Purpose of circuit components in this schematic

[UR_Correct]

I've been doing a project with condenser microphones, and I came across a schematic (see attached). I was wondering what some of the components do.

I know the 10uF cap across pins 1 and 8 are used to set the gain (just cap makes maximum gain) of the LM386. I was told the 470uF cap is used as a decoupling capacitor. From my understanding, this means it tries to disassociate the pin it's connected to from the rest of the circuit (i.e., quick changes won't affect that input). Does the 100uF cap next to the 10k steady the voltage to the microphone? Is the 220nF cap just standard when you hook up an electret? I've seen simpler schematics without the amplifier that usually connect like a 10uF straight to the microphone output.

And the 100k variable resistor; is that just for helping with gain also? Is it used to bias the amp? Are the 1k and 10k just used to limit the current to the mic?

I fear I might be wrong on all these explanations, so I figured I'd ask the wise folks here. Hopefully you guys can clear up my uncertainty. Thanks in advance for any help,
Mike

 

[vk6kro]

The microphone has a small amplifier built into it, so it has to have a DC power source.

The 1 k and 10 k provide this. The 100 µF and 470 µF are decoupling capacitors, so that any AC on the DC supply is cleaned up to give purer DC. The 470 µF also provides a necessary easy path to ground for the amplifier +ve supply connection.

The 220 nF carries the output signal to the pot which controls the level of the signal to the amplifier.

Only the 10 µF has anything to do with the amplifier's operation.

 

[Studiot]

The 470 µF also provides a necessary easy path to ground for the amplifier +ve supply connection.

That's an interesting concept, perhaps you'd explain further?

 

[vk6kro]

That is an very important concept.

A signal in an amplifier has to complete a circle just like the current in a DC circuit.
For example in this circuit:
[PLAIN]http://dl.dropbox.com/u/4222062/Signal%20paths.PNG
ignoring the DC currents, the signal current has to return to ground via the load resistor and the bypass capacitor.

If the capacitor was not there, the gain of the circuit would depend on the impedance of the power supply, which would be unpredictable. The output of some 3 terminal regulators would appear to be almost infinite impedance to a signal if they didn't have a bypass capacitor on the output.

Also, there might be several such circuits being powered by the same power supply, so the circuits would be coupled together by sharing the same load.

This leads to the possibility of feedback and oscillation between amplifiers. In audio amplifiers, it produces an effect called "motor-boating" which describes the resulting sound.

In VHF amplifiers, the quality of the bypass capacitors can affect the performance of the whole amplifier. Omitting them would be catastrophic. They are a vital link in the signal path of each amplifier.

In the diagram, I have given the pink circles a direction, but it is a direction that changes with the signal polarity.

So, always bypass your amplifiers as close to the cold end of the load as possible.

 

[sophiecentaur]

Don't we just call the 470uF power supply decoupling? Yes, I agree with all of the above but a shorthand name can be useful. :-)

 

[vk6kro]

Don't we just call the 470uF power supply decoupling? Yes, I agree with all of the above but a shorthand name can be useful. :-)

Yes, I did refer to that capacitor as a decoupling capacitor, but I thought I should mention the local function of making the amplifier work properly, as well.

It is not just there to clean up the cr@p that comes from the power supply. :)

I know people who routinely leave out all the bypass capacitors and then spend half a day trying to trouble-shoot the unstable mess that results. Then they do the same with the next project.

 

[UR_Correct]

Thank you so much, vk6kro! This helped a lot, and I appreciate you taking the time to respond. PF FTW!

 

[sophiecentaur]

Local decoupling is more for isolating sub circuits than interference suppression, I should say. Sloppy power rails are certainly a trap for the unwary constructor. Putting a unit in a copper box is not an end to your troubles by any means.