Electric Guitar/Violin Preamplifier

Magnetic pickups in musical instruments exhibit a relatively high output impedance, which can lead to a decrease in treble response when connected through long cable runs or to equipment with low input impedance. This preamplifier addresses these challenges by providing high input impedance and low output impedance. It is powered by either a battery or a DC plug pack. The input signal is AC-coupled to the non-inverting input of IC1a, a component of the TL074 quad op amp. This op amp features JFET input transistors, and its input impedance is determined by a 330kΩ bias resistor, which also establishes the DC level at this input to half the supply voltage (Vcc/2). This DC level is generated by a voltage divider consisting of two 10kΩ resistors, which is bypassed by a 47 µF capacitor to filter out noise and hum. IC1a is configured as a non-inverting amplifier with a gain ranging from 2 to 20, adjustable via VR1. The output of IC1a is routed to VR2 through a 22 µF capacitor, allowing for volume control. The audio signal is then sent to the non-inverting inputs of the remaining three op amps (IC1b-IC1d) connected in parallel to achieve a low output impedance capable of driving loads as low as 600Ω. Series 100Ω resistors at the outputs provide short-circuit protection for the op amps and prevent excessive current flow between outputs with slightly different offset voltages. The buffered output signal is AC-coupled to two output connectors using 47 µF electrolytic capacitors. For Output 1, a 47kΩ resistor sets the output DC level to ground, and a 220Ω series resistor offers additional short-circuit protection. Output 2 is similar but includes an additional potentiometer (VR3) for independent level adjustment, which can result in a high impedance (up to 2.5kΩ depending on the position of VR3’s wiper). The total harmonic distortion of this circuit is typically below 0.01% with the gain set to six. If a TL064 is used instead of a TL074, the current drain decreases, but output noise may increase. The input impedance can be further increased by raising the value of the 330kΩ resistor to accommodate high-impedance pickups.

The described preamplifier circuit is designed to enhance the performance of magnetic pickups in musical instruments by effectively managing impedance mismatches and signal integrity. The use of JFET input transistors in the TL074 op-amp configuration ensures minimal signal distortion and high input impedance, which is crucial for preserving the tonal quality of the instrument. The AC coupling at the input stage prevents DC offset issues and allows for a clean signal to be amplified.

The adjustable gain feature, facilitated by the variable resistor VR1, enables users to tailor the amplification to suit different performance environments or personal preferences. The subsequent volume control via VR2 allows for additional flexibility in managing output levels. The parallel connection of the remaining op-amps (IC1b-IC1d) not only lowers the output impedance but also enhances the output drive capability, ensuring compatibility with a wide range of audio equipment.

The inclusion of series resistors for short-circuit protection is a critical design consideration, safeguarding the op-amps from potential damage due to unexpected load conditions. The output capacitors ensure that the DC level is appropriately managed, with the resistors at the output providing a safe operating condition for the circuit. The potential for adjusting the impedance of Output 2 via VR3 adds versatility, allowing for various connection scenarios without compromising signal quality.

In conclusion, this preamplifier circuit represents a well-engineered solution for addressing common challenges faced by magnetic pickups, ensuring that musicians can achieve optimal sound quality and performance reliability.Magnetic pick-ups in musical instruments have a relatively high output impedance. This can result in a reduction in treble response when connected via a long cable run or to equipment with a low input impedance. This preamplifier provides a high input impedance and a low impedance output, solving both issues. It has adjustable voltage gain and can run off a battery or DC plugpack. The input signal is AC-coupled to the non-inverting input of IC1a, part of a TL074 quad op amp. This has JFET input transistors and the input impedance is set by a 330k © bias resistor which also sets the DC level at this input to half supply (Vcc/2). This is generated by a voltage divider comprising two 10k © resistors and bypassed by a 47 µF capacitor to reject noise and hum.

IC1a is configured as a non-inverting amplifier with a gain of between 2 and 20, depending on the setting of VR1. IC1a`s output is fed to VR2 via a 22 µF capacitor, allowing the output volume to be set. The audio then passes to the non-inverting inputs of the remaining three op amps (IC1b-IC1d) which are connected in parallel to provide a low output impedance; it will drive a load impedance as low as 600 ©.

The 100 © resistors in series with the outputs provide short-circuit protection for the op amps and also prevent large currents from flowing between the outputs in case they have slightly different offset voltages. The buffered signal is then AC-coupled to two output connectors using 47 µF electrolytic capacitors. For Output 1, a 47k © resistor sets the output DC level to ground and a 220 © series resistor provides further short-circuit protection.

Output 2 is similar but includes another potentiometer (VR3) to allow its level to be set individually. Note that this means the impedance of Output 2 can be high (up to 2. 5k © depending on the position of VR3`s wiper). The total harmonic distortion of this circuit is typically less than 0. 01% with the gain set to six. If a TL064 is used instead of a TL074, the current drain will decrease but there will be more noise at the output.

Finally, the input impedance can be increased by increasing the value of the 330k © resistor to suit high-impedance pick-ups. 🔗 External reference