vu meter question

[Mr Wednesday]

Hey Guys,

Just a question about VU meters - i got hold of some Coleman Vu meters and wanted to know the best way to integrate them into my set up. My audio outputs through my 2192 converters into my yamaha mixer for monitoring (still need to buy a passive monitor controller). Is it sufficient to just get a xlr splitter cable running from my converters into my yamaha and the other signal going to the VU's?
So splitting the stereo signal out of my converter into 2 stereo signals - one for Vu and one for monitoring.

Thanks
Duncan

[Chinagraf]

Hi Duncan, no, you need to put a buffer in between the xlrs and the meters or the meters will induce some dist0rtion into your outputs.
Something like this is perfect, but you need to get it power from somewhere, you can use a plug pack :
http://www.jlmaudio.com/shop/index.php? ... oductId=19
You'll probably also need to calibrate the meters, so you'll need to borrow a multimeter. Info on how to do that is on the net.
Cheers.

[Mr Wednesday]

Hey Andy,

Cheers for the reply - Is your comment based on assuming that audio can be passed through the coleman because it can't - the VU's don't have an XLR output just input only - so im not passing audio through, just directly to it - but i guess the fact that it doesn't have a buffer kit inbetween means the meters will still be inaccurate because the signal is distorting??? Does that make sense? Having said that, there are trim pots on the back and a switch for high and low inputs.

In terms of calibrating, can I not just play a 1khz sine wave through pro tools, out through my converters and then to the VU's? Adjusting the trim pots on the VU till for example 0VU = -18dbfs

Thanks mate.
Duncan

[Chinagraf]

Hi, I'm not a tech but I'll try to explain it with my limited skills..
it's not to do with the audio passing through the meters. It's to do with adding the meters to the output of your converters. The meters will load the audio circuit of your converters
(ie - the audio circuit will 'see' a different impedance than it was designed to ) in an odd way and it will introduce distortion into the audio outputs. Not fuzz box type distortion, but harmonic distortion. The amount will vary depending on the audio circuit, the impedeance it's designed to drive, and the meters in question. It may be so small that you can't hear it, but then it may not, or it may just make the audio sound a little different. So, generally it's better to use a buffer circuit in between the outputs and the meter as this isolates the outputs from the meter.

Re the calibration the idea of the VU is to have a standard reference that everyone uses. FOr pro level gear 0VU + 1.23 volts on a multimeter whne you connect it to pins 2 and 3 of the output xlr.
-18 dbfs on your system may equal 0VU, it may not for any number of reasons. So to calibrate you adjust your output till you have a reading of 1.23 volts on the multimeter, then trim your neter to read 0.

There are techs and much more technical members on here that can explain it better I'm sure, and if I've got anything incorrect hopefully one of them will chime in. It might be worth posting a question about this in
the DIY section for some more answers as well.

Cheers.

[rob]

First up, Andy's explanation of why raw, unbufferred, unpowered meters cause distortion is wrong. ..... Sorry Andy!
Dig around on my website and you'll find a copy of my AT article on VU meters. This will explain the distortion mechanism.
Never the less. These Coleman VUs are buffered, aren't they? Ie they need a power source to work? If so, your Y lead approach will be fine. If not, go for the JLM buffer.

[Mr Wednesday]

Hey Rob,
Thanks for the reply - yes the VU's have their own power supply.
Its is the whole boxed unit - maybe Andy thought i got hold of just the VU meter part themselves and not everything wired up and ready to go.
Cheers

[Chinagraf]

See, it told you someone who knew what they were talking about would chime in. I was referring to just a plain meter with rectifiers in it hung on the output, but if I get Rob's article, it's because the rectifiers themselves create a bit of distortion and some of this kinda leaks back up the wires to the other output, a bit like crosstalk?
Rob's aricle is here : http://proharmonic.com/articles/AT51_DIY_VU.pdf
Cheers.