I am working on a DIY Project from this YouTube video. I have bought the JLI2555 microphone capsule but I haven't managed to get it to work.

I connected a JFET with the gate of the capsule and source of the JFET to ground. I also put a resistor at the gate of the JFET and a negative DC voltage to bias it (circuit is below).

enter image description here

I would expect that when I measure the output with a voltmeter there would be some mV fluctuations as I yell into the microphone, but nothing seems to happen.

My questions are as follows:
Am I doing something wrong? Does the capsule really require a JFET or is it already built in? Does the capsule require external power? In the datasheet it says it works with an operating voltage of 1 V, but the YouTube guy says it doesn't require any voltage to operate.


1 Answer 1


According to data sheet, at least a JLI-2555BXZ3-GP has no FET built-in and it connects the FET differently. So based on that you did not connect it according to examples.

Also youtube guys may not have a clue what they are doing, so you can never be sure blindly believing what the videos say, you always need to confirm if something is correct or not.

So since the circuit has a FET it obviously needs power from somewhere to work, so your video is wrong. It needs power.

However, depending on where you connect it (e.g. mic input of mobile phone or PC) it may already provide power to the circuit.

  • \$\begingroup\$ Thanks for the answer. I am still not sure how would one go about and connect the capsel to be able to at least see it working? If I connect the jfet to the capsulenand provide a battery as a voltage source should I be able to see voltage fluctuations with a voltmwter if I talk into the mic? \$\endgroup\$
    – LEXOR AI
    Commented Apr 3, 2023 at 21:26
  • \$\begingroup\$ The fluctuations will be very small. It might be best to just attach the FET and plug it into phone/PC mic input. Mic inputs already support FET-amplified electret capsules, i.e. provide resistors and supplies and DC blocking audio input. \$\endgroup\$
    – Justme
    Commented Apr 3, 2023 at 21:31
  • \$\begingroup\$ I was planning on making an amplifier ciruit after the FET and then connect it to a mic input. I am just interested if I set the circuit up in a correct manner? So you connect the gate to te mic and the supply the JFET with 9 volts and a DC Bias and then it should work? Or is this not correct? Should I just follow this datasheet circuit on the internet? 1.5 Volts seems to small for JFET Vds voltage for example as stated in the datasheet? \$\endgroup\$
    – LEXOR AI
    Commented Apr 3, 2023 at 21:36
  • \$\begingroup\$ You don't need an amplifier because mic inputs are made for mic level signals. You need the capsule and FET buffer, zero other components, just wire it up to mic input \$\endgroup\$
    – Justme
    Commented Apr 3, 2023 at 21:40
  • 1
    \$\begingroup\$ Your 100 ohms resistor should be 2.2k to 10k depending on the specs of your Jfet. The measured drain DC voltage should be half of your 9V supply voltage. Then the output level will be about 0.01VAC when you talk near the mic. \$\endgroup\$
    – Audioguru
    Commented Apr 4, 2023 at 1:06

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