Microphone grounding problem...

[alk509]

Hey guys,

So I've pretty much finished my AKM78 microphone, but after everything is in place, there's still some annoying little bit of 60Hz hum coming out of the output. Here's the grounding diagram (ground connections in black):

Pin 1 of the output XLR and the ground of both the B+ and heater PSUs go to a single point in the chassis, which in turn is connected to AC mains ground. The case of the Tuchel mic connector is metal, so it's grounded where it touches the chassis, which in turn grounds the shield of the mic cable and the mic body itself.

I can't see anything wrong with my grounding, but please let me know what you think.

There are about 40mVp-p of crap coming out of the business end of the high voltage PSU. Can this be the culprit? Maybe some extra filtering is in order?

Any comments at all are welcome...

Please? :?

Peace,
Al.

 

[rodabod]

[quote author="alk509"]The case of the Tuchel mic connector is metal, so it's grounded where it touches the chassis, which in turn grounds the shield of the mic cable and the mic body itself.[/quote]

Check that this is definitely true. Some connectors / plugs have the shield insulated from the plug barrel / socket. I assume you've checked over with a multimeter to make sure the earth has good continuity throughout.

 

[CJ]

See if you can tell if it's 60 cycle or 120 cycle hum.

 

[ohhey]

How is the grill grounded ? That's been the most common source for me. Try an extra screen inside also just as a test to see if that helps.

My AKG solid tube had a hum till I determined the only ground for the screen was the body joint that didn't have a good connection. I just scraped some paint overspray and oxidation off the joint and put the mic back together and it was dead quiet.

 

[alk509]

[quote author="rodabod"]Check that this is definitely true. Some connectors / plugs have the shield insulated from the plug barrel / socket.[/quote]

I'm positive.

[quote author="ohhey"]How is the grill grounded?[/quote]

Through the body, which in turn is grounded through the shield of the cable. If I put the electronics inside a solid brass tube, the noise is sill there, so I don't think that's the problem...

[quote author="CJ"]See if you can tell if it's 60 cycle or 120 cycle hum.[/quote]

It does sound more like 120Hz than 60Hz. Let me scope it out and get back to you on that.

Peace,
Al.

 

[Gus]

check the heater and B+ for ripple/spikes with a scope.

 

[alk509]

OK, so I temporarily lifted the ground and the thing went NUTS with noise! Not just hum, but RF stuff as well. So I'm inclined to think it's not a grounding issue...

After reattaching the ground wire, I'm still getting about 40mVp-p of crap on top of my high voltage supply:

With the mic plugged in I get 60Hz crap on the audio output:

I've been sitting in front of this thing for hours now and I'm drawing a blank. I'm ready to take a break.

Any ideas?

Peace,
Al.

 

[rodabod]

I didn't take a look at your routing for the PCB, but (this is really obvious) make sure there are no conflicts with the noisy B+ voltage sitting alongside the mic signal path.

It might help you find the culprit if you filter the B+ a bit to confirm that it is definitely the source of the interference.

I had problems with a board which had noise problem and found that it had to be re-routed due to noisy Vs interfering (it was digital though - and very fussy).

Stay positive. Pun not intended. :razz:

 

[Gus]

what happens if you disconnect the ground at the XLR?

 

[alk509]

Hey guys,

[quote author="Gus"]what happens if you disconnect the ground at the XLR?[/quote]

That was actually the first thing I tried. There is practically no difference in noise level with the XLR pin 1 ground connected or lifted. I just realized, however, that I have NOT connected the XLR case (shield) to ground, which explains why I was getting some noise at the output even with the PSU turned off ... However, even with no XLR cable connected, there is still some noise at the output.

So things to try today:

- Tie XLR shield to pin 1.
- Add some extra capacitance to B+.

Anything else?

Thanks for the help, guys! :thumb:

Peace,
Al.

 

[jrmintz]

Try a different tube?