can this circuit hum?

according to Jakob Erland, this circuit has nothing in it that can produce hum,and i agree with him.
 
but i ask everyone again,
given
1) the connections are tight and good,
2) there is no ripple from the phantom dc rails
3) the 2 connections on left of schematic are joined to face(top) and backplate(bottom) of capsule

can this circuit cause hum of itself at the input of a preamp/mixer

thanks very much.  reuben

No.
The only reason that you will get hum is:
- Magnetic induction (produced by a mains transformer) in the output transformer.
- Gate connection of the FET not well screened.

RuudNL said:

No.
The only reason that you will get hum is:
- Magnetic induction (produced by a mains transformer) in the output transformer.
- Gate connection of the FET not well screened.

Click to expand...

thank u RuudNL that is encouraging.

because i'm about to buy a Focusrite Scarlett 2i2 48V phantom interface since my Behringer Xenyx 802 is humming when the mic connected.

to ensure gate of JFET is well screened,is embedded pcb traces enough screening?tx

I can't read the value of the gate resistor.
But with very high values, you will pick up everything!
I once made a microphone, and I shielded the capsule with wire mesh with openings of 1.2 mm.
But as soon as I came near the microphone, it started humming.
The openings of 1.2 mm. were simply too big to provide good enough Faraday screening!

RuudNL said:

I can't read the value of the gate resistor.
But with very high values, you will pick up everything!
I once made a microphone, and I shielded the capsule with wire mesh with openings of 1.2 mm.
But as soon as I came near the microphone, it started humming.
The openings of 1.2 mm. were simply too big to provide good enough Faraday screening!

Click to expand...

this gets me worried.ive seen cathedral pipes with bigger than 1.2mm mesh-and other mics,but i'm worried such thing could happen.

Rg=100M


btw, i like the recent mic you made with the dc to dc convertor inside

That is indeed a very open head basket!

i think the hum may be coming from the mixer; Behringer Xenyx 802 which converts +ve and -ve 14V AC into dc within mixer.
the power supply is the last pic

back

power supply

i mean is this mixer even grounded?

-for power it uses a large 'adapter' with transformer inside-but without rectification,with  2-pin wall plug.

It is not uncommon to use a voltage doubler/tripler to generate 48 Volts for phantom power.
I own (among others) a small Phonic mixer (for location recording) that uses a 2 x 16 Volts AC adapter, but the phantom power is completely 'clean'.
As long as the +48 V. is smoothed an stabilized good enough, there is no problem!

i do not know how 100Hz hum can be produced with my mic if it is not the mixer's fault. i do not think the mixer is grounded properly..

i mean i've employed in mic:

- capacitance multiplier
- capacitor/resistor network with f=1/(2*pie*R*C) well under 1 Hz
- zener diode on power rail

none prevents hum being at mixer,when mic amplifier is on
-when Rs,Rd or Coutput is disconnected-no hum

What happens if you increase the value of the 0.22 mfd capacitor over the supply voltage in your circuit?
(0.22 mfd is a very low value IMHO, I would choose something in the order of 47 mfd or so.)

in fact i have increased that to 47uF once, no change of hum but now it is 10uF.

Have you tried the mic on a different preamp? This exercise would definitely get to the root of the issue.

-James-

am going to soon.

Where is this circuit from?  What is the transformer?  Impedance & voltage ratio?

What is the capsule?  How is it mounted?

Why have you got a 1M/1M divider before the 33M feed to the capsule?

Can you post a pic(s) of the physical circuit circuit/layout/wiring?

Can you post an EXACT schematic with all components numbered and showing the capsule & wiring so we can sensibly advise?

How is the capsule earthed?  How is the PCB earthed?  How is the XLR earthed?  How is the case earthed?  Show all these on the schematic.

Have you tried another working P48V mike on the mixer?

...very good questions indeed...