Guitar preamp, based on an FET BF245 and BF256 type. Very easy to build, operate with 9 V battery. Unlikeprevious FET based Guitar pre amp which has gain of close to 1, which is presented here provide the gain (if all goes well).
Guitar preamp Input
The input impedance is mainly determined by the value of R1 and R2, the FET with an input impedance of several tens of MOhm. It is approximately 1 MB and corresponds more or less, even to the value of R2. The coupling between the sensor (microphone) and FET guitar is done through the capacitive coupling capacitor C1 associated with R2 forms a highpass filter whose cutoff frequency is low enough (about 30 Hz at -3 dB with a slope 6 dB / octave).
The preamp gain is determined by the value of resistors R3 and R4 respectively located in the branches Drain (D) and Source (S) of the FET. Potentiometer RV1, which provides a parallel, more or less pronounced on the capacitor C2 resistor R3, can make the variable gain which can vary from about 15 dB to about 25 dB. The maximum gain is obtained when the capacitor C2 is completely in parallel on R3, that is when the potentiometer RV1, mounted variable resistor, and a position of minimum resistance (cursor R3). The gain is minimal when the cursor is on the side of C2.
The amplified output is performed through the coupling capacitor of 4.7 uF C3, which is placed in parallel a second capacitor C4 6.8 nF. The second lowest value capacitor helps to let a little more treble. If you find this annoying, just delete it and let up only the capacitor C3. The resistor R5 located at the end of the circuit and wired directly in parallel on the output is not especially critical. It reduces the risk of plop when connecting or disconnecting the output of the preamp by setting the potential of capacitors C3 and C4 to a known value (here zero volts since R5 is connected to ground).
Bypass (Guitar preamp on / off)?
There are several ways to perform a bypass, making the preamp off and allowing the direct passage from input to output, avoiding the rewiring of cables AF when you do not want to use the device. The different methods are described on page Bypass effects (consider that this preamp is an effect).