Subwoofer Amplifier Circuit – High Power

The post explains a simple high power subwoofer amplifier circuit which could be used for driving high bass subwoofer speakers and home theater systems.

A Subwoofer is actually a loudspeaker which will develop audio signals of lower frequencies. The initial subwoofer amplifier had been engineered in 1970 by Ken Kreisler. It is fundamentally employed to enhance the bass quality of sound signals. In this article we structure a subwoofer amplifier generating sound signals at lower frequencies from 20 Hz to 200Hz along with an output power of 100W, useful to push a 4 ohm speaker.

Subwoofer Amplifier Circuit Theory:

Audio Signal is initially filtered in order to eliminate the high frequency content and enable just the lower frequencies to cross via it. This specific lower frequency transmission can now be amplified by using a voltage amplifier. This kind of lower power signal is subsequently amplified utilizing a transistor powered class AB power amplifier.

COMPONENT LIST
R1=5.6K
R2=5.6K
R3=120K
R4=27K
R5=15K
R6=3.3K
R7=330 Ohms
R8=33 Ohms
R9, R10=2.7 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:

In this article we developed a Sallen Key low pass filter making use of OPAMP LM7332. The cut off frequency had been believed to be 200Hz and the Quality factor is actually supposed to be 0.707. Additionally supposing the quantity of poles being comparable to 1 and value of C1 being identical to 0.1uF, value of C2 may be determined being 0.1uF. Supposing R1 and R2 to be same, the value can be obtained by replacing recognized values within the formula

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

This presents a figure of 5.6K per resistor. Here it is fixed at 6K resistors for R1 and R2. Given that we would like a closed loop gain filter, we do not require resistors in the non inverting pin, that is short circuited to the output pin.

Pre Amplifier Design:

The preamplifier is dependent on class A function of transistor 2N222A. Because the expected output power is 100W and load resistor can be 4 Ohms, therefore we call for a supply voltage of around 30V.

Supposing the collector quiescent current being 1mA and collector quiescent voltage being 1 / 2 of supply voltage, i.e.15V, the significance of load resistor could be worked out to be corresponding to 15K.

R5 = (Vcc/2Icq)

Base current is determined by, Ib = Icq/hfe

Substituting the figures, hfe or AC current gain , we have the base current being corresponding to 0.02mA. The bias current, Ibias is deemed being 10 times the base current, i.e. 0.2mA.

The emitter voltage can be deemed being 12% of the supply voltage, i.e. 3.6V. The base voltage, Vb can now add up to Ve +0.7, i.e. 4.3V.

Values of R3 and R4 tend to be subsequently determined as presented:

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

Substituting the values, we have R3 being corresponding to 130 K and R4 being corresponding to 22K

The emitter resistor can be determined being corresponding to 3.6K (Ve/Ie). On the other hand this specific resistance is embraced among a couple of resistors, R6 and R7, where R7 is employed as feedback resistor to minimize the decoupling influence of C4. Value of R7 is determined through the values of R5 and gain and identified being corresponding to 300Ohms. Value of R6 can now add up to 3.2K.

Given that capacitive reactance of C4 ought to be lower than the emitter resistance, we estimate the significance of C4 being comparable to 1uF.

Power Amplifier Design:

The power amplifier is fashioned utilizing Darlington transistors TIP142 and TIP147 in class AB mode. The biasing diodes tend to be picked in a way that their thermal properties are comparable to that of the Darlington transistors. In this article we select 1N4007.

Given that a sizable value of bias resistor is needed for any low bias current, we decide on R9 being corresponding to 3K.

The driver stage is employed in order to get a high impedance input for the power amplifier. In this article we work with a power transistor TIP41 in class A function. The emitter resistor, R8 is determined through the values of emitter voltage, Ve (1/2Vcc- 0.7) and emitter current, Ie (equal to collector current, i.e. 0.5A) and it is identified being corresponding to 28.6 Ohms. In this article we decide on a 30 Ohms resistor.

The significance of bootstrap resistor R10 must be in a way that it offers high impedance to the Darlington transistors. In this article we pick out R10 being 3K.

High Power Subwoofer Amplifier Circuit Operation:

The audio signal is filtered through the Sallen Key low pass filter making use of the OPAMP in a way that just frequencies under and corresponding to 200Hz tend to be transferred and excess blocked. This low frequency transmission is conveyed to the input of the transistor Q1 via the coupling capacitor, C3. The transistor functions in class A mode and delivers a increased variant of the input signal in its output. This particular amplified transmission is now transformed into a high impedance transmission by Q2 and it is presented to the class AB power amplifier. Both Darlington transistors work in a way that one particular performs for positive half cycle while other for negative half cycle, hence putting together a full period of output transmission. The emitter resistors R11 and R13 are accustomed to reduce any kind of distinction amongst the complementing transistors. The diodes are accustomed to guarantee nominal overlap distortion. This high power output signal is now accustomed to commute a loudspeaker or subwoofer of low impedance, around 4 Ohms. Observe that in this article we have made use of an 8 Ohm resistor intended for only diagnostic tests purpose.

Applications of Subwoofer Amplifier Circuit:

This circuit can be utilized with home theatre systems to operate subwoofers to be able to develop a top quality, huge bass music.
This circuit could additionally be utilized like a power amplifier intended for low frequency signals.