TillM

Re: Neumann M49 Clone : Build Thread Puck Style (TLM49 Conversion To M49 b-c)
« Reply #460 on: September 08, 2019, 05:26:23 PM »
I noticed a hum (at around 50hz) when I used my M49 and take a higher input gain from my pre as I usually do.
Is there any way to reduce the hum except the pin wiring 3 to 7 and 7 to three ?
It seams like a ground loop to me.
Appreciate any suggestions.


Delta Sigma

Re: Neumann M49 Clone : Build Thread Puck Style (TLM49 Conversion To M49 b-c)
« Reply #461 on: September 09, 2019, 07:05:47 PM »
I had issues with hum in my M49b. I had the PSU circuit board grounded by the 0V connection by the transformer at one of the PCB standoffs' chassis hole. If I moved the ground near the choke the hum would increase. I thought the case was too crowded (Hammond enclosure) so I rebuilt it in a larger case from dandeurloo. Same issue.

I turned out to be a ground loop because I grounded the PCB from the transformer side of the PCB to roughly centre of the chassis. I moved it to the mic cable connection to the PCB and grounded it on the 7 pin XLR mounting screw and the hum was gone.

Grounding is hard to describe so I made a rough sketch. I made a forum member mad once for suggesting a ground lift as shown in Neumann schematics; so to be clear:
1) the power supply chassis is always bonded to earth ground via ground connection at IEC outlet
2) the mic body is always bonded to the power supply chassis via mic cable shield (and sometimes extra "screen" conductor bonded to shield)
3) the Neumann grounding scheme (see U67/NU67 schematic) has 0V, screen/shield and mic body all bonded inside the mic
4) the U67/NU67 has a switch on the screen (pin 3) which is probably preferred over switching 0V
5) I put the switch on the 0V because the shield will always be bonded to the power supply chassis via XLR housing and I omitted the screen (pin 3 in U67 schematic) on my M49 to add relay control over omni only mode

TillM

Re: Neumann M49 Clone : Build Thread Puck Style (TLM49 Conversion To M49 b-c)
« Reply #462 on: September 10, 2019, 12:03:32 PM »
Thank you so much !
I will try it !

Delta Sigma

Re: Neumann M49 Clone : Build Thread Puck Style (TLM49 Conversion To M49 b-c)
« Reply #463 on: September 12, 2019, 12:40:51 PM »
Good luck! It's a frustrating thing to troubleshoot. I think the key is grounding the 0V to the chassis at the same location as the shield since they're connected in the mic. You have no choice with the shield because the mic cale connector is bonded to the chassis where it is mounted.

TillM

Re: Neumann M49 Clone : Build Thread Puck Style (TLM49 Conversion To M49 b-c)
« Reply #464 on: September 12, 2019, 02:11:12 PM »
the thing is, I've a M49c and a M49b.
The only problem with the 50hz hum makes the M49b version.

Delta Sigma

Re: Neumann M49 Clone : Build Thread Puck Style (TLM49 Conversion To M49 b-c)
« Reply #465 on: September 12, 2019, 02:46:17 PM »
I didn't have a problem with hum on my U67 or Elam 251. It wasn't until I built an M49b that I had an issue. It may be because of the choke. It may be because of the bias. Not sure why but changing the grounding corrected the problem. I can't remember where I grounded the 0V on my U67 but I believe it was at a PCB standoff. My 251 supply is grounded to a lug in the centre of the chassis. I referred to the U67 and 251 supplies when building my M49b supply and ended up with hum.

It's been a while since I've read it but Rane talks about the XLR "Pin 1 problems" here:
https://www.rane.com/note165.html

The difference to note is that your mic cable isn't coming into a preamp chassis that has a small power supply relative to the chassis. You're bringing your mic cable into a chassis that is 90% power supply components. My guess, is that the idea of having a supply ground (chassis ground to wall), 0V/signal ground and a shielding ground all going to different places in the chassis SHOULD work but doesn't seem to because of the choke.

There may be some benefit to experimenting with 0V/shielding connections in the mic body as well (i.e. not connecting 0V to the mic body - only connecting it to the power supply chassis) but I think we need someone to modify a mic that travels a lot to do experiments. The RF/grounding situation in one building may be different than another.

Unfortunately, my experience with grounding is in communications. I'm sure I have a lot of bad habits in audio grounding!


 

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