aviel

AKG C4000b hum
« on: January 10, 2019, 05:06:22 AM »
Hey,
I have a problem with my AKG C4000b.
There is a very subtle hum present, but it gets loud when you actually touch the capsule.
My preamp is well grounded, xlr pins 1 in the preamp goes directly to the chasis ground together with the psu and audio ground.

This problem isnt hapening with other mics (dynamic mics though).

What is the best practice regarding mic grounding? Is it correct to assume that the mic chasis is not well grounded?


Re: AKG C4000b hum
« Reply #1 on: January 10, 2019, 05:12:24 AM »
 Check the wiring in actual mic and make sure everything is assembled properly. Grounding issues can happen from misaligned components (like the base ring on a mic not being screwed on properly). Open it up carefully and take some pictures of the internals of the mic so people can tell you if they see something odd with it.

mhelin

Re: AKG C4000b hum
« Reply #2 on: February 28, 2019, 04:10:03 AM »
I have experienced lately exactly similar problem with my Violet Black Knight (mic designed by late Jurins Zarins). I don't think it had that hum until lately. I already replaced the electrolytic capacitors on both PCB's - on the other is the preamp borrowed from C414B-TL with cascode FET's, a FET current source and single BC547 driving the output (single ended, impedance balanced), the other is the capsule bias voltage generator (diode ladder). The bias voltage is connected to the  capsule backplate and the single diaphragm is connected directly to the FET gate and 1G resistor. The body shell is made of brass tube, the capsule mesh is single layer. The PCB's have outer ground track (wide) which connects to circuit ground close to section where the wires come from XLR connector at a single point. I can't find any problems there with the connections, and the mic PSU seems to load both 48V lines (pins 2 and 3) equally, and the pin 3 is terminated by a 24 ohms resistor in series with 47uF capacitor to circuit ground. The mic sounds fine except the LF hum, it's usable only for close miking some loud sources now. The body parts seem to have good contact with each other when the end cap is tightened. It was tricky to get it opened, also the mic has no metal rails but the two thick PCB's kind of form the internal structure supported by the body tube when the end cap is tightened (low cost desing). There must be ground loop somewhere. Should the shell be lifted from pin 1? Could there be some faulty component?
Mikko


 

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