# Is my microphone capsule dead?

I am using a JLI2555 microphone capsule. I tried making a circuit to see if the microphone capsule works. The goal was to measure voltage fluctuations at the output with a voltmeter as I yell into the capsule.

For this I used an op amp follower to match the output impedance of the capsule. As I yelled into the capsule, there were no voltage fluctuations at the output. (I am aware that a JFET should be used; I tried it with a JFET, and it also did not work; the JFET might be faulty though, but this is a different story).

My question is essentially — is my microphone capsule dead? It shows no signs of working whatsoever. Two 9 V batteries are used and a 1 MΩ resistor.

• What range was your multimeter set to? Commented Apr 8, 2023 at 16:28
• I tried AC 200V and DC 200mV. Commented Apr 8, 2023 at 16:29
• I suspect that it may be working then. Commented Apr 8, 2023 at 16:45
• No, it's because you are measuring DC and your multimeter will naturally have a slow response. That's why I asked you to say a word that begins with "p" because saying it naturally forces a column of air into the microphone and, that column of air is very low in frequency. Hold your hand an inch away from your mouth and say push, bush, dush and you will have the three most onerous consonant sounds that plague recording engineers. Commented Apr 8, 2023 at 16:54
• I think its working so I will mark your anwser as accepted. Commented Apr 8, 2023 at 17:05

OK, try saying the word "push" in front of the capsule and see if the flow of air from the "p" registers at DC. – Andy aka

The voltage on the meter keeps fluctuating. When I say push it does seem to go up a bit but I am not completely certain. it goes from 1.8mV to about 4 mV

I suspect that it may be working then. – Andy aka

Ok. After yelling push into it it does go up to about 100mV. The only question I have remaining is why does it slowly fall from 100mV to 0mV when I stop yelling? Is it because the voltage follower needs time for the negative feedback to bring it back? (What I am wondering is why does it not happen instantaneously)

No, it's because you are measuring DC and your multimeter will naturally have a slow response. That's why I asked you to say a word that begins with "p" because saying it naturally forces a column of air into the microphone and, that column of air is very low in frequency. Hold your hand an inch away from your mouth and say push, bush, dush and you will have the three most onerous consonant sounds that plague recording engineers.

I think its working so I will mark your answer as accepted.

The capsule is probably ok. The circuit shown in the OP will not work. This is an electret microphone that requires an external FET to function as shown in the data sheet. The electret charged with a sufficient voltage to drive the gate beyond $$\V_{GSth}\$$. So choice of gate threshold voltage is important. The 1.5 volt drain voltage is very low but can work.

The data sheet is very poor. For instance the Zout is speified at 2.2k. I suspect that this is the output of the impedance converter.

The data sheet makes no recommendation of what FET to use.

The triangle is a weird representation of an FET being used as a source follower.

EDIT, from the comments: The 2N4416 is provided. This video may help.

simulate this circuit – Schematic created using CircuitLab

• The FET can also use an OP amp though? Commented Apr 8, 2023 at 22:12
• They are using here a J-FET 2N4416 very close to the capsule as pre amplifier. youtube.com/watch?v=LoQu3XXIayc Commented Apr 8, 2023 at 22:14
• The output level of the Jfet is at mic level. Then your opamp must be a preamp and have a gain of between 50 and 200 times, not a follower. Commented Apr 8, 2023 at 23:49
• @Audioguru, So true, but without the specified JFET circuit, you might not get mic level. Commented Apr 8, 2023 at 23:57
• When its datasheet recommends the Jfet circuit then you should use it. This circuit has a Jfet follower with a gain of 1 for very high levels. Ordinary electret mics have an internal Jfet configured as common source with gain. It causes distortion at high levels but can be modified as a follower for lower output level and much lower distortion. Commented Apr 10, 2023 at 2:09