Here the circuit design for DIY guitar transistor preamp. It consists of a circuit of two transistors and four controls that can be vary the level of the treble, bass, gain, and total salidad circuit (volume). It can be used as buffer at the beginning of a chain of effects. Or combine it with the Marshal Guvernor and a power to put together a cute amplifier.

The circuit working with 9v dc power supply or battery.

Guitar Preamp Components list:

Nonpolar Capacitors

• 0.01uF
• 0.1uf
• 4700pF
• 0.047pf

Electrolytic Capacitors

• 1uf x 2
• 1uf x 2 (Tantalum)

Semiconductors

• 2N5089 (NPN or other) x 2

Resistors

• 1M x 2
• 47k x 2
• 2.2k x 2
• 100k x 2
• 4.7k
• 22k
• 1k

Potentiometers

• 250 Kohm linear x 2
• 250 Kohm log
• 10 Kohm linear

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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).

Gain
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.

Output
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).

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The Guitar pre amp circuit is based on a FET-type BS170, and require very little support components. The polarization of the gate (G, gate) of FET is provided by two resistors R1 and R2. Both resistors are also to set the input impedance of the assembly, which here is close to 500 ohms. The original scheme, the second resistor set at 92K each, and keep the input impedance of less than 50K, which may be too tight to plug the guitar.

BS170, As usual with this type FET or MOSFET power transistors smaller (TO92 plastic housing in particular), you should check the pin manufacturer in the document (datasheet), with consideration for the brand, because the manufacturers do not necessarily offer the same pinouts for the same component model. The BS170 is here using the Motorola brand and pinout is as follows.

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The guitar pre amp assembly does not really particular comment. The use of a type TL072 integrated circuit for the input stage is explained by the fact that it has a high input impedance (FET input) which allows to set the input impedance of assembly to the desired value, by 1Mohm, R1. Amplification is carried out by two separate stages to ensure a better stability and bandwidth when the gain is at its maximum. The completion of the two balanced outputs is facilitated by the use of line driver circuits specialized for this purpose in classical SSM2142. You will notice the presence of 680 ohm resistors on each output of the SSM2142. It is possible to do without it, provided to connect the terminals 7 and 8 together, and the terminals 1 and 2 together. The manufacturer instructions (datasheet) is not very precise about this, but I noticed better performance in distortion at high gain (> 50dB) … but in all honesty, through action, because the ear I have find any difference. Do as you wish.

Notes :
– Wire R7 and R3 as close to U1: A, R4 and R5 and the closest to U1: B
– If you see a parasitic oscillation when the gain is maximum, add a capacitor of 22pF to 120pF in parallel with the resistor R5.
– Minimize the cable length between the input connector and the input assembly.
– The SSM2142 can be replaced by DRV134, it has the same pinout and is a bit more efficient.
– The power supply must provide a dual voltage of + /-15V, and be perfectly stable.
thanks

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