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I have a JLI2555 microphone capsule connected to a JFET Transistor that is then connected to a sound card that goes into the usb port of my laptop. The idea is that I want to record my voice using the microphone. The capsule and the JFET are powered by the sound card through a TRS Plug.

The circuit looks as the above image enter image description here

The only difference is that the output is at V+ and there is no capacitor.

The TRS plug is connected to the JFET part of the circuit in the following way

enter image description here (The op amp represents the internal circuitry of the sound card)

The voltage of the sound card mic input has a voltage of about 2.85 volts. It is constant when I measure it with my multimeter, but when I connect it to the JFET and capsule it randomly drops to like 0.14 volts. It rises again later and drops again. When I speak into the mic the result is that I hear a buzzing noise and barely hear my own voice if at all.

Why does the voltage drops like this? Is it that the mic port does not have enough power to power a mic capsule like this?

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  • \$\begingroup\$ So you have the capsule, and additional JFET? Or you are just using the internal JFET in the capsule? Is the capsule connected correctly to TRS plug? It is unclear what you really have done to connect the items together, versus the diagrams that show how they should be wired together. \$\endgroup\$
    – Justme
    Commented Sep 22, 2023 at 19:27
  • \$\begingroup\$ The capsule is connected to the JFET because it does not have an internal JFET built in. The difference between my design and the first image is that the output is at V+ not the capacitor(I have no capacitor) (This is the place where the JFET is connected to the first pin of the TRS Plug). The ground of the JFET is connected to the last pin (ground) of the TRS PLug. The TRS Plug is then connected to the sound card that is then connected to the USB port \$\endgroup\$
    – LEXOR AI
    Commented Sep 22, 2023 at 19:38
  • \$\begingroup\$ Please show a JLI2555 datasheet that does not have an internal JFET. \$\endgroup\$
    – Justme
    Commented Sep 22, 2023 at 19:40
  • \$\begingroup\$ jlielectronics.com/content/JLI-2555BXZ3-GP.pdf \$\endgroup\$
    – LEXOR AI
    Commented Sep 22, 2023 at 19:41
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    \$\begingroup\$ Weird, the data sheet seems to give all the specs for the whole circuit as if the FET and everything was included like on normal electret capsules. But then the next question is, which JFET you used and how you connected it. Maybe the JFET is connected wrong? \$\endgroup\$
    – Justme
    Commented Sep 22, 2023 at 19:59

2 Answers 2

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I think you are using a wrong JFET for the job. That JFET is used for VHF radiofrequency signals and has current rating of several milliamps at zero Vgs.

Normally electret mics with built-in JFETs are listed around 0.5mA bias current.

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  • \$\begingroup\$ Do you know any such JFETS all i find are in the mA range? \$\endgroup\$
    – LEXOR AI
    Commented Sep 22, 2023 at 20:46
  • \$\begingroup\$ All JFETS have more than 0.5mA \$\endgroup\$
    – LEXOR AI
    Commented Sep 22, 2023 at 21:39
  • \$\begingroup\$ @LEXORAI Difficult, but MMBF5484 with additional 1 kohm between S and GND has a chance. \$\endgroup\$
    – Jens
    Commented Sep 22, 2023 at 23:14
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The circuit diagrams in the OP will not work. The DIY-Perks video shows that an external FET is required as does the data sheet for the JLI2555. The difference between the two is the DIY-PeRKs is a has a differential output, but the JLI2555 datasheet version is single ended.

DIY-PerksJLI2555 datasheet

The FET used in the video is obsolete, but the 2N4416A is still available. There are suggested substitutes on DigiKey. DigiKey offers many JFETs that may function for your application. I haven't the time to look closer, but don't rush in. Study prospective datasheets an make an informed choice.

The TRS connection in the OP will not work. Follow the electronics and make an appropraite choice.

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