VU Meters

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So your basic VU meters come in 2 varieties. Voltage driven or, more commonly, current driven. You need to read on it what the "full scale" range is. On most meters it'll say something slightly cryptic like: FS 50ua. Which stands for Full Scale: 50 micro-amps. It might say: FS 100mv if it was a voltage meter.

In any case, now that you've gotten the range right, you should go to Rod Elliots site, he's got a circuit for using analog VU meters. Project 55.
--
Danny
 
I've used the Sifam. The AL29B is an excellent product. I didn’t realize they had gotten so expensive. I know Selco US stopped carrying them.

Who is the new US rep?

A cheap and dirty way of wiring it would be to use a 10K 10-turn pot to calibrate it to zero VU when bridged across an audio line. This works assuming that the meters have built in rectifiers. One of the nice things about the Sifam is that they use very good diodes so that when they are bridged across an audio line they introduce very little distortion. ($70 bucks, they ought to)

T and H pads can also be used for isolation and calibration.

The Rod Elliot solution is nice too.
 
Do you think Germanium diodes will help, since they have a lower voltage drop?

I forgot the name of the company, but here's their email address.
Dwight (IKE) Moore [meters@ix.netcom.com]

It's $69.63 each for the AL20SQ Sifam meter.

Now, with the Sifam meters... do you just connect them across the audio lines without the need for addtl components? It will be line-level signals for a mic preamp output.
 
I used the AL 29 bridged across +4 dB voltage matched balanced lines.

The sources ran around 50 ohms and fed high impedance inputs.

To calibrate I would use a O Vu reference tone, verifying the level with a distortion analyzer. A 10 kohm pot was used to dial in the proper meter level. It’s not a very scientific approach, but it let us deploy a lot of meters (they were only half the price a dozen or so years ago) quickly for not too much money.

Using the ESP # 55 is a better approach. Having a high impedance op amp drive the meter as this circuit does gives nice isolation form the line that is being measured. The amp has a high impendence input making its presence on the line negligible.
 
These days it is very, very easy to do your own meter scales -- you can use PowerPoint, MSPaint -- if you do a Google "Group" search, entering "meter scales" you will find a couple links to a DOS program which automates the process.

It isn't difficult at all to convert a linear scale meter to log -- if you have the patience to wire up a log converter with a couple transistors -- making sure to temperature compensate -- or you can use something like the Analog Devices AD536 which can has a log out, or the several other Analog and Burr Brown (more expensive) log converter chips. When I first started with log converters I used the circuit in the National Semiconductor "Linear Applications" book. The cost for a hand-wired log converter is next to nothing.

azira said:
So your basic VU meters come in 2 varieties. Voltage driven or, more commonly, current driven. You need to read on it what the "full scale" range is. On most meters it'll say something slightly cryptic like: FS 50ua. Which stands for Full Scale: 50 micro-amps. It might say: FS 100mv if it was a voltage meter.

In any case, now that you've gotten the range right, you should go to Rod Elliots site, he's got a circuit for using analog VU meters. Project 55.
--
Danny

All analog meters are current meters. Even if it says "V" it's a current meter with the appropriate current limiting resistor.

If you come across some good looking meters (and there are scads of them on EBay) you can determine the internal resistance with the setup at the bottom of this post. Sometimes there will be an internal shunt which you can remove. Note that some meters are compensated for steel rather than aluminum panels.

dhaen said:
It's probably best to buffer the audio to the VU circuit. This will prevent the distortion that might occur due to the rectifier.

This is the one POOGE to the Heath IG5218 or IG18 audio oscillator which makes a real, real difference -- discussed in 1980 or something like that.
 

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