The 220nF cap will block DC and low frequency AC, but allow most audio frequencies to pass.

The 100k resistor is a potentiometer, it is a light load on the output of the mic, and allows controlling the level of input (as a volume control) to the amp.

Any capacitor will block dc but why use the 220nF and 100k ?

I understand that the fc = 1/2*pi*RC where fc = cut off frequency

From this I get 1/2*pi*220nF *100k = 7.23431554 hz
unless I calculated something wrong. Would this mean that it blocks all frequencies lower then 7.23 ...etc hz
If that is the reason why not get the RC combo close to 20hz that would filter more noise out I would think?
Unless I am missing something?

For example, the 470uf output capacitor into an 8 ohm speaker will produce a response that is -3dB (0.707V) at 42.5Hz.

using the above formula
I get fc = 1/2*pi*RC = 1/2*pi*470uf *8ohm = 42.328442 ...etc hz
so this would mean it cuts off any ac that is under this frequency.

If I am correct in all my calculations then these both are a form of high pass filters where everything over the cut off fc frequency is pass thru and everything under it is blocked out.

question -3dB (0.707V) how did you come up with this I can only calculate the fc why/where is the -3dB (0.707V) coming from?
Or the fc - 3dB in your formula???

Also 0.047uf and 10ohms is this just another filter to block stuff if
I calculate 1/2*pi*0.047uf * 10ohm = 338627.5384933943 hz =
approx 338.6 khz so it would block any frequency below this and let all frequencies higher then this go thru. But the voice is only in the range of 20hz - 20khz at most so I don't know how useful this R C is ?
Maybe it is the combination that does something or maybe I am missing some reason??? Either way I will look into the Boucherot cell. (my their lies the reason)

thanks

 

Any capacitor will block dc but why use the 220nF and 100k ?

I understand that the fc = 1/2*pi*RC where fc = cut off frequency

From this I get 1/2*pi*220nF *100k = 7.23431554 hz
unless I calculated something wrong. Would this mean that it blocks all frequencies lower then 7.23 ...etc hz
If that is the reason why not get the RC combo close to 20hz that would filter more noise out I would think?
Unless I am missing something?

Be my guest. Set the cutoff frequency to anything you like.

using the above formula
I get fc = 1/2*pi*RC = 1/2*pi*470uf *8ohm = 42.328442 ...etc hz
so this would mean it cuts off any ac that is under this frequency.

If I am correct in all my calculations then these both are a form of high pass filters where everything over the cut off fc frequency is pass thru and everything under it is blocked out.

Why don't you create a plot of the high pass filter?

question -3dB (0.707V) how did you come up with this I can only calculate the fc why/where is the -3dB (0.707V) coming from?
Or the fc - 3dB in your formula???

-3dB is a standard for determining the edges of a highpass/lowpass/bandpass filter. It's the half-power point.

Also 0.047uf and 10ohms is this just another filter to block stuff if
I calculate 1/2*pi*0.047uf * 10ohm = 338627.5384933943 hz =
approx 338.6 khz so it would block any frequency below this and let all frequencies higher then this go thru. But the voice is only in the range of 20hz - 20khz at most so I don't know how useful this R C is ?
Maybe it is the combination that does something or maybe I am missing some reason??? Either way I will look into the Boucherot cell. (my their lies the reason)

It's a Boucherot cell.
I already gave you a link to where you could find out about it.
It's in the datasheet for the LM386.

Why do you feel compelled to ask the same doggone questions over and over again?

Do you enjoy wasting our time?

 

Why do you feel compelled to ask the same doggone questions over and over again?

Do you enjoy wasting our time?

I am not. I am just get mixed answers or not full understanding some of what you are getting at.

I am not trying to waste your time. I really appreciate the help.

So for the 42.328442 ...etc hz won't it be just as good if I used a different capacitor then something that would reduce the 42hz down to a 20hz since are hearing range is between 20hz-20khz.
-3dB (0.707V) you said this is some type of center but what the hell does that mean since the cut off frequency means that it will let everything in above and keep out everything below this frequency.
Where is this -3db is that the error or fluctation that the cut off frequency has so some waves below may creep in at some times ???

And where did the 0.707V come from????


Also forgetting all this for a second it seems to me the whole amplifier circuit could be reduced to just using a mic. mic protection resistor , LM386 chip , gain capacitor 10uF (for 200 times) , and the output capacitors/resistors/speaker.

All the other stuff is for filtering out stuff. So in theory I should beable to hear my voice with alot of noise if I just didn't use the filter stuff?
I haven't tried it yet or even built the circuit yet but I got all the parts and tommarrow I am going to test this theory out.

One last minute question does the electric mic produce ac or dc changes in voltage/current when spoken into? I would think ac since the coils in the mic move back and forth over the magnet so you get both current/voltage moving in each direction depending on the what side of the magnet it is moving to right or left ,...etc

If this is true then the louder the sound ( = greater sound pressure) the greater the change in magnetic field which induces greater voltage/current in the coiled wire.

If I am correct with this then my ownly last question to end this thread for good (unless your answer opens up more questions)

Would be Boucherot cell is this nessary for the amplifier to work.
Because I would think this is just another filter.
Since the mic creates the ac sound wave and the gain cap magnifies it 200 times then the wave is sent to the output pin of the LM386 which then sends the amplified sound wave to the speaker.


So the whole circuit would be reduced down further to the bare min

whole amplifier circuit could be reduced to just using a mic. mic protection resistor , LM386 chip , gain capacitor 10uF (for 200 times) , and the output capacitors/resistors/speaker.

without the output capacitors/resistors/speaker just the speaker

Thanks you very much tommarrow is playing around with the circuits etc time

 

I understand that the fc = 1/2*pi*RC where fc = cut off frequency

From this I get 1/2*pi*220nF *100k = 7.23431554 hz
unless I calculated something wrong. Would this mean that it blocks all frequencies lower then 7.23 ...etc hz
If that is the reason why not get the RC combo close to 20hz that would filter more noise out I would think?
Unless I am missing something?

The 100k volume control is in parallel with the 50k input resistance of the LM386 amplifier. So ther total resistance is 33.3k ohms. The response is flat down to about 109Hz. The -3db cutoff frequency is 21.8Hz.

Lower frequencies are not blocked, they are reduced. 10.9Hz is at -6dB. 5.45Hz is at -12dB. 2.7Hz is at -18dB, etc.

(for the output capacitor) I get fc = 1/2*pi*RC = 1/2*pi*470uf *8ohm = 42.328442 ...etc hz
so this would mean it cuts off any ac that is under this frequency.

The output response is flat down to about 212Hz. 42.3Hz is at -3dB. 21.2Hz is at -6db. 10.6hz is at -12db, etc.

If I am correct in all my calculations then these both are a form of high pass filters where everything over the cut off fc frequency is pass thru and everything under it is blocked out.

They are very gradual filters so they just reduce the "blocked" frequencies. Both filters add their reductions.

question -3dB (0.707V) how did you come up with this I can only calculate the fc why/where is the -3dB (0.707V) coming from?
Or the fc - 3dB in your formula???

The formula has been around for a long time. -3dB is 0.707 times the AC voltage which is half the power. -3dB is a small noticeable reduction in audio volume. Most people cannot notice -1dB.

Also 0.047uf and 10ohms is this just another filter to block stuff if
I calculate 1/2*pi*0.047uf * 10ohm = 338627.5384933943 hz =
approx 338.6 khz so it would block any frequency below this and let all frequencies higher then this go thru. But the voice is only in the range of 20hz - 20khz at most so I don't know how useful this R C is ?

It is not a filter. It is a 10 ohms load at high frequencies where the inductance of a speaker causes it to be a high impedance. It prevents the IC from oscillating at a very high frequency.

 

It is not a filter. It is a 10 ohms load at high frequencies where the inductance of a speaker causes it to be a high impedance. It prevents the IC from oscillating at a very high frequency.

So the 0.047uF , 10ohm resistor are just to stop the LM386 chip from ossilating at very high frequencies. But if I took this out would it hurt the internals of the chip in anyway. (would high frequencies hurt the chip internals?) Because I would think this is not nessary if you just wanted to here your voice with alot of noise.

Lower frequencies are not blocked, they are reduced. 10.9Hz is at -6dB. 5.45Hz is at -12dB. 2.7Hz is at -18dB, etc.

so when I compute the fc in my formula it means at this frequency their is a reduction of 3db in the volume or strength at this frequency. If true then

Is this a significant amount I just don't have a frame of reference what is 0db , 1db , 2db ,...etc sound like. How high can the human ear take in increased db before it hurts or blows your ear drum? (maybe each frequency is different in the range of db it can contain? )

Same question but how much does it take decreasing db so your ear cann't hear it anymore ?

If 0.707V is half the ac I would think decreasing the sound by 3db will make the sound half as loud. Going by what you said.

Like I said if it is not to protect the inner circuitary of the LM386 chip
or protect the mic or add max gain 10uf 200X. Then it is not really need you would just here alot of noise when talking into the mic. So alot of the circuitary is unnessary provided you want a crap load of noise with your voice.
I will test this theory in a few hours.

Curious to know about the db sound question though?
And in my previous post does the mic create pulsating dc or ac sine waves when talking into it. I think ac if you read the last part of my post. If this is true then the capacitors are in noway creating the ac wave for the speaker the mic does that and all your doing with the capacitors/resistors are filtering , gain and protection. So in theory you would never need an amplifier chip or circuit if the mic only could produce an ac sinewave with enough voltage/current (power strength) to drive a speaker directly. Wondering if they sell mic's that can do it for an 8ohm 2watt speaker? Maybe these is where somebody mentioned crystal , cermic mic's or something dielectric mic don't know what ones are more sensitive or stronger then others though?

Thanks, all set with understanding the circuit and putting it together.

 

So the 0.047uF , 10ohm resistor are just to stop the LM386 chip from ossilating at very high frequencies. But if I took this out would it hurt the internals of the chip in anyway. (would high frequencies hurt the chip internals?) Because I would think this is not nessary if you just wanted to here your voice with alot of noise.

Yes, the LM386 might overheat and be destroyed if it oscillates at a high frequency. The battery will also be destroyed.

so when I compute the fc in my formula it means at this frequency their is a reduction of 3db in the volume or strength at this frequency. If true then

Is this a significant amount I just don't have a frame of reference what is 0db , 1db , 2db ,...etc sound like.

-3db is a noticeable small reduction in volume.

Same question but how much does it take decreasing db so your ear cann't hear it anymore ?

It depends how loud it was in the beginning.
A little LM386 amplifier is not very loud. If its sound was reduced 60dB then it would not be heard in a quiet room.

If 0.707V is half the ac I would think decreasing the sound by 3db will make the sound half as loud.

No.
-3dB is half the output power. its output voltage is 0.707 times. It is a noticeable small reduction in volume. Since your hearing's response to loudness is logarithmic then 1/10th the power sounds half as loud. 10 times the power sounds twice as loud.

Like I said if it is not to protect the inner circuitary of the LM386 chip
or protect the mic or add max gain 10uf 200X. Then it is not really need you would just here alot of noise when talking into the mic. So alot of the circuitary is unnessary provided you want a crap load of noise with your voice.
I will test this theory in a few hours.

You should make the amplifier as shown by me and as shown in its datasheet.

And in my previous post does the mic create pulsating dc or ac sine waves when talking into it?
I think ac if you read the last part of my post. If this is true then the capacitors are in noway creating the ac wave for the speaker the mic does that and all your doing with the capacitors/resistors are filtering , gain and protection.

All the capacitors are important. The amplifier will not work properly without them.

So in theory you would never need an amplifier chip or circuit if the mic only could produce an ac sinewave with enough voltage/current (power strength) to drive a speaker directly. Wondering if they sell mic's that can do it for an 8ohm 2watt speaker?

No.
A microphone has an output with a very low voltage and a very low current. An amplifier is needed to drive a speaker.

 

Thanks, got you on everything works pretty good.

No.
A microphone has an output with a very low voltage and a very low current. An amplifier is needed to drive a speaker.

Yes got you on this.

But the mic is creating the AC sine wave correct( at this point we have are voice converted into the equivalent electrical ac sound wave)? The only reason we cann't just have the mic directly connected to the speaker is because the volt/amps are to low to drive the speaker. But if their was a mic that had the correct volt/amps to drive a speaker then we wouldn't need an amplifier. Correct?

Also can you use speakers and microphones interchangable. I believe they work on the same principle like a coil of wire around a magnet moving induces a electric current in a wire. And a change in electric current in the coil of wire across the magnet changes the magnetic field which drives the speaker back and forth.

So in principle one could use the other interchangeable. Unless of course it is the sensitivity of the speaker that isn't going to match up with mic's preventing this interchangablity ? (i.e to much resistance in the coil wires of the speaker to make it much smaller then standard mic range you would really need an amplifier more powerful I guess if you used the speaker as a mic maybe you would need a preamplifier before the amplifer or something. ....)

Similarly i don't think you can use a mic as a speaker because it won't create enough sound pressure (but maybe the other way around will work speaker used for mic though)

Thanks this should probably be in a different thread ok no more

 

But the mic is creating the AC sine wave correct( at this point we have are voice converted into the equivalent electrical ac sound wave)? The only reason we cann't just have the mic directly connected to the speaker is because the volt/amps are to low to drive the speaker. But if their was a mic that had the correct volt/amps to drive a speaker then we wouldn't need an amplifier. Correct?

Correct, but who wants to carry around a microphone that's hot and weighs 20 lbs or more?

You have to realize that an LM386 is a very small and low-power amplifier. A public address system may be capable of supplying hundreds of Watts of power.

Also can you use speakers and microphones interchangable.

Sort of. Dynamic microphones are at a basic level like very small speakers. Piezo microphones are vaguely related to electrostatic speakers.

So in principle one could use the other interchangeable.

A dynamic mic is so small and the voice coil is of such small gauge wire that if you tried to use it for a speaker, you would likely burn it up in very short order.

Unless of course it is the sensitivity of the speaker that isn't going to match up with mic's preventing this interchangeability?

A loudspeaker is designed to be driven by a good bit of power supplied to it.
A microphone is designed to generate a small AC signal that needs to be amplified to drive a speaker. Microphones are very sensitive out of necessity; as speakers are relatively massive and rugged out of necessity.

 

Well I tried using my speaker as a mic in my computers soundcard mic port in. Worked fine.

Then I tried to use my computers mic in my speaker port of the sound card and play music thru it. It worked but I had to jam the mic so far into my ear just to hear a very low volume song. But it worked.

So in theory these 2 devices can be interchanged I would think?

However I would think the only useful way would be to interchange a mic with a speaker in the mic port in.

But the other way around just doesn't really work because the volume will be much to low for usefullness thru the mic. Plus the fact in some cases it may damage the mic like you said. <-- Didn't really think of this hopefully the mic will be ok it is still working now so I don't think it harmed it much?

That is another reason why subing a mic for a speaker is not really a good chioce. Though the other way around seems to work quite well unless I am over looking something???

 

Well I tried using my speaker as a mic in my computers soundcard mic port in. Worked fine.

Then I tried to use my computers mic in my speaker port of the sound card and play music thru it. It worked but I had to jam the mic so far into my ear just to hear a very low volume song. But it worked.

So in theory these 2 devices can be interchanged I would think?

However I would think the only useful way would be to interchange a mic with a speaker in the mic port in.

But the other way around just doesn't really work because the volume will be much to low for usefullness thru the mic. Plus the fact in some cases it may damage the mic like you said. <-- Didn't really think of this hopefully the mic will be ok it is still working now so I don't think it harmed it much?

That is another reason why subing a mic for a speaker is not really a good chioce. Though the other way around seems to work quite well unless I am over looking something???

You can use a hammer to install screws, too - but you'll get better results if you use a drill and a screwdriver.

When a hammer is only tool you know how to use, everything starts looking more or less like a nail.

A microphone is the proper tool to use for converting acoustical energy (sound waves) to electrical energy.

A loudspeaker is the proper tool to use for converting electrical energy into acoustical energy.

You might use one for the other, but your results won't be as good.

 

Got you.

Anyway I built the schematic by Audioguru but the only thing is it works fine.
However occasionally when I move the solderless breadboard or the speaker I get squeelying noise very loud and very unpleaseant sound.

Other then that everything is fine the voice is pretty loud even though the amplifier isn't the highest quality.

I am wondering how to remedy this and what is causeing it. I beleive it could be the mic maybe. I tried different speakers and all of them have it. So it is not a broken speaker. I replace the LM386 chip with a new one so it's not that. I am using all resistors at 1/2 watt if that makes any difference shouldn't though.
The capacitors/resistors are all the correct sizes and are in properly (polarized correctly)

The battery is brand new 9volt.
The only thing I had to sub for is the 100k log potientometor I didn't have it so I just used a fixed 56k resistor. I could try different values if that is the problem don't think so though?

The only other thing if it isn't the mic would be this
When I subed in the 56k resistor for the 100k pot should one side of the resistor go to ground?
Since I have the cap/resistor in series going into pin3 maybe they should be in parrellel like the digram looks like?
Wasn't sure if that was just the symbol for the pot or does the resistor go to ground and the cap/resistor in parrell?

I will try this see if it fixes it.
Either way the higher the resistance of your pot does that mean the higher or lower the volume? I would think higher when resistance is high because you have more of the current flowing into the pin3. If the resistance is low then path of least resistance say's that most of the current will go to ground quicker then it will go to the pin3 make the sound lower.

Thanks

Maybe it is just the fact that I am doing it on a solderless breadboard and didn't trim any of the capacitors/resistors legs. Or maybe the mic is to close to on of the cap legs. Or stray inductance/capacitance is occuring?

Update

No , forget what I said about cap/resistor in series or parrell it still has it either way though I would like to know which way it was meant?
This could only mean the squeely noise is coming from the mic or because the mic is to close to a capacitors leg or the legs are giving to much stray capacitance off ??? Either way if this is the problem then will it go away when I trim the legs and solder them on to a PCB board? <-(moving away from the solderless boards )

 

Get a big hammer, and give the breadboard a few good whacks.

That should stop that infernal squealing.

 

The squeely noise is acoustical feedback howling and is normal because your mic and speaker are too close together.

When the mic can hear the speaker then it picks up the sound and amplifies it in the amplifier. Then the same sound comes out of the speaker louder and is picked up and amplified over and over which makes a squealing sound.

Separate the mic and speaker, point them in different directions, turn down the volume control or put the mic in one closed room and the speaker in a different closed room.

 

You said your mic has a coil of wire and a magnet, just like a small speaker?
No, not if it is an electret mic (not electric mic) that is completely different. Look at electret mic in google. You described a dynamic mic.