FET Preamp In A Small Control Cavity

Last Updated 9/17/05
By Paul J. Marossy


 

Having used an FET preamp in several of my guitars now, I don't like playing any of my guitars that don't have a preamp in it anymore. I especially wanted to put one into my Ibanez S470FM guitar, but the control cavity is so small that a 9V battery can not fit in it. For a long time, I thought I would just have to live with this guitar not having a preamp circuit in it.

Recently, I had a thought about how I could add a preamp to the guitar making it compact as possible by using a small PCB and some small batteries. My initial thought was to use three CR2325 3V lithium batteries in series for 9V, but I found a 12 volt "N size" alkaline battery that is small enough to fit into the control cavity and an added bonus was being able to easily obtain a battery holder for it rather than having to fabricate something. I had to tweak stuff just a little bit, but it worked out really well.

Here is the unsuspecting victim, my Ibanez S470FM. It's a Saber type model with a mahagony body and a flame maple top. The color is "British Racing Green". It plays very well and sounds pretty good for a medium priced guitar. I really enjoy playing it.
Because the guitar has such a thin body, the control cavity is much smaller and shallower than an RG type Ibanez. This is the control cavity before I started the project.

Here is my self-designed PCB. It measures 1"x1.25". The trimpot is for adjusting the FET drain voltage, as I outline on this page . As I usually do, I used a socket for the FET just in case I ever need to replace it or want to try a different one on the circuit.

Here is the PCB installed in the guitar. It fits nicely in the dead space under the volume pot and next to the pickup selector switch. It's fastened to the guitar body with a single small wood screw.
Here is a view of the battery mounting. The battery holder is attached to the guitar body using some Super Glue. This battery does seem awful small, but it should actually last for a considerable amount of time. I'm thinking that the battery ought to last somewhere between 50-70 hours before the voltage drops down too low to be very usable.
It was also very convenient that the existing jack was already a stereo/switching type jack. All I had to do was connect the battery ground to the unused terminal and I was done.

This is somewhat of an experiment at this point, but I created this page to give some ideas on how to deal with a small control cavity such as this one. Another thing that could be done is to phantom power it using a stereo guitar cord and a seperate box with battery. Time will tell how long the battery will last, and if it really is practical or not.



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