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Why doesn't this preamp for electret mic work.

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yo2550

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View attachment 66123

I modified my ham earpiece by adding this preamp for the microphone(earpiece). It worked at the beginning for a few times but after that it did not work. I got some noise when I pressed the mic button of the earpiece without my voice.

I use 9V battery. I doubt that the voltage supply for the base is sufficient or over the limit of the 2N3904 but I'm not sure. I tried two of the 2N3904 and got the same result. I tried using 100k trimmer for base but it nothing is better. I do not want to waste, if it's burnt, them any more.

Please advise me how to make this circuit work.

Thanks.
 
It's not a great circuit, and lacks the essential decoupling capacitor across the supply - but it should still work.

If it doesn't you've either got a faulty part somewhere, or you've not connected it correctly.

Try measuring the voltages on the base and collector of the transistor, and the top of the microphone.
 
ok I think I use electrolytic capacitors which are not good for analog signal.
I have 6pf, 15pf, 30pf ceramic c. How can I put these in the circuit?
 
ok I think I use electrolytic capacitors which are not good for analog signal.
I have 6pf, 15pf, 30pf ceramic c. How can I put these in the circuit?

You can't, they are FAR too small, but electrolytics are perfectly fine for audio, despite what a few supposed audiophiles will tell you :D

But 0.1uF electrolytics are fairly rare anyway?.

Have you measured the voltages yet?.
 
It's not a great circuit, and lacks the essential decoupling capacitor across the supply - but it should still work.

If it doesn't you've either got a faulty part somewhere, or you've not connected it correctly.

Try measuring the voltages on the base and collector of the transistor, and the top of the microphone.

He said it worked in the beginning so it must have been connected properly at one time.

Have you tried another battery?
 
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A loose connection maybe?
 
View attachment 66127
You can buy them online.

The voltages while the transistor is on are as these: Vbe = 0.66v., Vce = 1.15v., Vcb = 0.44v.

The battery voltage now is around 8.4-8.6v.
 
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The electrolytic capacitor I use is polarized. Should I connect them back to back to form a bipolar capacitor then?
 
View attachment 66127
You can buy them online.

I know you can get them, but they are pretty rare - because they aren't any smaller than conventional capacitors of the same value.

If you've using those, fit them positive to the mike, and positive to the collector. You should also have a larger electrolytic (10uF upwards) across the supply rails.

The voltages while the transistor is on are as these: Vbe = 0.66v., Vce = 1.15v., Vcb = 0.44v.

The battery voltage now is around 8.4-8.6v.

Vce is rather low (for a 9V battery), increase the value of the 100K to make it higher, try 220K, 390K and 470K - it will work with 100K, but it means the output voltage is limited.
 
I tried adding another 100k resistor in series and I noticed less sensitivity of the mic. I removed it and use only 100k resistor and I hear louder sound from another ham.

Any advice to bring more gain or increase the volume of the signal?
 
Increasing the value of R2 will result in more gain. You seem to be experiencing the opposite? Increasing the value of either capacitor will result in more gain also. If you are experiencing less sensitivity with more gain it means the preamplifier is being overdriven by the mic, or the preamplifier has too much gain, and it is saturating, or the preamplifier is overdriving the following stage. The output will sound distorted.

What do you have connected to the output of the preamp exactly? In other words, what is the load's impedance?
 
Last try is I added a resistor and it works but got pretty low output sound. I'm not sure if the gain is too high or too low for the next stage.

These are the schematic and technical description of the radio.

**broken link removed**

**broken link removed**
 
More questions: Is the electret mic polarized? Does it really need a voltage supply when it's working?
 
These are the schematic and technical description of the radio.

If you are connecting to Jack J9-B, you probably don't need a preamp and can hook up your mic directly. There is already an internal mic for the radio that gets switched out when you insert a plug into that jack.

Edit: And if you still decide to use a preamp at that jack, you must cut the gain way down or you will overdrive the input at the radio.
 
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If you are connecting to Jack J9-B, you probably don't need a preamp and can hook up your mic directly. There is already an internal mic for the radio that gets switched out when you insert a plug into that jack.

Edit: And if you still decide to use a preamp at that jack, you must cut the gain way down or you will overdrive the input at the radio.

Yes. The J9-B is the mic jack. I tried connecting it directly but the sensitivity of the mic is pretty low for me. I need to speak too close to the mic to get a desirable level of sound. It could be that the electret mic has a bit high impedance for the circuit.

I will try to cut the gain so it won't overdrive the input of the mic circuit.
 
From what I recall, most electret mics have a fairly low output impedence, which is good. Check the specs on your mic to find out. It could be, too, that the voltage delivered to your mic by the radio is not correct for your mic. You can use a voltmeter, and with a plug inserted into the jack, measure the DC voltage coming out of the jack. Check that voltage against the voltage range required for your mic. If you still use your preamp, try cutting R2 down to 10K or less, maybe put a 10K pot in there to get the gain right.

It looks like you have to draw current from the power supplied at that jack to activate the transmitter (PTT), perhaps by putting a 2K resistor across the input in parallel with the mic with a pushbutton to switch the resistor in and out. I would expect that would be shown in the users manual for the radio. How are you planning to activate the transmitter? Maybe by enabling the VOX feature?
 
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More info: The inside(radio) mic is 2k ohm condenser. Does this help me in calculation of the gain and impedance of the earpiece?
 
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