LM386N-1 elecret mic amp

[joshthegeek1]

Hi,
I would like to build a simple intercom using two different instances of a microphone and a speaker. I have a few LM386N-1 chips, and a 5 volt supply. Is there an easy circuit to build a mic amp with that chip, and a 10k pot as the volume control. I have an 8Ω speaker and the mic is Jameco part #136574. Thanks in advance for the help!
Josh The Geek

 

[audioguru]

Your idea will produce long distance acoustical feedback howling because a sound will go around and around and go from end to end. You need a switch to disconnect your speaker when it connects your microphone.

Here is a circuit that can be used at each end:
If you use a 10k volume control then change the input coupling capacitor to 330nF.

 

[colin55]

Here's the circuit you need:

 

[carbonzit]

It looks like you simply tied both amps together, so they each amplify both mike signals. So what prevents feedback between each set of microphones and speakers?

 

[audioguru]

It looks like you simply tied both amps together, so they each amplify both mike signals. So what prevents feedback between each set of microphones and speakers?

The circuit posted by Colin was originally from www.redcircuits.com . Colin has the gound of the top circuit shorted to the +supply of the bottom circuit.
It should have each circuit connected together with shielded audio cable.
Colin shows backwards polarity of the 22uF capacitors at the emitters.

The 20k trimpots adjust a null (the signal at the collector cancels the signal that has the opposite phase at the emitter) so that your voice is not heard from your speaker. Then switching is not required to avoid feedback and the circuit is "full-duplex".

 

[Boncuk]

Here is the circuit containing a "private" switch which turns off the master microphone when it's not desired to have speech going out to the slave.

It uses a TDA7052, but can easily be modified by using an LM386.

Boncuk

 

[colin55]

Circuit mistakes have been fixed:
**broken link removed**

 

[Boncuk]

Circuit mistakes have been fixed:
**broken link removed**

Not completely: The electrolytic caps at the emitters are still connected backwards.

 

[colin55]

Cct Revision #3:
**broken link removed**

 

[carbonzit]

Still backwards, no? The left sides of the 22uF caps are going to be more positive than the right sides.

(Not that it makes a huge difference, but it's just wrong.)

 

[colin55]

You are right. Revision #3 did not come through


Here it is:
**broken link removed**

 

[carbonzit]

It's still wrong!

This reminds me of a similar problem we had several months ago where a revised image didn't show correctly because of browser caching or some such. I reloaded this page, but the image still shows the backwards cap on the emitter.

I'll try it tomorrow to see what it looks like then ...

 

[colin55]

Another try:
**broken link removed**

 

[Boncuk]

Another try:

Just turn them 180 degrees once, not twice.

One of the collector resistors might be omitted.

 

[colin55]

This website keeps looking for the old image. I have changed the name of the image:
**broken link removed**

 

[Boncuk]

This website keeps looking for the old image. I have changed the name of the image:

Looks good now. If you delete one of the collector resistors it's perfect.

 

[colin55]

No. You can't delete one of the collector resistors.

 

[Boncuk]

No. You can't delete one of the collector resistors.

I guess you can. One common collector resistor is good enough.

 

[colin55]

Ok. I'll remove one resistor.
**broken link removed**

 

[colin55]

Here's a simpler intercom:
**broken link removed**