Unknown VU meter or analog panel meters

I recently bought a few VU meters from eBay.
Does someone know how to measure them to find out what characteristic (range) they have ?

Are they VU meters, or are they just moving-coil meters?

Keith

Hi Lampas,

The meters you have will be of two types- either an "AC movement" similar or identical to a true-VU meter or else a standard "DC movement".

All meter movements are essentially the same- a coil is mounted freely in a constant permanent magnetic field. In a similar way to a motor, a current passing through the coil causes a turning motion- the magnetic field in the coil repels the permanent-magnet field. The needle is connected to the coil, and indicates the motion on the scale.

Because the direction of the current through the coil determines the magnetic polarity of the field, a plain "DC movement" can only measure a DC current- if you connect a DC meter up backwards (+ to- and - to +) the needle will attempt to go past "0" and peg violently backwards. Anyone with an analogue multimeter will be familiar with this when measuring a split-rail supply and forgets to swap the + and - probes!

If you apply an AC signal to a DC-movement, the needle will appear to hover just over the rest point (minimum reading). This is because the AC signal is effectively reversing the current through the coil so rapidly that the needle can't keep up. All meter movements are dampened by a spring, and so the needle can't keep up with these current reversals- it's alternately "pegging" past 0 and wanting to register a forward current. So it ends up vibrating just above 0. Not good.

DC movements are used everywhere though- the Gyraf SSL compressor requires a DC movement meter because it's measuring a DC "gain reduction" signal. Many tape machines use a DC meter because it has to be used to monitor input levels, output levels, bias etc. These meters are driven by a rectifier, buffer and level-control circuit.

Any DC movement will intrinsically measure a DC current. The amount of current passing through the meters coil to move the needle to its maximum deflection is termed its "FSD"- Full Scale Deflection. The lower end of this range is usually 50uA. Other common movements are 100uA, 500uA and 1mA.

To measure a voltage, a series resistor is added so that combined with the internal resistance of the meter (any coil of wire will have a resistance, and this has to be included in any calculations for adding "shunts" etc) when x-Volts is applied, the FSD of the meter is obtained.

For measuring larger currents, a parallel "shunt" resistor is connected across the meter terminals. The resistance of this shunt is determined so that when a current passes through the circuit, the current through the meter is small, and the larger current passes through the shunt. Ammeters measuring 1A or greater have shunts which are basically lengths of copper wire- 5A ammeters and larger have shunts which are just chunks of copper!

For a DC movement to measure AC a rectifier is needed. This is needed for AC ammeters and AC voltmeters. A traditional VU meter is an AC voltmeter. So to measure an AC voltage a basic DC movement has a bridge rectifier added (usually germanium or selenium- silicon rectifiers have too-large a voltage drop across them, and a significant amount of voltage (at audio-signal levels) would be lost across them. (At higher voltages a standard bridge rectifier can be used- if you want to measure an AC current and you only have a DC ammeter you can connect a chunky bridge rectifier across the meter terminals- + and - to the meter, and the AC terminals as the input.)

A series resistor is then added (usually 3k6 or 3k9- with an unknown meter apply a 1kHz signal of +4dBm and see which value gets you nearer to 0VU) to this DC movement-plus-rectifier, which is basically scaling our AC movement to function as an AC voltmeter.

A "true" VU meter has a distinct set of specifications. This includes the dB spacing of the scale, its "ballistics" (i.e. the time it takes for the "attack" and "decay" of the needle when confronted with a dynamic signal) I know there are some good posts on this here- do a search and I think PRR did some good stuff.

Okay, there's a bit of background.

The best way to test a batch of meters is to first weedle out the DC movements from the AC movements. The easiest way is to actually apply a small AC signal to each of them. You can use either a signal generator, or the output of a mixing desk/preamp etc. Connect the signal to the meter movement via a resistor of 3k6 (it doesn't actually matter what value at this stage- just a resistor of over 1k to protect the movement) Then bring up an audio signal from the desk/signal generator/soundcard etc. Bring it up slowly from nothing, and observe the meter needle. Use a 1kHz sine (preferrable) or music (workable!) If there is no movement of the needle at all, carefully bring up the level a bit more.

If the needle dithers about 0, even when you bring the level up, it is a DC movement- more about this later. If the needle moves up the scale in relation to the output level- great- it's an AC movement- ready to be made into some type of "audio level meter".

Once you've worked out which are DC-movements and which are AC-movements, you can then test to see which are scaled to some sort of VU-ish level. This time you need a 3k6 resistor. Connect it in series with the movement and apply a signal of 1kHz. Bring up the level until it reads "0VU" on the meter. Then measure the actual output voltage of the signal- either with another known meter, the VU meter on your console etc. It should read 1.23Vrms. If not, it doesn't meet "standard" VU/level meter specs but by adjusting the series resistor (use a 5k pot to set the value) you can get it to be a basic "audio level indicator" which can be useful for a particular application.

For speccing the DC movements, check my post here:

Mystery Meters!

Hope this helps,

:thumb:

Mark

EDITED 03/09/04 for sense.

Wow Mark ,that was an answer!!!
I don't know how many people needed that info,I certainly did.


There's two clear levels in which info changes hands here.One is betwen knowlegeable people in which sometimes only hints or references to a procedure are enough.The other level is for people with very basic knowledge but still able to enjoy DIY with the proper advice,basic knowledge that in time increases with your help.
Let me thank you for taking the time to help and enlighten the newbs.
Aharon

Excellent post Mark. You took the time to go into the variables... I didn;t have the time, so I just poked the question to see if the original poster was using the term "VU" meaning moving coil, -a common misunderstanding.

One suggested edit: When you say "hovering at (or above) zero" you should perhaps replace "sero" with "the rest position", since VU-scaled meters will have the rest position marked at minus infinity, not zero... and "zero" will be 3/4 of the way up the scale... many Behringer meters are exactly thus.

Keith

[quote author="Mark Burnley"]Hi Lampas,

.... which can be useful for a particular application.

For speccing the DC movements, check my post here:

Mystery Meters!

Hope this helps,

:thumb:

Mark[/quote]

Well Mark, thanx for the info I need, this is really good explanation!

[quote author="Aharon"]Wow Mark ,that was an answer!!!
I don't know how many people needed that info,I certainly did.
... Let me thank you for taking the time to help and enlighten the newbs.
[/quote]

:green: :green: :green:

correct,
this is why Mark was my unofficial assistant trainee Mod to my trainee Mod status over at the old place. Much of what we did and still do is aimed at the new and first timers.
Check Mark's write-ups at the Group DIY page ... these will move soon and I hope Mark will do some more to add to them at the new place.

excellent work Mark ... :thumb:

Keith,

Thanks for the heads-up. I had in my minds-eye a picture of a 0-10 "meter" at this point...edited and changed!

:guinness:

Kev- yes, I have some new stuff brewing. I'll let you know!

:thumb:

Mark