VU Meter Buffer DIY Kit Warning This document is distributed for educational purposes only. This equipment operates at potentially lethal voltages. Only trained, qualified personnel should operate, maintain, or service it. www.diy-tubes.com
The kit includes: PCB Resistors Electrolytic capacitors Connectors Diodes IC TL071 with dip8 socket Trim pot
This kit you to make cheap VU indicators really useful for almost any project in which you want to add the signal level indication. VU buffer isolates the signal lines isolates the Total Harmonic Distortion caused by the internal rectifiers in the VU meter, allows you to reach higher THD. The buffer is connected to the signal circuit at point where it is necessary to measure the signal level (input or output of the circuit, etc). Suitable for balanced/unbalanced and stereo/mono signals. VU Buffer can run on any DC voltage from 9v to 30v. Dimentions - 41x41mm.
The board has a small size - 41x41mm. and can be mounted directly on the mounting petals of the VU meter. Start soldering 3k3 и 47k resistors.
Solder 47k (5pcs). And now goes 3k3 (8pcs).
Mounting the diodes. Pay attention to the location of the diodes on the board. Carefully read the entire booklet - diodes D1 and D3 can be soldered in the opposite direction, in order to change the polarity of the scheme. I happens when you whant to connect positive of the schema to negative of the VU, and vice-versa. Diode D2 is always mounted in the same way. Take look at the photos. Resistor 100R.
Now solder dip8 socket. Please note where is the "key" (the notch of the socket).
Mount PLS connectors (optional). You can not to use these connectors and solder wires from power supply and signal path directly to PCB. Trim pot 4k7 (5k on the schematic). Used for adjustment of the signal level.
Solder electrolytic capacitors. Pay attention to how capacitors are oriented on the board - minus marked on the capacitor casing. Plus - longer capacitor terminal and is marked on the board. See the pics.
Usually before installing the chip it is recommended to connect the power and check the voltage at the relevant pins of the socket (set multimeter s hot probe to pin 7, minus to 4-pin). But, as the project is not complicated, just install the IC. Please note how TL071 opamp is oriented on the board. Keys of the socket and the opamp must match. Mounting is completed. Now we can connect the VU meter. Note that one of the terminals of VU meter is designated as plus. Second, respectively, minus (or ground). Sometimes we want to turn our buffer board and connect the board minus to VU s plus to suit VU in the case conveniently. To do so, let s change the orientation of the diodes D1, D3 so that the cathode (stripe on the body) start looking toward the plus contact of the VU meter. See the pic for VU installation variants.
And this one variant:
Now it s time to calibrate the VU meter. To do this we need the source of the signal. It can be a professional measuring instrument, or any signal generator you like. You can additionally control generator s signal level by a voltmeter which has a scale in decibels. For rough calibration you can use your favorite DAW or even a smartphone. The last variant we ll use to demonstrate a common approach. Install the signal generator software for a smartphone, any soft you like TesTone, for instance. Connect the headphone output to the audio input of the VU buffer PCB. Select: sine, 1kHz, 0dB. Apply power to the buffer. Turn on the generator and by adjusting the trimmer R15, set zero on the VU meter.
Now you can check how the reading of the scale changes, if we feed -6dB sine. Everything works. Again - for more accurate measurements use the appropriate equipment.
1 2 3 4 5 6 7 8 A A VU Buffer v1.2 B B GND R10 3k3 R11 3k3 GND GND C D audio+ VU+ D1 1N5819-B 1N5819-B D3 VU- audio- R2 R1 47k 47k GND 10u 10u C1 R15 5k 3 1 2 + C2 + 47k R4 47k R3 100R R5 3k3 R6 3k3 R7 2 3 8 7 4 1 3k3 3k3 TL071P 6 5 IC1 R9 R8 + C3 10u 47k R12 + + C5 10u C4 10u 1N5819-B D2 +9-12v 3k3 R13 3k3 R14 GND C D GND E E 1 2 3 4 5 6 7 8
41.5mm R14 D2 3 41.5mm 3 1