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Inline Meter

Peter F

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Wonder if anyone can help me out.


I want to make an LED Vu meter as a confidence check for recordings.

It needs to go "in line" with the audio signal, as shown in my little flowchart below:



Audio Source (L&R) ------------> Pete's LED Vu Meter -------------> Audio Destination


It has to work in this configuration.


I can find circuit diagrams for LED Vu meter in about 3 seconds using google,

oh look, here comes one now, nice and simple.




The challenge I lay down is this:

Pete's LED Vu meter(PLVM) has to sit in between the source and destination. The audio must not be affected by PLVM.

That's the bit I don't know how to do. If someone knows the electronics required to safely split or isolate the audio I would love them to share the information.


I'm sure it's blindingly simple but my GCSE Electronics is rather a distant memory.


For information, audio is at line level and unbalanced, metering accuracy is not critical, there are more accurate meters earlier on in the chain.





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The circuit is ok for putting accross the loudspeaker line, but no good at all for low level signal circuits - full scale needs 12V - line level is normally .775V - or just over a volt in some professsional spec equipment that runs at a higher output level. So this circuit design isn't much use for line level signals. You don't state what the application actually is only as audio source L&R. The circuit also is just a single channel, you'd need two for stereo.
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look at the LM3916 bargraph vu meter chip, go to National Semiconductors website and you'll find application circuit diagrams.


and there it is LM3916.pdf

page 18 is the only circuit I can spot that is working on line level input and that uses an LM307 opamp, so I am guessing I will shortly be doing the same.


Simon says I need a unity gain buffer amplifier to provide the isolation. I trundle off to my friend google again and it gives me


Quick peek at it and I can't help but notice how alike the circuit for a unity gain buffer amplifier is to the amplifier circuit using the LM307. Am I correct in thinking the LM307 amplifier would provide the isolation I require?


Other notes:

It will be an LED bargraph.

Any ballistics or standards you like, VU, PPM it doesn't matter, it's purely to show that there is some signal on the line, perhaps LED indicator would be a more appropriate label.

Budget is whatever. More important is being able to build something very compact, I'm talking matchbox size + a wall wart psu.




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Thanks Peter, never thought of the PDF.


Just had a quick look on Maplins (hideous) website, the LM3916 has been discontinued and they have no stock.

Have done a bit of searching and Farnell Electronics stock them. The National Semiconductor Part No is now

LM3916N-1 and Farnell's order code is 9486526

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The circuit that you mentioned uses two 3916s to expand the display and the op-amp acts as a multiplier so that the threshold for the second section is slightly above full scale for the first section. So to answer your question, no it is not a buffer.


If you go back to Elliot Sound there is a table giving different resistor values for different input voltages the bottom one (2V full scale) should work for you. There is a link for a calculator if you need different values.


Depending on what your “audio source” is the input impedance should be high enough to not need a buffer. However you could use one with variable gain to fine tune your display for best visual read out. The input rectifier and preamp/buffer at Elliot Sound has an input impedance of 100K ohms which should be plenty high enough for most applications.



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