Relay Switching Matrix Ground

[a soBer Newt]

Hello All I have a request from a client about making a switcher that will switch between his two tape machines and radar system and two different consoles. Image attached showing signal flow. My question is about how to handle grounding in this situation. Can I just tie each machines ground together and then switch that across to the output? Or am I better off to dedicate a relay for each channels ground.

 

[sodderboy]

First, you are dealing with cable shielding and not equipment grounding. Think of it that way and things get clearer.
This is typically done with an EDAC panel that gets designed, financed, built, and never used after a couple of months. I figure the cost of relay boxes will be 2-3x the cost of a panel because the terminations are not as simple and brainless. You don't have to think much until it is time to block.
Shielding depends on what you have now. How are shields tied currently? How I wire, the shields would be tied at all consoles and lifted at the opposite ends at equipment.
The bestest gold star way would be to switch the shields as well, and load unconnected tape machine inputs with 600Ω. I'm dealing with older stuff, and some things do not like to sit with open inputs. Especially in NYC. This would 100% mimic an EDAC panel with termination plugs for unused machine inputs. Anything less would have to be tested in situ first before scaling up. The first compromise would be linking all the CH1 shields together, CH2, and so on in a single box. You want to have single shielded twisted pairs connecting the equipment as best as possible. When you start to link shields between different equipment the quiet lo-noise floor balance can be disrupted.
If you are thinking of taking the existing wiring, cutting it, and wiring it to a box filled with relays don't do it. Just don't do it. Any serious place, like commercial or rockstar home studios would never do this. They have EDAC panels. Dusty EDAC panels.

Mike

 

[ccaudle]

The bestest gold star way would be to switch the shields as well

The shields would not successfully operate as shields for high frequencies if you do that.
The bestest way for decently designed equipment is that shields are shields and fully surround all of the signal connections all the time, i.e. tie the metal of all the chassis together.
Whether your equipment is decently designed or not will have to be inspected on a case by case basis.

 

[Tekay]

How about using ULN2001 or a similare circuit to drive the relays? No extra protection diodes needed and one port can control more than one relay.
I've used this circuit in several products together with 12v relays and a LM2596 adjusted to +10v to save heat and relay power.
I've used a 2k7 resistor in serie with the control inputs.

 

[ccaudle]

Can I just tie each machines ground together and then switch that across to the output?

Those devices all have balanced I/O, so the connections are signal hot, signal cold, and shield. Assuming that none of the equipment has a pin 1 problem (i.e. shield incorrectly tied into circuit reference node such that shield interference currents become noise in the circuit) you can tie the shields directly to the chassis of the switching matrix and switch the two signal connections with a dual pole relay.
If all the equipment has 3-pin power entry, you could selectively not connect the shield at the receive end for equipment which has a pin 1 problem.
Hopefully all of the equipment has the shields properly connected, because you might have to evaluate inputs and outputs separately for pin 1 problems.
Connecting any equipment which does not have 3-pin power entry would require separately tying the chassis of that device to the same earth potential that all of the other equipment is referenced to so that it does not have a high common mode voltage from power supply leakage.

That is essentially just restating what sodderboy recommended as well, but the only disagreement I have is the suggestion that also switching shields would be a good idea. If shields are useful it implies that the shield is protecting the audio signal connections from some interfering signal. If you then switch the shields, now you are taking that interfering signal and bringing it inside the box in a semi-uncontrolled way. That has a good chance of rebroadcasting the interference signal around the inside of the chassis, and keeping the interfering signal out of the chassis was the entire point of shielding to begin with.

 

[sodderboy]

The shields are already in the box, at the relays, what are you going to do with them ccaudle? How do you keep shield purity?
In a completely fresh design with these relay boxes in the middle, I would break all shields at the boxes and tie at sources and loads. All noise current drains to either end. Not my fave, but it works. But this wiring is already in place to some extent.
In the case of either box, switching shields always has the load end of a source shield(connected to source chassis) either connected to nothing, or to the shield of the cable going to the selected load. Both boxes. Always discreet cable runs. With my first-tier compromise the source shield of one channel would connect to the shields of the three load devices, vice-versa for the other box. This would have to be tested with modded mic cables connecting a few channels to make sure the existing noise floor does not change.
I absolutely disagree that relay switching cable shields does not effectively shield “high frequencies”. Please educate.
Mike