Power Amplifier Measured by VU Meter

The circuit was designed based on the functionality of LM3915 integrated circuit to produce a volume unit meter (VU Meter) that will be connected to the output of power amplifier.

• Power Amplifier – a type of amplifier built and designed for the purpose of delivering and supplying sufficient and maximum high output power to directly drive loudspeakers or loads, within a given percent of distortion
• Volume Unit – an entity equal to a decibel for expressing the magnitude of a complex audio; it states the level of complex, nonrecurring, and non-periodic signals such as music and speech
• LM3915 – a monolithic integrated circuit that detects analog voltage levels and powers ten LEDs, LCDs or vacuum fluorescent displays, supplying a logarithmic 3 dB/step analog display which can be calibrated logarithmically and is suitable for log measurements as logarithmic bar graph driver
• 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

The sensitivity of the circuit is the characteristic imposed in different output power with the adjustment of resistor R1 value. The value of R1 can be made précised for the circuit by placing it in series or in parallel to achieve the desired value. The LEDs used can be of round or square type to represent the visual indication. During the operation of VU meter, the switch S1 provides the option for bar or dot reading. A dot reading signifies an open (OFF) switch while bar reading signifies a closed (ON) switch. During the ON state, the LED can reach up to 150 mA which results to high consumption. In the case of amplifiers having two channels, two circuits should be made with one for each channel.

The circuit can be supplied form an existing power supply of power amplifier having 12 V. If the circuit will require more than 12 V, power amplifiers are still suitable which makes it possible to add one stage that is located in the discontinued line and with IC2 which will act as regulated stabilizer. Due to the large voltage difference between the input and output, a small heatsink is necessary to provide room for the increase in temperature. In lower levels, the voltage drop is controlled by R5 in the input of IC2. The resistor is placed with a distance in PCB because of dissipated heat after being regulated by IC2 with the aid of trimmer TR1, independent of IC1.

The VU meter can also be connected in parallel input of a receiver or amplifier. They were used in speech during the earlier times even before computers were introduced. In recording and broadcast studios, they are utilized to monitor power levels of audio to be able to regulate a signal into the optimal range for broadcast.

Source:users.otenet.gr/~athsam/vu_meter_for_power_amp_2.htm