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====== Bass Guitar Headphone Amplifier ======

{{:bass-headphone:pcb.png}}

This circuit is a bass guitar headphone amplifier powered from a single 9V supply. It uses a dual NE5532 op-amp together with a complementary transistor output stage to provide enough current to properly drive headphones while maintaining good audio quality.

The main purpose of this circuit is for practice playing; so you can slap away to your heart's content without annoying your neighbours, or other people in the house!

This isn't limited to just bass guitar; it can be used with an electric guitar, just without "overdrive". Honestly, a distortion pedal used in conjunction with this circuit would probably provide better overdrive than anything I can do.

The circuit is shown below.

{{:bass-headphone:schematic.png}}

**Fig. 1: schematic diagram of the guitar headphone amplifier**

The circuit in Fig. 1 can be divided into four main sections:\\
- Power supply / virtual ground\\        
- Input buffer stage\\        
- Voltage gain + volume control\\
- Headphone power buffer

The most important concept in this design is the creation of a “virtual ground” because the amplifier operates from a single 9V battery rather than a true +/- supply.

===== Power Supply and Virtual Ground =====

The supply enters at the left through the +9V terminal. Diode D1 provides reverse-polarity protection. If the battery is connected backwards, the diode blocks current and protects the circuit.

C1 filters the incoming supply and reduces ripple or noise. Resistors R4 and R5 create a midpoint voltage exactly halfway between the supply rails. Since the supply is 9V:

**Vmid = 2 / 9V = 4.5V**

This midpoint becomes the circuit’s virtual ground. Capacitors C3, C4, and C6 stabilize and filter this midpoint so it behaves like a clean reference point for the op-amps.

The rails are therefore:

**VCC = +4.5V above midpoint
VEE = −4.5V below midpoint**

even though only a single 9V battery/supply is used. The LED and resistor R2 act as the power indicator.

===== Input Stage (U1A) =====

The bass guitar signal enters through connector J1. R1 provides input biasing and sets a high input impedance so passive bass pickups are not heavily loaded. The value of 1 megaohm is typical for guitar and bass amplifiers. C2 is the input coupling capacitor. It blocks DC while allowing the AC audio signal into the amplifier.

The first op-amp section, U1A, is configured as a voltage follower (buffer amplifier). The output is directly connected to the inverting input, creating 100% negative feedback.

This gives:

**Av = 1**

so the stage has unity voltage gain.

Its purpose is not amplification but impedance conversion:\\
- very high input impedance\\
- low output impedance\\
- prevents tone loss from the bass pickup

R3, R6, and C5 help stabilize the input and reduce high-frequency noise or RF interference.

===== Volume Control and Coupling =====

The buffered signal passes through R7 into the volume potentiometer RV1. RV1 controls how much signal is sent to the second amplifier stage.

C7 AC-couples the signal into the next stage while blocking DC offsets between stages.

R8 provides bias reference for the second op-amp input.

===== Second Gain Stage (U1B) =====

U1B is the main voltage amplifier stage. The non-inverting input receives the signal through C7. The inverting input receives feedback through R10 and R9. This is a non-inverting amplifier configuration.

The gain is approximately:

**Av = 1 + R10 / R9**

Using the component values R10 = 47k and R9 = 1k the gain becomes:

**Av = 1 + 47 / 1 = 48**

So this stage provides substantial voltage gain for the bass signal.

C8 bypasses R9 at AC frequencies and shapes the low-frequency response of the amplifier. This capacitor helps improve bass response by increasing low-frequency gain.

===== Output Stage (Q1 and Q2) =====

The op-amp alone cannot supply enough current to drive headphones properly, especially low-impedance headphones. So the circuit uses a complementary emitter-follower output stage made from Q1 and Q2. These transistors form a Class-AB push-pull output buffer.

The op-amp controls the transistor bases, while the transistors provide the heavy output current required by the headphones.

The output stage has:\\
- high current gain\\
- low output impedance\\
- voltage gain approximately equal to 1

Its purpose is current amplification, not voltage amplification.

===== Biasing with D2 and D3 =====

Diodes D2 and D3 provide the bias voltage between the transistor bases.

Together they create approximately:

**2 × 0.65V = 1.3V**

This keeps both transistors slightly conducting even with no signal present. That operation mode is called Class-AB biasing.

The purpose is to reduce crossover distortion that would occur if both transistors turned completely off around zero crossing.

===== Emitter Resistors =====

R13 and R14 are small emitter resistors.

They:\\
- improve thermal stability\\
- help current sharing\\
- reduce the chance of thermal runaway\\
- linearize transistor operation

===== Headphone Output =====

The headphone signal exits through R15 and R16.

These resistors:\\
- isolate the amplifier from difficult headphone loads\\
- improve stability\\
- limit short-circuit current\\
- reduce the risk of oscillation

The output is referenced to the virtual ground system rather than true earth ground.

===== Overall Operation =====

The signal flow is thus:\\
- Bass guitar input\\
- High-impedance input buffer (U1A)\\
- Volume control\\        
- Voltage gain stage (U1B)\\        
- Class-AB transistor current buffer (Q1 and Q2)\\        
- Headphones

The design combines:\\
- op-amp voltage amplification\\
- transistor current amplification\\
- virtual-ground single-supply operation

to create a compact headphone amplifier, from a bass guitar, capable of driving headphones cleanly from a simple 9V supply.

===== Simulation test results =====

{{:bass-headphone:fr.jpg}}

**Fig. 2: frequency response curve**

{{:bass-headphone:noise.jpg}}

**Fig. 3: total noise**

===== Conclusion =====

TINA-TI simulation of this showed around 28mW of power in to 32 ohm headphones with around 0.02% total harmonic distortion (THD), and a frequency response of 10Hz to 22 kHz.

All resistors should 1% metal film, 1/4W type for low-noise. Electrolytic capacitor voltage ratings should be minimum 16V. Maximum supply voltage should not exceed 12V DC.

I have also designed a PCB for this which the gerber files are available [[https://www.patreon.com/posts/bass-guitar-158765597|here]].

You can download the TINA-TI simulation {{:bass-headphone:headphone-amp-v2.zip|here}}.