Acoustic instrument amplification is still a nascent field with a myriad solutions and no silver bullet. Put another way, there is no ideal solution for acoustic guitar such as the wide-diaphragm condenser microphone used in studio recording. In fact, many musicians tell me that if you want a true representation of your instrument, go with a traditional mic/mic stand pointed around the 12th fret and be prepared for feedback.
Even so, electronic pickups have been steadily improving- even in the last decade. For this reason, among others, we do not cut big holes in the instruments and install large 'barndoor' preamps or battery compartments which may be removed later.
Blackbird uses the rechargeable Simple Jack preamp from MiSi, which offers the improved sound and convenience being battery-free mated with the industry standard Fishman Matrix pickup. L.R. Baggs Lyric internal mic systems have also garnered quite a following. None of these systems are perfect. For instance the L.R. Baggs Lyric requires a 9v which is easily installed in El Capitan with access through the large sound hole. K&K also makes some nice passive undersound board transducers such as the pure mini, which sound great and don't require preamplification yet still have a strong signal. These however can be more difficult to handle in high SPLs (loud venues like playing with a drummer!) compared with undersaddle piezo pickups.
Finally the image above shows the RMC Acoustic Gold mated with RMC's Polydrive II preamp for the ultimate in piezo sound with each string resting on a separate pickup. RMC is a bit involved and expensive system, but offers superior string balance and dynamics. While that piezo 'quack' can be dialed back with the Polydrive II, its still audible. We could install the Polydrive onboard, but then what happens down the road when you want the latest greatest version? Not to mention the large unit/barn door that robs acoustic tone. It is especially important for RMC to be kept up-to-date because of guitar synth functionality which Roland changes every few years. All told there are some great options out there now, but we'll still see lots of innovation and therefore new form factors and technologies. This means that the current crop of systems will eventually be antiquated with future systems getting closer to the holy grail of a studio level sound with less feedback and easier interface. The instrument on the other-hand will not go out of date (hello pre-war Martins) so keeping the holes to a minimum opens the door to future possibilities.
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