Amplifier Circuit Diagrams

100W Subwoofer Amplifier Circuit

A subwoofer is a speaker designed to reproduce low-frequency audio signals. The inaugural subwoofer amplifier was pioneered by Ken Kreisler in 1970. Its chief objective is to elevate the bass fidelity of audio signals. We have developed a subwoofer amplifier capable of generating audio signals within the low-frequency range of 20 to 200 Hz, and it delivers an output power of 100 watts to drive a 4-ohm load.


  • Subwoofer Amplifier Circuit Principle
  • Circuit Diagram of 100W Subwoofer Amplifier
  • Circuit Components:
  • Subwoofer Amplifier Circuit Design:
    • Audio Filter Design:
    • Pre Amplifier Design:
  • Power Amplifier Design:
  • Subwoofer Amplifier Circuit Operation:
  • Applications of Subwoofer Amplifier Circuit:
    • Limitations of the Circuit:

Subwoofer Amplifier Circuit Principle

The audio signal undergoes an initial filtration process to eliminate high-frequency components, permitting only the low-frequency signals to pass. Subsequently, this low-frequency signal undergoes amplification through a voltage amplifier. The amplified, yet low-power signal is then further boosted through a transistor-driven Class AB power amplifier.

Circuit Diagram of 100W Subwoofer Amplifier

Subwoofer Amplifier

Circuit Components:

R1 6K
R2 6K
R3 130K
R4 22K
R5 15K
R6 3.2K
R7 300 Ohms
R8 30 Ohms
R9, R10 3 K
C1, C2 0.1uF, electrolyte
C3,C5,C6 10uF, electrolyte
C4 1uF, electrolyte
Q1 2N222A
Q2 TIP41
Q3 TIP41
Q4 TIP147, PNP
D1, D2 1N4007
Dual Supply +/-30V

Subwoofer Amplifier Circuit Design:

Audio Filter Design:

We utilized the OPAMP LM7332 to create a Sallen Key low-pass filter. The desired quality factor was set at 0.707, with a presumed cutoff frequency of 200Hz. Likewise, when the number of poles is 1 and C1 has a value of 0.1uF, the value for C2 can be calculated as 0.1uF. This calculation can be performed by substituting the known values into the equation, assuming that R1 and R2 are equal.

Q / (2πf_c * C2) = R1 = R2

This results in both resistors having a value of 5.6K. In this particular case, R1 and R2 are both 6K resistors. There is no need for resistors at the non-inverting terminal, as it is shorted to the output terminal to achieve a closed-loop gain filter.

Pre Amplifier Design:

The preamplifier is based on class A operation of transistor 2N222A.  Since the required output power is 100W and load resistor is 4 Ohms, here we require a supply voltage of 30V.

 Assuming the collector quiescent current to be 1mA and collector quiescent voltage to be half of supply voltage, i.e.15V, the value of load resistor is calculated to be equal to 15K.

R5 = (Vcc/2Icq)

Base current is given by, Ib = Icq/hfe

Substituting the values, hfe or AC current gain , we get the base current to be equal to 0.02mA. The bias current, Ibias is assumed to be ten times the base current, i.e. 0.2mA.

The emitter voltage is assumed to be 12% of the supply voltage, i.e. 3.6V. The base voltage, Vb is then equal to Ve +0.7, i.e. 4.3V.

Values of R3 and R4 are then calculated as given:

R3 = (Vcc – Vb)/ Ibias  and R4 = Vb/Ibias

Substituting the values, we get R3 to be equal to 130 K and R4 to be equal to 22K

The emitter resistor (Ve/Ie) is calculated to be 3.6K. However, this resistance is shared by two resistors, R6 and R7, with R7 serving as a feedback resistor to mitigate C4’s decoupling impact. The value of R7 is discovered to be 300Ohms when the values of R5 and gain are added together. The value of R6 is then 3.2K.

We determine the value of C4 to be 1uF since the capacitive reactance of C4 should be less than the emitter resistance.

Power Amplifier Design:

The power amplifier has been constructed employing Darlington transistors, specifically the TIP142 and TIP147, operating in class AB mode. To ensure thermal compatibility with the Darlington transistors, biasing diodes of the 1N4007 type have been chosen.

Given the necessity for a substantial bias resistor to achieve low bias current, we have opted for R9 with a value of 3K.

The driver stage serves to furnish the power amplifier with a high-impedance signal. In this scenario, we’ve employed a TIP41 power transistor in class A mode. The value of the emitter resistor, R8, is 28.6 Ohms, determined by the emitter voltage (Ve), which is half of Vcc minus 0.7, and the emitter current (Ie), which matches the collector current at 0.5A. For this purpose, we’ve chosen a 30 Ohm resistor.

For ensuring a high impedance in the Darlington transistors, the value of the bootstrap resistor, R10, is set at 3K in this instance.

Subwoofer Amplifier Circuit Operation:

The Sallen Key low-pass filter serves the purpose of audio signal filtration, permitting frequencies at or below 200Hz to pass while effectively filtering out higher frequencies. This filtered low-frequency signal is introduced to the input of transistor Q1 through coupling capacitor C3. Operating in class A mode, Q1 amplifies the input signal, generating an amplified version at its output. Subsequently, Q2 transforms this amplified signal into a high-impedance form, which is then directed to the class AB power amplifier.

The tandem action of the two Darlington transistors results in the production of a complete cycle of the output signal, with one conducting during the positive half-cycle and the other during the negative half-cycle. Emitter resistors R11 and R13 are employed to equalize the behavior of the matching transistors. The diodes are incorporated to ensure minimal crossover distortion. This high-power output signal is then employed to drive a low-impedance loudspeaker or subwoofer, typically around 4 Ohms. It’s worth noting that an 8 Ohm resistor has been used for testing purposes.

Applications of Subwoofer Amplifier Circuit:

  1. This circuit can be used at home theatre systems to drive subwoofers to produce a high quality, high bass music.
  2. This circuit can also be used as a power amplifier for low frequency signals.

Limitations of the Circuit:

  1. The filter circuit tends to increase the DC level of the audio signal, causing a disruption in the biasing.
  2. The use of linear devices causes power dissipation, thus reducing the efficiency of the circuit.
  3. It is a theoretical circuit and output contains distortion.
  4. The circuit doesn’t provide any provision to remove noise signal and thus the output may contain noisy disturbance.

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