meter nightmares still (G1176 specific)

Hey,

I'm having real trouble understanding the whole VU meter, dB(and associated suffixes), Vrms caper - even after reading the great threads here that covers such subjects and also googlin various info from here n there...really isnt clicking on how I should go about setting stuff up - I know it should but i'm a simple soul when it comes to basics...

First off this is G1176 specifc as I want to calibrate it all proper and make sure the meter is behaving as it should etc..

So to begin with and rather then asking 1001 q's and waffling on I'm just gonna ask a q at a time to make sure I understand the process at it progresses...

Right then, 1st q:

Before using the meters I should first make sure they hit 0VU with a 1.23Vrms input signal right (When measuring the voltage i'd have the dmm set to VAC and look for the 1.23V ye?)? So from my soundcard output (600R balanced) I tie pin 3 to 1 and this goes to the meter - input, pin 2 goes to the meter + input via a 5k trimmer connected in series. Then I apply a signal and trim till I get 0VU. With the resistance measured on the pot this is the value resistor I'd use inplace of the 3K6 resistor on the 1176 pcb...?

Meters I'm using are the 500mA SM14's with a diode bridge added...

Once i get this part I can start calibrating this thing proper I hope...

Thanks

When measuring the voltage i'd have the dmm set to VAC

Click to expand...

Be aware that many DMM`s do not have that great a frequency response. You may want to check the specs of your meter & make sure that the signal your feeding into the G1176 is at a frequency that your meter can read accurately.

Hey Rob,

I have a HP 971A and Fluke 110 which I was testing with - sending a 250hz signal from the comp soundcard...also got that audiotester app if it helps...

> Meters I'm using are the 500mA SM14's with a diode bridge added...

500 MILLIamps??? That's way too big.

no meter required. just use ears.

I go with that anyways CJ - it'd just be nice to understand is all...and I know for a fact my 1176 aint calibrated correct...

PRR, okay so these meters are no good - in this post http://www.groupdiy.com/index.php?topic=2439&highlight=sm14+1176 theyre mentioned as being okay for GR but not so coo for VU...

> okay so these meters are no good

No, you typed "mA", I was too lazy to look it up and check you.

It is in fact rated for 500MICROamps. Not true-VU but will work here.

> I've got some proper Sifam VU's that i could still test with but nothing is clear

A VU meter should read "zero" for a +4dBm signal. +4dBu is 1.2283 Volts.

> for instance I just sent a 0dB, 250hz signal direct from the soundcard out and measure 0.979VAC

Sound card's "zero" seems to be 1V, not a traditional broadcast-audio reference but very logical.

> When connecting the sifam vu across this the 5K trimmer is turned to about 790R for the needle to land on 0vu...even tho it's spec'ed as needing an ext 3K6 resistor...when the trimmer is set to 3k6 the needle goes to around -2.5vu...

VU meter "zero" is 1.2283 volts. 0.979 Volts should read 1.97 dB lower, or -2dB. Half-dB error is not a big problem. I'd say use a 3K3 resistor and you are more than good enough for Rock-N-Roll.

Those 500uA meters may need 1K4 or so to read "zero VU" on a 1.23V signal. And that is kind of a heavy load for some outputs.

Now the other question is: the 1176 uses the meter as a DC meter to measure some function of FET bias to display Gain Reduction. Can that circuit pull a meter that needs 2.5X the current of a proper VU meter?

okay, turns out the app i was using to send the signal didn't effect level so I've just tried again with the app routed to a mixer and adjusted the level till 1.23(ish)VAC on output was reached...

with a 3k6 res and the sifam vu - this hit the 0VU mark more-or-less, whilst with the SM14 a 2k resistor was pretty much on the money...

so to start with I guess swapping out the 2x3k6's to 2x2k's (on the G1176 pcb and meter switch pcb) is in order ye? then I can start calibrating n whatnot...

cheers :thumb: