Build an Analogue VU Meter with a normal Voltmeter

[Richard Crowley]

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.

[dannyf]

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.

[Richard Crowley]

1) Choose which circuit you want to use.
2) Select the power supply that works with that circuit.

[dannyf]

Most audio power amps are dual-railed.

and it is not difficult to convert a dual-rail circuit to single rail.

[Niklas_R]

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.

[Richard Crowley]

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.

[Niklas_R]

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.

[Richard Crowley]

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.

[Niklas_R]

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.

[Niklas_R]

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?

[Richard Crowley]

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

[dannyf]

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@10mv
7ua@100mv
31ua@1v
37ua@2v
43ua@5v

Pretty logrithmic if you ask me,

Notice that the input signal is DC biased.

[Niklas_R]

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.

[Niklas_R]

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?

[dannyf]

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.

[Niklas_R]

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.

[Niklas_R]

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.

[Richard Crowley]

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

[dannyf]

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.

[Richard Crowley]

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.

Yes, 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".

[Niklas_R]

Thank you all for your fast replies! 

We were assuming that you were talking about a separate meter/circuit for each channel (left and right).

Well, 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.

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?

I will do so this weekend.

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.

I 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?

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.

How exactly would I do that?

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

I 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?

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.

Could 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?

[Niklas_R]

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.

[Richard Crowley]

Well, 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.

The 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.

I 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?

You 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.

How exactly would I do that?

You have identified the differential input circuit that I was suggesting.

I 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?

Yes, 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.

Could 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?

Yes, 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.

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.

Yes, 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.

[Niklas_R]

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 out 

I 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 =)

[Niklas_R]

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!!!!