A low power audio amplifier can be easily manufactured at a minimal cost, if we use a suitable integrated circuit. Here, we present a small stereo audio amplifier that is able to deliver up to 20W of power at each audio channel, and has been especially designed to be used in low-power car audio systems.
The amplifier circuit is based on TDA2005, which is a high-quality stereo integrated amplifier. As a stereo amplifier, TDA2005 contains two independent amplifiers. Each one of them, when powered from 12V, can deliver up to 5W into a 4 ohms load. There is also the possibility of combining the two amplifiers to create a single amplifier that can deliver an output power that is about 20W, which is four times the output power of a single ended amplifier. The combination of the two internal amplifiers of the TDA2005 into one is called "bridged connection" or "bridge configuration".
The amplifier we present here uses the bridge configuration. In a bridge configuration, the combined amplifiers are able to deliver twice the maximum current of each separate amplifier. Given now that power PL, at a given load RL, is proportional to the square of the current i, (PL = i2RL), the output power is fourfold.
The TDA2005 is a class B dual audio power amplifier, specifically designed for car radio applications. It provides a high current capability (up to 3.5 A) and can drive very low impedance loads (down to 1.6 Ω in stereo applications). Indeed, it is able to deliver more power in a 2 Ω load rather than 4 Ω. This means that instead of a speaker of 4 ohms, you may also use two speakers of 4 ohms, connected in parallel. Such, you’ll get a speaker of 2Ω and the maximum output power would be around 30W, considerably larger than the 20W that the amplifier can deliver at 4 ohms.
The electronic schematic of the 20W amplifier (bridge configuration) is presented here:
Please notice, that the presented schematic is a monophonic amplifier (supports a single audio channel). By combining two identical monophonic circuits in a single board, we are able to build a stereo amplifier. In the stereo amplifier, a mono amp is used for the right (R,) and the other mono amp for the left (L) audio channel, respectively.
How to build the 2x20W stereo amplifier
In order to build the 2x20W stereo amplifier 2x20W, you‘ll need a printed circuit board. You may use the board’s artwork provided below, and etch it by yourself:
The printed circuit board has been designed to host two identical-mono amps. The two of them, form the stereo version.
The individual components listed in the schematic should be placed and soldered on the board according to the assembly instructions on the image below:
There are two TDA2005 integrated circuits in the board. Both of them should be mounted on a large metal heat sink (heat radiator), as indicated in the assembly guide, and also shown in prototype’s photos. The heat sink may be grounded directly (connected to the negative pole of the battery) and no insulation is needed. All you have to do is to mount the IC’s directly into the heat sink, by using screws. You may also use some thermal grease (also called CPU grease, heat paste, heat sink compound, heat sink paste, thermal compound, thermal gel, thermal interface material, thermal paste, or grey goo) as an interface between heat sink and the ICs . The grease will eliminate air gaps or spaces (which act as thermal insulator) from the interface area and will improve heat transfer.
Test and use
Immediately after installation you may directly power up the amplifier and test it on speakers or on a dummy load. At 14V, current consumption is expected to be about 70mA at rest (with zero input) in each channel. A higher current value will be a sign of self-oscillation and instability problems. Probably, you will not experience instability problems, but if you face any, due to unexpected feedback from the cables, you may fix the problem by connecting a capacitor of 100pF between the amplifier’s input and ground.
Note that there is no polarity at the outputs. Each output is balanced. Finally, if you decide to install the amplifier in a car, remember to definitely use a fuse of 2A (in series to power supply), for short circuit protection.