Hi Everyone
After my Project with the Phantom Powered Big Condenser Capsule and Balanced Output I started a new Project for a similar Microphone Preamp for a Large-Diaphragm Condenser Microphone.
But I got stuck with some noise in the Audiosignal after I connect the Condenser Capsule.
It's not the GND-Lift Noise, this I've already fixed.
My Problem is that, after I connect the Large-Diaphragm Condenser Capsule I have some Noise between about 160 to 2500 Hz.
If I disconnect the Capsule it is silent. I tried two different Condenser Capsules with the same Result.
I also replaced the LSK 489 B JFET with the one of my other Mic, where it worked silent, with the same Result.
I changed also the 1 Giga-Ohm Resistor with another an the other Components with the same Result.
There must be an fault in the circuit, but I don't have an Idea to fix that...
The Q-Point and VGSOff I got from the Datasheet:
The Equation for RD and RS:
RD: (V+ - (Volt Q-Point + (VGS(OFF)*-1))) / IDSS (A)
(48-(9+2.2)) / 0.0075 = 4906 Ohm
RS: (VGS(OFF)*-1) / IDSS (A)
2.2 / 0.0075 = 293 Ohm
I Uploaded on my Webserver an Example-Wav of the Noise: https://www.netcoast.ch/Noise.wav
I Produce the 48 Volts with a Boost-Converter (5 Volt to 48 Volt), but I also tested it with the Phantom Power from my sound mixer with the same result:
Thank you for your Help!
Best Regards
Lukas
After my Project with the Phantom Powered Big Condenser Capsule and Balanced Output I started a new Project for a similar Microphone Preamp for a Large-Diaphragm Condenser Microphone.
But I got stuck with some noise in the Audiosignal after I connect the Condenser Capsule.
It's not the GND-Lift Noise, this I've already fixed.
My Problem is that, after I connect the Large-Diaphragm Condenser Capsule I have some Noise between about 160 to 2500 Hz.
If I disconnect the Capsule it is silent. I tried two different Condenser Capsules with the same Result.
I also replaced the LSK 489 B JFET with the one of my other Mic, where it worked silent, with the same Result.
I changed also the 1 Giga-Ohm Resistor with another an the other Components with the same Result.
There must be an fault in the circuit, but I don't have an Idea to fix that...
The Q-Point and VGSOff I got from the Datasheet:
The Equation for RD and RS:
RD: (V+ - (Volt Q-Point + (VGS(OFF)*-1))) / IDSS (A)
(48-(9+2.2)) / 0.0075 = 4906 Ohm
RS: (VGS(OFF)*-1) / IDSS (A)
2.2 / 0.0075 = 293 Ohm
I Uploaded on my Webserver an Example-Wav of the Noise: https://www.netcoast.ch/Noise.wav
I Produce the 48 Volts with a Boost-Converter (5 Volt to 48 Volt), but I also tested it with the Phantom Power from my sound mixer with the same result:
Thank you for your Help!
Best Regards
Lukas
You might want to use a regulated low voltage rail for the FET, say 10V, rather than drive its drain direct from an un-decoupled 48V rail. 48V is good for biasing the capsule, but JFETs are best run at a lower voltage to reduce the possibility of impact ionization (IIRC). Using a zener with plenty of decoupling for a 10V rail perhaps, possibly then an RC filter before the drain resistor. But its just a guess.