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Some help with mic preamp to vu meter

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st1f1

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hello, this is my first time posting here, and i have a problem.. i would like to build a tree-like vu meter for a gift. i made one with lm3915. the problem is that when i plug in my mic preamplifier, almost all 10 led turn on and stay that way.. can someone help me please, this is my first time doing something like this? max output from preamp is 500mV with 47 kOhm resistance, and i think that vu works, when i plug it into output for my computer speakers it works perfectly...
 
You forgot to post the schematic of your LM3915 circuit and preamp for me to see if it has an input resistor to ground and an input coupling capacitor.

The LM3915 is a DC voltmeter so if your preamp feeds DC to it then it shows it.
 
Schematic for preamp: **broken link removed** (page 12), i am sorry i can't find an english version, and this is the lm3915 circuit: Circuits-Lab.com » Blog Archive » VU meter circuit using LM3915. i also made an lm386 amp for the preamp, vu didn't work properly, so i thought there was something wrong with the preamp, so i made one with another lm386...but it seems there is too much noise.. i was also wondering if i could use this schematic http://www.instructables.com/files/orig/FX5/67GS/FLROK12R/FX567GSFLROK12R.gif but use lm3915 instead of lm3916? thank you
 
need some resistor

i think your project its ok. but only need a resistor between the output of the preamp mic and the input of VU meter circuit.
coz i think the output of the preamp mic is too big for the VU meter circuit.
i suggest you to give 10kΩ or more resistor for it.

or have u try he preamp circuit into a amplifier? is it work? i dont think so :(.
if is it not work you may need to find other preamp circuit.
im sorry my english is bad :D
 
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i gave it a quick try with 50 - 80 kohm, but still, 6/10 leds is on all the time. thank you for your time. i guess i should buy a new lm386 and lm3916 and make that circuit i was asking about earlier
 
The input of the LM3915 is a PNP transistor. Its input bias current causes it to float high. When it floats high then it turns on all the LEDs. Use a 22k ohms resistor from the input pin 5 to ground so that the input is 0V without a signal.

Do not connect a resistor in series from the output of the preamp to the input of the LM3915.

The preamp has a fairly high gain. The LM386 with the capacitor between pin 1 and pin 8 also has a lot of gain. Then the total gain is too high and it makes noise. Remove the capacitor at pin 1 and pin 8 of the LM386.

Like most Instructables, this one is also wrong. The LEDs will be extremely dim.

The LM3915 has equal steps. The LM3916 has big steps at the bottom and small steps at the top. I like the LM3915.
 
thank you

i'm not sure if i did this corectly, i put the resistor between output of the preamp and it's ground, is that ok? it needs around 70 kohm to stop all the leds flashing, but then with me screaming at mic i get only 5 leds to light up. is that because preamp output is 0,5V and max input for lm3915 is 1,2V?if that is so, then i need a different preamp?
i tried disconecting pin 1 and 8 from the capacitor before, but it never changed anything.. should i try this one with resistor as well?
too bad that is a false schematic, i saw it on youtube and it worked kinda like i want it to work.. does anyone have a full schematic with lm386 for mic preamp and lm3915 vu , everything put together? chirstmas is coming and i'm affraid i'm runing out of time, it seems i wont be able to get this to work
 
The max input bias current of an LM3915 is only 0.1uA. The first LED lights with an input of 56mV so a resistance to ground at the input must be less than 56mV/0.1uA= 560k. Your circuit needed 70k so maybe it was picking up mains hum or noise.

The LM3915 has logarithmic steps. With the 10k pot at minimum then its 10th LED lights with an input of 1.25V.

Your transistors preamp has a low max output level. An opamp like a TL071 would have a higher output level.

An LM386 is a power amp, not a preamp. Without a capacitor from pin 1 to pin 8 its gain is 20. With the capacitor its gain is 200 and it is noisy.

Do you have an electret mic with 3 wires like in the preamp kit?
 
yeah i know that lm386 is an amp, but i also saw schematic for its use as a preamp to an electret mic... so i made one preamp, one amp ( i had two lm386 ), and i bought one preamp ( the first schematic with transistors). i tried different setups of this three chips and none of it worked. i tried with gain 20 and 200, it was mostly the same... i guess i'm doing something wrong. well the mic in the preamp kit had only two wires. i have another one from an old telephone handle. why?thank you very much for your time
 
The preamp kit shows an electret mic with 3 wires on the schematic. It must have a jumper wire added from NF to + to work with a 2-wire electret mic. The + that feeds the 10k resistor that powers the mic is not filtered so the circuit will probably oscillate.

I must go away for a couple of hours now then I will post the schematic for an opamp preamp.
 
it shows a three-wire mic, but i got a two-wire one, red and black. along came a little book on how to solder the kit, it showed how to solder a three-wire mic (page 11) and how to solder a two-wire mic kit (page 12) . so i put black on - and red to + and nf. i'm not sure if that is allright.. thank you, i'm so happy you are willing to help me
 
You connected your mic correctly.

Here is a mic preamp that uses a low noise opamp. The 470k trimpot has a high value to make the max gain 215. If you don't want such a high gain then a 100k trimpot makes a max gain of 46.

The max output level is 3.3V when the 9V battery is new.
 

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thank you for your help audioguru, but i was really hoping for something simpler, even if i'm making this as a gift for my girlfriend i just think i lack experience to make that circuit, since i'm having troubles with simpler lm386 circuit. is there any way to get somekind of lm386 preamp to work with lm3915? you mentioned that LEDs would be dimm with that circuit that has lm386 for a preamp and lm3916 for vu, but it seems that it works:YouTube - Microphone LED VU Meter.. so could i just replace lm3916 with lm3915? i already have all the components that it requres, only 3915 instead of 3916.. if that circuit doesn't work, i will do my best to make yours, since i really want to make this thing to work. if that doesn't work, i'll just buy another plug for computer speakers and connect that to vu..
 
You can use an LM386 as a noisy mic preamp if you want.
Here is the circuit:
 

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Lm386 with Lm3915 Question audio gurus..

Hi I wanted to create an audio decibel meter using an lm386 with an lm3915.

I can connect a speaker to the output of the lm386 and hear the input at the output with a speaker. I need to know what i need to merge the two IC's so that they can work together. I have a decibel meter for calibration but at this point it is useless. I connected both grounds together as to make a common ground. Input to Pin 5 on the lm3915 is the output of the lm386 but I'm really not getting anything..Any ideas or help would be greatly appreciated..Thanks..
 
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The LM3915 displays the AC or DC voltage.
But the output pin of an LM386 is a DC voltage that is half the supply voltage. Then the LM3915 will show a high voltage even if there is no signal.

Use a coupling capacitor to block DC but pass AC.

Use an low noise audio opamp as a preamp, not an LM386 little power amp that has hiss.
 
How do you interface an lm3915 with an Atmega32 microcontroller?

all I have to work with is port A on the atmega 32.. The microcontroller is set up to accept information already to portA.. The Led bar can be removed if necessary to interface to the atmega32. for the atmega a logic one is within 3 volts. zero is below.
 

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Why are you using an LM386 power amplifier instead of an opamp?
The display will look like a dim blur if you do not use one of the peak detector circuits shown in the datasheet for the LM3915.

Your micro-controller does not have an input. What do you want it to do?
 
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