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Overview The circuit consists of preamplifier, tone controls, and regulated DC power supply, while providing a power output of 18 Watts for 8 Ohm load and 30 Watts for 4 Ohms load. Terminology - BC550 – an NPN general purpose transistor with low current and low voltage used for low noise stages in audio frequency equipment
- IRF530 – an N-channel enhancement mode silicon gate power field effect transistor with advanced power MOSFETs designed, tested, and guaranteed to withstand a specified level of energy in the breakdown avalanche mode of operation, used for applications such as switching regulators, switching convertors, relay drivers, motor drivers, and drivers for high power bipolar switching transistors requiring high speed and low gate drive power
- 2N3819 - an all purpose JFET which offers good performance at mid to high frequencies and a low cost type; has low noise and leakage, very high system sensitivity, high speed switching capability, wideband high gain at 100 MHz, high low-level signal amplification and high quality of amplification
- LM317 – an adjustable 3-terminal positive voltage regulator capable of supplying in excess of 1.5A over an output voltage range of 1.2V to 37V and requires only two external resistors to set the output voltage due to its internal current limiting, thermal shutdown and safe area compensation, making it essentially blow-out proof
Circuit Explanation The design of this audio amplifier includes few parts for delivering proper output power without giving up its quality. Only three transistors and a few resistors were used in the power amplifier section while the capacitors are in a shunt feedback configuration. A regulated power supply is necessary in order to ensure overall stability and achieve such performance. This provides a guaranteed predictable output power into different load impedances while keeping the hum and noise of the preamp to very low levels. A single rail supply is required for the amplifier with a very good DC voltage regulator which is capable of more than 2 A of supply at 40 V. In the preamp circuit, the volume control was situated at the input to obtain a very high input overload margin where it provides a very low noise bootstrapped circuitry while having FET-input implemented. A treble and bass stage control was added after a unity gain. To drive the mini MOSFET power amplifier properly, a low output impedance and excellent total harmonic distortion up to 4V RMS should be featured at this stage. The sensitivity and overload margin of the preamp were designed to handle the most modern audio programme. The input connectors and source selecting switches were arranged depending on the choice of the construction.
The addition of adjustable regulator chip LM317T to the PNP power transistor makes a regulated power supply section more powerful. This will cause the mini MOSET amplifier to be driven to full output with much more than the required power. A suitable heatsink should be used with Q2 & Q3 and Q1 & IC1 to lessen the heat dissipation. If the loudspeaker has an 8 Ohm nominal impedance, a power transformer with secondary winding at 36V and 50 VA is required.
Application
The mini MOSFET audio amplifier may be applied with tape recorder, CD player, tuners, iPods, and computer audio outputs to continually improve the audio quality. Other models were used with audio-video receivers, automotive sound systems, and home theatre entertainment systems. Source:redcircuits.com/Page123.htm
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