Ground Connections for Poctop's mics

[dmp]

This is how I am connecting the grounds for Poctop's mic builds (U67, M269, D47)

Seems to work without any noise/ ground loops, but I haven't been in a challenging environment. Several years ago I setup to record with my U67 build and it was unusable due to a ground loop hum - sadly I can't get back there to test.
I think this follows the Neumann schematics exactly EXCEPT always connects p3 and the mic body to chassis & IEC ground (more safe). Moves the lift from XLR p1 to chassis (which is key to kill the ground loop) - but I haven't added the switch, I always have p1 lifted.
The ALT P1 LIFT would give added fault protection while blocking a ground loop.

 

[ccaudle]

Is the PSU chassis conductive, for example galvanized steel? Pin 1 is defined as the shield connection, so if the chassis is conductive metal, connect pin 1 directly to the chassis at the output connector. Since there is no need for return current on the output like there is for phantom power, no need for that connection to connect anywhere else.

 

[dmp]

Is the PSU chassis conductive, for example galvanized steel?

yes

connect pin 1 directly to the chassis at the output connector.

While this seems the 'best' way to do it, possibly could can create a ground hum in some locations?
For instance, if you have the mic in a live room on a different circuit as the preamp in the control room, would a difference in ground level (causing current to flow through pin 1) create a hum?

This p1 lift functions like the lift on a direct box.

 

[ccaudle]

possibly could can create a ground hum in some locations?

Yes, if the receiving device does not properly terminate the shield directly to chassis (aka "pin 1 problem"). In that case the preferred solution is to lift at the receiver end. There is an audio engineering society paper that goes into the mathematical details of why lifting the shield at the receiver end is preferred to lifting at the sender end, it makes a larger difference the longer the cable.

If you really want a lift at the microphone end, the better configuration would be to locate the switch directly next to the output XLR, and the switch connects the pin 1 to the XLR shell or not. If you are using this only in your own studio the preferred solution would be to connect pin 1 directly to the shell with the shortest possible connection with no switch, and just correct any connecting equipment which causes noise with that connection. Understandably you sometimes have to compromise if you are using equipment in other studios where you do not have control over other equipment used.

One suggestion I have seen before is to connect the cable shield directly to equipment case, and in cases where you need to disconnect the shield at one end, keep a cable adapter that has back-to-back female to male connectors, without pin 1 connected internally.

 

[dmp]

Yes, I know where you're coming from - I've read the RANE notes and am familar with the "pin 1 problem". I'm not trying to revisit the theory of grounding etc.
In the Neumann schematics (i.e. u67) you'll see there are two jumpers in the psu for grounding. I'm trying to conform Poctop's builds to this (and though it's been discussed in various threads, I don't think there is a diagram). There is already a SHLF jumper to connect / lift pin3 to audio ground. I changed the other jumper to only lift pin1 on the output xlr instead of pin 3 and pin 1, because I don't want to lift the mic body from chassis / IEC ground.
Maybe having pin1 to chassis would have been fine in the 'bad' environment (with the SHLF jumper out). I don't know and can't go back there to try it.

One suggestion I have seen before is to connect the cable shield directly to equipment case, and in cases where you need to disconnect the shield at one end, keep a cable adapter that has back-to-back female to male connectors, without pin 1 connected internally.

That's a great thing to have when doing any on-location recording.

 

[ccaudle]

I think you can make a ground lift adapter with some parts that Switchcraft sells, but you can also buy them off the shelf:
Hosa ground lift adapter
Available at Amazon, Sweetwater, B&H Photo, and Sam Ash online.