Grounding Schemes in Microphones

[C12VR]

I notice that most people recommend the use of separate paths for signal ground and shield/chassis ground until the common ground point in the amplifier chassis. In microphones, however, the signal return and shield are often homogenized. Is this a function of efficiency, i.e, being able to use fewer pins and thus a cheaper, thinner cable, or is it a genuinely proper practice? Won't this lead to a ground loop?

 

[Newmarket]

You have to consider phantom power where that is used. A mic expert will likely be along soon....

 

[C12VR]

True, but what about a tube mic where phantom shouldn't be a major concern?

 

[MicUlli]

Typically mic signal lines are symmetric. This leads to high common mode rejection ratio. A "ground loop" requires a second conductor carried from the microphone to the amplifier. This is not the case at all in practice because the mic body is clamped by an isolating mic holder.
Therefore seperated conductors for GND and SHIELD/CHASSIS are simply not necessary.

 

[ccaudle]

the signal return and shield are often homogenized

Typically microphones used balanced audio connections, so the signal and signal return are a twisted pair. For phantom powered microphones the shield is used as power return, but I am not aware of any microphones which use shield for signal return. At least none that are useful for recording, probably headset or phone microphones may do that, but that isn't generally a topic on this site.

 

[Matador]

I think in an ideal world, in a tube mike, one would send B+ and B- (power supply high voltage positive and negative) and H+ and H- (heater DC supply positive and negative) separately through the cable, to terminate into the power supplies analog 0V node. The shell of the mike body would then connect to the microphone cable shield, which is common with the PSU's metal chassis, which connects to the PSU 0V point in the PSU. In theory, a large radiated noise source can sum with the B- and be amplified inside the mike frontend if the chassis/shield and B- are intermingled.

However in practice, compromises are made: for example, with DC heaters, H- and B- are connected together without issue. In many designs, the mike shell is connected to B- as well, without issues as well.

In the C12 design from Alctron, I experimented with breaking the mike shell/headbasket from B-, and couldn't measure any audible improvement, even with the mike shell placed close to EMI sources like ballast's for neon tubes, CRT monitors, etc. I think the trade off between reduced susceptibility to EMI and minimizing ground impedance is probably hard to measure in that specific case.

 

[ccaudle]

couldn't measure any audible improvement

Is the transformer for that design in the microphone, or in the power supply? Transformers help eliminate a lot of issues around return paths.

 

[Matador]

In the microphone itself: and agreed, it definitely shrinks the problematic areas.

 

[tomas.borgstrom]

Even with a cable with separate Shield and H-/B- Neumann always connect them:

- In the microphone
- In both cable connectors
- In the PSU

I do the same and have no issues since I started doing so.