I would not plan on adding a built in preamp on my next mandolin. It is easy enough to hear the instrument unplugged and un-amplified without having to design and build a complicated preamp and take up the space for a battery and the circuit. It actually has a descent sound un-amplified while still being quiet enough to play without disturbing too many people.
Smaller Main Cavity
Since there isnít a need for so much room in the main cavity without the preamp and battery, I would make the cavity for the pickup much smaller. This may make connecting the cavities more difficult since I was barely able to drill between cavities as it was. A routed slot between the back and main body may be necessary to connect the pickup to the potentiometers. A second pickup in the bridge position would nicely fill up the rest of the cavity if you decide to go that route.
Pickup Mounting Options
As an alternative to mounting the cavity from the back like I did, you could also mount the pickup in a cavity routed into the top. The pickup in this placement could be trimmed with a small cavity cover on the top of the mandolin. Another option is to simply mount the pickup without any trim directly into the top of the instrument. This style seems to lend a high end, custom made look due to the high level of craftsmanship required to pull it off.
Other Pickup Issues
Almost all guitars have at least two pickups while many have three which can all be used in different modes for an array of tones. The one neck position pickup on this mandolin is rather limited in comparison. Multiple pickups will definitely be worth experimenting with on a future mandolin.
The mandolin could be made more versatile with a hotter pickup. The pickup I have is rather wimpy at a resistance of 6.4K compared to its over-wound EMG branded counterpart at 11.0K. This pickup is good for jazz and acoustic like sounds, but I havenít been able to make some of the more extreme tones such as the heavy overdrive and distortion used in metal and rock, not that too many people use electric mandolins to play heavy metal.
Although this pickup has done a descent job, there are more expensive models that are better made and would have a better tone if your budget is not an issue. Although I havenít tried them first hand, I have seen Dimarzio rail style hum-bucking pickups on professionally made electric mandolins. There is also the higher end EMG-HZ S3 pickup I mentioned above which is also a dual coil, hum-canceling, blade pole piece design.
Output Jack Placement
The one ergonomic design flaw I have discovered in playing the mandolin is the position of the output jack. Since the mandolin is so small, I tend to rest it on my knee where the jack is on the lower bout instead of its waist. I had though about a number of different positions: facing outwards from the top may have obstructed playing without an angled cable (plus the body wasnít thick enough to accommodate the whole jack in that position); there wasnít enough room to fit a Stratocaster style jack; it would have been difficult to get the wires to where they need to go and may have interfered with the player with the jack on the other side of the mandolin. The current position of the jack wouldnít be a problem when standing, but I usually sit to play (mostly because I have yet to find a strap I like.)
Alternative Construction Methods
Despite some aesthetic issues, the construction method I used on this mandolin was relatively effective. In part, I constructed the mandolin this way due to my familiarity with the mortise and tennon neck joint. I also settled on this method since I wanted to get started with building as soon as possible and I couldnít find the wood I needed for other methods. As it turned out, some parts of this mandolin ended up looking awkward, for example, the joint running down the side of the mandolin resulting from attaching the back. To alleviate the awkward joint, I would suggest using a much thinner piece for the back (about 1/8 inch) if you use the same construction method. The joint could then be covered with binding and paired with binding on the top edge for a more finished look. The corner of the neck joint on the side without the cut out was also a bit awkward. The binding, paired with a thinner back and the addition of a button would make this section appear better trimmed. Depending on the design I choose, I will likely try a different method for my next electric mandolin, although this method is workable if you canít find the ideal piece of wood.
Depending on whether you plan on using the mandolin for travel, you may also like to try out the standard tuner placement on the headstock. The reversed tuner position is slightly harder to tune with, but makes it less likely that the peg head will break off, a common weak point on many stringed instruments. Still, it isnít necessary if you donít need to keep the mandolin compact for travel.
A Larger Pattern
A larger body would have made for an easier instrument to hold and a less cramped fit for the various electrical components. If the mandolin isnít going to be used for travel, it would be better to use a larger pattern. Many of the components had to be tightly packed into their cavities making for difficult design and installation. The potentiometers were the most difficult to fit into the body. I almost ended up using more expensive and harder to find mini potentiometers due to the space issues, but was able to fit full size pots with some well though out packaging.