VU meters in guitar amps

[guitarfredy]

Hello, i want to put a simple vu meter into an lm386 based amp but i'm not really sure how to do it. I have found countless schematics on the web, but i think there is much more to it than just putting bits together. How would i attach it to the amp? and howwould i calibrate it? the scematics do not really go into detail about component values, voltages, and numbers. Could someone perhaps point me in the right direction, to a web site hopefully, so i could learn about the thoery and the maths behind such circuits?

Thank you.

[teemuk]

http://www.sound.westhost.com/articles/meters.htm

I'd guess the thing where to start is finding out the specs of your meter...

[Enzo]

Welcome to the forum.

What do you REALLY want to accomplish here? Do you really want some accurate audio measurement? If so, of what? The speaker signal? The signal at the master volume? Officially VU meters measure something that your guitar amp doesn;t have - a balanced line out. Now a level meter might not be officially a VU meter, but maybe close enough for you? That would be a meter that simply measured from zero to all the way up - whatever that might be.

Or do you just want something to indicate louder and softer - eye candy in other words? Watch the signal dance along?

I don;t see that actual VU readings serve much purpose on a guitar amp, but they could make a nice decorative addition. I like the rack tuners with the "Cylon" LED sweeping back and forth - visually appealing.

If you only want visual appeal, then you can build most any LED ladder display - steal a circuit from some amp that has one. You can trim the thing to respond at a suitable level when you play. Or you could use a real analog meter, though those are hard to see from any distance. Those things could be connected most anywhere in the amp and the drive circuit appropriately massaged into place.

LED strips can be driven a number of ways. There are chips designed to do exactly that. The LM39xx family - LM3917 for example. Or a couple quad voltage comparator chips and a resistor ladder would work.

SO maybe define the mission a little further?

[guitarfredy]

It's purely for visual effect, doesn't have to be accurate. Though i would like it to display the dynamics of the guitar. It would also be a nice idea if, as the gain increased, a display of some sort could fade from colour to colour, almost like a channel indictor with green for clean, orange for crunch and red for distortion for example.

My electronics knowledge is very small. I have made some kits and small circuits from schematics, but i dont really know much theory.

Thanks for all the help so far!

[Enzo]

OK, so it is decorative. An LED meter seems more appropriate than a real meter then. A row of LEDs can be any color you want. Look at many mixers - the low level ones may be green, the middle level yellow, and the top couple are red.

[guitarfredy]

i found this on the net: http://www.national.com/mpf/LM/LM3916.html. Will it work with my 1/2 watt little gem? Do i need to worry about the wattage of resistors and the voltage of the LEDs? how would i calibrate it? and where exactly do i connect it to on the little gem? i am going to guess pin 5 on the lm386 to pin 5 on the lm3914 and the other speaker terminal goes to pin 4 on the lm3914. is this right?

Also, i can only get my hands on an LM3914 or LM3915, will these work instead of the LM3916?

Finally, i am having problems with my little gem amp. I can't use a power supply over 7 volts with out the IC blowing. Why is this? Could it be the 1/4watt resistors i have used?

Thanks!

[guitarfredy]

ok i have done a bit more work, and my 2 main problems now are:
1) where do i put the VU meter in the amp? should i have the tip from guitar to go direct in to pin 5 of the LM3916 and then have pin 4 of the LM3916 to go to pin 2 of the LM386 which would mean sending the signal through i load of crap it doesnt want to go through? or how about as i said before, pin 5 to pin 5 and pin 4 to -ve so that it is parallel to the speaker? or do i have it in series after the output of the lm386 to the speaker? or would this be too much for the lm3916? grrrr...i wish i knew more about electronics, i really do.
2) i need to split 24v into 3v for the LEDs, 9v for the amp and 12v for the vu. Whats the best way of going about this? i was going to use some potential dividers but i have been told that i should use a dc-dc convertor. How ever, they are expensive and i cant afford one.
Sorry, i feel as though i am "milking" you guys to get an easy answer, its just that i am pushed for time trying to make a present for someone for christmas. think the best idea is to get her something else and work on this later.

Thanks guys for you help though.

[Steve Conner]

That's exactly what you're doing, milking us for an easy answer. Therefore I won't even remotely try to answer all your questions.

I'd never even heard of the LM3916. You want the LM3915 which is the logarithmic one. (so many dB per LED) The 3914 is linear, which is for things like battery voltage and so on.

I'd start by connecting the meter input to the speaker output through some sort of potential divider.

To get your voltages, use regulators. Better still don't use 24V to start with, use 12 so you can connect the power amp straight to it. The LEDs don't need a 3V supply, the LM391x is capable of regulating the current through them so that they can be run off the same supply as the chip itself. IIRC.

[Loogie]

Sorry

"It appears we have appointed our worst generals to command forces, and our most gifted and brilliant to edit newspapers." [Robert E. Lee, 1863]

Companies would make more energy efficient products if they assigned the job to the engineers instead of the lobbyists. [Steven Chu]

[guitarfredy]

Thankyou for your help, i appreciate it.

[Enzo]

The 3916 is the VU meter one. But this is eye candy, so it doesn;t matter how accurate it is, just so long as it looks good while playing.

You can tap into the signal path most anywhere, and then make the signal voltage appropriate with a voltage divider if necessary.