building your own meters..

Mbira

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I know there have been lots of threads on meters, but I haven't seen this addressed. I am currently a starving college student-but I'm a tech at the EE lab and we have all these cool old BIG meters. There are lots of different types. I'd like to build a nice big stereo VU meter that could switch from rec. level to mains level. Is it doable to redo the components of a meter to measure VU and just print up a nice template to glue onto the meter? Thanks.
Joel
 

PRR

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> Is it doable to redo the components

Yes.

Don't expect VU ballistics. A PPM is more workable.
 

Mbira

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Thank you PRR. I have seen this:

http://sound.westhost.com/project55.htm

Any other reading that anyone would suggest?

Is a PPM meter essentially like a VU but doesn't have a buffer (wrong word?) so it reacts quicker to transients, etc?

thanks, Joel
 

Mbira

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OK, now the newbee fun begins...These are westinghouse milliamperes meters. They go from 0 to 30 milliamps. What do I do? :roll: Do I just gut them and build something from scratch to tell these little needles how to move? Are there any pcb layouts for such fun?
Thanks and cheers as always.

Joel :guinness:
 

Mbira

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Here's a couple of pics of the meter.

http://rattletree.com/DIYpics/7.jpg

http://rattletree.com/DIYpics/8.jpg

I don't even know where to begin with this. If you guys think it's not worth it and I'm wasting my (and your) time-let me know. Thanks.
Joel
 

microx

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In theory you could make a massive VU meter but it would only be an ornament. It's what PRR is telling you that you should consider. The ballistics or response characteristics would be such that you would never read a true peak, it would move so slowly.
PRR also mentioned PPM, Peak Prog Meter, which has a much faster rise time and much slower decay. It is a meter that reads full scale to indicate zero and so the suspension of the meter movement works to assist a peak reading.
If you have to have a big meter then include a clip LED to get you out of trouble.
Steve
 

Mbira

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Thanks,
So are there any schematics or better yet layouts for a PPM meter? It doesn't seem like this milliamp meter has much on it. It just seemed to have the +- leads and a fuse, and a needle. What would I need to remove to make this happen?
Thanks.
Joel
 

mcs

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For a schematic look in the LM3916 datasheet. I may make a PCB layout for that (or a similar) circuit soon.

Best regards,

Mikkel C. Simonsen
 

PRR

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> Is a PPM meter essentially like a VU but doesn't have a buffer (wrong word?) so it reacts quicker to transients, etc?

No. Aren't they teaching you anything in that EE lab?

Put one of these old meters on an audio signal generator. Start with a very low frequency, like 10Hz, and sneak-up the level until the needle moves. This should happen at a very low voltage, less than 1 volt.

Because the input is alternating current, the needle will swing both ways. Do NOT try to get a large deflection, just enough to see. You can read "level" by eye-balling the edge of the blur of the wavering needle. That is awkward. Also as you increase the frequency, the blur-width for constant input level soon decreases, and will be very small above 100Hz. Over-powering the heavy needle is not the answer: the forces needed to cause a large swing will actually throw the moving parts out of their pivot, ruining the meter.

So it is good for DC, poor for 10Hz AC, and useless for audio.

What if we turn the AC into a varying DC, so the needle does not try to swing both ways? Get a bridge rectifier. Now both + and - swings of signal flow as + through the meter, so ut just swings "up". Now you probably have a pretty useful AC meter from around 20Hz up to 5KHz.

One problem: the input is very low voltage and fairly high current. Probably less than 1 volt (maybe 0.1V) DC for full meter deflection, at 0.030 Amps, or around 33Ω. Maybe useful for headphone jacks, or low-level speaker amps. Way too heavy a load for an line level.

Also the 1V or 0.1V voltage sensitivity isn't accurate: the meter was calibrated for current, not voltage. The falling voltage response at the top of the audio band is due to coil inductance causing rising impedance, or less current for a given voltage. Also the meter voltage is low compared to the 0.6V dead-zone of Silicon rectifiers (1.2V for a Bridge).

Put a resistor in series with the movement. Ideally we want resistor voltage drop to be much-much greater than meter movement or rectifier voltage drop, so the resistor dominates the voltage sensitivity. If the diode/meter drop is around 1V and we want the total to be 100 times higher, we want about 100V sensitivity. This may be awkward, and we don't need 1% accuracy. Try 10X, or 10V sensitivity. 10V/30mA= 333Ω in series with the meter and rectifier. For signals from around 1V to 10V the meter input impedance is around 330-340Ω. Low, but we could dedicate an amp to it. The 10V full scale sensitivity is awkwardly high, though we could ask that amp to boost it.

You see why, in days when amps were expensive, they did not use 30mA movements on audio meters. The VU meter is only a 0.2mA movement and still causes significant loading on 600Ω lines. You might use a 30mA on speakers, but you won't see much below 1V signal because common Silicon rectifiers drop a large part of a Volt. You could find Germanium or Copper-Oxide rectifiers with lower drop, and indeed that's what VU meters use. But they are typically rated 100mA and melt easy. They would be OK with 30mA of steady tone, but audio peaks are over 10X higher, so there is a large risk of burn-out from 300mA peaks in 100mA diodes.

About those audio peaks: you already know from AC tests that this needle won't move much faster than 100 milliSeconds (10Hz). In audio metering we are often interested in large transients that are 50mS to 1mS long. SO this meter will read the longer-term average, averaging-in the transients, which will hardly change the reading on the heavy needle.

The VU meter compromises. It uses a higher force/mass movement than the average utility meter, so it will respond to sub-50mS peaks with an extra high twitch. It smooshes 1mS transients, so when using a VU meter to avoid peak clipping you must keep the meter peaks far (10dB) below the system overload. The VU meter is a passable "loudness" meter, and gives a guide for setting average level so most peaks won't clip. It never actually shows peaks.

No mechanical meter can show audio peaks. Just too heavy.

But tubes and transistors can respond faster than audio peaks. And capacitors can hold the levels of those peaks long enough for a heavy needle to indicate.

So most good audio meters are a full-wave peak detector, a leaky sample-hold, and buffering so the meter movement does not load the hold-cap.

It is possible to pick the peak detector and leaky-hold rates to simulate a VU meter on a lesser movement. I think that is dumb: the VU is a terrible meter, it was just the best they could do in 1937. Today I consider VUs more decorative than useful. More modern thinking meter specs use a shorter peak-catch time, often a longer hold. Also audio swings over 1,000:1 while it is hard to read more than 20:1 on a needle-meter. Most recent good-meters use a log-amp to compress a wide range of signal into the usable part of the scale.
 

Kev

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[quote author="PRR"]... the VU is a terrible meter, it was just the best they could do in 1937. Today I consider VUs more decorative than useful.[/quote]

oh yes ... and the whole question of loads and levels we live with today is down to the choices made back then.
Outside of Mic's and guitars and their pre-amps ... and amps and their speakers the world of audio signals doesn't have to be based on these choices ...
Unless you want to include these old units in your system.

so PRR's next comment could be most important ...

[quote author="PRR"]... More modern thinking meter specs use a shorter peak-catch time, often a longer hold. Also audio swings over 1,000:1 while it is hard to read more than 20:1 on a needle-meter. Most recent good-meters use a log-amp to compress a wide range of signal into the usable part of the scale.[/quote]

yep and whether you do this with a needle or LEDs it probably doesn't really matter.
As with the comment above it might be good to go back to first principles and look at why we want a meter or indicator in the first place.

Not suggesting we don't ... just suggesting that there may be other things more useful to know.

I find the MOST useful indicator when working with a Digital unit like a DAW is the Headroom indicator. There is one sure thing you know about in digital audio ... 0 dBFS.

It is possible to build this type of thing into a Mic-pre. You could set up an indicator to light and/or stay ligth until you release it ... when the signal gets to within 3 dB of clipping.

If the light goes OFF then you must lower then gain. This may be over-simple for all situations, especially for those going for some of that drive/saturation factor.

I tend to use PT input meters in this way. Most of my DIY Mic-pres are set to be below clipping when PT lights up. Yes I am waisting a small amount of Headroom from my Mi-pres.

Back to the point.
A meter or indicator can be set up to be useful to you.
It is your meter so you can develop a system to help tell you what you want to know.
True VU is only needed when you want to line up with existing equipment to existing audio standards. You should have at least a couple of these things in your studio.
 

kent

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yes, yes and yes to all the stuff above. My extra $.02 is that VU's ARE good for mastering. Unless you're one of those Finalyzer freaks. Then it's like WDEF radio - full deflection ALL the time!

cheers,
kent
 

Mbira

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I humbly bow to you all. Thanks PRR for the post.

Aren't they teaching you anything in that EE lab?

Not much. I mainly check out projectors to professors and teach grad-student TA's how to use a DMM-no shit. Quite honestly-this is my education here. I should be paying you guys my tuition.

why we want a meter or indicator in the first place.

To answer this question honestly I'd have to say:

1) I have access to a couple of free meters.
2) I think they look cool and would look cool in my rack
3) Back to one-I'm broke right now and I'm trying to do projects that I'll learn with that I can do with parts available at the EE lab here...Unfortunately that removes any type of iron, etc.
Not to be a sob story but that's the way it is right now...
at least till I get my student loans again in January-then it's back to the other stuff!

I find the MOST useful indicator when working with a Digital unit like a DAW is the Headroom indicator. There is one sure thing you know about in digital audio ... 0 dBFS.

Actually what I was imagining was stereo PPM meters with 2 LED's per channel. One that would flash at a clipping level, and one that would stay on if clipping had occured until you reset it. Now that' would be cool...

Joel
 
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[quote author="PRR"]
Because the input is alternating current, the needle will swing both ways. Do NOT try to get a large deflection, just enough to see. You can read "level" by eye-balling the edge of the blur of the wavering needle. That is awkward. Also as you increase the frequency, the blur-width for constant input level soon decreases, and will be very small above 100Hz. Over-powering the heavy needle is not the answer: the forces needed to cause a large swing will actually throw the moving parts out of their pivot, ruining the meter.
.[/quote]
Everybody say out loud - Simon (aka UK03878) you are a muppet
I had three old VU and PPM meters that I got out of the store cupboard last night and hooked up to a signal generator
I set the signal generator to output at about 0.775V (bloody impossible with an old farnell one I had - but I got close enough)
I then hooked up the ground and out of the signal generator to each of my "raw" PPM/VU meter terminals
At 10kHz - I heard a faint musical buzzing - I thought from the signal generator
I pushed the frequency down to 100Hz and the buzzing stopped - but the needles where flying lefty and right - thats what the sound was - the needles going far left then back right and bouncing against the stop needle on the left of the movement
I then scratched my head for agaes - came in the morning - looked up PRRs post and smacked my head with a DUH!!!!!
Repeat AC, AC, needle swings both ways, AC, AC...

oh and bump to this brilliant PRR post
 

Kev

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:?
" Simon (aka UK03878) you are a muppet "

:grin: :grin: :grin:

we're all muppets from time to time ... and if youre not ... then your not trying hard enough
and probably NO FUN AT ALL.

:cool:
 

CJ

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I came across this semi related article.

Let me know if you want more eye strain, I can dredge up the rest.


surplus_meter_a.jpg
 
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