XLR for DC voltages

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warpie

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Feb 7, 2009
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Hello, this might be a silly question but I've seen some gear using XLR connectors to transfer DC voltages from an external psu into the actual unit, which I guess it's fine if the DC voltages are within the connector's limits. However, somehow you have to also make sure that the unit is shielded so the earth (gnd?) from the psu should also pass through the XLRs. Is this safe is there's a fault?
 
Use 4 pin connectors (at least different then standard 3 pin to avoid misshaps) .
Usually power lines do not need to be shielded unless there is almost no current flow (high impedance) or if the unit to be powered is extremely sensitive (microphone, photon counter etc.).
 
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Use 4 pin connectors (at least different then standard 3 pin to avoid misshaps) .
Usually power lines do not need to be shielded unless there is almost no current flow (high impedance) or if the unit to be powered is extremely sensitive (microphone, photon counter etc.).

Thanks,

So you mean that the remote unit (not the psu) should not be connected somehow to psu's chassis (i.e. to earth)? I was under the impression that both the external unit and the psu (of course) should be bonded to earth.
 
Grounding and shielding are 2 different topics. Grounding is important but only at 1 spot to prevent loops.

I realise that. My question is purely from a safety perspective.

My understanding is that both the psu's chassis and the remote unit's chassis should be connected to earth. So the dc cable between the psu and the external unit should also carry the "earth". Is it safe to use n XLR in this case?
 
This is not required but it is best practice. You could even use a small value resistor or 2 diodes antiparallel or a capacitor to ground from the chassis in case you get ground issues.

An XLR connector can be used in case of low currents.

A solid state preamp from a respected company with seperate PSU connected via mini XLR (plus/ minus and ground).
 

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This is not required but it is best practice. You could even use a small value resistor or 2 diodes antiparallel or a capacitor to ground from the chassis in case you get ground issues.

An XLR connector can be used in case of low currents.

A solid state preamp from a respected company with seperate PSU connected via mini XLR (plus/ minus and ground).

So, the preamp is not earthed at all, only the psu, right? I wonder whether this is a safer solution...
 
3 wires. Plus/minus AND ground.
So ground can be earthed in the psu or in the preamp. But as maybe other gear is grounded too (phono stage, record player or power amp) it is sometimes better not to ground (ground lift switch can be implemented) . As said before only 1 ground otherwise a chance of ground loops exists.
 
Do you own this preamp? If you probe between these two cases (psu and preamp) do you have continuity on this specific product?
 
You will often see 5 pin XLRs or 5 pin DINs used as well.

The EljinTek and Meanwell bipolar "desktop" (brick) supplies also provide a third output usually +5 or +48V. That output has its own 0V return. The XLR shell is also sometimes tied to frame ground which is a total 6 conductors.
 
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Hello, this might be a silly question but I've seen some gear using XLR connectors to transfer DC voltages from an external psu into the actual unit, which I guess it's fine if the DC voltages are within the connector's limits. However, somehow you have to also make sure that the unit is shielded so the earth (gnd?) from the psu should also pass through the XLRs. Is this safe is there's a fault?

Yes, and there is no fault.

I am assuming that your preamp is a microphone preamp. In which case you have more reason to make sure that the unit chassis is connected to earth as pin 1 of input/output XLRs must be connected to the chassis.

The unit also requires protection against EMI/ESD. So, again the unit chassis must be earthed. Using the circuit (or analogue) ground for EMI/ESD between the unit and power supply is asking for trouble.

Do not connect the circuit/analogue ground to earth/chassis at the unit. Bring them separately to the power supply and tie them together at the chassis/earth stud point inside the power supply.
 
I realise that. My question is purely from a safety perspective.

My understanding is that both the psu's chassis and the remote unit's chassis should be connected to earth. So the dc cable between the psu and the external unit should also carry the "earth". Is it safe to use n XLR in this case?
Yes.

Cheers

Ian
 
Hello, this might be a silly question but I've seen some gear using XLR connectors to transfer DC voltages from an external psu into the actual unit, which I guess it's fine if the DC voltages are within the connector's limits. However, somehow you have to also make sure that the unit is shielded so the earth (gnd?) from the psu should also pass through the XLRs. Is this safe is there's a fault?
There are three different "Grounds" that come into play here;
1) SAFETY Ground. This is the 3rd pin on the AC connector. This MUST connect to the chassis of any device that also has the live voltage from the line cord in it, so in this case, the remote power supply. It must connect via a separate, dedicated screw, BTW. That's so you should never, in the course of servicing the device, have to remove the ground screw to access another part (which is why it's a no-no to use one of the transformer mounting screws as the safety ground screw, for example). The whole purpose of the safety ground in this enclosure is to ensure that if a live connection were to contact the metal chassis of the device in question, it would short to safety ground and trip the associated fuse/circuit breaker, rather than energize the chassis. Note that this is NOT necessarily the fuse on the device, but the protection circuit in your mains supply, which could be either a fuse/breaker at your service entrance, or simply the breaker on a power bar if you happened to be using one...just the protection device upstream of the power supply. Note also that this safety ground connection is meant to protect against live wires inside the device from energizing the chassis such as wiring errors, or mechanical issues like you're tightening the lock nut on a round mains switch and it turns 45 degrees and one of the terminals contacts the chassis.
2) "Power Supply Ground", aka "0v" (zero volts). This is the 0v reference for the DC power supply voltages, whatever they may be. This should connect to Safety Ground at one point inside the device, typically as close to the power supply as possible. Note that I'm just laying out the concepts here, there are more knowledgeable folks on this forum that may chime in with more precise detail.
3) CHASSIS Ground. This is the case of the device, which in a unit that was powered by a mains plug would be the same as the safety ground. In a unit powered by a remote power supply, this should still connect to the safety ground in the power supply, BUT, ideally through a separate conductor from 0v.
The best way to accomplish this using, say, a 4-pin XLR for a mic pre-amp that runs on bipolar DC and has 48v phantom power would be to use a four conductor shielded cable. There's no industry standard (that I'm aware of, anyway) for how the 4 pins would be assigned, but I would do it like this:
Pin-1: 0v
Pin-2: -15v (or whatever the +/- supply rails are, 12, 15, 18, 24 volts, doesn't matter)
Pin-3: +15v (see above)
Pin-4: +48v
Case/Shell: Overall shield
That keeps your 0v and chassis grounds separate until they meet in the power supply, screens the power supply cable and screens the chassis of the device.
 
Thank you gents!

So, we can simply connect pin 1 (for example) directly tho the solder lug of the XLRs both on the psu and the unit like we would do in an audio connection, right?
 
Yes
I would still use a ground lift switch in the psu to the mains ground connector just in case a loop is made via other equipment.
 
Yes
I would still use a ground lift switch in the psu to the mains ground connector just in case a loop is made via other equipment.
I would not do that. For the two reasons I mentioned above.

However, if one is concerned with ground loop, then we can wire things a bit differently.

Do not tie the safety earth and DC (circuit) ground at the power supply, and again bring them separately to the equipment case. Tie the safety earth to the equipment chassis permanently. You can then install the ground lift switch at the equipment end to cut the connection between the circuit ground and safety earth.

This is certainly not the best way of doing it, but at least the equipment will still have permanent safety and EMI/ESD protection.
 
Hello, this might be a silly question but I've seen some gear using XLR connectors to transfer DC voltages from an external psu into the actual unit, which I guess it's fine if the DC voltages are within the connector's limits. However, somehow you have to also make sure that the unit is shielded so the earth (gnd?) from the psu should also pass through the XLRs. Is this safe is there's a fault?

Not necessarily the case. Eg if the external psu is double insulated Type 2 then there is no PEC conductor. Audio screen connections are made to the 0V of the power input. Ideally in a way that does not result in noise currents flow through the signal 0V on the PCB etc. Example - Soundcraft Spirit series (simple external transformer). Notoriously bad connector choice (I know 🙄).
 
If it is DC, there is a standard for it with the 4-pin XLR connector. On Sennheiser, AKG, etc. microport receivers this is used. I have serviced theater microports (and filmmakers) for decades. Look for it online: 4 pole XLR pinout standard. Good luck !
 
hello, i don't want to open a new topic, but i'm in the process of designing my first external psu for a tubes project. i don't know where to search for a 6 or 7 pin connector, a panel mounting one with his associated cable. can you address me to some product in the eu?
maybe farnell, reichelt or mouser?
thank you very much
 
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