FET biasing in condenser microphones [video]

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MagnetoSound

MagnetoSound

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Joined
May 12, 2008
Messages
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Location
Southern England
Apologies if this has been posted before (I did a quick forum search and it didn't come up), but I just stumbled on this rather cool video on FET biasing for condenser mics, courtesy of a fellow called Karl Adams. Perhaps some of you are familiar with him, I hadn't heard of him before.

The guy has some nice hardware, the analyzer in particular, but even if you don't have the same gear you can follow the method, and most of us can afford software analyzers so hopefully this video will at least help out the beginners among us.

https://m.youtube.com/watch?v=SHD4Rrwqy_Q


[EDIT: perhaps I should have put this in The Lab. Mods please move it if you feel it is appropriate. Thanks.]

 
It might be my mistake, but I think I hear him say:
"Okay, so I've dug up a seventy three ohm resistor and popped it in, in place of that trimpot". (35:21)
 
analyzers are expensive, getting one that goes down as low as his, are super expensive,
Drool......nice video,  thank you.
 
Well, to be fair, you could achieve pretty much the same thing (at least in this particular case / application) with a cheap little 1-2 channel interface and a spectrum analyzer plugin in your DAW of choice :)
 
Khron said:
Well, to be fair, you could achieve pretty much the same thing (at least in this particular case / application) with a cheap little 1-2 channel interface and a spectrum analyzer plugin in your DAW of choice :)
Click to expand...

+1
 
Khron said:
Well, to be fair, you could achieve pretty much the same thing (at least in this particular case / application) with a cheap little 1-2 channel interface and a spectrum analyzer plugin in your DAW of choice :)
Click to expand...

+2

(like I said.)

 
Thank you for the video!

It's also strange he doesn't disconnect the mic capsule, and he doesn't use a metal enclosure to protect Hi-Z area, as recommended by guru friends.

When the metal body from a mic is removed and  the P48V is turned on, a lot of humm noise is present on the audio.

How would this affect the biasing or THD adjusting process?

HL

 
Even if there's hum, that's gonna be decidedly in the lower-frequency areas (50-100-150-200-250Hz etc, or 60-120-180-240-320Hz), and the amplitude of the harmonics "should" taper off, the higher you go.

If you use, say, a 1kHz test-signal, you only care about its harmonics, and the overlap of hum harmonics with the 1kHz harmonics should be minimal, i'd think :)
 
Khron said:
Even if there's hum, that's gonna be decidedly in the lower-frequency areas (50-100-150-200-250Hz etc, or 60-120-180-240-320Hz), and the amplitude of the harmonics "should" taper off, the higher you go.

If you use, say, a 1kHz test-signal, you only care about its harmonics, and the overlap of hum harmonics with the 1kHz harmonics should be minimal, i'd think :)
Click to expand...

Yes, not only that but the amplitude of the hum is going to be dwarfed by comparison with the input signal levels (say 1v-2v) you are using to get near clipping at the FET.
 
When connected this way i never experienced hum. It appears if you feed signal to FET through a capacitor.
 
If you inject the test tone right into the gate of the FET from a low-impedance source, then the HiZ portions of the microphone are effectively bypassed.
 
Please don't forget that some oscillators have DC offset, which is good to block with capacitor.
 
homero.leal said:
A 1000p polyestirene will be good to use?

Thank you!
Click to expand...

Yes, as many topologies have this value at the input.
Also, you can use value closer to capsule capacitance like 47pF-82pF.
 

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