A&M uA meters

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synthi

synthi

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Joined
Jun 9, 2004
Messages
560
Location
Madrid (SPAIN)
I got some vintage looking round meters, those are A&M instruments UA Dc meters, -7_0_+7.
Do you know if they can be converted for using them as VU or PPM?
Thanks,

Synthi.
 
Hi Synthi,

They sound like "centre-zero" meters- does the needle sit in the middle of the scale when no connections are made to the meter?

If you need to work out the sensitivity of the meter (Full Scale Deflection), here's a post I made in the old forum that may help:

Aah, mystery meters...

Well, when I get some unknown meters I usually set up a little jig. The chances are they are "straight" DC meters. These can come in any range from 100uA to 1mA. These are the basic "movement" sensitivities- any higher and you can bet that they're a basic 100uA/500uA/1mA movement with a "shunt" resistor added in parallel with the movement...

To test for the sensitivity, it's always safest to assume it's a 100uA meter. Things you need for testing: DMM set to 2000uA range, 1M LIN pot (or a fixed resistor), 1.5V battery, other value fixed resistors.

Connect the +Ve wire from the battery to the +Ve lead of the DMM. Then connect the -Ve lead of the DMM to the end-track terminal of the 1M variable resistor. Then connect a 1M fixed resistor to the wiper of the 1M pot. Connect the other end of this 1M fixed resistor to the "+Ve" terminal of the unknown-meter to be tested. Finally, connect the -Ve terminal of the meter to the -Ve terminal of the 1.5V battery.

Okay, before connecting the battery, make sure the wiper of the pot is hard-against the end of the track not connected. The pot is being used as a series resistor, and setting it here means that the full 1M of resistance is in the circuit. Connect the battery, and then slowly rotate the 1M pot. You'll see the needle of the meter move. If it appears to "peg" backwards- swap the connections to the meter- it's polarity is reversed, and the meter could be damaged by applying more current!

There will be a setting of the pot where the needle will point to the max. position on the scale- this is the Full Scale Deflection, or FSD. The value of the current flowing through the meter will be the current registered on the DMM. If this setup doesn't move the needle much, it obvioulsy has a higher FSD than 100uA. Swap out the 1M fixed resistor for 470k, 100k, 50k, 10k, 4k7, 1k etc and adjust the pot until you get the FSD. The series resistor helps to protect the meter in case you go "too far" with the pot!
Click to expand...

(Thanks to Jay (Fallout) for saving and sending this to me!!)

All "proper" VU meters are carefully designed with a rectifier and resistive pad so that 0VU=+4dBm, and their mechanical construction allows the VU characteristics of attack and decay (the ballistics) to adhere to the standard. So to make a microammeter or milliammeter into a VU meter needs, at minimum, a rectifier (usually a germanium diode to minimise voltage drop) and a resitor, and at most an entire buffer/rectifier/driver circuit. This can be based around an op-amp and a handful of other components, but it will only be a visual representation of audio signal level, rather than a true VU meter.

A PPM meter is different again- it has a very fast attack, and a slow release. A PPM meter is always driven by an active circuit, which has rectification and time-constant elements. There are different scales of PPM too, depending on where you are in the world.

One good project for a centre-zero meter is a phase meter. This compares two channels of audio signal and measures the overall phase-difference between them. This is very useful in the studio and workshop, as it is a quick way of making sure that material is mono-compatible (where's the reverb gone :shock: ) and that you haven't messed up balanced lines. A bit of listening to in-phase and out-of-phase audio soon trains your ears to spot it though. I'm often in shops or bars where you know the speakers are out of phase and it kinda hurts your head (or is that just me :roll: )

Anyway, someone posted a good circuit for a DIY phase meter- I'll have a look in my archive.

Hope this helps,

Mark
 
[quote author="Mark Burnley"]I'm often in shops or bars where you know the speakers are out of phase and it kinda hurts your head[/quote]

Well, I'm often in bars where the *beer* hurts your head... but I still know what you mean! -I've even been in HI-FI stores where some of the speakers have been 'out-of-phase' (i.e. polarity reversed on one channel)... and it feels like my head's inside out... I wonder how people can "not Notice"!

Keith
 
It's the same 95% who thinks MP3 is the pinnacle of the digital revolution.. :roll:

When I go into a bar my head always comes out outta phase :green:
 
yes, we are always trying to gain some degree of quality for our sound and then the world takes the Mp3 as a reference for audio quality....
Why the entire world seems to have shoes instead of ears!!????

Synthi.
 

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