Calibrating VU meters

[George S.]

Gene, here's the formula from Watts Law.
P(w)=Vrms(v)squared/8(ohms).
Watts equal RMS output at the speaker jacks squared, divided by 8.
O.K., so how do we use this?
We hook up a signal generator to the amps input using BNC to RCA cables, and a RMS AC voltmeter to the speaker jacks.
We select a 500 Hz sine wave on the signal generator, and increase amplitude (voltage peak to peak of the sine wave) while we monitor the RMS output on the AC voltmeter.
Once we've reached the desired output, we calibrate our VU meter with resistors to that output.
Photos follow.
If I've made a omission or error, please correct me you all. I'm a hobbyist.

 

[George S.]

A 400 S1 WOPL showing 20 Vrms output, which equals 50 watts. The VU meters have the correct resistor installed to show 0dB. Note the Vpp on the signal generator. Stock settings for 400 S1 meters are 50 watt output at 0dB.

 

[George S.]

The same 400 S1 WOPL showing 40 Vrms output, which equals 200 watts. The meter signal circuit has a different resistor installed to recalibrate the meter to 0 dB for the increased output.
Note the increased amplitude of the sine wave driving the amp to higher output.

 

[George S.]

And, a 400 S2 WOPL with WOAD Cylon meters calibrated to 40 Vrms output, which is 200 watts.
My 700 Pro WOPL with WOAD Cylon meters are calibrated at 350 watts.

 

[George S.]

So, use the signal generator and RMS meter to set the desired amp output, where you want the VU meters to show 0 dB.
Then, calibrate your VU meters to that signal level by changing the resistor value.
Easiest way to calculate the resistor value for analog meters is using a decade resistor box. In the photo it's the small box marked EDFM, available on eBay. Here I'm using it to calculate a voltage dropping resistor for the LED's I added to the S2 amps.
WOAD Cylon meters have precision trimmers, PL Cylons have normal trimmers.

 

[George S.]

Gene, your software based signal generator should work fine. 500 Hz sine wave, amplitude Vpp turned down low. Bring the amplitude up slowly while monitoring Vrms output.
That equation is a variation of Watts Law for AC voltages.
Hope those cool digital meters your playing with have built in trimmers. If your pipes are 4 ohms then use 4 in the equation and you can get a really good calibration for those particular speakers.
Have fun!

 

[gene french]

the digital vu meters do have pots....and thanks for explaining how to do it...
sounds like a fun project....i kinda set them to display the red light sabers in the 20 watt neighborhood according to the phase linear meters...
20 watts is a lot of power....very loud...they hit yellow in the 10 watt range...i have another set ordered to put on the other wopl....i want them more modern lookin....i do like the digital meters...
i have a graphic equalizer should be here today....hafler....i will remove the dsp (overkill) and put the eq on the voigts....that is being driver by one of my wopls...when i set that up...i can tweak and dial in the meters....the big 4 way speakers will retain their dsp as well as the bedroom system....
thanks mr. george!!!!
keep up the good work!!!!

 

[laatsch55]

[QUOTE="gene french, post:
20 watts is a lot of power....very loud...![/QUOTE]


LOL!!

 

[gene french]

[QUOTE="gene french, post:
20 watts is a lot of power....very loud...!

LOL!![/QUOTE]
i have delicate ears....

 

[laatsch55]

Amen Gene...

 

[gene french]

Amen Gene...

lolololol