building a simple pad - help please!

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building a simple pad - help please!

Postby Martin » Wed Mar 14, 2007 11:05 am

i have some VU meters that i'd like to connect up, all the outputs that i can use to connect them to are +4 and these things are -10...

assuming they will still give an accurate reading, i'd like to make a simple pad to reduce the levels

i've done a bit of research but most things i've found are designs for making mic pads, not so much line level stuff... soldering etc is no problem, just need to know what value resitors to use and where to put them!

any help would be great...
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Postby Martin » Wed Mar 14, 2007 11:35 am

i found this

http://www.uneeda-audio.com/pads/

which is helping... still not 100% how to solder it across the lead but trial and error it will be!
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Postby astrovic » Wed Mar 14, 2007 11:51 am

Also have a look at http://www.rane.com/par-a.html#attenuator under "attenuator pad" (scroll down). In the L pad section there's a guide to resistor values for particular dB loss.

I would have thought, to make an unbalanced L-pad...you'd solder R1 from the incoming line to the + terminal of the meter, and R2 between the + and - terminals. So at the + terminal, you will have 2 resistors attached to it, which can make for some awkward soldering!

Also look at http://www.sengpielaudio.com/calculator-db-volt.htm for voltage conversions...looks like you need to drop 7.78 dBu to go from +4 to -10. Which would be about a 1.5k resistor if I read that table in the Rane document correctly. Another way you could do it, I suppose, is put a trimpot (maybe 2k) as R2 - that way you can adjust it to get the precise level of attenuation you want rather than resistor guessing.
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Postby chris p » Wed Mar 14, 2007 12:08 pm

I assume you want to pad a balanced signal?

1 x 5k resistor between IN+ and OUT +
1 x 5k resistor between IN- and OUT -
1 x 3k6 resistor between OUT+ and OUT -

That should do it.
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Postby astrovic » Wed Mar 14, 2007 12:18 pm

of course, I could have just said "wait till Chrisp or Rob comes along", but I thought I'd have a go at it :oops:
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Postby Martin » Wed Mar 14, 2007 1:11 pm

the VU meters just have RCA input... its an old Ta@#$% mu-40 unit that i'm trying to put to use

RCA, so unbalanced...


i can get it to show 0 with a +4 test tone by putting a 28k resistor in series on the + (or - for that matter)

im not sure how accurate that is though... putting a resistor across the + and - didn't seem to do anything

thanks for the help guys!! keep it coming...
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Postby chris p » Wed Mar 14, 2007 2:54 pm

For unbalanced,

1 x 10k resistor between your source signal and the RCA centre prong
1 x 3k6 resistor between the RCA centre prong and the RCA sleeve
and of course the gnd wire from the signal source to the RCA sleeve.

A bit of careful soldering and heatshrink tubing and you could build it into your connector!

If your around NW Sydney I'd be happy to meet up and help you if required (but not this weekend, sorry!)

Good luck.

Oh, and Astro, the difference between +4 and -10 is 11.8db - which is somewhat confusing because they used different base voltages (dbu vs dbv) for their standards.
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Postby astrovic » Wed Mar 14, 2007 3:16 pm

Now I get it:

-10 dBv = -7.78 dBu
+4 dBu = 4 dBu (duh!)

Put them together you get a difference of -11.78 dBu

(or you could use the formula you put up the first time).
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Postby rob » Wed Mar 14, 2007 5:44 pm

chrisp's pad formula's are correct

the thing is 'though the VU meter you have has an input impedance that will present itself effectively in parallel with the 3K6 resistor thus giving a lower combined value. Thus you'll end up with a bit too much attenuation and the meter will read low.

You say a 28K resistor in series with the input gives a correct reading. Besides not being sure where you got the 28K resistor as it's not a standard value, this hints at the easiest solution. You could simply put a 50K trimpot in series with the input and calibrate it to read 0VU at +4dBu in.

Is your source balanced? It sounds like the VU meters are unbalanced in, thus unless you float the grounds you'll be unbalancing your outputs when connected to the meters. This may or may not be a problem

The most bullet proof solution is to use the JLM VU driver kit ( which i developed for them :) )

http://jlmaudio.com/VU%20Buffer.htm

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Postby Martin » Wed Mar 14, 2007 8:29 pm

wow thanks heaps guys!

yeah i knew 28k sounded funky but the multimeter i was using was also funky and i'm not too good with colours to read the little bars (red, organge, brown all look the same...)

i built it into a connecter quite neatly, will post a pic when i get a chance

i do have some 10k and 3.6k resistors so i'll try that method and give it another go!!

the source is balanced, the meter box inputs are all unbalanced RCA in, and i can switch them between -10dBv or 0dBu, too hot on both settings at the moment!

rob that kit look great! the LED's are what i had in mind to do to this box...

maybe i'd be better off making new drivers for the things? if the meters are compatible... i'll find out the part numbers and see what happens

building kits is no problem, i enjoy it :D
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Postby Jeremy H » Thu Mar 15, 2007 8:10 am

Hey guys,
I actually did this on my m3500 Ta@#$% desk - changed the resistor values on the little pcb meter mount of the master VU. One for each meter and one for each peak led

I don
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Postby rob » Thu Mar 15, 2007 8:56 am

Changing the resistors in the drive to the meter won't change it's ballistics.
The ballisitcs are determined by the mechanical construction of the meter. Different meters adhere to the standards for a VU meter to various degrees.

Two things matter, the overshoot and the time response or if you like the attack and decay of the meter. You can read my article in Audio Technology for more on this.

But, here's a test you can do if you have some DAW system at hand. Create a wave file using your software's internal tone generator that consists of a burst of 1kHz tone that is 300mS in length, repeat or loop this tone in 1 second intervals. Add to this a tone at the beginning of the file that is 1kHz for say 10 seconds. Thus you have some audio that is 1kHz all at the same level for 10seconds, then silence for 1 second, then a burst for 300mS, then silence for 1 second, then a burst for 300mS, then silence for 1 second and so on and on.....

Play this out to your meter and either calibrate the meter or adjust the send level to the meter such that the initial 10 second tone reads 0VU. Now, watch the meter when the bursts of tone arrive. For a meter that meets the standard, the needle will just rise to the 0VU mark before the tone burst ends. It also won't momentarily jump past the 0VU mark before settling back to the 0VU mark ( this is overshoot if it does ). Most meters are not slower than the standard and so should get to 0VU in the 300mS.
Next edit or recreate a new file with shorter bursts, say 200mS. See if the meter gets to 0VU within the time of the shorter bursts. If if does the meter is reading faster than the standard, ie it's attack and release time is too fast and as a useful VU meter will be too responsive. The faster it responds the more it moves from reading VU to reading peak levels. You can play around with the length of the tone bursts to find the time period that will just allow the meter to read 0VU before the tone ends. For a true VU meter this will be 300mS.

One of the things the VU driver circuit on the JLM site allows for is to add a capacitor across the meter to effectively slug the attack time, slowing down the drive to a fast meter to better approximate a true VU meter's response. This adding of a cap can't be done with many meter driver circuits.

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Postby astrovic » Thu Mar 15, 2007 10:37 am

That's some fantastic advice - thanks Rob
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Postby Martin » Thu Mar 15, 2007 10:38 am

part number on the meter is "teac 5296006101" which doesn't bring up any results in a search so i've no idea about that one!
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Postby Jeremy H » Thu Mar 15, 2007 11:10 am

Good stuff Rob - thank you
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Postby Martin » Thu Mar 15, 2007 2:52 pm

sucess!!! thanks for the help guys... that pic is with a +4 test tone running through them
its great to have these meters in a room full of digital gear

luckily the master outs of the desk were free so just used them!

Image


LED converstion is next, just need to find out how much voltage is running through those lamps
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Postby astrovic » Thu Mar 15, 2007 3:43 pm

Good stuff! And nice VU's, too
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