im having trouble getting an electret mic to behave properly

Thread Starter

abdelrahman k.

Joined Mar 16, 2024
15
hello, im building a microphone with preamp circuit and the audio has too much strange anomalies, this is the design im following:Screenshot 2024-03-06 025235.png
i replaced R6 with a potentiometer to troubleshoot easily and removed the LED part completely.
Now the anomalies i was talking about are that every time i power the circuit on and dial R6 to keep the noise down the mic signal becomes weak unless i speak very close to it, doing that triggers something that makes the noise take over completely and no voice signal can be heard at all.

Im using a mic capsule with no embedded transistor so i had to add my own and im guessing its the problem.
it seems like talking very closely to the mic makes the signal go over a certain threshold and the transistor just gives up (thats my theory)

the capsule im using is the jli-2555 and the transistor is 2n4416 which is a VHF JFET.
also i found that shorting the gate of the transistor with ground seems to "reset" the transistor.

can you guys suggest another transistor that i can replace the 4416 with to try to pinpoint the problem?
 
Last edited:

Ian0

Joined Aug 7, 2020
10,076
What is the voltage at the junction of R2 and C3?
Please redraw the circuit including your JFET and any biassing components.
My guess is that is is not correctly biassed.
 

Thread Starter

abdelrahman k.

Joined Mar 16, 2024
15
What is the voltage at the junction of R2 and C3?
Please redraw the circuit including your JFET and any biassing components.
My guess is that is is not correctly biassed.
i couldnt measure the voltage at the junction you mentioned, it kept fluctuating.
heres the drawing you requested:Untitled.png
 

Ian0

Joined Aug 7, 2020
10,076
You need a resistor between JFET gate and ground. It depends on the capacitance of the capsule, but the value should be somewhere around 100MΩ to 1GΩ.
If you don't have one to hand, use the largest value you have which will probably be 10MΩ. It will be good enough to get the bias sorted out, but might not have much bass response,
 

Thread Starter

abdelrahman k.

Joined Mar 16, 2024
15
You need a resistor between JFET gate and ground. It depends on the capacitance of the capsule, but the value should be somewhere around 100MΩ to 1GΩ.
If you don't have one to hand, use the largest value you have which will probably be 10MΩ. It will be good enough to get the bias sorted out, but might not have much bass response,
this is all the provided data in the datasheet of the capsule
 

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Thread Starter

abdelrahman k.

Joined Mar 16, 2024
15
You need a resistor between JFET gate and ground.
abut this part, i dug around and found a 3.5M and it definitely made things better, i'll buy a higher value and put it instead,
im
trying to eliminate hissing noise that appears only when i speak, what could be the source of that?
 

Ian0

Joined Aug 7, 2020
10,076
abut this part, i dug around and found a 3.5M and it definitely made things better, i'll buy a higher value and put it instead,
im
trying to eliminate hissing noise that appears only when i speak, what could be the source of that?
Now you've done that, you can measure the voltage at the junction of R2 and C3 and get the bias correct.
 

Alec_t

Joined Sep 17, 2013
14,384
You need a resistor between JFET gate and ground.
The 2N4416 is n-channel. Wouldn't a resistor normally go in series with the source, to reverse-bias the gate-source junction of the jfet?
The 2N4416 datasheet gives the Vgs cut-off threshold as in the -2.5V to -6V range. A jfet with that -6V could be an issue if the circuit supply is only +5V.
 
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Audioguru again

Joined Oct 21, 2019
6,765
The very old 2N4416 is obsolete and many conduct too much current when its gate resistor connects to ground. Most 2N4416 Jfets would need a very high gate resistor value connected to a negative voltage. Use an electret mic that has matched mic, Jfet and resistor. Many electret mic Jfets use a 10G ohms resistance to ground.

The old LM358 is never used as an audio preamp because it produces too much noise (rumble and hiss), produces crossover distortion and has a poor high frequency response and poor slew rate. Use an opamp rated for audio.

EDIT: The datasheet for the electret mic shows the Jfet as a Source-Follower (output from the source pin) for use in a very loud drum or piano (see Linkwitz electret mic modification in Google).
 
Last edited:

Ian0

Joined Aug 7, 2020
10,076
The 2N4416 is n-channel. Wouldn't a resistor normally go in series with the source, to reverse-bias the gate-source junction of the jfet?
The 2N4416 datasheet gives the Vgs cut-off threshold as in the -2.5V to -6V range. A jfet with that -6V could be an issue if the circuit supply is only +5V.
Yes, it would, but in order to determine the value of the source resistor, the gate needs a DC path to ground.
With the gate properly connected a few values of source resistor could be tried to get the drain voltage to mid-supply.
Vgs cutoff can vary from -2.5V to -6V, so perhaps the correct values is better determined by measurement than by calculation.
We are aiming for about 250uA drain current.
 

Ian0

Joined Aug 7, 2020
10,076
It's overbiassed. The transistor is fully switched on and the drain voltage is as low as it possibly can be.
You need to add resistance between the source and ground. That will make the gate voltage lower than the source voltage, which will start to turn off the JFET.
Try 1k to start with, and see if you can get the drain voltage reasonably close to mid-supply.
When you have done that connect a big capacitor across the resistor (say 100uF)
 
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