By Paul Marossy
Here is a cool guitar mounted FET preamp that you can mount in almost any electric guitar control cavity. One day while poking around on the web for a guitar preamp circuit, I stumbled across this FET preamp, a very simple and small circuit with good performance. It has a very high input impedance of around 3 megohms, about 3dB gain, low noise and if overload occurs, it does so gracefully. This is because the FET behaves rather like a 12AX7 does in a preamp section on a guitar amplifer. If you want to know more details, follow the link above.
I built it per the schematic, with two exceptions: Instead of the 2.2K resistor on the source of the JFET, I used a 10K trimpot wired as a variable resistor. This makes it a lot simpler to get the right voltage at the FET drain, rather than just putting a bunch in the circuit until you find one that falls in the correct voltage range, as suggested by the circuit's creator. The other change that I made was adding a 4.7uF electrolytic capacitor bypass cap in parallel with the source resistor. This increases the gain of the circuit some, but if you want more gain still, you could try using a 10uF cap instead.
Here is some pictures of my project:
Here is the first victim of my new surgical procedure, an Ibanez EX370FM electric guitar that I bought back in late 1992. It's a good sounding, very playable guitar for the money I spent for it.
Here is a close up of the body. Ibanez called this finish "antique violin". I did two modifications to it: I installed a DiMarzio FRED pickup at the bridge (I was really into Satriani at the time, and that's what he used in some of his guitars), and a pickup option switch between the volume and tone controls. This allows me to also have neck and bridge active at the same time, which gives a nice tone, kind of a mellower neck pickup sound or a fatter bridge pickup sound.
Here is the FET preamp board installed in the control cavity. I had just enough room to install it and a 9V battery. I built it on a piece of perfboard, since it's such a simple circuit.
Here is a better view of the preamp board. It is held in place by a small angle bracket. Power to the circuit is switched on and off via a 1/4" stereo jack. That means when you plug a cord into the guitar, the power is turned on. When you pull the cord out, the power to the circuit is turned off. This kind of switching arrangement is standard for most stompboxes.
Here is an angle view showing how it just fits in the space available. I used the smallest parts in physical size that I had in my parts bin to make the board as compact as possible. I used some resistors in series to get the right amount of resistance for some of the resistors, since I didn't have them. The next board I make will utilize a PCB of my own design, and will be more compact still.
The only negative thing about this circuit is that this little preamp would not be practical on a Strat style electric, because it would require you to remove all the strings, and thirteen screws to remove the pickguard to get to the battery. I used to have a '77 Fender fretless P-Bass that had a preamp installed in it, done by the previous owner, and it was a pain to change the battery! It was the same process as a Strat would be... For a Strat style guitar, an external box of some sort may be a better way to go.
After using this little preamp for a few months, I have to say that I really like having it in my guitar. It just seems to have so much more "authority" now compared to before I put this circuit in there.
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