DIY 48V phantom mic humming

[reubenm]

hello all, thankyou for looking in here. my phantom powered diy mic with a 77pF capsule makes a 0dbV loud 100Hz hum at minor mixer input gain ,the mixer: Behringer Xenyx 802.
    i have had a capacitance multiplier circuit introduced into the circuit at:after the 2 -2.7kohm phantom power tapping resistors and before the first rail capacitor ;there was no reduction of hum .this proved the Xenyx did not have AC ripple so i removed it ,and the circuit is left as the schematic shows.
    when i removed the source resistor of 5kohm with bypass capacitor 1uF,the amplifier is dead,and the hum goes away,therefore i said it was the amp doing something AC-wise to the Xenyx input. the resistor Rd is 47kohm,the gate resistor Rg is 100Mohm ,the jfet is Fairchild 2N4393.it runs at Vd ~35V from rail 45V [Behringer] .power coming in, the first capacitor on rail is 10uF.
    when i grip the ground of the XLR connector [bottom, mic] ,the hum attenuates,and the awesome tone of the mic in its fullness and happy can be heard through my monitors. what must i do to remove the hum or what causes it. thanks,
                                                                                                                                                                                      reuben.

 

[MagnetoSound]

Certainly a grounding/screening issue.

You are testing with the body cover on, right?

 

[reubenm]

Hi, yup with the body on

 

[reubenm]

The current consumption after the first cap is measured 10mA

 

[MagnetoSound]

when i grip the ground of the XLR connector [bottom, mic] ,the hum attenuates,and the awesome tone of the mic in its fullness and happy can be heard through my monitors.

Did you check for continuity between XLR pin 1 and the mic body/PCB ground?

 

[reubenm]

Did you check for continuity between XLR pin 1 and the mic body/PCB ground?

i did, it is very good

 

[MagnetoSound]

The current consumption after the first cap is measured 10mA

That's an awful lot of drain current.

 

[gyraf]

1) there is nothing in the microphone that can produce "hum". So it must be from outside - a shielding problem (if the mic is built as drawn)

2) 10mA is too much, as mentioned above. You won't get anywhere near the specified 45V polarisation voltage for the capsule if you draw this much current. The KM84 draws around 1-1.5mA iirc.

Jakob E.

 

[reubenm]

i'm honored to have both MagnetoSound and Jakob E of Gyraf replying.

i just shorted JFET source with a 1uF capacitor to ground and the massive 0dbV hum was replace by equally loud 'white noise' type sound.then , i disconnected mic and attached a 270k ohm resistor across XLR pin 2 to 1,and then pin 3 to1 and there was NO NOISE.

2. i measured that 10mA current draw with a cheap[cheapest] analog voltage and current measuring tool with a rotary dial i forget what its called. it may be measurement- since given the load values, it must be impossible that current draw.my total load by the mic is Rd + Rbias= 47k + [500k+2.7M] =47k and 3.2M ,,, should be under 1mA.

 

[reubenm]

*2.it may be wrong measurement

 

[reubenm]

1) there is nothing in the microphone that can produce "hum".

thats what i'm thinking;theres only a few parts in the mic.

 

[reubenm]

i run the 2N4393 drain at=35V ,is that causing the hum? no matter what i've tried -

-disconnecting the capsule,
-shorting JFET source with cap,
-bypassing Rs with a bigger cap at f=<1Hz,
-removing one leg of the capsule bias
-using a voltage divider to have Vd=22V
-making Rg smaller ;it was 500M ohm now 100M ohm
-using 0.1% phantom power tapping resistors

there is still 0dbV hum at mixer

 

[MagnetoSound]

Is the primary well grounded at the bottom?

Check ALL of your ground paths throughout the mic. Seems like you have a bad earth somewhere between the XLR and the capsule.

 

[reubenm]

Is the primary well grounded at the bottom?

Check ALL of your ground paths throughout the mic. Seems like you have a bad earth somewhere between the XLR and the capsule.

funny you said that [i'm sorry the schematic was misleading]
my transformer-it is epoxy encapsulated 14:1 ,and the primary:secondary  =  200K:1K center tapped,giving dual 50K:250

ohm capability,i use half of it only for 50K:250 ohm,the remaining two pins i hacked...but slightly stick out the

bottom[1mm]; on to the pcb;but the transformer is separated by pcb from ground plane which is underneath,the power rail on top.
  can this transformer be used for DC/phantom power?

 

[reubenm]

here is the revised schematic

 

[MagnetoSound]

OK, so pin 2 is the ground? That's fine, just check it is properly grounded.

You say the hum goes away when you disconnect the source resistor ... I still think you have an open ground somewhere - probably on the capsule side of the FET.

Check the backplate grounding cap. Measure from pin 1, all along the ground plane of your PCB right up to the capsule. You are looking for a break in the ground plane or a bad solder connection around the capsule area.

 

[reubenm]

OK, so pin 2 is the ground?

pin 1 is groud. will check trace as you said.

 

[MagnetoSound]

Reuben, your schematic shows the centre tap grounded. That's pin 2 in your pinout diagram.

It doesn't matter which pin is ground, but it is something of an obstacle to troubleshooting when the pictures and words don't match. 

 

[reubenm]

sorry about any confusion.

 

[reubenm]

So i have removed transformer and replaced with a DI 10:1 =20k:150 transformer,but the hum remained. i'm starting to think this is a problem with the Behringer mixer so that  Jakob E: 'there is nothing in the mic which can cause hum'.