AF Amplifier with Up/Down Volume Setting
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his project is simply made up with TDA8551. Its a small audio amplifier from Philips Semiconductor (now NXP Semiconductor) with integrated volume control. When operated from +5v, it delivers a nominal output power of more than one watt into 8 ohms. It can also be used over a supply voltage range of +2.7v to +5.5v, with correspondingly reduced output power.
The output volume can be adjusted from -60dB to +20dB in 64 steps, using a set of up and down push buttons. The shared UP/DOWN input for the up and down switches has three states. If it is 'floating', which means that both of the switches are open, the volume remains unchanged. A pulse to earth decreases the volume by 1.25dB, while a positive pulse increases the volume by 1.25dB. When the power is switched on, the internal counter takes on the -20dB setting.
An additional input (MODE) allows the amplifier to be switched from the operating state to the mute or standby state. If this input is held at the earth level, the amplifier is operational. If +5v is applied to this pin, the TDA8551 enters the standby mode, in which the current consumption drops from the typical operational level of 6mA to less than 10uA. Finally, the MODE input can be used as a mute input by applying a voltage of 1 to 3.6v to this input. This voltage can be provided by a connection to the SCR pin, which lies at the half of the operating voltage and to which a filter capacitor is connected.
The loudspeaker is connected in a floating configuration between the two outputs of the bridge amplifier in the TDA8551. This provides the desired output power level, inspite of the low supply voltage. For headphone applications, which do not need as much output power, you can connect the headphone between earth and one of the outputs, via an electrolytic couling capacitor. You can make a stereo headphone amplifier in this way, using two TDA8551 IC's.
The TDA8551 is housed in a DIP8 package. The SMD version is the TDA8551T, in an SO8 package. The datasheet can be obtained from the http://www.nxp.com/
Noise-Driven Sound Generator
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he sound generator shown here uses its built-in loudspeaker as a microphone when it is in the stand-by state. As soon as it detects a noise that exceeds an adjustable threshold level, it becomes active. This small, clever circuit can be used as an alarm generator that reacts to noises. You can also use it to help you locate an object in response to a loud sound, such as clapping your hands or whistling loudly.
The gate at the bottom of the schematic diagram acts a as linera low-frequency amplifier, due to the negative feedback resistor R3. It receives its input signal from a loud speaker LS1, which acts as a microphone when T1 is cut-off. The amplified output signal from this gate passes via C1 to a second 4011 gate, which triggers a monostable formed by the two gates located before and after C5 and R5. The DC threshold level is applied to pin2 of IC1 via R2. This DC voltage is superimposed on the signal from C1. Retriggering of the monostable is prevented by the combination of R4, C4 and the first gate, in addition to which the sound generator (IC2) prevents the loudspeaker from acting as a microphone once it has been activated.
Low-Noise Microphone Amplifier
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ere's the first electronic project I would be sharing for our hobbyist, students and other techies. I suggest to try all the electronics projects I published on the "bread board" first before implementing them on the PCB. If the schematic doesn't work in the first place that would be the most fun as you have chance to debug the circuit and learn as you go along.
As the signal from a microphone is too weak for a standard line input. This low-noise DC-coupled microphone amplifier provides a solution for anyone who wants to connect a microphone to his/her Hi-Fi installation. Below is the schematic diagram of a Low-Noise Microphone Amplifier.
A differential amplifier is built around T1(MAT-03E), which is a low-noise dual transistor(if you got other alternative parts, please try it). The combination of T2 and LED D1 forms a constant-current source for the input stage. A low-noise op-amp(OP-270E) amplifies the difference signal that appears at the collectors of the dual transistor. The result is an analog signal at line level. The bandwidth of the amplifier ranges from 1Hz to 20kHz. Within the audio range (20Hz to 20kHz), the distortion is less than 0.005 percent. Since only half of the OP-270E is used, the remaining op-amp could be used in the output stage of a stereo version.
The amplifier can be powered from a stabilized, symmetrical supply with a voltage between +/-12V and +/- 15V. Such supply voltages are already present in many amplifiers.
