Build an Analogue VU Meter with a normal Voltmeter - Page 1
Forum section: electronics / projects

author: Niklas_R
author_rank: Contributor
timestamp: February 17, 2016, 05:47:18 pm
content: Hello =)I want to build an anologue VU-meter to clamp onto my stereo (probably the audio-cables running to the speakers). It does not have to be perfect for professional measurements or anything.On one of the videos I saw this peakmeter:(I added the potentiometer such that I can configure how long a certain value is held, my calculations say I need one between 40kOhm and 950kOhm. I could of cause choose a bigger capacitor in order to use a smaller resistor.)I assume I need to build this circuit in order to get the current max-voltage on the alternating signal that is driving the speakers. Is this assumption correct?Q1: What do I connect to ground? The black speaker cable? I ask this, because I do not want to distort the audio-signal and am not quite sure how speaker-cables are connected internally.Q2: On the same note, the red speaker cable is probably connected to the +side of the first amplifier correct?Q3: Do I have to add another logarithmic amplifier to get a logarithmic scale on the display (an analogue voltmeter () )? How would I accomplish that?Sorry I am still very much a beginner and English is not my first language. I am open to learn new stuff, so if you can explain how I can accomplish my goal that would be great. If you can explain why I am on the completely wrong track here, that would be just as good.

author: Paul Price
author_rank: Super Contributor
timestamp: February 17, 2016, 06:47:32 pm
content: Some stereos have bridge outputs, so neither the red or the black or at ground and grounding either of them could damage the amp.You can easily check the red and black wires with a ohmmeter to see if one of the wires is ground.Otherwise, the VU meter circuit and your observations about the circuit are accurate and it should work well.If there is a bridge output amp you are working with you can just use ground and either the black or red wires to feed into your VU meter.You may need to add a pot, perhaps a series resistor to the speaker output, and a biasing network with a series capacitor at the input to not overload the input if the circuit is using a rail to rail op-amp with a single supply. Also, you will have to find a suitable value of series resistor at the output to match the sensitivity of the analog meter you are using.With the circuit as you shown, some essential input-coupling components are missing, but as it is shown, you would need at least to supply the op-amp with +V and -V power supplies.

author: Niklas_R
author_rank: Contributor
timestamp: February 17, 2016, 07:16:06 pm
content: QuoteOkay, since I will use this probably on two different stereos, I think I will take this option, just to be on the safe side. So I use the ground of the power supply to connect to the ground of the capacitor and feed the red cable into the +side of the first op-amp. I have a technical understanding problem here: How can I be sure, that the signal on the speaker cables is referenced to the same ground that I would get from a power supply? Would I have to measure that out with an ohmmeter? The power-plug of the stereo has no ground-connection. I am an absolute beginner in that regard.Quote.Could you go into detail here?QuoteI left those out to make it more readable. I plan on providing -15V and +15V in order to get an output between 0 and 15V which seems to be the range for many analogue voltmeters.

author: GamerAndds
author_rank: Regular Contributor
timestamp: February 17, 2016, 09:14:30 pm
content: Why so complex? If you just want something to bounce then go old skool! LOLThis is off a phase linear amplifier. (I own one.) The meter circuit is simple and will never go out of spec. Either use D 20 or use the zDiode I drew in. The range switch can be omited as you probally have only one load connected to it and you are building custom and other than that if its a bridge type amp just double up your resistances. Youll probally want to throw in a ADJment POT to help tuning it.

author: Richard Crowley
author_rank: Super Contributor
timestamp: February 17, 2016, 09:54:50 pm
content: From the data sheet/application note for the National LM391x series of LED meter chips...

author: Niklas_R
author_rank: Contributor
timestamp: February 17, 2016, 10:18:09 pm
content: Thank you both a lot for your responses!@GamerAndds: Thank you for that graphic! I have some trouble understanding your graphic. What is the "-" I have to connect ground to? Is the Circle with +- the meter? In that case, where does Output go to? If it is a Voltage-source, I do not understand its effect. I do not find an amplifier, so will this have an effect on my signal?@Richard Crowley: That you a lot for that! Could I put the output to an analogue voltmeter? Would I need another logarithmic amplifier? I guess the input is just the red speaker cable and Ground is some ground from the power supply in the device? Or what is ground?As I said, I am still a beginner.

author: Richard Crowley
author_rank: Super Contributor
timestamp: February 17, 2016, 10:39:09 pm
content: No, you cannot drive most meters with these circuits.We need to know exactly what meters you have so we can recommend something suitable.Are these the common DC milliAmp (or microAmp) meters?What is the full-scale range of your meters?

author: Audioguru
author_rank: Super Contributor
timestamp: February 17, 2016, 10:47:21 pm
content: I designed and built a Sound Level Indicator project where I use an opamp driving a transistor as the peak detector. The opamp has inputs that still work when their voltage is all the way down to the negative supply voltage so a negative supply is not needed and the inputs work down to 0V. The transistor is the rectifier and amplifies the current so the capacitor is charged to peak levels very quickly. The supply voltage is low enough (regulated at 5V) that the reverse bias emitter-base voltage rating on the transistor (6V) is never exceeded. The output is linear. It drives an LM3915 bar graph LED driver that has logarithmic levels. The LM3915 has a range of 30dB. I added an automatic gain control circuit that boosts the range to 50dB. Here is my peak detector:

author: Pjotr
author_rank: Frequent Contributor
timestamp: February 17, 2016, 11:04:28 pm
content: Note that a VU meter signals average signal level.notpeak level.This for a 200 uA meter:

author: Paul Price
author_rank: Super Contributor
timestamp: February 17, 2016, 11:44:50 pm
content: This is a perfectly functioning VU Meter with the least number of components for connection at the speaker outputs and it will give you average audio output readings.The meter could be a sensitive micro-amp meter or even a milli-ampere meter instead of a voltmeter.Adj. the pot(rheostat) to calibrate.Adj. the capacitor value up or down in value to adj. attack and decay response of VU meter readings.

author: GamerAndds
author_rank: Regular Contributor
timestamp: February 18, 2016, 02:24:41 am
content: Quote from: Niklas_R on February 17, 2016, 10:18:09 pmOkay- the negative point goes to wherever the negative of the speaker is on the amplifier, the positive to the positive side of the speaker. Yes, the circle with tge +/- on it is the actual meter. It is polarity essential. A cap can be used across the meter to help for a slowerer effect. I'm not sure what you are talking about the signal?

author: Richard Crowley
author_rank: Super Contributor
timestamp: February 18, 2016, 03:14:32 am
content: If you are just looking for something to move the meter, I recommend the circuit suggested by Paul Price.It is cheap, easy, and quick. And it has a reasonable chance of working whatever kind of meter you have.If you were looking for something that was calibrated and had traditional VU meter ballistics, etc. that would be a different matter.

author: Niklas_R
author_rank: Contributor
timestamp: February 18, 2016, 10:07:36 am
content: Wow, so many good answers! I will try to reply to everyone.Quote from: Richard Crowley on February 17, 2016, 10:39:09 pmI have not bought any meter yet, so I am open for everything.Quote from: GamerAndds on February 18, 2016, 02:24:41 amI do not want to distort the signal that comes out of my stereo in any way. I wanted to know if your circuit would guarantee that.Quote from: Audioguru on February 17, 2016, 10:47:21 pmSo what would I need to do in order to drive some analogue meter with that?Quote from: Pjotr on February 17, 2016, 11:04:28 pmIs LS Out the red line from the speaker?Quote from: Paul Price on February 17, 2016, 11:44:50 pmThis looks really nice and easy. But it looks as if it takes all its power off the speaker-cables. I (as a beginner) think that it could have a negative effect on the signal of the stereo. It is important for me that the audiosignal is not influenced by the meter. Could you explain to me if/why I am wrong or if I have a point, how to ensure that the signal does not get influenced?

author: dannyf
author_rank: Super Contributor
timestamp: February 18, 2016, 11:55:19 am
content: Two things to consider.1 do you want a half wave or a full wave rectifier.2 do you want it to be logarithmic? If not, do you want it to be a precision rectifier (aka it can detect low input signal as well)?As to impact on the audio signal, it will have an impact. You can minimize that with a buffer.

author: Richard Crowley
author_rank: Super Contributor
timestamp: February 18, 2016, 01:53:08 pm
content: Quote from: Niklas_R on February 18, 2016, 10:07:36 amIf you have not decided on a meter (or LED array, etc.) yet, then trying to select a circuit is premature. You cannot select a ciruit when you don't know what you are working with. Select a diaplay (meter, LEDs, etc.) FIRST.QuoteYou are dealing with SPEAKER LEVEL. That has VERY VERY LOW source impedance. You cannot build a meter circiut that will introduce ANY distortion into the audio signal unless you try very hard and do something silly.QuoteThe data sheet has all the suggested circuits (as most chip data sheets do). Also, there are literally hundreds of implementations of LM3915 VU meters on the internet and demonstrations on YouTube. HUNDREDS and HUNDREDS! Go to YouTube and search for VU meter or LM3915 or a similar key word and you will find enough different ideas and variations to watch for several days, non-stop.QuoteYou have 1000x more power available in the speaker signal than it will take to operate a meter. maybe 10000x. Unless you are trying to light up big incandescent lamps or something silly, this is absolutely not a concern at all.QuoteYou will have to work really hard and do something really stupid to actually introduce distortion into the speaker signal. Repeating: This is absolutely NOT a concern if you are only driving ordinary meters (or LED array).Quote from: dannyf on February 18, 2016, 11:55:19 am@Niklas_R said he only wants to have a visual indication, not a laboratory-grad measurement tool. He doesn't need to concern himself with any of those details. Unless he also wants to use this as some sort of learning experience for electronics. In which case this not exactly a good starting point.QuoteYes, that would be true if we were talking about line-level audio signals. But the speaker level signal source impedance may well be <1 ohm so there is virtually NO danger of distorting the signal with ordinary circuits.

author: retrolefty
author_rank: Super Contributor
timestamp: February 18, 2016, 02:28:15 pm
content: Below is a useful VU project so you can at least know what you are dealing with and a good example to follow.http://sound.westhost.com/project55.htm

author: GamerAndds
author_rank: Regular Contributor
timestamp: February 18, 2016, 02:49:25 pm
content: Quote from: Niklas_R on February 18, 2016, 10:07:36 amNo it will not affevt the putput signal of your amplifer. The vu meter used a very small amount of current from the speaker- but is negligible. Paul Price circuit is pretty much the same- I would only like to add a diode OR Zener diode across the actual meter coil. This will protect from reverse polarity. It can also shunt excess current.

author: Niklas_R
author_rank: Contributor
timestamp: February 18, 2016, 03:34:29 pm
content: Thank you for your replys, you help me a lot!Quote from: Richard Crowley on February 18, 2016, 01:53:08 pmI do at least know that I want an analogue display (no LEDs). But I thought that it could be a mistake to select a shitty meter and then trying to build something around it to make it work better. But you are right, I should have the meter before designing the circuit.Quote from: Richard Crowley on February 18, 2016, 01:53:08 pmWow that is really good to know. I was afraid I could ruin the sound. But if that is the case, greatQuote from: retrolefty on February 18, 2016, 02:28:15 pmI will look into that right now!Quote from: GamerAndds on February 18, 2016, 02:49:25 pmThank you for that tip!

author: Niklas_R
author_rank: Contributor
timestamp: February 18, 2016, 04:24:27 pm
content: Quote from: retrolefty on February 18, 2016, 02:28:15 pmI have now finsihed reading it and I have some questions. Maybe you can help me? We talked about different types of amplifiers here. And if they do not output a steady voltage on the black speaker-line, then this will mean that ground in the circuit in that link is not steady. Is that a problem? Or will it not matter at all because it just changes the reference point for the potentials in that circuit? As you can see I still have some problems dealing with high frequency signals and separated systems with different power supplies.Secondly: The author did not specify if he designed the resistance for 50µA meters or 100µA meters, so I am not sure for which of the meters the values are laid out.

author: Richard Crowley
author_rank: Super Contributor
timestamp: February 19, 2016, 04:16:54 am
content: Quote from: Niklas_R on February 18, 2016, 04:24:27 pmThe signal is across the two speaker leads. Whether the "black speaker-line" is "steady" or not makes no difference. Furthermore, it is not clear why you would think it was "not steady".QuoteWow, I have no idea what that means. What "high frequency signals"? What "separated systems"? What "different power supplies"?QuoteYou will have to change the series resistor depending on how much voltage is on your speaker leads, and depending on exactly which meter you are using. But this is trivial to do with an ordinary 5K or 10K pot.

author: Niklas_R
author_rank: Contributor
timestamp: February 19, 2016, 08:11:38 am
content: Quote from: Richard Crowley on February 19, 2016, 04:16:54 amI was refering to this:QuoteQuote from: Richard Crowley on February 19, 2016, 04:16:54 amWhat I mean is, I still have some problems when I have to deal with two systems that have two independent power supplies and no common ground. And in this case they are only connected by the audio-signal where I cannot be sure that I can connect the black line to ground or not as mentioned above.

author: Richard Crowley
author_rank: Super Contributor
timestamp: February 19, 2016, 11:12:11 am
content: You can use a very simple passive circuit to do what you are asking. There is 10000x more power in that speaker signal than you will need to simply move the meter needle. And you don't need sophisticated circuits for logrithmic conversion, etc. Therefore you don't need an active circuit that requires external power.So that means that there is no danger from connection to a speaker signal where neither side is ground-referenced. Because the circuit is passive and self-contained. So it is completely independent.Even if you use a solution that requires a powered active circuit, the local power would be "floating" and NOT hard-connected to mains ground-reference. While those cautions you quoted are valid in SOME cases, they do not apply to your particular situation.

author: dannyf
author_rank: Super Contributor
timestamp: February 19, 2016, 12:16:24 pm
content: Put the meter inside of a bridge and put that bridge inside the feedback loop of an opamp, and call it a day.It doesn't need to be so complicated.

author: Richard Crowley
author_rank: Super Contributor
timestamp: February 19, 2016, 03:04:04 pm
content: Put the meter inside of a bridge and put that bridge between the speaker wires.It doesn't need an opamp or a feedback loop. And call it a morning.

author: Niklas_R
author_rank: Contributor
timestamp: February 20, 2016, 05:37:50 pm
content: Quote from: Richard Crowley on February 19, 2016, 11:12:11 amSo I could connect ground of the power supply for the opamps to the black speaker cable and it would work no matter how the stereo is built-up internally? All under the assumption that I would build an active meter.But it seems like a passive meter would be the easier way.

author: Richard Crowley
author_rank: Super Contributor
timestamp: February 20, 2016, 07:46:47 pm
content: Quote from: Niklas_R on February 20, 2016, 05:37:50 pmYes. The circuit that Paul Price posted is all you need. This whole thing has been vastly overblown as often happens on the internet.

author: Niklas_R
author_rank: Contributor
timestamp: February 21, 2016, 10:27:09 am
content: But in order to understand the matter completely, I'd still like to know if I could connect ground of the power supply for the opamps (in the complicated circuit) to the black speaker cable and it would work no matter how the stereo is built-up internally?

author: Richard Crowley
author_rank: Super Contributor
timestamp: February 21, 2016, 02:42:59 pm
content: Quote from: Niklas_R on February 21, 2016, 10:27:09 amASSUMING that the power supply is ISOLATED from mains power or ground, then YES you can connect it to either side of the speaker cable.

author: dannyf
author_rank: Super Contributor
timestamp: February 21, 2016, 04:35:45 pm
content: QuoteAnything can "work", depending on your definition of that "work" means.The "passive meter" presented earlier presents an uneven load to the circuit it is attached to: a) the current flow is one direction only; and b) the current flow only happens during the peak. In other words, it presents the capacitor + meter to the circuit only briefly, during the very peak of the input signal. It also has an issue of exposing the meter (typically a sensitive instrument) to very large drive current during peak signals.The first issue can be addressed with a bridge rectifier; the 2nd issue can be alleviated but not eliminated by attaching a large value serial resistor to the diode.If you want to go down that path - which I don't recommend - combine the above: put the meter in a bridge, and put a serial resistor with the whole thing. the serial resistor will protect the meter and then provide some calibration capability to your circuit.

author: Niklas_R
author_rank: Contributor
timestamp: February 21, 2016, 08:46:01 pm
content: Quote from: Richard Crowley on February 21, 2016, 02:42:59 pmHow isolated are we talking here? It would still be connected over the magnetic flux inside the transformer that I'd need for the power supply.Quote from: dannyf on February 21, 2016, 04:35:45 pmWhat would you recommend instead?

author: Richard Crowley
author_rank: Super Contributor
timestamp: February 21, 2016, 09:28:24 pm
content: Quote from: Niklas_R on February 21, 2016, 08:46:01 pmELECTRICALLY isolated. The magnetic coupling of a typical power transformer (whether the big "heavy-iron" traditional mains-frequency transformer, or a new switch-mode power-supply (SMPS) small high-frequency transformer. The magnetic coupling provides complete electrical isolation.QuoteHe is recommending this circuit as shown in the ESP article:http://sound.westhost.com/project55.htmBut replace that fixed 3.9K resistor with perhaps a 10K ohm pot. That would give you a wide range of adjustment to accommodate your speaker levels and whatever meter you select.If you were connecting this to a line-level signal, then the concerns about distorting the signal would be valid. But since you are connecting to the speaker leads, worrying about loading and distortion is equivalent to being concerned about overloading your utility power by plugging in a night-light.

author: Niklas_R
author_rank: Contributor
timestamp: February 21, 2016, 09:44:54 pm
content: Okay that sounds reasonable. I guess I will try that one out and if it does not work as desired I can still try another circuit. I have no rush. Thank you all for your patience and thanks for the explanations!

author: dannyf
author_rank: Super Contributor
timestamp: February 21, 2016, 09:48:43 pm
content: QuoteI wouldn't use any of the passive meters without trying them out on your particular setup.the passive meter in question here is essentially shorting the output to a capacitor periodically: a typical setup, it is shorting it for about 80us, and the current going through the diode is about 0.1amp @ 10Vpp sine. Or equivalent to about 100ohm, vs. your 8ohm load.But more importantly, no one can predict how it will impact your amplifier - remember the amplifier is a feedback device and typically unstable into a capacitive load - that capacitive load is a 10uf capacitor here and it is only switch in / out very quickly.Again, if you have to go down this path, isolate that capacitive load from your amplifier through a large resistor. That resistor's value depends on the meters used and output power. But it wouldn't surprise me if you end up using something like 200K resistor there: 25w into 8ohm load, and 50ua coil meter. minimum "pulsing" current as you would see in the "passive meter" posted earlier. The resistor value will go up down depending on your amp + coil meter, obviously.However, the meter's frequency response is uneven.Again, people use opamps here for a reason.

author: Niklas_R
author_rank: Contributor
timestamp: February 21, 2016, 09:57:15 pm
content: So you'd say something likehttp://sound.westhost.com/project55.htmis the only real way to do this?

author: Richard Crowley
author_rank: Super Contributor
timestamp: February 22, 2016, 01:18:35 am
content: All the cautions that @dannyf are applicable if you are trying to make a meter with some sort of calibration that will operate from a modest signal source such as a line-level audio signal source.However your stated requirements are quite different:1) You want to operate this from your speaker leads. That means the available signal is EXTREMELY STRONG AND ROBUST because it has a source impedance quite possibly less than 1 ohm and a quite high power. Therefore any danger from distorting the signal from a high-impedance load like your meter just doesn't exist, by definition. Because the power amplifier is providing a signal 10000x greater than what your meter needs. The meter draws such an infinitesimal fraction of the total power going to the speaker that the amplifier will neither know or care whether it is connected or not.2) You want a meter waving the needle only for entertainment purposes, and not for any actual "meaurement" function. Therefore there are NO actual "calibration" requirements as we would expect for a practical test instrument.If I weren't actually participating in this discussion, I would be sitting back and laughing at how enormously overblown this discussion has become for such a simple requrement. Alas, this often happens on the internet where simple things grow enormously complex for a variety of reasons. Please remember that when reading and participating in discussions like this.Now, go out and find some meters you like, and then we can discuss practical circuits. I really don't see any point in continued discussion of theoretical options when we have nothing practical to work with here.

author: Niklas_R
author_rank: Contributor
timestamp: February 22, 2016, 09:21:58 am
content: I ll probably buy this meter:http://www.elv.de/einbaumessmodul-analog-strom-dc-100-a.html. That site is in German, so I will translate the specs as good as I can.Specs:Range100µAVoltage0,2VInner resistance2000 Ohmwidth70mmheight60mm

author: Richard Crowley
author_rank: Super Contributor
timestamp: February 22, 2016, 01:37:30 pm
content: Good. You can probably pop the clear plastic cover off and replace (or overlay) the scale with something resembling a "VU meter" type scale. As discussed in the ESP page. You can find a wide variety of VU meter type scales using Google image search, etc.

author: Niklas_R
author_rank: Contributor
timestamp: February 22, 2016, 03:56:31 pm
content: The clear front seems to be click-locked on (I do not know how to say that in English, but I guess you understand what I mean) so I should be able to change the scale with something else.In the circuit with the 4 diods I should replace the fixed resistor value with a potentiometer of a 10K range you said. What about the Capacitor? As far as I understand, the higher the capacitance the slower the indicator will descend back to 0. What value seems logical here?In my opinion, it would make sense to add a large cap (I am thinking about 10µF) with a parallel potentiometer (470kOhm?) to tune the descend. If I did not calculate anything terribly wrong, then this yields a circuit with C=10µF, R=1,999kOhm (2000Ohm of meter) and this yields a time constant of 0.01999s so it would only have a 50% drop between two peaks in a 50Hz sound (the lowest frequency that matters in music). On the other hand a large cap will pull a lot of ampere in the loading process, so a smaller one would be better for this purpose. I am not quite sure what to use here.Do you have an idea?

author: Richard Crowley
author_rank: Super Contributor
timestamp: February 22, 2016, 04:56:53 pm
content: This is like cooking. You can make recipes in your head and say I need so much of this and that quantity of that, and maybe 3g of salt. But you don't know for sure how it will taste until you actually try it. You don't know how salty the meat (or whatever) will be. So good cooks leave their option open until they are actually cooking and can sample what it tastes like.In a similar manner, some analog meters like that are very loose and the meter moves quickly. But some are quite damped and the needle moves much more slowly. Until you actually have the meter in your hands, you cannot predict how much capacitance you will need (if any). That is why few of the circuits show a specific value for the capacitor. Because it depends on the meter, the program material (the audio) and what your objectives are for the motion of the display.

author: Niklas_R
author_rank: Contributor
timestamp: February 22, 2016, 05:16:49 pm
content: Okay. Then I will make a test-circuit and see how it works. Thank you. I will write again when I tested it all.

author: dannyf
author_rank: Super Contributor
timestamp: February 22, 2016, 09:54:46 pm
content: "h a parallel potentiometer (470kOhm?) to tune the descend"It will be effectively shunned by the coil meters relatively low resistance.Put a pot in serial with the meter to do that, at the cost of lower sensitivity. The best way is to experiment with the capacitance.

author: Niklas_R
author_rank: Contributor
timestamp: February 27, 2016, 03:42:39 pm
content: Ok I've built it all up now. A big plus is that it looks really cool in my opinion, when the music is loud and the meter moves.The non-logarithmic scale is a big negative though. For normal volume levels the needle is not moving at all - even without potentiometer. I first thought that maybe my diodes had a problem with that high frequencys or something Idk so I tried out Schottky-Diodes with the danger of higher reverse amperage, but that did not help. With relatively high volume settings the needle uses the whole scale and even touched the right side of the meter's frame. That is not satisfactory, as I listen to louder music than that sometimes and I do not want to adjust the potentiometer depending on the loudness of the stereo.What I will do now is building the more complex circuit posted above and using my bench-top power supply for the 15V. If that works nice I will use that circuit and look for a small 15V power supply I could buy/build into that.

author: Richard Crowley
author_rank: Super Contributor
timestamp: February 29, 2016, 11:34:25 am
content: QuoteYes, the output is floating. The capacitor is there for noise-reduction purposes because that is a switch-mode power-supply (SMPS).QuoteThat isn't really the right question. It is not a matter of "how high", but "how much conductance". In any case, the output from virtually all supplies is quite sufficiently isolated for an application like yours (or 99% of other types of circuits).That power supply is probably vast over-kill (do you have that idiom in German?). You can use most any random discarded (redundant) small "wall wart" power supply for such a non-critical circuit. You don't really need 15V for such a simple circuit. It might run on power as low as 5V since it takes very little power to move the meter needle.

author: Niklas_R
author_rank: Contributor
timestamp: February 29, 2016, 12:28:46 pm
content: Thank you for that reply.I'd prefer to use a 15V source though, because then I do not have to redo the whole circuit again. However, I am aware that the power supply is overkill(no German 1:1 translation) because of the 1A it can supply. The circuit will never pull 15W. But it is only 9€ and I wouldn't need to pay shipping. Additionally I could easily mount that think inside a case (together with the circuit and the meters) and add a very simple on/off switch. I really just do not like wall-warts

author: ruffy91
author_rank: Regular Contributor
timestamp: February 29, 2016, 12:46:16 pm
content: The equivalent to "overkill" in german is "Mit Kanonen auf Spatzen schiessen" which translates to "shoot with cannons on sparrows". Not really more friendly than the english version.

author: Niklas_R
author_rank: Contributor
timestamp: February 29, 2016, 07:50:36 pm
content: I guess I should have looked at the schematics before writing all this, because then I would have noticed, that the opamps are inverting in order to rectify the full wave. That is why they need +15V and -15V. A small 30V Psu is a bit too much for that task I think. It would also be more expensive. So I think I need to modify the schematics. Or use a completely different circuit.Oooor I could just use the 15V power supply, connect 15V to 0V with two 100k resistors and use this as a +7.5V and -7.5V power supply. I think I reached the point where I really have to understand the complete schematics and how the resistors and capacitors were calculated, before I can go on. I will write again when I have thought of something.

author: dannyf
author_rank: Super Contributor
timestamp: February 29, 2016, 09:59:14 pm
content: Your issue seems to be more related to the diode threshold more than anything. If so, using shortly or germanium diodes would help.I tend to think a true log rectifier is more helpful as it broadens the range of the vu meter.It is fairly easy to do in the analog domain. It is very easy to do that in the digital domain, as you can write software whichever way you want.As is, the meter have you is the opposite of what you want. If I were you, I would hang it on the output of a log amp (resistor on input and diode on the feedback).

author: dannyf
author_rank: Super Contributor
timestamp: March 01, 2016, 12:51:23 am
content: Here is one for you to play with.It is essentially a log V/I precision converter.V3 is the input (ac) signal, and R1/D1/D6 performs a logrithmic conversion of that signal and feed it through the opamp. The bridge rectifier + the meter (R2 + a cap - not drawn) + the opamp form a precision rectifier. R3 controls the sensitivity of this whole set-up -> about 100ua@5v.From the simulation, you see that it follows a log relationship vs. the input voltage; and it is symmetrical. So if you were to feed it an ac signal, the output would be DC.The design requires a two rail power supply -> this can be fixed.If you need one channel, you can simplify it by splitting the function in to two opamps: 1) precision rectifier and 2) log amp.Hope it helps.

author: Niklas_R
author_rank: Contributor
timestamp: March 01, 2016, 06:54:02 am
content: Thank you a lot!I could just use a 15V or 18V power supply and create an artificial ground in the middle for supplying +V and -V.But how did you simulate this on the PC? I.e. which software did you use?I read somewhere that log amps with diodes are very temperature dependant, is that a problem here?

author: Niklas_R
author_rank: Contributor
timestamp: March 02, 2016, 06:55:15 pm
content: The more I read into this power supply business, the more difficult it seems. What would you recommend?

author: Richard Crowley
author_rank: Super Contributor
timestamp: March 02, 2016, 07:35:38 pm
content: First, select a circuit that you want to use.In English we say: "You are putting the cart ahead of the horse."Or in other words you are trying to do the second step before the first one.You can't select an appropriate power supply until you know what you are trying to power.This is why there are 71 listings for "VU meter driver board" currently on the US Ebay website.

author: dannyf
author_rank: Super Contributor
timestamp: March 02, 2016, 09:47:08 pm
content: Yes, the diodes forward voltage will change with temperature. However that kind of changes @2mv per C is nothing to be worried about for your application here.

author: Richard Crowley
author_rank: Super Contributor
timestamp: March 03, 2016, 04:01:58 am
content: 1) Choose which circuit you want to use.2) Select the power supply that works with that circuit.

author: dannyf
author_rank: Super Contributor
timestamp: March 03, 2016, 11:46:35 am
content: Most audio power amps are dual-railed.and it is not difficult to convert a dual-rail circuit to single rail.

author: Niklas_R
author_rank: Contributor
timestamp: March 03, 2016, 06:16:34 pm
content: Okay I calculated a bit with my stereo. It has 20Watts output. With a 4Ohm speaker this leaves us with max. 2.2VAC on the channel. Since the power probably uses effective voltage instead of peak voltage, this yields peaks of ca. 3VAC. My other stereo is more powerful, but also lacks a watt-rating, but it seems unlikely that I ever want to have more than 9VAC on the speaker cables, since I do not want to go deaf. This makes be believe that the big powerful circuit of project55 is a bit over the top. Although the PPM/VU switch is nice.I think I will go with dannyf's circuit. Which means I need a +-9V power supply.I still would like to know how you simulated that circuit dannyf.

author: Richard Crowley
author_rank: Super Contributor
timestamp: March 03, 2016, 06:40:29 pm
content: The power supply for your meter circuit has NOTHING to do with the power available in the measured signal.You are NOT relying on ANY "power" from the measured signal.The fact that in this particular case, the measured signal actually has some power behind it is completely irrelevant and immaterial.Remember that MOST VU meter circuits are measuring circuits that have almost ZERO POWER (because they are measuring low voltage from a rather high impedance)At this rate, you will analyze this thing to death until you are bored with it, and you will never build anything.The fact is, you can get yourself four 9V batteries and likely power the circuit for a year.That will give you + and - 18V, which is more than adequate for ANY circuit you select.

author: Niklas_R
author_rank: Contributor
timestamp: March 03, 2016, 07:31:20 pm
content: I was calculating the volts on the speaker-lines because I wanted to know, if it is in the range of voltages this circuit can measure. Maybe I misunderstood the circuit, but as far as I understand the connections of the op-amp, it would not be able to distinguish a 10VAC signal from a 20VAC signal in this setting, as both are above its rails. I did not know which voltages will be on the speaker-cables, so I thought I calculate it before.I do not want to use batteries though. It is not an option for me.

author: Richard Crowley
author_rank: Super Contributor
timestamp: March 03, 2016, 07:39:27 pm
content: Any circuit you choose will be able to measure thousands of volts. It is not limited by the power supply voltage.The circuit it self operates on a low voltage (perhaps 5V at the most).If you want to work with a larger measured signal voltage than that, then you attenuate the incoming signal with a voltage divider or a potentiometer.Remember that the meter movement itself needs a very very small amount of power to move the needle.You could easily operate a circuit like this on batteries for a year or two.

author: Niklas_R
author_rank: Contributor
timestamp: March 03, 2016, 08:23:30 pm
content: You are totally right, I misunderstood that circuit! I feel very stupid now. But at least I understand it now.I do understand that because of the little power consumption 9V blocks could feed this for a long long time.But after I was certain that I wanteded a logarithmic scale and certainly needed a power supply of some form, I thought about putting this together with a cinch-switch (that project at least is/was a no-brainer). I did throw some LEDs in that switch (only soldering is left to do for that part (and of course taking the right LED-resistors)). So that would need some more power. I would have written about this earlier, but it did not seem to matter.So I am looking for a power supply. I could of course build something like this:(found here:http://www.hobby-bastelecke.de/projekte/netzteil_9vsym.htm)myself. There are many tutorials online on how to build a +-9V power supply. Or I buy one if I find a cheap solution.I guess all my questions have been answered now.I'd still love to know which software dannyf used.

author: Niklas_R
author_rank: Contributor
timestamp: March 03, 2016, 09:44:24 pm
content: Okay I decided now. I hope this double-post is ok.I will take a Gerth 4818-2 transformer (http://cdn-reichelt.de/documents/datenblatt/C500/4808%5B1%5D.pdf)I will connect it to this circuit:(found here)I will use 500mA fuses though. The transformer is not short circuit proof.And then I will build dannyf's measurement circuit. Twice. One for each channel (I have two amperemeters).Did I miss anything important here?

author: Richard Crowley
author_rank: Super Contributor
timestamp: March 03, 2016, 10:44:30 pm
content: Yes, that looks good.Those 78xx/79xx regulators need 2V higher on the input than what you get from the output. (called the "Dropout Voltage")So 9V regulators need 11V input.Your 9V-0-9V transformer and those capacitor filters (C1, C2, C3, C4) will provide about 12.7 V DC for each polarity (+ side and - side)So that gives you a comfortable margin of safety if the mains voltage droops.There are also complete power supply boards like this, assembled and tested for probably less than what that transformer costs.http://www.ebay.com/itm/252207145176

author: dannyf
author_rank: Super Contributor
timestamp: March 04, 2016, 12:20:27 am
content: if your amp is single rail, it would be crazy to build a dual-rail power supply for your vu meter: converting it to single rail is a piece of cake.Here is a simple try - it is essentially the same as I showed earlier, using a half wave rectifier rather than a full wave rectifier.Its output current vs. input Vpp:0.7ua@10mv7ua@100mv31ua@1v37ua@2v43ua@5vPretty logrithmic if you ask me,Notice that the input signal is DC biased.

author: Niklas_R
author_rank: Contributor
timestamp: March 04, 2016, 08:50:41 am
content: The transformer block is not that expensive (5€). And I think it would be nice to build it myself and learn something while doing that.I do not know the inner functionalities of my amp/amps. One is old as stone and has no manual anymore (and I couldn't find one), the other one has a shitty manual with no further information. Since your first circuit would work with both, I guess I'll be on the safe side, just builing that, no? It is not like it has more parts or anything. And building the power supply sounds fun and interesting.

author: Niklas_R
author_rank: Contributor
timestamp: March 05, 2016, 09:09:51 pm
content: I am ordering right now and I happen to notice that the LT1001 is very expensive, the NE 5532 Opamp only costs a tenth of that and it would work just as well, wouldnt it?

author: dannyf
author_rank: Super Contributor
timestamp: March 05, 2016, 09:36:24 pm
content: I just wanted to note that the only reason I used LT1001 here is that it is the first part of choice in the sim program I was using. Any opamp, likely the lowly NE5532, or TL0xx, or even LM324 should work here. I really apologize if my use of LT1001 has caused any confusion. Hopefully it isn't too late.

author: Niklas_R
author_rank: Contributor
timestamp: March 05, 2016, 10:21:45 pm
content: Thank you for the explanation. I just wanted to make sure, that there was nothing special in the LT1001 that I missed. I decided for the NE5532 now.EDIT: I will probably finish it in the coming 10days. I had a lot to do right now. First achievement: It works with batteries as of yet.

author: Niklas_R
author_rank: Contributor
timestamp: March 07, 2016, 10:17:08 pm
content: I just realized that I made a huge mistake here, that would have been fixed, if I had told directly that the VU-Meter has to be stereo (have to amperemeters).I do not know if my stereo has bridged output (it is stoneold and I cannot find a manual in the house nor online). If it does, the black speaker cables are not at the same potential, which means that I cannot float GND of my power supply around as intended. It cannot be at two different voltages at the same time. I am not home for the next days, but when I am, I will try to find out if the amp is bridged, by measured voltage-difference between the black cables with my multimeter.But what if they are not at the same level? How could I still rescue this?I could use the cinch output of the stereo, but that is not amplified and therefore does not reflect the loudness.

author: Richard Crowley
author_rank: Super Contributor
timestamp: March 07, 2016, 11:09:34 pm
content: We were assuming that you were talking about a separate meter/circuit for each channel (left and right).Do you KNOW for sure that you power amplifier has bridging output and not ground referenced?With the amplifier disconnected and unplugged, can you measure the resistance between the black terminal and chassis ground?If you want to combine the speaker audio into a single signal, you could use relatively high-value "summing resistors" (like perhaps 1K) to combine the two speaker level audio into a summed L+R signal. Even if you had bridging outputs from your power amplifier, you could use a 1K resistor from each black terminal to the "ground" of your meter circuit, and then a 1K resistor from each red terminal to the "input" of your meter circuit.Now, this isn't a conventional way of doing things, and it wouldn't produce laboratory-grade results. But then that isn't the requirement here. Such a high impedance as 1K will make no discernible difference to the speaker audio signals and it doesn't put the amplifier at any real risk.If you really wanted to do it "properly" you could use differential circuits on the input which would take the difference between the black and red wire for each channel. Or you could use a pair of inexpensive audio isolation transformers something like these...http://www.ebay.com/itm/2pcs-JAMES-16645-Telephone-Line-Matching-ADAPTOR-AUDIO-TRANSFORMER-600-600-OHms-/121434713336

author: dannyf
author_rank: Super Contributor
timestamp: March 07, 2016, 11:13:18 pm
content: It doesn't really matter if is is bridged or not as your meter is singled ended: all it needs is one signal input plus ground.In a bridged set up, the other output is out of phase so as far as the meter is concerned it doesn't add additional information - it adds more amplitude.So for this project, you need to find one output wire, it doesn't matter which one, plus ground and you are set.

author: Richard Crowley
author_rank: Super Contributor
timestamp: March 07, 2016, 11:17:41 pm
content: Quote from: dannyf on March 07, 2016, 11:13:18 pmYes, that would probably work because each side (red/+ and black/-) is essentially a ground-referenced output, but each of opposite phase.You would want to select the same sample for each side. Both red or both black. Else the considerable amount of audio that is common to both sides will cancel and your meter will only indicate "separation" and not "audio level".

author: Niklas_R
author_rank: Contributor
timestamp: March 08, 2016, 08:58:01 am
content: Thank you all for your fast replies!Quote from: Richard Crowley on March 07, 2016, 11:09:34 pmWell, yes I did. But that circuit we discussed relies on the fact that the black wire will be the reference point for GND. So If I build two circuits I'd also have to build two power supplies in that setup.Quote from: Richard Crowley on March 07, 2016, 11:09:34 pmI will do so this weekend.Quote from: Richard Crowley on March 07, 2016, 11:09:34 pmI am not sure if I understood this correctly. I want a "Left GND Right" signal, but this way I'd get a mono signal or not?Quote from: Richard Crowley on March 07, 2016, 11:09:34 pmHow exactly would I do that?Quote from: Richard Crowley on March 07, 2016, 11:09:34 pmI would use these across the speaker wires, correct? Meaning that black and red drive the primary coils of the transformers. Then I would connect the secondary coils at one point and would obtain a common-ground, correct?Quote from: dannyf on March 07, 2016, 11:13:18 pmCould you explain this in detail? I did not understand it completely. The "In a bridged set up, the other output is out of phase so as far as the meter is concerned it doesn't add additional information - it adds more amplitude." part I understood, since in a bridged output black would carry the inversion of red. But where do I get the ground? I could at a voltage divider between black and red of two 1K resistors and the middle would be ground. This could work in a bridged setup but obviously not in a non-bridged output (which is no problem, I could add a switch for that or something). Is this what you meant?

author: Niklas_R
author_rank: Contributor
timestamp: March 08, 2016, 03:33:28 pm
content: After googling "differential circuit" I made this. The amplitudes are arbitrary and just for the sake of testing.Would this work or is this bullshit?I'd feed the outputs "left" and "right" into two builts of dannyf's original circuit.The first is for a bridged, the second for a non bridged output.

author: Richard Crowley
author_rank: Super Contributor
timestamp: March 08, 2016, 07:26:09 pm
content: Quote from: Niklas_R on March 08, 2016, 08:58:01 amThe black wire would be the reference for GND if the outputs from your amplifier are ground-referenced.Building two circuits does not require two power supplies.When you measure the resistance between each of the black terminals (left and right) and chassis ground, be sure to measure it BOTH ways (exchange the leads on your meter). That way you can be sure that you have a hard connection vs. measuring through some semiconductor and giving a false indication.QuoteYou can use the resistors to form a "summing circuit" to mix the Left Speaker audio and the Right Speaker audio into a single L+R signal.QuoteYou have identified the differential input circuit that I was suggesting.QuoteYes, that is correct. And the "hot" side of the secondary winding of each transformer can be connected to a pair of resistors to sum the audio together into a monaural L+R signal.QuoteYes, if you have two balanced, differential signals, then you don't really have a ground reference. But if you use differential input circuits, then you don't need a ground reference from the source, you create your own local ground reference.QuoteYes, that is the kind of circuit we are talking about. That is the active-circuit equivalent of using isolation transformers.You don't really need R1/R2/R3/R4 if we assume you have a conventional audio power amplifier, whether differential or ground-referenced.If the amplifier is ground-referenced, then the "-" side of "V2" and "V3" are ground by definition, and "V1" doesn't exist.If the amplifier is bridged/differential, then the center points of V1/V2 and V3/V4 are ground, and "V5" doesn't exist.The two circuits are essentially identical. So if you use a differential input, it will work with either type of amplifier.If the amplifier is bridged/differential, then you have no external access to the amplifier's "ground" reference.

author: Niklas_R
author_rank: Contributor
timestamp: March 08, 2016, 07:37:58 pm
content: Okay then I will built that differential circuit and have a universial vu-meter that works with bridged and non-bridged amplifiers. I would have built it right now, but I was sent the wrong opamps. 5534 instead fo 5532. Only noticed that after I burnt my finger on that thing shorting outI will write again when I have built it all up and tested it.EDIT: I will finish it in the next 10 days or so. It already works with batteries =)

author: Niklas_R
author_rank: Contributor
timestamp: March 28, 2016, 10:38:46 am
content: 10 days was almost spot on. It took a lot more work than I expected, though.The VU-meter works perfectly. I replaced the 4.7k resistors with 1k resistors and 4.5k potentiometers in order to tune the meter to the range I need. This makes the dB scale wrong, but that is not important for me.The whole box still needs a paint-job, but that is probably not as interesting for this forum.How the interface works:For the cinch-switch: The three switches on the side control the 4-fold cinch-switch. The 4 LEDs on top of each other indicate which cinch input is selected.For the meter: The shift-switch on top controls the backlight of the meters (off, half, full illumination).The stereo-amp is balanced btw. So it's very good that the whole circuit was designed for balanced/bridged speaker-output.I want to thank you all for your help!!!!

author: Richard Crowley
author_rank: Super Contributor
timestamp: March 28, 2016, 01:20:38 pm
content: Looks good. Congratulations on a successful project!