Wiring a VU meter

[danielzink]

Hey all,

I've seen a couple of amps lately (ie: ashdown) that have VU meters wired into them.

Anyone have any experience doing this ?

I have a couple of "project" amps that I'm always fooling around with and I think it would be a hoot to have a big ol' VU meter mounted on one of them.
Whether it does anything functional is beside the point. The fact that the needle would move and the meter wouldn't affect the sound the amp in any way are the primary concerns.

I was wondering what kind of meter I would need and where it would be wired into ?

Thanks, Dan

[chocopower]

Hi there!!

i was looking for something similar... maybe those ones....

http://es.farnell.com/jsp/search/res...isGoback=false

[danielzink]

i was looking for something similar... maybe those ones....

http://es.farnell.com/jsp/search/res...isGoback=false
Today 09:39 AM

mmmmmmmmmm....not really...

Your link shows all digital meters.

I'm trying to wire in something more along the lines of these:







Thanks, Dan

[Old Tele man]

...just remember that most (but not all) VU-meters assume a 600-ohm source load, so you're probably gonna have to come up with a voltage-divider circuit having a 600-ohm resistor in either the "top" or "bottom" postion.

[danielzink]

Originally Posted by Old Tele man

...just remember that most (but not all) VU-meters assume a 600-ohm source load, so you're probably gonna have to come up with a voltage-divider circuit having a 600-ohm resistor in either the "top" or "bottom" postion.

That's the kind of info I'm looking for.

Any ideas on how to do that ?

[Ampclutz]

Many old tape recorders had VU meters. Maybe you could find something by looking at those schematics and layouts.

I have an old one somewhere around here that has VU meters.

AC

[petemoore]

http://sound.westhost.com/project55.htm

[Enzo]

I think if you have a piece of equipment called a VU meter, that 600 ohm thing might pertain, but in the circuit, it is just a voltmeter. True VU meters have a controlled ballistic response to the needle, but that is mechanical rather than electrical. The stuff on amps today is just eye candy. it takes nothing much to drive it, so try a low voltage meter. AC volts for direct drive from a signal, or you could rectify the signal and drive a DC meter. For that matter you could certainly steal a meter from a dead tape deck or something similar, then it would already have the VU scale printed on it.

SInce you want it for decorative reasons, who cares how accurate it is?

[Steve Conner]

I put meters on all my homebrew amps! I like to have it reading the cathode current of the power tubes, because that's actually useful.

What I do is connect a resistor of 1 ohm or so between the tube cathodes and ground, and connect the meter across that. I add another resistor in series with the meter, to adjust the scale, and also to give it a little protection in case a tube shorts out. Though if the 1 ohm resistor blew open, I doubt anything could save the meter, so I use an oversize power resistor there too.

BTW, the ballistics depend on the source impedance of the electric circuit, too. If you drive a VU meter from a circuit with higher than the rated impedance, the needle will overshoot more. A short circuit gives the highest mechanical damping, and some people recommend shipping meters with their terminals shorted to stop the pointer flailing around so badly.

[bob p]

Originally Posted by Steve Conner

A short circuit gives the highest mechanical damping, and some people recommend shipping meters with their terminals shorted to stop the pointer flailing around so badly.

fwiw, I transport speakers with a shorting plug in place.

[Steve Conner]

[Old Tele man]

...from Wikipedia:

"Volume Unit (VU) defined: The reading of the volume indicator shall be 0 VU when it is connected to a 600-ohm resistance in which is flowing one milliwatt of sine-wave power at 1000 cycles per second."

"In professional audio, a popular unit is the dBu (see below for all the units). The "u" stands for "unloaded", and was probably chosen to be similar to lowercase "v", as dBv was the older name for the same thing. It was changed to avoid confusion with dBV. This unit (dBu) is an RMS measurement of voltage which uses as its reference 0.775 VRMS. Chosen for historical reasons, it is the voltage level at which you get 1 mW of power in a 600 ohm resistor, which used to be the standard reference impedance in almost all professional low impedance audio circuits."

...also explained in most collegiate electronic textbooks.

[danielzink]

This is all great - but in the spirit of my original post.

Man......I just really want a needle that moves up and down in relation to something within the amp.......

It doesn't even have to mean anything......


Dan

Originally Posted by Old Tele man

...from Wikipedia:

"Volume Unit (VU) defined: The reading of the volume indicator shall be 0 VU when it is connected to a 600-ohm resistance in which is flowing one milliwatt of sine-wave power at 1000 cycles per second."

"In professional audio, a popular unit is the dBu (see below for all the units). The "u" stands for "unloaded", and was probably chosen to be similar to lowercase "v", as dBv was the older name for the same thing. It was changed to avoid confusion with dBV. This unit (dBu) is an RMS measurement of voltage which uses as its reference 0.775 VRMS. Chosen for historical reasons, it is the voltage level at which you get 1 mW of power in a 600 ohm resistor, which used to be the standard reference impedance in almost all professional low impedance audio circuits."

...also explained in most collegiate electronic textbooks.

[Old Tele man]

...simple "wiggle-meter" can be either:

(a) a hi-Z voltmeter (so it won't load down the signal path) at input to PI

...or...

(b) a milliampmeter (with small shunt capacitor to 'smooth' out the "crest-factor" peaks) in cathode of output tube(s).

[danielzink]

Originally Posted by Old Tele man

...simple "wiggle-meter" can be either:

(a) a hi-Z voltmeter (so it won't load down the signal path) at input to PI

...or...

(b) a milliampmeter (with small shunt capacitor to 'smooth' out the "crest-factor" peaks) in cathode of output tube(s).

[Old Tele man]

...uh, just exactly WHAT do you want that meter to "show you"?

1) that something is happening inside?
2) that something is happening in the preamps?
3) that something is happening in the PI?
4) that something is happening in the power tubes?
5) that something is happening in the power supply?

...or what?

[danielzink]

Originally Posted by Old Tele man

...uh, just exactly WHAT do you want that meter to "show you"?

1) that something is happening inside?
2) that something is happening in the preamps?
3) that something is happening in the PI?
4) that something is happening in the power tubes?
5) that something is happening in the power supply?

...or what?

Okay - now realize - this is not a smart aleck answer but.....yes....

to simplify. Just for grins - I'd like a VU meter on the front of the amp - jumping around in realtion to something.

It's not for diagnostic purposes, it's not for anything - other than to have a big 'ol meter on the amp doing something.


Thanks for all the replies, Dan

[petemoore]

Here;'s what I'd do.
But I have free VU's laying around.
Can't hurt to try...
Connect guitar pickup to VU see what happens, probably little to nothing.
Then figure it's a 600ohm input, and that'll certainly dump signal, so use a buffer with low output impedance, I'd say try an LM386 which is able to drive a speaker, but that's probably overkill.
Use an active booster/buffer...breadboard.
Use a voltage divider [volume control], ease it up until the meter jumps.
I have pegged these things and they seem to take a good bit of current and still work.
Points being...You'll probably need a buffer to drive the VU, and will want one to drive a signal split anyway to prevent signal loading.
Guitar into buffer might make it jump.
Guitar into gain stage into buffer I think would do the trick.
I'd keep all the parts as opamps or discretes ...or...
try the slick trick described above which utilizes the voltage across the cathode resistor of a preamp stage to do the VU coil driving...only question I have is how much does a current draw influence cathode voltage, er...how much would that 'matter'...I think not much, seems like an elegant way to assign a VU to an 'already there' driver, haven't tried it to comment....I'd check/reread before building what I forgot about a voltage reducing divider between the tap and VU.

[bob p]

Originally Posted by Steve Conner

hey steve, i really like the red lettering on black. is that a lighting artifact, or is that the real color? it looks like you had 2 layer black on red plastic professionally engraved. or did you use some sort of lettering kit? looks good!

[Ampclutz]

Ok, Here we go,

I'm looking at an old tape recorder with a VU meter. This meter is wired between the speaker out jack's tip lug and ground. The meter has 10K resistance and no series or parallel resistors connected. It is really butt simple. I think the speaker is supposed to be 8 ohms.

This thread got my curiosity going so I just had to check and this is a modular recorder and I already have the amp out.

So, this is one way that will do something for sure.

Hope this helps,

AC

[petemoore]

I'm looking at an old tape recorder with a VU meter. This meter is wired between the speaker out jack's tip lug and ground. The meter has 10K resistance and no series or parallel resistors connected. It is really butt simple. I think the speaker is supposed to be 8 ohms.
I'd have half guessed connecting it directly to the speaker [well..'10k vs. 8k'] would bury the pegged needle.
So anyway it is like driving a speaker, tape player amp probably isn't as powerful as guitar amps, a series resistor would probably remedy that.
Looks 1/2 like an attenuator design I was dreamin' up..

[Ampclutz]

I'd have half guessed connecting it directly to the speaker [well..'10k vs. 8k'] would bury the pegged needle.
So anyway it is like driving a speaker, tape player amp probably isn't as powerful as guitar amps, a series resistor would probably remedy that.
Looks 1/2 like an attenuator design I was dreamin' up..

That may be a typo [well..'10k vs. 8k']... 10,000 vs. 8 ohms. or a 1250 to 1 ratio. This amp has a single end el84 output section for what... 5 watts output maybe. So a big amp could overdrive the meter. A trim pot would fix it probably

AC

[petemoore]

Looks 1/2 like an attenuator design I was dreamin' up..
Had about the same dream...lol.
And why not? wiggling inductor coil is something I dream up about too much...instead of all the speaker suspension stuff, all really needed is a coil alignment movement and some damping, that could all be done [aligned etc.] using a pendulum on a small bearing...instead of 'whole speaker style' [dual suspension/cone etc.] speaker attenuator, seems it'd be alot easier to mess with the damping and everything else if you could take it apart to mod it instead of having to glue together a whole new modified prototype dummy coil suspension system.

[CarlZ]

Leave it to me to throw a monkey in the wrench! But here's a thought...how bout on of those old "eye" tubes?! Don'e even ask me what the designator on the things are though. You used to see them in a lot of old reel to reel tape decks from the 60's as clipping indicators. Might be kinda cool with the little glow in the dark stripe bouncing all over the place as you play, especially if all you want is some sort of eye candy on the amp face. Would be a lot cheaper than buying a meter too.

-Carl

[petemoore]

We had a Sony with them.
Very eyecatching eye tubes, very cool tube mojo candy, and can be read quite easily from a distance. IIRC everyone who has ever seen them for the first time had numerous comments / questions.

[Steve Conner]

Hi guys,

Bob: It's clear plexiglass with the lettering laser engraved and edge-lit by red LEDs. It looks great in the dark. The laser can cut out all the holes too.

Magic eye tubes are cool. I used to have one wired to my speaker output so it would jump around in time to the music. The problem is that they have a relatively short life before they go dim, and aren't produced any more, so you'll have trouble finding good ones. And if you did, you might be better selling them to vintage radio enthusiasts...

A guitar pickup or PI output will not do anything to a VU meter. They just aren't sensitive enough, and with an impedance of only a few k Ohms, they would load the hell out of the signal. (The output impedance of your PI is maybe 100k plate-to-plate...) So the best idea is probably to hook it across the speaker terminals, with a 10k ohm series resistor and a small bridge rectifier, since all these meters want DC. 10k ohms is just a guess for the resistor value: you'll need to adjust it by trial and error depending on your meter, and the power output of your amp.

[bob p]

thanks!

[danielzink]

Originally Posted by Steve Conner

Hi guys,
the speaker terminals, with a 10k ohm series resistor and a small bridge rectifier, since all these meters want DC. 10k ohms is just a guess for the resistor value: you'll need to adjust it by trial and error depending on your meter, and the power output of your amp.

A rectifier like any of these ?

http://www.radioshack.com/sm-diodes-...8.2032230.html


Then I'll need guidance as far as wiring the rectifier 'eh ?
I can figure out how to bridge the speaker terminals with a 10k resistor

Thanks, Dan

[danielzink]

Anyone have a chance to look at those rectifiers ?


Thanks, Dan

[Old Tele man]

...yes, they'll work fine, but they're definitely "over-kill" current rating wise!