Meter Calibration on Console

I had noticed my VUs are off a bit between each of the busses on my console so I thought I would get out the ol' multimeter and do some calibration.  It appears my VU meters are pretty close (about -.5db off at 1.28 VRMS) and I realize I can adjust them but what dawned on me was that my bus faders are all just a bit different as far as being at, above or slightly just below unity to get them to read 1.28 VRMS.

Now I know I can adjust the position of my VUs to represent 0db VU at 1.28 VRMS, but the issue is more the faders... do I just have to live with them not being perectly in line at unity to measure 0db VU or is there some minor adjustment that can be made?

I am really just nitpicking here I guess.  For reference the console is an old Soundcraft Series 2 (circa 1978) and the faders are Penny & Giles.

man-bot said:

I had noticed my VUs are off a bit between each of the busses on my console so I thought I would get out the ol' multimeter and do some calibration.  

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Beware of the fact that your multimeter may not respond well to a 1kHz tone. You need to check its accuracy before doing anything on your mixer.

It appears my VU meters are pretty close (about -.5db off at 1.28 VRMS)...

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Is it a typo? +4dBu is 1.228Vrms. That may explain the 0.5dB off...

...and I realize I can adjust them but what dawned on me was that my bus faders are all just a bit different as far as being at, above or slightly just below unity to get them to read 1.28 VRMS. 
Now I know I can adjust the position of my VUs to represent 0db VU at 1.28 VRMS, but the issue is more the faders... do I just have to live with them not being perectly in line at unity to measure 0db VU or is there some minor adjustment that can be made?

Click to expand...

Depends if the cause is the gain of mix amp + output amp or the fader law (or both). You must check the gain by putting all faders in top position. Any discrepancy here can be adjusted by trimming the gain stage's FB resistor. You would need to install a trimmer or select resistors while under test.
Then, you should check the nominal attenuation (typically 10dB); any difference here is due to the fader law. P&G faders are factory trimmed for excellent accuracy (I think they guaranteed +/- 0.2dB), and there's not much you can do if you're not satisfied with it. There are ways to correct fader-law, but they work for single-track faders; IIRC, faders used in the Series 2 are of the ladder type, which does not lend itself to law-steering.

Sorry that was a typo!  I should have said 1.228VRMS - and that is with the VUs reading about -.5db off.  Again no biggie in reality.

Re: correcting the faders... for the amount of work it will take and being 1/32" to 1/16" off of unity I think I can live with it.  I have to run through all the bus faders, but for the most part they don't deviate more than 1/16". 

Thanks!

faders wear out. how old is this console?

The console has dates from 1978-1979 on the boards inside.

At some point the output boards were modded using a completely different design with a daughter board attached, so depending on the design/components I assume there could be some variance from board to board (which could explain what I am experiencing - but all appears to be done with good workmanship).

All in all the faders seem to function well... its just if I set them all to unity (based on the mark on the board) the VUs are all off a bit from each other.

In the end it really doesn't mean a whole lot I guess - but I would like to see some uniformity and know that it was really uniform and not just visually aligned (without giving different VRMS at each output).

again, faders wear out. they will not output the same. they will work, they will just be off. move the faders (remove fader and install in other channel) from channel to channel and see if the level stays with the channel or follows the fader.

Unless you have really spendy faders, they will have tolerances just like pots, and that is probably part of what you are seeing. 

Well - shortly I'll be going through the output section throughly and I may have to experiment with a fader swap.

Thanks for all the replies!  Makes sense...

If you don't need any "gain in hand" you can lower the fader makeup gain by 10 db (or whatever value is above unity) and just calibrate the faders to zero at full up.  The markings won't match anymore, but they are all inline again.