Here, we present a Stereo Volume Unit (VU) meter. It uses some passive electromechanical devices and common electronic components. That kind of device is better known as a “Needle VU Meter” and it is based on passive microampere-meters (150μΑ-DC) fed from full-wave diode-rectifiers. A “Needle VU meter” can be used as an alternative for a common LED-VU meter, to display a representation of an audio signal level in any audio equipment.
Referring to the electronic schematic, we use a microampere-meter for the Left, and another one for the Right audio channel. The masses of the needles and also C4 and C7, cause a relatively slow response, which in effect integrates the signal, with a rise time. The integrators averaging out peaks and troughs of short duration, and reflect more the perceived loudness of the signal. Many audio enthusiasts will agree that this is a useful characteristic for applications where the meter-indication must not reflect instantaneous features of the signal but mostly its peak level which, in many cases, must not pass a defined limit.
The meter was designed to be powered from a single +12V power supply. Its input impedance is high enough (about 50K), for not to overload the circuits of any signal source. R4 and R8 resistor-trimmers are used for adjusting the sensitivity of the meter for the left and right channel, respectively. Increasing or degreasing the values of C4 and C7, will slow or speed-up the meter’s response (affect the integrators).