PCBs are available for this project on PCBWay as a shared project.

Fig. 1: the complete schematic of the preamplifier

The top half of this schematic is a single-channel op-amp preamplifier built around one half of an NE5532 dual operational amplifier. It operates from a simple split power supply generated from a single DC input source and provides adjustable signal attenuation together with moderate voltage gain.

Power enters through connector J1. The incoming DC voltage passes through resistor R1, while capacitor C1 filters low-frequency ripple and supply fluctuations. Zener diode ZD1 clamps the positive supply rail to approximately +12V, while ZD2 clamps the negative rail to approximately −12V. Resistor R2 performs the same function for the negative side. Capacitors C3, C4, and C5 provide local decoupling and filtering, ensuring the op-amp receives a stable and low-noise supply voltage. This arrangement forms a simple regulated dual-rail supply for the NE5532.

This arrangement was chosen so the preamplifier could be added to an existing amplifier's power supply rails - with the use of Zeners only, the noise floor would be considerably higher than if it was further regulated with 78XX and 79XX devices. However, I wanted to keep the PCB simple in layout - and it's up to the constructor whether or not you include the Zeners and R1 - R2, or use shorting links for the resistors, exclude the Zeners and run the board off of a regulated +/-12V supply.

The audio signal enters through connector J2 and passes through coupling capacitor C101. This capacitor blocks any DC voltage from the source equipment while allowing the AC audio signal to continue into the amplifier. Resistor R101 provides an input reference to ground and establishes the input impedance seen by the signal source.

The signal then reaches potentiometer RV1A, which is the volume control. Because it is a linear potentiometer connected between the incoming signal and ground, it operates as a variable voltage divider. The wiper of the potentiometer picks off an adjustable fraction of the input signal and feeds it into the non-inverting input of the op-amp through resistor R103.

The important detail is how RV1A interacts with resistor R102. Resistor R102 is connected from the lower end of the potentiometer to ground, meaning the potentiometer is not connected directly across the full input signal and ground as in a conventional volume control. Instead, R102 modifies the taper and operating law of the linear potentiometer.

A purely linear potentiometer changes resistance uniformly with shaft rotation, but human hearing responds logarithmically to sound level. Without compensation, most of the audible volume change would occur over a very small portion of the control rotation, making adjustment feel abrupt and unnatural.

By adding R102, the linear potentiometer behaves more like a logarithmic or “audio taper” potentiometer. The resistor reshapes the voltage divider relationship so the output signal changes more gradually over the control rotation. This gives smoother and more natural volume adjustment using a cheaper and more readily available linear potentiometer rather than a true logarithmic one.

The wiper signal then passes through R103 into the non-inverting input of U1A. Since the signal is applied to the non-inverting input, this stage operates as a non-inverting amplifier. The inverting input receives feedback from the output through resistor R105, while resistor R104 connects the inverting input to ground. Together, R104 and R105 determine the closed-loop voltage gain according to:

Av = 1 + R105 / R104

With R105 = 10k and R104 = 2.2k, the stage gain is approximately:

Av = 1 + 10 / 2.2 = 5.5

This means the amplifier increases the input signal voltage by roughly 5.5 times while maintaining the original signal polarity.

The amplified output signal leaves pin 1 of the op-amp and passes through coupling capacitor C102, which blocks the op-amp’s DC offset from reaching external equipment. Resistor R106 isolates the op-amp from capacitive cable loads and improves stability when driving long audio cables or reactive loads. The final AC output signal then appears at connector J3.

This preamplifier was designed to provide gain only adjustable with a single volume control - therefore it has no tone controls. This means the frequency response of the circuit remains flat between 20Hz to 20kHz. For what it is, it performs very well - and I have used it in a 5W test amplifier build (which I will create an article about later). And when I say “test” amplifier, I mean an amplifier that is used to test other pieces of audio gear that require an amplifier.