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

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

    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!

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    Supporting Member nevetslab's Avatar
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    Quote Originally Posted by OwenM View Post
    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.

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    Lifetime Member Enzo's Avatar
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    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.


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    Old Timer olddawg's Avatar
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    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.

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    Lifetime Member Enzo's Avatar
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    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.

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    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?

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

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    Quote Originally Posted by olddawg View Post
    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!

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    Old Timer olddawg's Avatar
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    Quote Originally Posted by OwenM View Post
    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.

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    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...

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    Supporting Member nevetslab's Avatar
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    Quote Originally Posted by OwenM View Post
    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.

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    Junior Member Wal_zz's Avatar
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    Quote Originally Posted by OwenM View Post
    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.

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    Junior Member Wal_zz's Avatar
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    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.

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    Quote Originally Posted by nevetslab View Post
    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!

    Quote Originally Posted by Wal_zz View Post
    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!

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    Supporting Member nevetslab's Avatar
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    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.

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    Quote Originally Posted by nevetslab View Post
    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.
    Ah ok, thanks, learning point noted, I had assumed (hoped) the 2 wire's would correspond to either side of a normal transformer secondary, but it makes sense that it's one line of 15VAC and a ground, thinking about it. I think I'll find a way to use a +V and ground op amp like the LM324. I assume I'll only be able to get 7v ish from my 15v ac supply when half-wave rectified...

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    I'd use the last circuit from the the link in post 10. It uses a single dual op-amp (TL072) and it doesn't need a +/- supply. It will run off a single 9V battery so you could use a regulated DC wall wart.

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    Junior Member Wal_zz's Avatar
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    Owen,

    The smoothing capacitor will charge to the peak value of the 15v AC, which is around 21v without any load. With a load applied the DC voltage will drop to a value dependent on the current drawn by the Op Amp. A better idea would be to use a full wave bridge and use the spare op amp in the TL072 as a rail splitter which generates +/- voltages from the single ended supply. Unfortunately, as I am new here I don't yet have privileges to post images or other files to clarify what I'm saying.

    The circuit shown in figure 8 of the article is almost exactly what you need. Instead of using a 9v battery supply feed the circuit from 15v AC > Full wave bridge > 1000uF Smoothing capacitor. The net result is you will have a +/- 10v supply for the Op Amp. I hope this is clear for you, if not feel free to ask more questions... and have fun!

    Walter

    PS: Here is some bedtime reading for you about rail splitting and virtual grounds.

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    Quote Originally Posted by Wal_zz View Post
    Owen,

    The smoothing capacitor will charge to the peak value of the 15v AC, which is around 21v without any load. With a load applied the DC voltage will drop to a value dependent on the current drawn by the Op Amp. A better idea would be to use a full wave bridge and use the spare op amp in the TL072 as a rail splitter which generates +/- voltages from the single ended supply. Unfortunately, as I am new here I don't yet have privileges to post images or other files to clarify what I'm saying.

    The circuit shown in figure 8 of the article is almost exactly what you need. Instead of using a 9v battery supply feed the circuit from 15v AC > Full wave bridge > 1000uF Smoothing capacitor. The net result is you will have a +/- 10v supply for the Op Amp. I hope this is clear for you, if not feel free to ask more questions... and have fun!

    Walter

    PS: Here is some bedtime reading for you about rail splitting and virtual grounds.
    Thanks Walter and Dave! Great info all round.. Ok, I think I'm getting my head around it. The final circuit (fig 8) of the article I posted seems to be the way to go. I think throughout today, pondering it, I've been down several dead ends working it out and now I think I'm there but I'm a bit confused on the terminology!

    So as I only have a 15VAC supply, NOT a transformer with a 15v secondary, I can't use the standard bridge rectifier found on amps to get my + and - . But I don't need it for my proposed circuit (fig 8), as the second half of the op amp is set in such a way that, I can see, is doing some sort of half-way voltage dividing to the source voltage to make a virtual ground of some sort. I'm not sure I get exactly how, yet! But I guess from it's feedback to the signal input and its perhaps floating the circuit midway and causing the op amps to behave as though they are halfway in between the single positive voltage and ground, giving a + - situation as far as the op amps see it... I may be way off, but doesn't really matter for now as I'll learn more of that as I build it and it might be a bit beyond my comprehension right now. I'll read the rail splitting and virtual grounds link you posted Walter! Either way, the main point is the circuit can be fed with +9v ish and Ground.

    So I might just get a 9v DC, or dig one out of the shoebox of spares and build this circuit more or less as is. I'll probably remove the meter voltage-range selector switching and, I imagine, I will need to omit the meter rectifiers, as the meters I am probably buying have one in. (would the meters have the cap C1 in them too, if not what's the way in which it is usually accommodated?)

    I think I understand the principle of using my 15vAC supply and rectifying it, but I'm confused at the term full range bridge when I only have the single ended, one half of what you would normally have with a 15v transformer secondary, supply. You mention that I could use a full wave bridge, which to my understanding needs either end of the transformer supply and always creates a positive and negative voltage, but then say that it will be single ended however, and an op amp could be used to create the + and - supplies (which I gather is exactly what's happening in the proposed circuit). So I think I'm missing something here, sorry! But that's all good, I don't have a variac or bench DC power supply yet so I'm going to take the opportunity to build a few bridge rectifiers at a nice safe 15v and get to know them a bit better.

    thanks for all your time everyone! I'm really excited to start getting my head around this!

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  20. #20
    Junior Member Wal_zz's Avatar
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    Quote Originally Posted by OwenM View Post
    So as I only have a 15VAC supply, NOT a transformer with a 15v secondary, I can't use the standard bridge rectifier found on amps to get my + and - . But I don't need it for my proposed circuit (fig 8), as the second half of the op amp is set in such a way that, I can see, is doing some sort of half-way voltage dividing to the source voltage to make a virtual ground of some sort. I'm not sure I get exactly how, yet! But I guess from it's feedback to the signal input and its perhaps floating the circuit midway and causing the op amps to behave as though they are halfway in between the single positive voltage and ground, giving a + - situation as far as the op amps see it... I may be way off, but doesn't really matter for now as I'll learn more of that as I build it and it might be a bit beyond my comprehension right now. I'll read the rail splitting and virtual grounds link you posted Walter!

    So I might just get a 9v DC, or dig one out of the shoebox of spares and build this circuit more or less as is. I'll probably remove the meter voltage-range selector switching and, I imagine, I will need to omit the meter rectifiers, as the meters I am probably buying have one in. (would the meters have the cap C1 in them too, if not what's the way in which it is usually accommodated?)

    I think I understand the principle of using my 15vAC supply and rectifying it, but I'm confused at the term full range bridge when I only have the single ended, one half of what you would normally have with a 15v transformer secondary, supply. You mention that I could use a full wave bridge, which to my understanding needs either end of the transformer supply and always creates a positive and negative voltage, but then say that it will be single ended however, and an op amp could be used to create the + and - supplies. So I think I'm missing something here, sorry! But that's all good, I don't have a variac or bench DC power supply yet so I'm going to take the opportunity to build a few bridge rectifiers at a nice safe 15v and get to know them a bit better.
    Clarifying a few points in the order you gave..

    - The wallwart supply is simply a 230v to 15v transformer in a box that provides 15v AC out with no other internal components.
    - You can use any standard bridge rectifier or you can build your own using discrete parts.. your choice.
    - The second half of the TL072 is generating a virtual ground, effectively dividing the incoming DC of approximately 20v into a +/- 10v supply for the Op Amp.
    - Running your project from a 9v battery to start with is fine and you can switch to Mains power down the track.
    - You can remove redundant parts that won't be required for your application. R3, R4, S1, D1, D2.
    - Leave the meter rectifier (D3-D6) in place. That way you can use a meter with or without an internal rectifier. Capacitor C1 controls meter ballistics (the speed at which the needle moves) You may find that 100uF is slightly too large and the needle is sluggish on music signals when you finally do a real world test.
    - Don't forget to use a smoothing capacitor after the bridge when building an experimental supply. (A 1000uF 25v would be suitable) This won't be required in the final circuit as C2 and C3 will perform the smoothing function.
    - That tutorial on rail splitting is obviously beyond you at the moment so revisit it later after you get your head around the foregoing stuff.

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  21. #21
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    Quote Originally Posted by Wal_zz View Post
    Clarifying a few points in the order you gave..
    Awesome thanks, I think, if my assumption that the wall-wart is NOT a normal transformer is wrong, then I understand it all more or less (at least for my level) and everything is set for me to go forward once I get the meters themselves. Thanks so much for your time! I think I didn't understand post #15 and started thinking of my AC supply as something more complex than a transformer.

    All other points I'm clear on, cheers, and I'll nibble away at that rail splitting tutorial once I'm really clear on the stuff on my plate at present!

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    Junior Member Wal_zz's Avatar
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    Good to hear!

    History of the VU meter

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    I knocked up a schematic in LTspice, as I figured I may as well learn a few more skills on the way! Does this look kind of right? I had a question...

    R7 and R14 (replacing R2 R3 and R4 in the original) were in the original circuit to achieve a varying input sensitivity from 500mV to 50V, my signal is probably going to be 1v RMS at most. Am I right that as the R2, R3, R4 resistance increases in the original then that shifts circuit sensitivity to a higher voltage range? If so should I have a slightly higher resistance for my R7 and R14 (left and right), to achieve a shift from 500mV max to about 1v max, or will that be easily adjustable via my R5 and R12 trim pots? I'm still grappling with how the rail splitting OpAmps U1-1 and U2-1 (my circuit) are operating and not sure if that's something to worry about!

    The 7500 resistance on the meters is just from what the article I posted quoted as a nominal impedance at 0Vu, I don't expect it's particularly accurate for every meter and it's just a rough figure!

    I only have a 2200uF cap available for C1, I simulated on a rectifier app and seemed to be ok, but am I causing a problem using a higher than rated filter cap?

    All the best in these odd times!

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    Thanks guys, I made a....nice looking level meter that may or may not be relatively close to some kind of VU standard! It works great, I realised the error in the schematic I posted above, removed those diodes but that should have included their wire too. I spent an hour trying to see why that would work until I got rid of it and it all made sense, ha!

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    VU Meter Schematic opamps ps sorted.pdf

    I know I didn't need the smoothing cap(s) on the power supply, but made it an exercise in making a simple power supply as well as the buffer/meter circuit.. Only had 470uf caps up to voltage hence two! Tested it plenty and learned a load, about opamps in particular, which has been a sticking point for me...

    cheers for the help!

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  25. #25
    Junior Member Wal_zz's Avatar
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    Pleased to see the finished project Owen, you've done an excellent job!
    Now that many weeks have passed I'm curious if it is now in use, what the reactions of the boys were, and how it is helping keep audio levels under control.

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    Quote Originally Posted by Wal_zz View Post
    Pleased to see the finished project Owen, you've done an excellent job!
    Now that many weeks have passed I'm curious if it is now in use, what the reactions of the boys were, and how it is helping keep audio levels under control.
    Thanks, and thanks very much for your help! You were very patient with me and thanks to you and others I feel I fully understand the circuit, at least, on the 'advancing beginner' level! I had a lot of fun making a nice box for it and practicing my quasi French polishing on it too

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    Unfortunately we didn't get to test it in anger as our end-of-term was focused on frantically getting international kids home (it's a boarding school) and since then we haven't been able to have anyone back. But I talked about the project with the kids most involved with doing the parties and they are keen to make a similar thing with me when we get back to school, so I'll keep this one for my studio setup and they can learn about putting together the same thing and learn a bit about the principles involved. I'm currently teaching a beginner electronics theory course with them online (they are very much at the begging, we are just looking at ohms law and common components, I have no ideas above my station of teaching beyond this stage!) and this will serve as a nice little project when they get back to school, along with the little veroboard guitar pedal layouts I've been playing with. I'll probably try and do the version with the 9v battery supply, or at least build the board with them and then finish it and add the power myself, but they will get a lot from watching it being built I think and may get them more into the subject, hopefully!

    Don't worry, I'm a music teacher and this is just a side lesson while we are all locked down and bored, I wouldn't let anyone touch anything with the potential to even give a mild shock. They think just seeing an op amp chip is the coolest thing and telling them how you can use it to make something happen has fired their enthusiasm!

    If nothing else it's been great just using it as a guide for my home setup, even my partner has gotten used to looking over to the meter and then turning up the TV, rather than just cranking the amp straight away (giving me a shock when I go to play music from my computer later), and she's the most technophobe person I know of

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