100W Subwoofer Amplifier Circuit Construction and Working

In the year 1970, the term subwoofer was by “Ken Kreiser”. A 100W Subwoofer Amplifier is a loudspeaker that generates low-frequency audio signals. The subwoofer amplifier circuit is used to enhance the quality of the audio signals. Here, this article gives an overview of a designing of a subwoofer amplifier that generates audio signals at a low frequency ranging from 20Hz-200Hz and with 100W o/p power, which drives a 4Ohm load.

Subwoofer Amplifier Circuit
Subwoofer Amplifier Circuit

The working principle of this 100W subwoofer amplifier circuit is, when the high-frequency signals are removed, then the audio signal will be filtered. It permits the low-frequency signals to flow through it, then this low-frequency signal will be amplified using a voltage regulator and low power signal is amplifies using a transistor to determine class AB amplifier.

100W Subwoofer Amplifier Circuit Diagram

The required components for 100W Subwoofer Amplifier Circuit Construction is; 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

100W Subwoofer Amplifier Circuit
100W Subwoofer Amplifier Circuit

Circuit Design of Subwoofer Amplifier

The circuit design of the Subwoofer Amplifier mainly includes three designs such as auto-filter design, pre-amplifier design, and power amplifier design.

Designing of Audio Filter

Here, a Sallen key LPF is designed with LM 7332 Op-Amp. Both Q factor and cutoff frequency are assumed to be 0.707 and 200Hz. And also, assuming the C1 value to be equal to 0.1uF and the number of poles to be equal to 1. The C2 value can be calculated to be 0.1uF. Assuming R1 & R2 are similar, and by substituting known values into the following equation the value can be found.

Audio Amplifier
Audio Amplifier

R1 = R2 = Q/(2*pi*fc*C2)


The above equation gives 5.6K value for resistors R1 & R2. Here 6K resistor is selected as resistors R1, R2 But, we require a closed-loop gain filter do not need resistors at the –ve terminal, that is shorted to the o/p terminal. Designing of Pre Amplifier Designing of Pre Amplifier

Designing of Pre Amplifier

The designing of the pre-amplifier depends on the operation of class-A transistor 2N222A. The required load resistor is 4 Ohms & output power is 100W. Here the required supply voltage is 30 volts.

Assume the collector quiescent voltage to 15 volts and collector quiescent current to 1mA. The calculated RL (Load Resistor) value is 15k.

Pre Amplifier
Pre Amplifier

R5= (Vcc/2lcq)

Base current Ib= Icq / hfe

By substituting the values of AC current gain or hfe. Then, we can get the base current 0.02 mA. The bias current is assumed as ten times the base current. Assume the emitter voltage to 12% of the supply, that is 3.6volts. The base voltage Vb is equal to emitter voltage Ve +0.7 volts that are 4.3 volts.

Resistors R3, R4 values are calculated by using the following equations.

R3= (Vcc-Vb) / Ibias

R4= Vb / Ibias

By substituting the above values we get the R3 value, which is 130K & R4 value is equal to 22K.

The emitter resistor value is 3.6 K (Ve/ Ie) and it is common between two resistors R6 &R7. Here, resistor R7 is used as a feedback resistor to decrease the decoupling effect of C4. The value of R7 resistor is calculated from the values of resistor R5 and gain & found to be equal to 300 ohms, then the resistor R6 value is equal to 3.2 K. The capacitive reactance of C4 must be below the emitter resistance, the value of C4 is equal to 1uF.

Designing of Power Amplifier

The power amplifier is designed with Darlington transistors such as TIP147 & TIP142 in the mode of class AB. The selected biasing diodes properties are equal to the Darlington transistors. Select 1N4007, then the largest value of bias resistor is necessary for a low bias current, select resistor R9 which is equal to 3K.

Power Amplifier
Power Amplifier

The main function of the driver stage is to offer a high impedance i/p to the power amplifier. A TIP41 power transistor is used in class A mode. The emitter resistor ‘Re’ is given by the values of emitter voltage’ that is, 1/2Vcc- 0.7. & emitter current ‘Ie’ is equal to collector current ‘Ic’ that is 0.5A. Here, the bootstrap resistor R10 is used to offer high impedance to the Darlington transistors. The value of R10 is 3K.Operation of Subwoofer Amplifier Circuit

Operation of Subwoofer Amplifier Circuit

The audio signal is filtered by the LPF (Low Pass Filter) using the operational amplifier. This low-frequency signal is given to the i/p of the Q1 transistor through the C3 coupling capacitor. The operation of this transistor is in class A mode & generates an amplified version of the i/p signal at its o/p. Then, this signal is changed into a high-impedance signal by transistor Q2 and is given to the class AB power amplifier.

The working of two Darlington transistors are, one transistor behaviors for a +Ve half cycle and remaining transistor behaviors for a -Ve half cycle, then generates a full cycle of o/p signal. The R11 & R13 emitter resistors are used to reduce any difference between the matching transistors. The crossover distortion is ensured by using diodes. This high power o/p signal is used to drive a loudspeaker, about 4 Ohms. Subwoofer Amplifier Circuit Applications.

Subwoofer Amplifier Circuit Applications

A subwoofer amplifier circuit using IC is used in home theaters to make subwoofers to generate a high bass and high-quality music. This 100w subwoofer amplifier circuit is also used for low-frequency signals as a power amplifier.

Limitations of Subwoofer Amplifier Circuit

This circuit tends to increase the level of DC of the audio signal producing a disturbance in the biasing.

  • This circuit tends to increase the level of DC of the audio signal producing a disturbance in the biasing.
  • The main purpose of linear devices is, it affects power dissipation and reduces the circuit efficiency.
  • The subwoofer amplifier circuit is theoretical and o/p of this circuit contains distortion.
  • The circuit doesn’t deliver any provision to eliminate noise signal & thus the o/p may have noise.

This is all about the 100w subwoofer amplifier circuit working with applications. We hope that you have got a better understanding of this concept. Furthermore, any queries regarding this concept, please give your feedback by commenting in the comment section below. Here is a question for you, what is the function of the 100w subwoofer amplifier circuit?

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