Want to make a standalone VU meter module, could do with advice

[OwenM]

Hi there

I want to add a VU meter to a DJ mixer output to a school setup.

Kids use their phones when they're DJ-ing, they're all different levels at max-vol, they don't understand the 'gain', 'master', 'power amp volume' controls and how they interact (though I am working on that!)

The DJ mixer gives out about 1v at a nominal 'max' signal. We want to set the power amp controls and hide them away, and add a VU meter in the DJ box so they just know they can drive the DJ mixer up until it hits about 0DbVU and that's the volume limit we're allowed to operate at. And also 'when the little kids are around we should go fo -3DbVU' or similar. Some visual reference for 'all out party' compared to 'conservative volume' which means no-ones arguing about 'how loud it was last week' etc...

So, it ain't high fidelity. I Just want to add an inline VU meter on the DJ mixer outputs, so the kids can have the gain on max and the master vol on 1 and still know when we are reaching 'the neighbours will complain' threshold by the VU meter. If their high-output phone distorts the DJ mixer then that's for another lesson... All things considered, if its set 'correctly' I'd like it to be a pretty decent transference of signal after the VU meter to the power amp.

So I am gonna buy a couple of eBay VU's, just the meters, and I want to put them in a box with a simple op-amp buffer and a signal input-output that are directly connected. Is that the right way to go? The near-infinite resistance of the op amp in should do next to nothing to the audio signal, I figure. I'll work out power for the backlights, but do I need an amplifier circuit for the VU needles, considering I'll be wanting the 1v output of the mixer to register about ODbVU on the other side of the op amp. In a simplistic approach I feel that 1v on the op amp in should end up being 1v out if setup as a buffer but I know VU's are gonna move by current, not voltage. I've seen various buffered VU circuits that have some kind of current amp to make the meter movement work but I'm not sure if I can use ANYTHING, or if I need to follow a sort of standardised VU meter current amp schematic after the buffer..

Will a meter have a given current that moves the needle to the 0DbVU position, and I have to make an amp that makes 1v give that current to the movement?

Any thoughts?

Thanks in advance, as ever!

[nevetslab]

Hi there

I want to add a VU meter to a DJ mixer output to a school setup.

Kids use their phones when they're DJ-ing, they're all different levels at max-vol, they don't understand the 'gain', 'master', 'power amp volume' controls and how they interact (though I am working on that!)

The DJ mixer gives out about 1v at a nominal 'max' signal. We want to set the power amp controls and hide them away, and add a VU meter in the DJ box so they just know they can drive the DJ mixer up until it hits about 0DbVU and that's the volume limit we're allowed to operate at. And also 'when the little kids are around we should go fo -3DbVU' or similar. Some visual reference for 'all out party' compared to 'conservative volume' which means no-ones arguing about 'how loud it was last week' etc...

So, it ain't high fidelity. I Just want to add an inline VU meter on the DJ mixer outputs, so the kids can have the gain on max and the master vol on 1 and still know when we are reaching 'the neighbours will complain' threshold by the VU meter. If their high-output phone distorts the DJ mixer then that's for another lesson... All things considered, if its set 'correctly' I'd like it to be a pretty decent transference of signal after the VU meter to the power amp.

So I am gonna buy a couple of eBay VU's, just the meters, and I want to put them in a box with a simple op-amp buffer and a signal input-output that are directly connected. Is that the right way to go? The near-infinite resistance of the op amp in should do next to nothing to the audio signal, I figure. I'll work out power for the backlights, but do I need an amplifier circuit for the VU needles, considering I'll be wanting the 1v output of the mixer to register about ODbVU on the other side of the op amp. In a simplistic approach I feel that 1v on the op amp in should end up being 1v out if setup as a buffer but I know VU's are gonna move by current, not voltage. I've seen various buffered VU circuits that have some kind of current amp to make the meter movement work but I'm not sure if I can use ANYTHING, or if I need to follow a sort of standardised VU meter current amp schematic after the buffer..

Will a meter have a given current that moves the needle to the 0DbVU position, and I have to make an amp that makes 1v give that current to the movement?

Any thoughts?

Thanks in advance, as ever!

I haven't gone looking to see just what is on ebay in way of inexpensive VU Meters. The 'real' VU meter, such as Weston, API, Altec, Honeywell, others are based on sitting in a 3600 ohm circuit, and have built-in full-wave rectifiers. Not sure what the inexpensive ones are.....they could require you put the rectifier circuit together, and have to do the calibration/scaling, as they're simple DC meter movements with a traditional VU meter scale (+3, +2, +1, 0VU, -1, -3, -5, -7, -10db and an end mark, which might be -20dB).

I've been spoiled rotten having Precision Integrating Sound Level Meters, whose analog meter have at least 25dB range, often 30dB or 50dB with linear dB graduations. Basically TRUE RMS converters driving Log Converters to yield fast response, variable damping, and produce a much wider range than the traditional Analog VU meter as you're considering. Also Measuring Amplifiers with meter circuits offering 50dB Log range, 20dB Linear range (with VU-like scales), plus offering Peak Reading.

Now, in the used signal processing market, you'll find compressor/limiters which have meters with often wider range than a common VU meter, done with LED indicators....GRN for the -30dB thru -10dB range, YEL for -8dB thru 0dB, and RED for the range above, which may go 10dB above '0VU. There is an IC that National Semiconductor made (LM3915) having a 30dB range, in 3dB steps that many companies have made use of. Some gear will also have Peak indicators besides the VU meter indicators. You might find some old dBX gear or similar with useful meters (all would normally be 1 or 2 rack units tall, 19" wide panels. Finding something that needs repair could be a place to start, depending on your expertise in electronics repair.

There are no doubt VU Meter kits out there if you search long enough. I haven't looked. I had picked up a Tektronix 760A L/R & L+R linear LED meters that give both VU and Peak reading simultaneous, as well as having an XY scope display of the waveform (waveform monitor for use in broadcasting)....I think I had paid maybe $150 for it. Overkill for your needs, but, there are many options out there for metering. I personally like seeing a wider dB range than the limited range an Analog VU meter yields. That's just me, of course.

You might post some ebay links to the particular items you're considering so we can have a look and comment on further, depending on how much info the seller is providing. At any rate, a worthy project, with many ways to go. Lots of used stuff out there that could be cannibalized and adopted for the task.

There may be old retired Teac or similar tape recorder electronics having usable meters with their meter circuits that could be hacked and repackaged into your small box.

[Enzo]

So you don't need a VU meter, you just want a level meter. You don't really care that it accurately measures decibels. Right?

"VU meter" means two things. One VU meter is a piece of equipment. It is a calibrated meter. It has a specified impedance, they even have specified mechanical attributes, right down to the inertia of the needle. The other VU meter is the indicator itself. Like the ones on your old stereo. They may also indicate the accurate decibel reading, but only as a part of a whole circuit. The actual meter would not be a VU meter standing on its own.

And for that matter, we as a group often use "VU meter" as a general term for anything that bobs up and down with the music.

SO really, you want a volt meter stuck on the output of your mixer. And you want a scale on it made out to be useful to your group.

There is an old product by Conn, the strobe tuner folks, called a Dynalevel. It is a visual VU meter. It would do exactly what you are looking to do, at least as I see it.

[olddawg]

Why not make a calibrated ladder with a green, yellow, and red LED? You drive it with a transistor or op amp then use resistors and a trim pot. Cheap, quick and effective. In a small project box.

[Enzo]

I like to scrap old stereos and VCRs for catharsis, and I often save the LED VU panel. Typically they stand alone with a four wire connector: ground, V+, left signal, right signal.

They make the LM3915 family of ICs which will directly drive a string of LEDs in meter fashion.

[OwenM]

Thanks for all the interesting input guys! Loads of food for thought... And yes Enzo, you're completely right and I'm not sure why I hadn't thought of it before, I just want a level meter with 1v corresponding to somewhere at the end of the scale, not a VU meter at all. No correctly exponential gradient over a range of different sources necessary at all!!

I WOULD kind of like it to be with a needle movement, the kids were excited at it being 'old style' and I think it's nice for them to see some physical real thing, as most data they see these days is from screens of course. The LED thing would certainly be cool and I had thought about going that way. Would be fun to build something with an LM3915. So I'll probably go that way real quick if the needle meter proves fiddly..

For now though , I'd be happy with just something like you see on the front of some lower end sound cards or ashdown bass amps.

Was thinking something like this - https://www.ebay.co.uk/itm/Panel-VU-...oAAOSwtyxeaGtg

So 630Ohm resistance on the movement and the meters at 500Micro amp? I guess that means 500micro amps to move the needle fully? If I could buffer with an op amp, build a little rectifier and some kind of amp to get current in that region, plus a few bits for calibration would I be roughly on the right track of 'something I can get to point at 0 when I get 1v on input' with this sort of thing?

[olddawg]

VU Meter 2x Panel VU Meter Warm Back Light Recording & Audio Level Amp with Driver Board https://www.amazon.com/dp/B07SFHV85Y..._NuaDEb01DC7K2

[OwenM]

VU Meter 2x Panel VU Meter Warm Back Light Recording & Audio Level Amp with Driver Board https://www.amazon.com/dp/B07SFHV85Y..._NuaDEb01DC7K2

Thanks! Yeah I was thinking about one of those, that was what got me thinking in the first place, really. But really I'd rather understand the process a little and try and build something, though only to the limit that people are willing/enthused to explain a few of the tricky corners with it, that is! I've had limited success with those super cheap boards, they sure do the job well when they work! But with the data on the listing being less than ideal I was left wondering wether I could get my 1v signal to hit 'what that meter calls 0dbVU' on them with calibration.. As the meters+board specs are pretty similar to just the meter alone I'm wondering how useful those specs really are?

I just want to get my head around some of the fundamentals really.. So I need a buffer to leave the input signal alone, a rectifier to turn the audio signal to DC, a movement with a given DC resistance and current value (current value relating to max level displayed?) and a transistor to turn the audio signal/op amp low current to something that will move a movement.. I have all those bits and a veroboard to populate I'm just wondering what the theory is, roughly!

[olddawg]

Thanks! Yeah I was thinking about one of those, that was what got me thinking in the first place, really. But really I'd rather understand the process a little and try and build something, though only to the limit that people are willing/enthused to explain a few of the tricky corners with it, that is! I've had limited success with those super cheap boards, they sure do the job well when they work! But with the data on the listing being less than ideal I was left wondering wether I could get my 1v signal to hit 'what that meter calls 0dbVU' on them with calibration.. As the meters+board specs are pretty similar to just the meter alone I'm wondering how useful those specs really are?

I just want to get my head around some of the fundamentals really.. So I need a buffer to leave the input signal alone, a rectifier to turn the audio signal to DC, a movement with a given DC resistance and current value (current value relating to max level displayed?) and a transistor to turn the audio signal/op amp low current to something that will move a movement.. I have all those bits and a veroboard to populate I'm just wondering what the theory is, roughly!

Go to a thrift store and buy an old cassette recorder or whatever with a VU or two for next to nothing and salvage parts.

[OwenM]

I think I've found an answer to my question, the circuits on the page posted below seems to show how to get an op amp into the range that it will provide enough current to drive the meter, at least from the authors experience. I'm a little lost as to how opamps affect current in various configurations so I'm gonna build this (I have all the parts) and measure/learn the results I get! The author is talking about the same cheap meters I've been looking at on eBay, though I'm all up for coming across something I could salvage and use too!

http://objectivesounds.co.uk/article...her-ac-meters/

One question though, is there a standard input or V+, V- voltage that op amps use if not stated explicitly? The author doesn't denote the V pins on the op amp...

[nevetslab]

I think I've found an answer to my question, the circuits on the page posted below seems to show how to get an op amp into the range that it will provide enough current to drive the meter, at least from the authors experience. I'm a little lost as to how opamps affect current in various configurations so I'm gonna build this (I have all the parts) and measure/learn the results I get! The author is talking about the same cheap meters I've been looking at on eBay, though I'm all up for coming across something I could salvage and use too!

http://objectivesounds.co.uk/article...her-ac-meters/

One question though, is there a standard input or V+, V- voltage that op amps use if not stated explicitly? The author doesn't denote the V pins on the op amp...

Nice article! I only scanned it briefly, but it shows all the key ingredients and points needed. And, it looks like the final rendition would be a winner. How are your breadboarding skills, as I presume that circuit doesn't have a predesigned & fabricated PCB that just needs to be stuffed.

As for power supply voltages, anywhere from +/-12V to +/- 15V would be more than adequate. You can find pre-built small current bipolar supplies that just need the appropriate power xfmr. All of these aspects head to your ability to 'cobble' materials together safely that results in a small box as perceived that can be plugged in and work. As you're in the UK, you're at the upper end of AC mains voltage (230-240VAC), so attention to safe assembly & wiring practices are paramount.

[Wal_zz]

Thanks! Yeah I was thinking about one of those, that was what got me thinking in the first place, really. But really I'd rather understand the process a little and try and build something... I was left wondering whether I could get my 1v signal to hit 'what that meter calls 0dbVU' on them with calibration.. As the meters+board specs are pretty similar to just the meter alone I'm wondering how useful those specs really are?

I just want to get my head around some of the fundamentals really.. So I need a buffer to leave the input signal alone, a rectifier to turn the audio signal to DC, a movement with a given DC resistance and current value (current value relating to max level displayed?) and a transistor to turn the audio signal/op amp low current to something that will move a movement.. I have all those bits and a veroboard to populate I'm just wondering what the theory is, roughly!

Those meters are pretty small. I suggest looking at a larger rectangular unit like this https://www.ebay.co.uk/itm/Audio-Amp...r/202204775907

It has an illuminated scale, inbuilt rectifier, an impedance of 3.6K ohms, and is calibrated so that 0VU corresponds to 0.775v. (0dBm/1mW/600ohm line) A simple buffer comprised of a one transistor amplifier (2N2222 etc) or an integrated circuit (LM386 etc) will drive it. A trimpot at the input to the buffer will allow you to calibrate 0VU on the meter scale to your chosen level.

You can buy a pair of LM386 modules, complete with gain preset for a mere 0.99P!! It would cost you way more than that to buy the parts. https://www.ebay.co.uk/itm/2PCS-LM38...o/253098292859
Or, you can use the bits you have and build something with those.

The theory is: Audio from the mixer > Buffer amp with adjustable gain/loss > Meter with internal rectifier. If a schematic would help let me know and I'll post something.

[Wal_zz]

If you decide to go with the Op Amp driver as described in the article you quoted, you can substitute an LM324 in place of the TL072 and power the device from a single rail supply rather than a split supply using two batteries. The LM386 module mentioned above would be my preferred option for driving the VU meter. It can be powered from a single 9v battery or even better, 4 AA alkaline cells in a battery holder for longer battery life. An interesting bit of trivia: The LM386 was invented by Ernie Leroy Long at Motorola in 1969. It was originally produced for part of a fuel injection system for a Ford Car.

[OwenM]

As for power supply voltages, anywhere from +/-12V to +/- 15V would be more than adequate. You can find pre-built small current bipolar supplies that just need the appropriate power xfmr. All of these aspects head to your ability to 'cobble' materials together safely that results in a small box as perceived that can be plugged in and work. As you're in the UK, you're at the upper end of AC mains voltage (230-240VAC), so attention to safe assembly & wiring practices are paramount.

Great info thanks! Yeah, I hear you on building something from mains from scratch.. I mean if it were just me using it I would feel very comfortable with doing so, but I don't feel comfortable with building something and then leaving it at a school to get used an abused for maybe a decade after I move on! So I don't think this is the project to do that on... However, I'm very comfortable with veroboard or perfboard so not worried about building later elements of the circuit.

However, quite amusingly, the little DJ mixer I got for the school shipped with the wrong wall-wart psu, it was sent with PSU with a 15vAC, 200mA secondary So if I'm correct I can easily bridge rectify that to get my +-15v rails.. And then I just have a low voltage box with a 2.5mm power socket and a properly designed wall wart somewhere out of reach.. So that's definitely plan A!

You can buy a pair of LM386 modules, complete with gain preset for a mere 0.99P!! It would cost you way more than that to buy the parts. https://www.ebay.co.uk/itm/2PCS-LM38...o/253098292859
Or, you can use the bits you have and build something with those.

The theory is: Audio from the mixer > Buffer amp with adjustable gain/loss > Meter with internal rectifier. If a schematic would help let me know and I'll post something.

And thanks Wal_zz and welcome to the board! Really cool info, I'll look into all of that too.. I MIGHT just go for the tl072 just because I have those in my drawer and I have this power supply and to start with I'm keen to see how easily it comes together. But excited to learn about the LM324, I'm gonna order some of those for sure. I want to start with making them with op amps because I plan to use this project to to learn as much as I can, fiddle with bits and see the result and try and get my head around practical op-amp usage I know its a really simple just using them as a buffer, but I want to clear that hurdle first. My skills at amp fault finding and basic component use are getting somewhere, but the next layer or so, op amps and practical transistor circuits (used as just more than class A/B amp or switch) are still very vague for me!! For that reason I'll probably try the transistor-as-a-buffer too. I mean, the UK is in increasingly more strict lockdown and the beer will run out soon , gotta make some use of the time!

Those LM386 boards are insanely cheap though, I want to start learning about more specific-purpose IC's next, so I think I'll probably order some of those too to play with, and may end up using them in the final build.

Cheers for the heads up on the size too, Yeah they do look pretty piddly, thinking about! I'll go for the ones you posted, or try salvage something!

[nevetslab]

With only 2-wire 15VAC, you won't be able to create +/- supplies with a bridge. You can only produce half-wave rectifier based bipolar supply, as one lead will have to be ground. With the full-wave bridge, you have no ground.