LM386, MIC Help required

[gkulkarnii]

Hi All,

I am trying to build a clap recognizer. As a first step i am trying to amplify the output of electrut microphone. My circut is attached, which i found in this forum. I am getting amplified output as 0.65 volts which does not vary with "whatever sound i make!" and fades with time i.e after sometime the output becomes zero. I am checking the ac output in multimeter. What can be the possible problem?

I tried to break the circuit and just check whether stand alone microphone is giving desired output!. Circuit attached. Samething with this also. Output (1.3v) doesnt vary with sound and fades away.

I have checked the connections, capacitor polarity, LM386 connection, all are fine.

FIrst time when i connected circuit i connected all electrolyte capacitors in reverse directions. Capacitors might have gone bad, is that why this behaviour? Also while soldering microphone it got heated up (2 mics i have soldered) will that damage mic?

Im not electronics guy. Please help me move ahead. I will appreacite all your suggestions.

Thanks,
Guru

 

[poor mystic]

That is odd electronics. Without having made a careful analysis of the circuit, I can see from the outset that some of the values (10uF on the i/p cap) and circuit topology (floating input on LM386) are surprising to say the least.
I hope there are better circuits than that around on the web.

PS
I'm sorry, that might not be quite clear. I think that the design is so bad that it should be thrown out, and a better design found.

 

[gkulkarnii]

That is odd electronics. Without having made a careful analysis of the circuit, I can see from the outset that some of the values (10uF on the i/p cap) and circuit topology (floating input on LM386) are surprising to say the least.
I hope there are better circuits than that around on the web.

PS
I'm sorry, that might not be quite clear. I think that the design is so bad that it should be thrown out, and a better design found.

Hi... Thanks for the reply. Okie... I will check for any other circuits on the same. But again, i may not know whether that circuit is a proper one or not. Please share the link or circuit if you know one.

Thanks,
Guru

 

[mgrass]

add input bias resistor

I've looked at the data sheet for the LM386. For your set up, I think that you need to add a 10k resistor from the input pin to ground. If your gain is not high enough you can add a cap across the two gain pins. Check the data sheet. If you have a scope check the output from you condenser mic. If you don't have a scope, run the signal to the input on your computer. Make sure you run it through a cap to your computer so you have dc isolation.
Can't really agree with comment about "a crappy design", becuase the design appears to be pretty much in line with the sample circuits in the LM386 data sheets. Do a google search for "LM386 Data Sheet".

 

[poor mystic]

Thanks mgrass for the feedback
the deficiencies I perceived in the design are adequately addressed (in my mind at least) by your input resistor. That looks like a very cheap fix and is probably what the designer intended. In my opinon.

PS in fact, let's get it right. mgrass' post is excellent.