what to do with 48v winding - hairball's toroid

toneboner · Jan 14, 2013

Where I'm not likely to kill myself or blow something up, I will conduct experiments to find the best solutions. But I just can't pass up this opportunity to show off my ignorance: for the same reason you describe keeping phantom 0v away from the chassis (via direct path anyway) , my instinct is to also keep the psu common (what you refer to as regular signal 0v) away from the chassis

ruffrecords said:
1. Connect phantom PSU 0V to chassis at the power supply just like the regular signal 0V. The problem with this is that all the phantom supply return current flows through the chassis along with all the other crud current the chassis is picking up

But that would be wrong, right? Because you don't want the preamp boards 0v floating with respect to chassis ground? Until yesterday I thought the 0v from regulated side of my psu would only go to preamp. But standard practice it seems is to also have it go to chassis (perhaps also where IEC earth is tied). The Green pre layout provides the option to bring 0v to pin 1 of xlr out as well. Is that functionally the same thing?

Is this why rane put a question mark in the diagram?!!!

toneboner · Jan 14, 2013

I think i'm confusing signal ground in the pic (or -16v for green pre) with 0v. Or I'm just confused! See, this is why I just wire it up and throw the switch - easier to poke around and troubleshoot than to make sense of all this conceptual mumbo jumbo!!!

Andy Peters · Jan 15, 2013

toneboner said:
Is this why rane put a question mark in the diagram?!!!

The question mark indicates that the impedance (or, more simply put, the path) between the chassis and the circuit common is not known.

-a

ruffrecords · Jan 15, 2013

toneboner said:
Where I'm not likely to kill myself or blow something up, I will conduct experiments to find the best solutions. But I just can't pass up this opportunity to show off my ignorance: for the same reason you describe keeping phantom 0v away from the chassis (via direct path anyway) , my instinct is to also keep the psu common (what you refer to as regular signal 0v) away from the chassis

ruffrecords said:

1. Connect phantom PSU 0V to chassis at the power supply just like the regular signal 0V. The problem with this is that all the phantom supply return current flows through the chassis along with all the other crud current the chassis is picking up

Click to expand...

But that would be wrong, right? Because you don't want the preamp boards 0v floating with respect to chassis ground? Until yesterday I thought the 0v from regulated side of my psu would only go to preamp. But standard practice it seems is to also have it go to chassis (perhaps also where IEC earth is tied). The Green pre layout provides the option to bring 0v to pin 1 of xlr out as well. Is that functionally the same thing?

Is this why rane put a question mark in the diagram?!!!

Pretty much yes.

The thing to remember is that the chassis and screen should only every carry interference currents. That's how a screen works by providing a separate path to ground for all the crud. Now, at some point you do have to connect the two together so the BIG question is where do you do this?

The answer comes in two parts.

1. You do it at one point and one point only because if you don't you create an earth loop. That's why unbalanced inputs/outputs with connectors that short signal 0V to the chassis are such a PITA. That's also why you DON'T connect pin 1 of XLRs to signal 0V.

2. You do it so that the chassis and signal 0V share as little common cable as possible.

Usually this means you connect them together in the power supply where the safety earth enters via the mains socket. As regulations say the safety earth must be connected directly to the chassis as close as possible to the connector then it makes sense to make that same saftey earth chassis connection point the place where you connect signal 0V to the chassis. This is sometimes called a or the start point.

In the 'good old days' many studios had a designated 'Technical Earth' which was often a big lump of copper buried in the ground. The signal 0V and the chassis would only be connected together close to this point. The attached Neve Technical Note describes a 'Technical Earth' although it does confuse things a bit by referring to signal 0V as B- and chassis as signal earth.

Cheers

Ian