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VU Meter Wiring over Output
Old 31st January 2017
  #1
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VU Meter Wiring over Output

Hi there, I had a quick question about VU Meters and wiring. I know there have been a few posts about this but couldn't find a definitive answer. I have a few old VU meters out of an old Carvin live sound board and would like to wire them up in some kind of housing to have the overall output of my interface go thru them so I can monitor my whole mix as I work. My question is, Ive heard you need some kind of buffer circuit so as to not distort the signal as it passes thru the VU. I also heard the VU meters come in two different types. I was curious if anyone has a schematic for this buffer circuit and is it standard or are all meters different? I will attach a picture of the meter as it already has a few diodes and a resistor on the back and Im not sure if this may already be a part of the old buffer circuit. Any help on this would be greatly appreciated and If I missed an obvious post with the answer, my apologies. Thank you so much for your time and help!

Sincerely,
David Mills
Attached Thumbnails
VU Meter Wiring over Output-vu-front.jpg   VU Meter Wiring over Output-vu-back.jpg  
Old 31st January 2017
  #2
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Hi, David - The board on the back of your meter is indeed the "buffer"...it's ready to accept an audio signal. I wouldn't advise sending your main outputs "thru" the meter if your main outputs are actually performing another function like feeding a recorder, a distribution amplifier, or your monitor inputs - drive the meter from another dedicated source that mirrors your main outputs.
Old 31st January 2017
  #3
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Isn't there a difference between a buffer and a vu driver? I was told something very similar as David... Vu adds distortion when it's connected to the output... But add a driver (different than what he has now) with a couple of resistors and an opamp 1st voila! No more problems.

Wouldn't that be ok to tap off his main outs?
Old 31st January 2017
  #4
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I should have been a bit more clear...rectification, integration time setting and trim can be internal or external to the meter case, and you're correct, Enginefire, there is a difference, and yes, an actual buffer would be OK to tap off the main outs with...although I personally would not, feeling that main outs are to be loaded once at the end of the line. An inexpensive mains powered headphone amp with balanced input would probably be the quickest way to plumb those meters if an alternate set of outputs from the OP's interface aren't available.
Old 1st February 2017
  #5
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Awesome thank you ripple_fx1 that is very helpful I will have to try that here in a few, I guess it would make more sense to just send another output instead of using the main output. Now just one more question if ya dont mind, I see there are three connections would I just hookup the (+), and (-) of a balanced ouput to the lower two terminals? And does it matter if it is a balanced or unbalanced signal goin to the VU? I heard somewhere that they usually take unbalanced signal but obviously my interface sends balanced, easy enough to get around. But I figure you can just connect the (+), and (-) leaving the ground null? Also am I correct in assuming the lower two terminals are for the audio signal and the center is for the light? These were out of the board when I got them or I would have just traced them back at the time. Again thank you for takin the time to help me on this! I know it has been discussed before but never could find a real definitive answer, very much appreciated!

Sincerely,
David Mills
Old 1st February 2017
  #6
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I'd assume the orange/white & blue/white daisy chain is the lamp circuit. Looks like input to the meter is the solid orange, one of which appears to share a tab with blue/white, which seems to point to an unbalanced feed with audio and lamp ground shared. See if you can determine if this is in fact the case, and report back.
Just a bit of line level audio applied to the suspects should bounce the meters a bit. Start with a low dose so you don't slam them over.
Old 1st February 2017
  #7
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Radardoug's Avatar
 

Theres no buffer on that board, just a 4 diode bridge, and a terminating resistor or two.
Old 1st February 2017
  #8
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The first thing you should do is to read up on vu meters and buffers on the web - there is a lot of information on them. Vu's are always connected unbalanced - the problem is when the output is balanced, this put's out twice the unbalanced output level. So, the buffer should contain a trimmer in order to compensate for the level difference.
Old 2nd February 2017
  #9
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Yes you are correct ripple_fx1 the solid orange one is the meter input. I have sent some signal to them and most of them seem to work. They do not seem calibrated correctly which probably has to do with the balanced to unbalanced connection. Also they dont seem calibrated to each other (one is a few db from the other) and the front trim knobs dont seem to work. What would be the correct way to wire them from a balanced output to them, is it pretty straight forward or do you need the trimmer and buffer as posted. Thank you again for all of your guys' help!

Sincerely,
David Mills
Old 2nd February 2017
  #10
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If I were going about it, I'd try to determine what the lamp voltage was, and then try to source a headphone amp that requires an outboard power supply of the same single-ended (+ voltage and 0 volt ground) DC voltage that it takes to drive the lamps. You see where I'm going with the power thing, yes? A balanced input headphone amp so that it doesn't unbalance the convertor output driving it, in case you do decide to strap it to the main outs...and at least a pair of headphone outputs so that you can grab the L output from one headphone amp, and the R output from the other headphone amp and trim your uncooperative meters with the output level knobs. Radio Design Labs "Stick-On" or ebay a used Rane or Presonus or one of the dozens of raw headphone amp cards that are probably a google search and a $10 investment away. Of course, if you do have the spare convertor outputs, and your DAW and workflow are such that you're able to put the equivalent of an aux send on your master bus and assign it to the spare outs, you're pretty much done as long as the convertor outs can throw enough power into the meters to get them bouncing - sans illumination - to your satisfaction.

Last edited by ripple_fx1; 2nd February 2017 at 04:18 AM..
Old 2nd February 2017
  #11
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Geoff_T's Avatar
 

Hi

All the 14+ years I was at Neve they would wire the Sifam VU meters directly across the balanced outputs in series with a 3.6Kohm resistor (the signal via the meter source switching) and only a few companies, EMI in particular, insisted on the B286 buffer amp that had an input transformer and a 748 IC.

They always measured distortion with the meters switched away from the outputs but I once measured it with and without the VU...

Without the VU = 0.025% THD
With the VU = 0.050% THD

So I really don't think you could hear the difference and neither apparently did Neve for many years.

The VU's measure the level of signal they are across, whether it's balanced or unbalanced.





Quote:
Originally Posted by Vitalis View Post
The first thing you should do is to read up on vu meters and buffers on the web - there is a lot of information on them. Vu's are always connected unbalanced - the problem is when the output is balanced, this put's out twice the unbalanced output level. So, the buffer should contain a trimmer in order to compensate for the level difference.
Old 3rd February 2017
  #12
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When you say they placed it directly across the balanced output, are you saying across the + and - , or across + and ground - please explain.
Old 3rd February 2017
  #13
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Geoff_T's Avatar
 

Hi

The VU is a two terminal passive device devoid of any connection to ground.

You put a series resistor of 3.6Kohm to the positive terminal and connect that to either the + phase of balanced or the signal of unbalanced.

You then connect the negative terminal to the - phase of a balanced signal or the shield/0v/ground part of the unbalanced signal.

It would work exactly the same way as your putting a Fluke multimeter across either signal.

A +4dBu balanced signal is exactly the same, level wise, as a +4dBu unbalanced signal and both would read 0VU on the meter.



PS With hindsight, I can see the point you were making if a circuit is electronically balanced and uses one side as an unbalanced feed it would be 6dB lower, but it's always best to describe the set up because the VU, like the Fluke, just tells it like it is. If the level is too low, an IC buffer amp would be needed.


Quote:
Originally Posted by Vitalis View Post
When you say they placed it directly across the balanced output, are you saying across the + and - , or across + and ground - please explain.

Last edited by Geoff_T; 3rd February 2017 at 08:40 PM..
Old 4th February 2017
  #14
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Well, that is what I said - That the signal from the balanced side will be twice that of the unbalanced side. Back in the old school (Bell Telephone) days, things were done as you say, that is, that VU's were connected after the output transformer with no buffer. But, as time moved on, and ears became more aware, people could detect that .025 percent distortion and decided to elliminate it with a buffer circuit. Anyway, the purpose of the VU is not to read voltage levels but volume units. In other words, the VU is used to tell you how loud one signal is relative to another, as opposed to telling you how many volts a signal has. It has been common practice for many, many years now that VU's are connected unbalanced - as any modern schematic will demonstrate. One should always use a buffer with a trimmer in order to compensate for the meter being placed before the balancing circuitry.
Old 4th February 2017
  #15
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Hi
There is so much mis information in this thread.
The meter shown has no internal rectifier and you can see the 4 diodes. The 680 Ohm resistor is an odd value to be using but it could well reflect the source level that is feeding it. Maybe the meter movement is not a 'standard' rating or the signal level is actually not +4 but an internal -2dB or something.
A VU meter as has been said is a 2 terminal device and will read the same across a balanced or unbalanced signal AS LONG as you understand HOW your signal is balanced. If it is from a single transformer secondary it will not read at all well if you hook your VU to hot and GROUND for example. You have to understand the various flavours of 'balanced' circuits. You only 'lose' 6dB if you have a 2 amplifier output technology that is not 'cross coupled'. Transformers and cross coupled electroniuc designs will maintain the correct level into a balanced or unbalanced 'load'.
The rectifier does indeed distort the signal line but the AMOUNT depends on the source impedance. From a 600 Ohm output SOURCE the distortion can be 0.3 percent when you are hitting the meter with a high signal level but if it is from a SOURCE with very low impedance (a couple of Ohms) the distortion will be negligible. In Geoff's Neve example the output impedance must have been pretty low, perhaps a few tens of Ohms so the result distortion was deemed 'acceptable'.
A 'Standard' VU meter to proper specifications should have a 3K6 resistor feeding it to indicate +4dBu at '0VU' scale marking. There are a lot of cheap meters that LOOK like a VU but simply aren't. A REAL VU is darn expensive. The Sifam 'top range' meters that meet all specifications are £100 a time, but for a meter that reads accurately from 10Hz to over 100KHz within a small fraction of a dB, being a mechanical device with the correct rise and fall times is pretty good.
The bottom line is you SHOULD use a 'buffer amplifier', an amplifier with a gain of 1 which has well balanced inputs so the line does not get distorted. A simple way is to use a TL072 wired as 2 'unity gain followers' (pin 1-2 and 6-7) with a symmetrical power supply and 470K or other high resistance 'biassing' resistors at the inputs (pin 3 and 5) to ground.
Matt S
Old 4th February 2017
  #16
Stuffing a diode bridge and coil on audio outputs isn't a great idea, a 35 cent 071 opamp is the buffer solution for that.
Old 9th February 2017
  #17
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Wow thanks guys, a lot of great info here! I got them working for the most part by hooking of the (+) to the center terminal and the (-) to the right terminal (in the pic) and It seems to bee working but not dead on accurate to the signal levels in the daw and on VUs in some other equipment but pretty darn close! Thanks again for all of this help!
Old 3rd August 2020
  #18
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roger's Avatar
 

Old thread but I’ll try to wake it up.
I just built a stereo RCA pre and I’m hooking up the sifam VU meters across the outputs. I bought a cheap buffer circuit off eBay and for some reason it’s mega noisy.
I’ve ordered a buffer circuit from JLM but just now I hooked up one of the meters directly to the output xlr and.....it works. It works really well AND I can’t appreciate any distortion at all by ear on vocal and acoustic guitar. I haven’t hooked up the 3k6 resistor or anything.
If it works should I just run with it?
Cheers
Attached Thumbnails
VU Meter Wiring over Output-3278c38f-34c3-4215-9b5f-95a587feb140.jpg  
Old 4th August 2020
  #19
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just wiring a 'naked' meter across the output suggests you have not got the hang of this.
saying even a 'cheap' buffer circuit is 'Noisy' also suggets that you had not connected it properly and you need to define what you are calling 'noise' and where it was experienced.

If it works for you then just leave it but I doubt it is actually 'correct'.
Matt S
Old 4th August 2020
  #20
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roger's Avatar
 

Quote:
Originally Posted by Matt Syson View Post
just wiring a 'naked' meter across the output suggests you have not got the hang of this.
saying even a 'cheap' buffer circuit is 'Noisy' also suggets that you had not connected it properly and you need to define what you are calling 'noise' and where it was experienced.

If it works for you then just leave it but I doubt it is actually 'correct'.
Matt S
I want the meters to just show me signal at my outputs so I know from across the room that I’m up and running (VU as “virtually useless” ha!). I’m not worried about db’s. I’ll just use my ears.
I wired up the eBay £4 buffer board and powered it with a 12vdc wall wart; the idea to protect the output signal from distortion introduced by the meter’s components - but it introduced a loud buzz into the signal. I triple checked the wiring. Earthed everything that made sense but couldn’t get rid of the terrible noise. I do have the fancier JLM buffer boards coming but I was just shocked that strapping the sifam directly across the output XLR worked so well (for my purposes) and was so clean/quiet.
I’ve only READ about the need for the buffer so expected nastier distortion. I just don’t hear it. What will the 3.6k r do?
Def newby at this - you’re right!
Old 4th August 2020
  #21
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Ah the perils of using cheap and nasty 'wallwart' supplies.
Equipment manufacturers that use them design in proper filtering in their gear and specify particular make and design of wallwarts which they have tested and know do not 'upset' their gear. That is partly why buying them from the gear manufacturer is more expensive. They have done the 'research'.
The JLM buffer will also cause a problem if you use a rubbish supply for that. The JLM board itself would be fine.
SOME Sifam meters have the necessary 3K6 resistors built inside.
The amount of distortion that a 'naked' meter will add to the audio signal depends on the output impedance of the unit. If the output impedance is 600 Ohms then distortion can be 0.1% or so, depending on signal level.
Across an output impedance of say 10 Ohms the distortion would be a lot lower.
Matt S
Old 4th August 2020
  #22
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roger's Avatar
 

Quote:
Originally Posted by Matt Syson View Post
Ah the perils of using cheap and nasty 'wallwart' supplies.
Equipment manufacturers that use them design in proper filtering in their gear and specify particular make and design of wallwarts which they have tested and know do not 'upset' their gear. That is partly why buying them from the gear manufacturer is more expensive. They have done the 'research'.
The JLM buffer will also cause a problem if you use a rubbish supply for that. The JLM board itself would be fine.
SOME Sifam meters have the necessary 3K6 resistors built inside.
The amount of distortion that a 'naked' meter will add to the audio signal depends on the output impedance of the unit. If the output impedance is 600 Ohms then distortion can be 0.1% or so, depending on signal level.
Across an output impedance of say 10 Ohms the distortion would be a lot lower.
Matt S
Makes total sense. Cheap eBay psu! The buffer board is prob fine. Sigh.
Jlm sell a filtered 12vdc psu. I’ll get 1 of them too.
0.1% ain’t too bad. Haha!
Thanks for your help Matt!
Old 4th August 2020
  #23
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The buffer board itself would not 'add' any noise but it is the combination of powering it with a supply that is obviously electrically connected to your preamp and that supply can radiate and conduct nasty 'noise' totally unrelated to your audio signal.
0.1% distortion can sound terrible depending on the harmonic content.
Matt S
Old 4th August 2020
  #24
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roger's Avatar
 

Quote:
Originally Posted by Matt Syson View Post
The buffer board itself would not 'add' any noise but it is the combination of powering it with a supply that is obviously electrically connected to your preamp and that supply can radiate and conduct nasty 'noise' totally unrelated to your audio signal.
0.1% distortion can sound terrible depending on the harmonic content.
Matt S
Yep. I understand. Better psu needed for buffer board.
So maybe drum hits that 0.1% could get ugly.
The meter actually says on the label “3k6 resistor required”. What does that do, Matt?
Old 4th August 2020
  #25
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The 3K6 sets the current required to make the meter show 0VU when 1.228 Volts is applied to the meter when in series with the resistor.
Otherwise the meter will still read something but not in a useful way.
The meter movement itself is more sensitive than it needs to be but having the 3K6 resistor helps swamp the non linear effects of the rectifiers inside the meter.
Matt S
Old 4th August 2020
  #26
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roger's Avatar
 

Quote:
Originally Posted by Matt Syson View Post
The 3K6 sets the current required to make the meter show 0VU when 1.228 Volts is applied to the meter when in series with the resistor.
Otherwise the meter will still read something but not in a useful way.
The meter movement itself is more sensitive than it needs to be but having the 3K6 resistor helps swamp the non linear effects of the rectifiers inside the meter.
Matt S
Great! Thank you! Makes sense!
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