The various acoustic guitar pickups available each have a unique "voice." The characteristics of the signal coming from a pickup are a function of at least three properties of the pickup itself:
In addition, most modern pickups have special signal handling requirements, and thus must be used with a specialized preamp. Many pickups are sold as active pickup systems that include not only the pickup itself, but also a preamp module that mounts in the guitar (usually powered by a 9V battery in the guitar, but in some cases remotely powered). These onboard preamps typically provide three functions:
Through these functions, and especially through EQ, the preamp module plays an important role in determining the "voice" of a pickup system.
In some cases you may want to use a particular pickup with a preamp other than the one provided with the pickup by its manufacturer. You may want to change your pickup without changing the electronics you are using; you may want to remove an internal preamp and rely on an external preamp (for more control or to eliminate the nuisance of a battery in the guitar); or you may want to keep your current pickup but change the onboard electronics (e.g., to use a remotely powered unit). In these cases it may be useful to you to know the gain and frequency response of the preamp you are replacing or the one you are going to use in order to help you set up your system to preserve your previous tone or to improve specific aspects of that tone. The plots here are presented for this purpose.
Shown above are the EQ curves of four onboard preamps (the type mounted on the inside end of the endpin jack). The preamps measured are the Fishman Acoustic Matrix Natural preamp (the model previous to the current model, with a rectangular box rather than a tubular one), the LR Baggs iBeam active preamp, the B-Band Core99 preamp for their UST undersaddle pickup, and the B-Band 2150 preamp for their AST soundboard accelerometer pickup. The curve for each pickup is identified in the legend. The Core99 preamp has four internal DIP switches that the user can set to adjust its response. The lower Core99 curve is with all switches off. The gain reduction switch moves the whole curve down 10 dB. A low cutoff switch extends the low end response by an octave (this may be especially useful for baritone or bass guitars). There are also low enhancer and hi enhancer switches; the upper Core99 curve shows the Core99 response with both of these activated.
Important Caveat: Acoustic guitar pickups are circuit elements themselves, with a frequency-dependent impedance (resistance to current flow). The purpose of the buffer stage in onboard preamps is to eliminate the effects of this frequency dependence from the signal. However, the pickup impedance may itself play a role in determining the frequency response of the pickup/preamp system. In particular, it may affect the low frequency behavior, and in the case of the B-Band pickups, the overall gain of the system may be reduced somewhat from what these measurements show. The measurements were made with a standard test setup, using a low impedance signal source so that only the effects of the preamp circuitry itself are shown, not any subtle pickup/preamp interaction. Keep this in mind when using these curves
If you are using an external preamp, you should be aware of its inherent frequency response if you are trying to duplicate the behavior of some other unit. Most of the more sophisticated units (e.g., the LR Baggs Para Acoustic DI, or rack-mounted preamps) have a flat response when their controls are set to their "no effect" position. But some simpler units have a built-in EQ curve so that their frequency response is not at all flat even when the controls are set to have no effect.
As an example, the plot above shows EQ curves measured for an LR Baggs Gigpro preamp with its bass and treble controls at their "flat" (12 o'clock) positions (and with the gain at 12 o'clock and the volume at maximum). The Gigpro has a low cut filter (the "trim" knob) that cannot be turned off; the upper curve was measured with it is set to its lowest setting (least affecting the tone), and the lower curve is for its highest setting. The response is obviously not at all flat; there is a low end emphasis and a broad dip at 1.5 kHz. The Baggs engineers have presumably settled on this curve as a good "default" setting for producing a pleasing tone from their pickups. In particular, you will notice that the lower curve resembles the curve for the iBeam preamp in the figure above (but with the low end cutoff a little higher than for the iBeam preamp). Thus if you were to use the Gigpro with a passive iBeam pickup, you would not need to adjust the bass and treble knobs much to best duplicate the active iBeam tone (the trim should be adjusted, however). On the other hand, if you were to use a Baggs Para Acoustic DI preamp, you would have to make significant adjustments to do so.
I hope these notes help you "voice" your pickups!
Tom Loredo, June 2001