We present a special Volume Unit Meter circuit, which resembles a retro electromechanical meter device. You may place it at the front panel of your power amplifier. The specific VU meter uses a logarithmic scale of 3 dB/step, and gives a real view of the output power of the amplifier. It can be powered from a voltage between 10 and 13V, and can be connected to any amplifier having an output power between 1 and 200W.
The VU meter is made from 20 rectangular LEDs. The LEDs are placed in a semicircle, and they produce a nice visual effect . The effect gives the feeling that the rectangular LEDs rotate around a central axis (like the needle of an electromechanical device).
The circuit
The VU meter uses only two integrated circuits; a LM.3915 and a MC.1458. The LM3915 is a Dot/Bar LED - bar graph driver, and the MC1458 is a dual operational amplifier. One operational amplifier is used as an ideal rectifier, and the other one is used to produce a reference voltage of 4V. The rectifier produces a DC voltage, which is proportional to the amplitude of the input signal.
Figure 1. Retro VU meter schematic
The input signal can be normally picked up from the cable of a loudspeaker. The R1 trimmer is used to adjust the sensitivity of VU-Meter. Normally, you may wish to adjust it in such a way so that the last pair of LEDs will normally light up during a maximum volume peak. The J1 jumper is used to select DOT display - mode, or BAR display -mode, when placed at BA or BC position, respectively.
How to build the VU meter
For making things easy, we have designed an appropriate printed circuit board. The board has copper on both sides, and you may easily etch and drill it. Copper on solder-side is shown on figure 2, and copper on components - side is shown on figure 3.
You must place and solder all the components on the PCB, according to the assembly guide of figure 4. After building and testing the circuit, you may place it on an amplifier’s front panel.
Figure 2. Retro VU meter - PCB (solder side - mirrored)
Figure 3. Retro VU meter PCB (components side)
Normally, all components are placed in one side of the PCB – the one that is shown on figure 4. However, it is more convenient to place the J1 jumper, the R1 trimmer and the power terminal, at the copper side. This way, you will be able to make adjustments, even when the VU meter is attached on a panel.
Figure 4. Retro VU meter - assembly (copper lines on solder side are shown)
Since the circuit is monophonic (supports a single audio channel), you may wish to build two identical VU meters, to use them for a stereo amplifier.
Connect the amplifier
If you have an amplifier which has an unbalanced output (uses ground on speakers), just connect the VU meter’s input to the cable of a loudspeaker by making sure to connect the ground wire of the VU meter (which is the negative power terminal) to the ground wire of the loudspeaker, and the signal wire of the VU meter, to the signal wire of the speaker.
If you have an amplifier which has a balanced output (does not use ground on speakers), connect the signal wire of the VU meter to any of the two wires of the loudspeaker, and do not use the second wire of the loudspeaker. Then, connect the ground wire of the VU meter (the negative pole) to the ground of the amplifier (which usually goes to earth).
