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  #1  
Old 05-10-2009, 12:19 AM
tron tron is offline
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Default audio amplifier using lm386 HELP!

Hello

I am trying to make an audio amplifier. I am using an LM386 audio amplifier. I want to pick up a sound using a small microphone (link below) and hear the sound through a set of head phones. I found the circuit below online:



for the output, i hooked up a trs connector (for stereo output). the ring and tip leads are connected to pin three, and the sleeve is grounded.

fo the 250uF cap, i used a 250uF ELECTROLYTIC capacitor, is this okay?

my problem:
i hooked up the microphone to the left side of the schematic (the ground going to the dot on the left, then i have the microphone hooked to power).

the potentiometer is also hooked up, the middle pin is grounded, then one side goes to pin 3 and the other side to the negative side of the microphone.

what happens is i just hear a noise. excuse the explination but it makes a 'farting' sound and when i turn the pot, the sound either gets higher or lower. i play a song on a speaker nearby and i cannot hear it. is there anything i should check first? i breadboarded everything and its still not working. heres a pic of the breadboard i made:



the microphone i used:

question about the mic: both cables in the black heatshrink have to be grounded right?

thanks
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Old 05-10-2009, 03:00 AM
lofi lofi is offline
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mic black to ground, red to pot maybe?
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Old 05-10-2009, 03:04 AM
lofi lofi is offline
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pots have input, output and ground i think

if so then

left mic in
centre pot out (pin 3 in the 386)
right to ground
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Old 05-10-2009, 03:14 AM
lofi lofi is offline
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power to pin 6 on the 386 and pin 4 to ground
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Old 05-10-2009, 03:59 AM
Audioguru Audioguru is offline
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Your microphone is an electret type. It won't do anything if it is not powered and connected correctly. It looks like a 3-wires one from RadioShack but they don't know and don't say how it is supposed to be connected.

I would guess that the black wire is ground, the red wire is from a decoupled positive voltage from 4V to 10V and the white is the output that might need a coupling capacitor.

Next time buy a microphone from a parts source that has connections instructions.
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Old 05-10-2009, 04:55 AM
David Bridgen David Bridgen is offline
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Quote:
Originally Posted by tron View Post
for the output, i hooked up a trs connector (for stereo output). the ring and tip leads are connected to pin three
I shall assume that you mean pin five.
Quote:
fo the 250uF cap, i used a 250uF ELECTROLYTIC capacitor, is this okay?
Of course. It's the olny type of that value which you are likely to find.

The picture of the microphone doesn't help much. A view, or sketch, of the rear would have been better.

Neverthless, it looks as if there is a single screened cable (what you refer to as "both cables in the black heatshrink"), and one single (red) unscreened wire connected to it.

If that is so, the red wire is to bias trhe electret capsule, and the screened cable carries the signal on its inner, white, core.

This is how the whole thing should be connected:




The symbols hanging below the diagram of the LM386 to which you refer are Earth symbols, i.e. they mean a connection to the Earth.
If you run this from a battery source, then those symbols are not appropriate. Unless, of course, you really are going to connect it to Earth.
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Old 05-10-2009, 07:40 AM
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eblc1388 eblc1388 is offline
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@tron,

The resistor on the breadboard at the LM386 output after the 0.05uF capacitor looks like a 10KΩ to me.

The correct color band for a 10Ω resistor is "Brown-Black-Black".
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Old 05-10-2009, 08:00 AM
David Bridgen David Bridgen is offline
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Yes, it does doesn't it.

Neverthless, 10k is not the right value to use there.

Although 10 ohms is often used, 1 ohm is the value I have seen more often.
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Last edited by David Bridgen; 05-10-2009 at 08:26 AM. Reason: Punctuation
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Old 05-10-2009, 11:40 AM
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Bill_Marsden Bill_Marsden is offline
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Quote:
Originally Posted by David Bridgen View Post
The symbols hanging below the diagram of the LM386 to which you refer are Earth symbols, i.e. they mean a connection to the Earth.
If you run this from a battery source, then those symbols are not appropriate. Unless, of course, you really are going to connect it to Earth.
I'll have to strongly disagree with that statement, it is a convention. I've worked for Collins Radio, Alcatel as a lead tech for 25 years, and have a AA degree, that is how I was taught and how most of the schematics I've used were drawn. It is also how I draw the many I have drawn for this site.

Ground is a tricky subject, it can be the chassis (though not always, especially with plastic cases). It might be better to say it would be the lead you would connect to earth if you were going to, but in the world of power supplies, especially dual, it is the reference by which the other voltages are measured, nothing more.

A note to the OP: You can also replace that electret mic with a dynamic microphone or an 8Ω speaker, which also acts like a dynamic mic.
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Last edited by Bill_Marsden; 05-10-2009 at 11:49 AM.
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Old 05-10-2009, 01:46 PM
David Bridgen David Bridgen is offline
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Quote:
Originally Posted by Bill_Marsden View Post
I'll have to strongly disagree with that statement
Your disagreement is noted, but it is still wrong, sorry.
Quote:
Ground (sic) is a tricky subject
You have never said anything more true in your life Bill.

Read this http://www.davidbridgen.com/earth.htm

I read all the British monthly "Radio" magazines since around the mid 50s. Every now and then one or the other of them would publish an article on various conventions. These included not only circuit representatrions but abbreviations of commonly used units. The Earth symbol was always as I described, with successively short horizontal lines.
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Last edited by David Bridgen; 05-10-2009 at 01:52 PM. Reason: Addendum
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