, because each stage is limited in how much gain it can produce, dynamic range is reduced. This lessens the impact and punch of your highly dynamic acoustic instrument.

The preamp is the first amplification stage, and as such, is the most important. All signal that follows will be a very close match to this first stage. A preamp that is correctly matched to your pickup will provide much more accurate frequency response and reduce extraneous interference. The condenser mic, since it has a voltage applied to it at the source, can really be seen as having no impedance, and offers the added advantage of strong signal to the preamp. Just about any preamp will work with a condenser mic, and all mixing boards are set up to deal with the relatively low output from a dynamic mic. As most dynamic microphones are low impedance devices, they present the little resistance to current flow and consequently have wide and flat frequency response.

Since early days of amplification, tube amps had been the norm. Of course, a tube requires "heating" a filament so these are necessarily high impedance amplifiers and match up very well with magnetic pickups. The rub here is that the higher the source (pickup) impedance, the more frequency response anomalies appear unless the amplification stages are designed to compensate. However, most tube preamps have a generally sweet tone (based on the 12AX7 preamp tube) and you can be pretty well assured of a good sounding rig if you employ one of these with your magnetic pickup.

Piezo devices are ultra-high impedance transducers and as such present the player with a dilemmahow to match the pickup with the preamp. This is why the typical "acoustic/electric" guitar has an active preamp on the guitar itself. There is so much resistance to the flow of current to and from the pickup, an amp must be very close to accept the signal, or any semblance of accurate frequency response will be lost. In addition, the phase shift that is present with saddle 'ducers in particular, contribute to the typical "quack" we hear from these devices. It is the rare (and expensive) preamp that can deal with this phase shift problemand since manufacturers aren't willing to publish this specification, it is up to you to educate your ears as to which will sound better to you. If you are planning on using a saddle transducer by all means listen to everything, from onboard preamps to outboard "blenders" (used with dual source systems).

Once the signal has been conditioned with the preamp stage (which matches impedance and adds gain), it is now compatible with just about every amplifier available. You can send the signal to a direct box, which will split it up for both stage monitoring and house PA. Or, you can run direct to equalization, to help correct for poor harmonic response and feedback reduction.

EQ is probably the most overused and misunderstood function in the amplification chain. For definition purposes, equalization is the selection of a frequency (or group of frequencies) and then cutting or boosting that frequency to suit the amplification needs of the moment.

To put it bluntly, if it isn't right to begin with, EQ will not correct the problem. While modification of your signal with slight or general tweaking of the EQ curve may help, it in and of itself cannot correct poor tone quality. You will discover that IF you have your pickup and preamp system sounding good without EQ in a low gain setting, you will probably not need very much of it at all.

However, when you start running your gain up to compete with crowd noise or other instruments, finely controlled equalization can really help you to "cut through" the mix. The typical 3-band EQ found on most amplifiers and PA channels is a broadband EQ that is easily designed and can be very effective for overall tonal quality. The low and high frequency controls are shelving curves(where all frequencies either below or above a given "knee" frequency are either boosted or cut). Lows are generally shelved (rolled off or boosted) at 100 hertz, and highs at 10K hz. These rolloff points are arbitrarily chosen by the designer for the specific application; acoustic guitar amps may have differing points chosen. The midband control(s) are a peak/dip curve (the frequency point chosen is the midpoint of the curve).

The graphic EQ uses a peak/dip curve, but has more than several, at typically equidistant points along the frequency response curve. The 10-band, or octave graphic divides the frequency spectrum into the 10 audible octaves, with the center frequency reacting more to the control fader than the outer frequencies. The 15 band graphic, or 2/3 octave, divides the audio spectrum into 15 equal parts, and the 31 band into 1/3 octave increments. The more the control is moved from "flat" the sharper the response curve becomes. Of course, since the frequency and "bandwidth" are fixed, the frequencies that need to be adjusted may be on the edge of the response curve and the user will be adjusting more of the sound than desired.

This brings us to the parametric EQ, where the frequency point, bandwidth and amount of boost/cut can be defined. This is perhaps the most useful of the equalizers for acoustic amplification purposes, as problem feedback frequencies can be pinpointed and removed without much effect on surrounding frequencies. On good quality units, the bandwidth will be adjustable to 1/12 octave (one note) through at least two. There will typically be at least three complete parametric bands to choose from, and they will have overlapping frequency responses, so the user may define two or more problem frequencies that are close together.

The modern acoustic electric amp will have a "notch filter", which is essentially a narrow band, variable frequency, cut-only EQ. This will usually help with either the secondary or primary feedback node.

Next Chapter: EFX and signal modification

To learn more about Dave and the guitars he builds, please visit http://www.electrocoustic.com

Dave Wendler is a luthier at Ozark Instrument, and holds a US patent in the field of acoustic instrument amplification technology.