Passive Vu Meter - What am I missing?

[astrovic]

Hi all

I decided I needed a passive Vu meter for use in testing, and thought I'd have a go at the one on Kev's website: http://www.diyfactory.com/data/passive_vu_01.pdf

I think Kev has recommended it as a good first project for noobs (like me!) - famous last words!

I've been reading up on dB, dBV, dBu etc today, learning some and forgetting half But anyways, I've got to this (highly simplified) point - if I put a 0dB signal into the input of my passive Vu meter device, I should read 0.775 VAC at the signal input point, (unbalanced signal, testing VAC across the hot and grund pins of the input) right?

And at the meter itself, if that 0dB signal is passing through to the meter (via an appropriate resistor/trimmer as per the schemo) correctly, I should read 0.775 VAC at the meter's terminals, right? AND I should see the needle on the Vu meter sit at 0dB, right?

Well, I've got 0.775 VAC everywhere I think I should have it as per the above. I can adjust the VAC at the meter's terminals by playing with the trimmer quite nicely. What I can't do is get the Vu meter needle to move!

Any thoughts on where I've gone wrong? Is it back to school for me for more theory? :razz:

Thanks for any help - I know it's simple stuff, but we all gotta start somewhere!!

 

[nielsk]

VU meters need to see DC, so if your is not internally rectified, put a bridge on the input. This project would benifit from a buffered input stage & a meter driver with calibration......

 

[Guest]

First time I fed a signal generator into my VU meter I wondered why it stuck at 0 and buzzing
It didn't - move the frequency dial - it actual is going from +0.775V to Minus 0.775V - but the meter stops at 0 - very very quickly
Scratched my head for a while and then had a duh moment

 

[NewYorkDave]

First of all, is it a real VU meter? Having a "VU" scale is no guarantee of that.

 

[SSLtech]

A little search will yield a lot of knowlege... This has to be one of the most-asked questions.

Keith

 

[astrovic]

[quote author="NewYorkDave"]First of all, is it a real VU meter? Having a "VU" scale is no guarantee of that.[/quote]

Thanks for the tip Dave - I'll tackle that angle.

Cheers

 

[Kev]

[quote author="astrovic"]
- if I put a 0dB signal into the input of my passive Vu meter device, I should read 0.775 VAC at the signal input point, (unbalanced signal, testing VAC across the hot and grund pins of the input) right?[/quote]
err ... yep
first
Let me be picky
0dB ... only in specific situations does 0dB actually mean a specific value.
cool I used specific twice on one sentence

you need to specify ( to be specific ) to correctly identify what you are talking about.

Here I think we are talking about 0dBu or 0dBm ... u - unterminated and m terminated with 600ohms

see the Rane site and the Analog Devices site for more detailed and speficic references to dB values and RMS and Peak voltages

What I can't do is get the Vu meter needle to move!

at all !!
there could be a problem

As Dave said,
meters are different and so we need to know what meter you have ... ???
my drawing shows a Koretsu KM-66 which is a very good VU meter and even then some say it is not REAL VU

 

[astrovic]

A cheap one. I am quickly forming the view that my cheapness is the culprit here.

Thanks for picking me up on slack nomenclature, Kev - will be more diligent in the future :green:

Yes, I did just want to write nomenclature...

 

[Kev]

[quote author="astrovic"] Thanks for picking me up on slack nomenclature, Kev - will be more diligent in the future [/quote]

no _ men _ cla _ ture

ooooo ... :shock:

I'll have to google that and try to use it in a sentence before the day is out
:grin: