JFET preampA preamplifier with Junction Field Effect Transistor (JFET) makes your guitar sounds great, operating in similar manner to the vacuum tubes. This circuit is very simple to build and can be used as a booster.
Designators and components Step 3 of 5
BRIEF INTRODUCTION FOR JFET PREAMPLIFIER DESIGN
In the circuit, the value of R1 represents the input impedance. For guitar applications, 1 MOhm is a common choice. You can also use 2.2 MOhm or even 10 MOhm, you guitar will become brighter; however the noise will be higher. The resistance R2 attenuates the high frequency (in combination with C1), you can use 33k or 64k Ohm. This resistor is optional; you can omit it by placing a jumper. The capacitor C1 attenuates the high frequency. You can use from 100pF to 330pF to make the guitar less bright. By placing C2, you can obtain all the possible gain of the transistor, however it will introduce a small amount of noise, you can omit it if you need very low noise preamplifier. Finally, C3 is the output capacitor, it blocks the low frequency signal, you can increase the value to let pass more low frequency signal. Capacitors C1 and C2 are optional; however you need to place C3.
The values of Rs and Rd define the gain and bias of the preamplifier. In class A amplifier operation, the output signal needs to swing from V+ and GND. For clean audio applications, it is recommend set the bias point at half value of +V. For example, if you use 9V power supply, set the bias at 4.5V. However, for guitar applications, it is common to add some harmonic content to the output signal. In this case, you can set the bias point closer to GND or V+ and let the signal saturate easier. If you set the bias point below 4.5V, the saturation will produce hard clipping. If you set the bias point above 4.5V, the saturation will be softer and smoother (tube sounding). By placing trimmers on the PCB, you can easily adjust the preamplifier behavior and find the sound you like. The bias voltage can be measured by a voltmeter over the pad Rd on the PCB. You can also obtain the voltage Vs by measuring on the pad Rs. For simplicity, you can only use one trimmer and fix the value of the other resistor. If you want to use only one trimmer, you should fix the value for Rs and use trimmer for Rd.
On the internet you can find much information about FET guitar preamplifiers. Here a couple of great references:
COMPONENT LIST (Some suggested values)
With this PCB you can design your own JFET preamplifier and easily bias it by adjusting the trimmers. You can use many types of FET transistors (n-type) such as 2N5457, J201, 2N5458 or MPF102. In the table you can find some recommended components, however free feel to experiment and create your own preamplifier.
The component value "-" means that you dont need to place anything.
Note: If you use a filtered power supply and the circuit still presents hiss noise it maybe comes from the trimmers, you can substitute them for better ones or use regular resistors.
GENERAL DESCRIPTION OF COMPONENTS
In some cases the component value is "jumper". You can use a small wire or component leg to create it. They are mounted on the component place (diode, resistor, capacitor... )
The resistors are ¼ Watt metal type. You can either use a multimeter or the color bands to obtain their values. Resistors don’t have polarity; you can assemble them in any direction.
Electrolytic capacitors have the value printed on them. The polarity is also is marked for the negative lead, additionally the longer leg is the positive and shorter the negative. Assemble the electrolytic capacitor according with the positive polarity (+) marked in the PCB, that is, the longer leg of the capacitor is passed through the pad marked with +.
The polyester capacitors have three numbers, read as picofarads (pF), the first two are the 1st and 2nd digits and the third is the multiplier code. This type of capacitors doesn’t have polarity; you can assemble them in any direction.
The ceramic capacitors have three numbers, read as picofarads (pF), the first two are the 1st and 2nd digits and the third is the multiplier code. This type of capacitors doesn’t have polarity; you can assemble them in any direction.
Potentiometers can be mounted directly on the PCB. For a correct orientation, the potentiometer shaft is marked on the PCB.
Transistors are three terminals components and their model and name are printed on them. To indicate the correct assembling position one side of the transistor is flat and the other one is round, in the same way, the PCB is marked with round and flat side.