Today we are going to discuss about a Low cost, compact Audio Pre-Amplifier with Loud speaker. By using this device you can hear your voice loudly with a better quality. The circuit provides an output power of about 0.5 watts by just 5V supply. It can be used as a part of intercoms, walkie-talkies, pocket radio receivers etc… all the components used in this project are readily available in the market at low cost.
IC 1 – BEL1895
T1 & T2 – BC148 NPN
Loud Speaker 4Ω 1W
For better understanding we can divide the circuit into two sections
1) High gain Mic Pre-Amplifier
2) A.F Power Amplifier
High Gain Mic Pre-Amplifier
It consists of a condenser mic, a variable resistor and two NPN transistors (BC148) with some passive components such as Resistors and Capacitors.
- Condenser Mic
It is basically a transducer, used to convert audio(speech) signals to electrical signals. The commonly used condenser mic’s are cylindrical in shape in which its top most part is enclosed by a sensitive diaphragm. The diaphragm vibrates with respect to our voice, hence the carbon granules filled inside it. This offers a variation in output resistance which in turn varies the voltage directly. This minute variation in electric voltage with respect to our voice is amplified in the further stages.
- Variable Resistor
It is a 10kΩ variable resistor, used to control the gain of audio pre-amplifier by adjusting this we can increase or decrease the output gain.
- NPN Transistors T1 & T2
The circuit uses two stage RC coupled amplifier to increase the gain of signal coming from condenser mic. The transistors are biased by a voltage divider network to provide proper stabilization.
A.F Power Amplifier
Audio frequency power amplifier section consisting of IC BEL 1895, it is a monolithic audio power amplifier designed for AM radio applications. It delivers 1Watt output power at 6V supply voltage. BEL1895 ic exhibits low distortion and noise with an operating range of 3-9V. The output is taken from pin1 which drives the loud speaker.