voltmeter into vu meter?

G

Guest

Guest
i found a few simpson voltmeters (very beautiful), not sure what range , so on an so forth,

is it possible to turn a volmeter into a vu meter ?

or is it turning an apple into an orange?.

any one try ?

thanks

driller45
 

gyraf

Well-known member
Moderator
Joined
Jun 4, 2004
Messages
10,118
Reaction score
608
Location
Aarhus, Denmark
Driller,

Try searching for the previous posts on this. It's been covered several times, I think.

Jakob E.
 

amorris

Well-known member
Joined
Nov 5, 2004
Messages
525
Reaction score
2
Location
Orlando
you could build a diode rectifier circuit for the audio input and find a reasonable dc range for it, but linear vs log thing (dc sensing would be linear voltage swings and audio metering would want log sensing)would probably get you in deeper water.
 

NewYorkDave

Well-known member
Joined
Jun 4, 2004
Messages
4,378
Reaction score
18
Location
New York (Hudson Valley)
And the meter ballistics aren't the same.

If you have no use for voltmeters, put them up on evilBay. Simpsons are first-class meters and I'm sure someone would be happy to buy them.
 

PRR

Well-known member
Joined
Jan 30, 2010
Messages
11,144
Reaction score
1,031
Location
Maine USA
> simpson voltmeters (very beautiful)

Simpson made thousands of different types of meters. Need better clues.

> not sure what range

MeterMark.jpg

Look on the lower-right of the dial. Many generic meters, even with custom scales, had the actual meter sensitivity marked. This one is 200μA "F.S." (Full Scale). There actually are no voltmeters, they are all current meters. A voltmeter will be a current meter plus a resistor, which you can sometimes swap-out to get a different volt range, or remove for current measurement.

If unmarked, use a 9V battery and 100K and 10K resistors. Find a resistor that gives a large needle reading: more than 20% but not off the top. Use Mr Ohm's Law to figure the current (assume the meter resistance is negligible, maybe 200 ohms). If 9V/100KΩ = 90μA gives a reading at 45% of full scale, then full scale must be about twice that or 180uA, and really 1/0.45 or 2.2 times higher which is 198uA which is surely 200uA rated.

> is it possible to turn a volmeter into a vu meter?

Any meter can be an audio meter.

But how much audio power do you need to drive it? Jakob (I think) had a 200 AMP meter. If we assume it drops 0.1 Volts at full current (fairly typical), then it needs 0.1V*200A= 20 WATTS of power to move it to full scale. You need a small power amp! And the 0.1V/200A= 0.000,5Ω impedance is awkward. If padded-out to a comfortable 4Ω load, it looks like 160,000 watts! You could adapt a spot-welder transformer to transform the push-pull plates of a 2x6L6 guitar amp to 0.1V 200A, but the frequency response is uncertain, probably not as good as we want.

In practice, we used to drive 100uA meters directly off 600Ω lines with "small" loading and distortion. (All suitable meters are DC, the AC-DC conversion puts an ugly load on the audio which distorts it.) "Small" was ~1dB and 1%THD. We no longer like to see THD numbers that high. So we usually want some form of buffer amp (more parts to DIY). With a dedicated buffer, meters up to 1mA are reasonable (can use 741/TL071 cheap-chips).

But the real obstacle: Dynamics. Any meter will read a steady tone. For eggample that VTVM in the picture is very accurate on steady tone.... IF you wait 5 or 10 seconds. And when the tone stops, it takes even looonger to fall back to zero. The original design has a lot of filtering on the AC ranges; I added even more for doing frequency response tests below 20Hz. If I put it on dynamic music, it hardly budges. If I put it on steady tone, some wind instrument solos, it will rise and fall so slowly that I really don't have a clue what the audio levels are. Peaks are missed, and soft passages usually hidden in the decay from a preceding loud passage.

Most basic meters can't be this bad. In fact most are "too lively": they overshoot and fall back at rates that don't really suit musical audio or eye-response. Most meters are made for steady or slow-change measurements.

SO the short answer is: yes you can make an audio meter with almost anything; but no, you probably won't be able to make music/speech audio judgements from it unless it was made to be a VU or VU-like meter.
 

PRR

Well-known member
Joined
Jan 30, 2010
Messages
11,144
Reaction score
1,031
Location
Maine USA
> linear vs log thing

FWIW: all standard VU meters are essentially linear.

Look at the dB marks. -20VU to -19VU are a hair apart, +2VU to +3VU is a big gap.

Look at the percent marks (any real VU meter has a % scale). These are essentially equally spaced all the way.

There is a small kink at the bottom due to rectifier turn-on voltage. 10%/-20VU is further left than 10% of the distance from the rest position to 100%/0VU. For AM broadcasting, and for many loud-part checks, that's fine. AM levels should stay well above -20VU to cut through AM radio static, and if you are watching loud parts then you are most concerned with -6VU to +3VU, -10VU and down does not have to be accurate.

That was the state of the art in 1938, and is still sorta a comfort level for DIY. Post-war, european broadcasters developed precision rectifier dB-scale meters. The BBC PPM is most famous in english-speaking lands. These are great for ensuring you don't overmodulate a transmitter or saturate a tape. With experience you learn where it should twitch in soft passages. But they are non-trivial to build, and rarely DIY-ed.

Interestingly, Sir Rupert and friends did a PPM in 5 transistors:

PPM.gif

big image with booster amp

This seems like a lovely project for someone to layout and get boards made. It is small, so once the setup fee is paid the boards are cheap. They'd fit easy in an envelope. Charge a few bucks to cover PCB setup cost and post-office hassle, and a lot of folks here could put surplus meters to work.
 
G

Guest

Guest
thanks you very much , this information is very appreciated.

if that is the circuit , i will lay it into pcb format and post it .

takes me a second to memorize it and make an efficient lay out .

but sound like fun.



driller 45
 

Latest posts

Top