With the addition of a few passive components it is possible to build a low cost stereo amplifier featuring a good impulse response, ideally suited for connection to a small speaker. The project here features a simple design for easy construction; it is relatively inexpensive. The design is based on a LM1875T. The amplifier system been described here produces 20 Watt power at very low distortion. The described Amplifier has inputs for audio sources such as CD player, MP3 player and FM/AM tuner. Other controls for the described amplifier system are Bass, Treble and Volume control. The amplifier puts out a surprising amount of power, considering that it runs from a 16V AC 1.25A transformer. One reason this system performs so well is that it is based on the National Semiconductor LM1875T 20W audio amplifier IC. This IC has inbuilt thermal protection so that even if you abuse it or short out its output, it won’t be damaged. The power amplifier circuits are very close to the Students’ Amp circuit but inevitably there are component differences to provide different gain and so on. 23
3.2 Features and Specifications of the Audio Amplifier System Total harmonic distortion plus noise: typically <0.03% Signal-to-noise ratio: 93dB (96dB A-weighted) with respect to 10W into 8Ω Channel separation: –72dB at 1 kHz Input sensitivity: 500mV RMS for 10W into 8Ω Input impedance: 8.3kΩ 3.3 Block Diagram Fig 3.1: Shows the Block Diagram for the Amplifier System 24 PREAMP TONE CONTROL POWER AMPLIFIER OUTPUT POWER SUPPLY
3.4 Design Stages Preamplifier Stage Fig. 3.2: Shows the Circuit Diagram of the Pre-Amplifier The Preamplifier stage amplify the audio signal before passing it to the tone control stage for proper separation of the desired frequency, where it will then distribute the signal in equal proportion to the power amplifier stage. Tone Control Stage Fig. 3.3: Shows cct Diagram of the Tone Control 25
The Tone Control circuits are often found in most high fidelity amplifiers. All tone control circuits produce “boost” in one region of the audio spectrum by reducing response at all others. This arrangement uses gain reducing negative feedback to achieve the desired result. Power Amplifier and Output Stage Fig. 3.4: Shows Circuit Diagram for Power Amplifier The power amplifier / output stage comprises of the power amplifier integrated circuit type LM1875T. Only a few external components are required by this single Power Amplifier integrated circuit in other to deliver the required output. The above mention chip contains one low distortion amplifiers that will be used in the driving of stereo speakers in bridge tied Load (BTL) configuration, with rail to rail swing on the outputs and is inherently stable with fixed gain. 26
Power Supply Stage Fig. 3.5: Shows the Circuit Diagram of the Power Supply The power supply for the amplifier is derived from a standard mains power. In this design power supply will be taken from 2 amp mains rectified DC power supply. The supply will make use of a mains transformer with 15Vcenter taped secondary windings. The secondary winding will feed to a bridge rectifier and a filter capacitor that would help decouple the supply line.
- Spring '14