DIY VU meter bridge

 I was in need of some VU Meters for my home studio setup and, as usual, I decided to build instead of buy.  I started by reading up a bit on just how VU meters and audio presenters work and then found a few schematics to work from. The schematics all  had plenty in common and most resembled any other simple buffer or preamp board, utilizing the TL071 or TL072. I began thinking about other opamps I had laying about and thought of the humble NE5532.  Since I had a few NE5532 buffer boards on hand from a previous project, I decided to see if they would fit the bill. Spoiler: They did!

First, a bit of VU theory: VU meters of the past were low level AC devices. They required a sesnisitve Germanium diode bridge(sometimes built inside) and resistor to calibrate to an accurate level of +4dbu on the VU scale.  Simply strapping one across your audio outputs with the required 3.6k resistor would yield the appropriate information on the meter. Is anything ever that easy?  No, it is not.  In such a setup you'll get an accurate reading, but you'll also likely encounter a notable amount of distortion.  

My diode boards are based on this>>>   https://michaelfidler.com/articles/practical-vu-meter-circuits/900cavuds360.png

I used everything in the schematic from C2 to the meter.  Everything else on the schematic was provided on the ne5532 board.

How much distortion is acceptable is a personal question each engineer must answer for themselves.  Or, you could use a buffer, such as an opamp. By buffering the meters from the loudspeakers, you can get the best of both worlds.  Note, that with the buffer, you still need the diode bridge to get the meter working.  Also, You can wire up a "Thru" patch to the inputs of the meters and have no discernable penalty to the audio.  This is great a solution and should you need to run your meters on your main stereo bus, you should add the "Thru" jacks into your meter build.  

However, since I have plenty of outputs to spare, I simply run a second stereo bus to another set of outputs on my recording interface.  So, my meters just have an input, and they work great.  

The Audio path looks like this:  Input jacks> 100k Pots> 5532 board> Diode board/cap>Meter +. (Note: Ground to audio input ground)

So now my needles are jumping, I needed to get them glowing.  Since Teac used 8v lamps in their meters, I knew I needed to get there without spending too much hard earned cash, or valuable time.  Enter the $15  variable voltage regulator.  With a simple input of 12v, AC or DC, I could get to 8 volts, or even a little less to increase the life of the bulbs.  

The Power supplies A &B look like this:  A.  DC 12v wallwart> 5532 board.  B.  DC 12V> Adjustable power regulator board (set to 7 or 8 volts) > 8 volt meter lamps.

Tests of the voltage regulator from 24vdc down to 7.41vdc.

I also made a case out of mahogany wood to make my meters "pop!"

All that was left to do was calibrate the meters from a 1k sine wave at 1.228v.  

When the input pots are set to "0" you're good to go.  

And that's pretty much it!  Plug in your inputs and select your program material- you're ready to "see" what you're hearing.