If you are looking for a compact and inexpensive add-on audio amplifier for your digital surround sound system or computer, then this subwoofer amplifier circuit is suitable for your needs. It delivers 15W per channel in stereo mode and about 29W in bridge mono mode from a box as small as a house brick.
Many low-cost DVD players availabe in the market do not have in-builtaudio amplifiers. To enjoy stereophonic sound, you have to buy separate left and right speakers. Moreover, to play a DVD in Dolby Digital format, you would probably need five amplifiers and speakers, one for each of the channels.
Like many audio projects published earlier in EFY, subwoofer amplifier circuit can upgrade an audio system according to your personal taste. It helps a low-cost DVD player to play excellent Dolby Digital (AC-3) surround sound.
Dolby Digital ‘5.1 channel’ systems have separate channel outputs, with the centre–front signal quite different from the front, left and right ones. Many DVDs have main screen dialogues on the centre-front track, so to hear dialogues, you need a centre front amplifier and speaker. Besides, there are separate rear channel signals with Dolby Digital for MPEG and DTS encoding, and to hear them you need two separate amplifiers and speakers. The subwoofer track (low frequency effect, LFE) is quite discrete in Dolby Digital encoding. It delivers most of the grunt for explosions. So, if you watch an action movie without a subwoofer, you miss out most of the sound effects.
In brief, in order to enjoy Dolby Digital format movies, one needs three extra speakers and matching amplifiers apart from stereo system. Getting extra speakers at affordable price is not a problem, but it is not the case with add-on amplifier for a stereo or bridged mono system.
The circuit described in the article provides two 15W RMS power amplifier channels, with low distortion noise and a good frequency response. The input sensitivity is good, too. You need around 200mV input to produce full output, suitable for use with ‘line’ output on DVD players with in-built Dolby Digital decoders.
In fact, one or two brick amplifiers can be used with the DVD player to get an effective output from your surround sound system. This amplifier is also suitable for enhancing multimedia in a computer, allowing you to play external speakers.
Subwoofer amplifier circuit
The subwoofer amplifier circuit is simple. It consists of power supply and audio power amplifier sections. The simple scheme of the two channels to be driven from one of the inputs facilitates a hassle-free bridge mono mode operation.
This subwoofer amplifier circuit consists of a TL072 low-noise JFET dual operational amplifier (IC1) and two LM1875 (IC2 and IC3) power amplifiers.
In stereo mode, each half of the TL072 operates as a non-inverting input amplifier, with a gain of about 2.8 times as determined by feedback resistors R5 and R6. In front of each input amplifier there is a low-pass filter for RF suppression (R1/C1 and R15/C12), followed by volume controls VR1 and VR2 and blocking capacitors C2 and C13.
Switch S1 is used to select bridge mono mode operation. When it is switched to mono position ‘M‘, the non-inverting input of IC1(B) is connected to the ground, making the left input of the brick ineffective. However, at inverting terminal of IC1(B), the cold end of resistor R13 is disconnected from the ground and connected to R7. It is now converted into a unity gain inverting stage and the output of IC1(A) is connected to IC1(B) through R7. This converts the front end to produce dual phase drive signals from the right channel input. In stereo mode, each of the amplifier stage drives the right and the left speaker connected across pin 4 of IC2 and IC3, respectively.
Each input stage drives one of the LM1875 power stages (IC2 and IC3) with traditional circuitry. Coupling capacitors C7 and C14 provide DC blocking, with the power stage voltage gains set to around 18 by feedback networks R9/R10 and R20/R21. Resistors R8 and R19 provide biasing for the non-inverting inputs, while C8/C15 provide AC grounding for the feedback networks.
R11 and C9, and R22 and C18, are Zobel impedance stabilising circuits used across power amplifier output to ensure maximum AC stability.
Output stages operate from DC rails of ±21V, which are provided from the power supply, using 21V-0-21V, 2A centre-tapped transformer.
Power supply circuit
The transformer drives the bridge rectifier module BR1 and four 2200μF capacitors to provide +21V and –21V DC supplies. The left and the right channel input stages formed by the TL072 op-amp require ±12V DC supplies for the operation. While +12V is obtained from resistor R12 and zener diode ZD1, –12V is obtained from resistor R23 and zener diode ZD2.