Basic Layout/Wiring Question re: Outboard PSU

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I have a Hagerman Bugle PCB stuffed and tested (sounds good powered by 9V batteries) and the PSU PCB to go with it, also stuffed. I have two small Hammond steel chassis with holes drilled, PCBs mounted on standoffs.

Now I'm trying to figure out how to connect the audio box to the +/-15VDC from the PSU box.

I have an abundant supply of leftover 3-pin XLR microphone cables. I'm pretty sure they can handle 15VDC at 50mA max. That's only 750mW of power. I figure a 1m run of 24ga copper wire is good for that without heating up. So assuming it's okay to use these cables as connectors for the DC power from PSU to audio circuits, here's my question:

Both the PSU and audio PCBs have pads for +15VDC and ground, and -15VDC and ground.

On the schematics for both the PSU and audio circuits, the ground pads are connected to a common ground, so they match the PCB layouts.

If I use a 3-pin XLR cable to connect the PSU to the audio chassis, is there any harm in connecting both 0V 'ground' pads to Pin 1 on the XLR connectors (the braided shield), with +15V on Pin 2 and -15V on Pin 3?

It seems to me that it won't matter if the 0V connection between the PSU and audio chassis is carried on a single wire, but I thought I'd check with more experienced people to see if I've overlooked something.

Thanks in advance.
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PS - There will be a chassis ground stud on the PSU chassis to which the green wire from the 3-conductor AC cord will be connected as the safety ground. That's the easy part. The rest isn't so obvious to me...
 
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Let's see if I can clarify the question with an illustration.

My understanding is that the audio circuits should be grounded only at their inputs.

However, if I do that, the power transformer center tap will be left floating, and so will the 0V (ground) connection to the audio chassis. Only the PSU chassis will have a bond to actual ground through the AC plug's 'Earth' connection.

What if the power transformer CT is connected to the safety ground connection? That will be the one and only connection to ground for the 0V connection. Like in the first untitled attached drawing. But then the entire audio chassis is left floating. Is that the way it should go?

In the drawing entitled GROUNDING SCHEME 'A', the safety ground is accomplished in the usual way, with the earth lead from the AC mains plug directly connected to the metal PSU chassis. This time, the power transformer center tap is only connected to the PSU chassis' 0V connector going to the umbilical, which connects to the audio chassis' 0V connector, which is connected to the metal audio chassis at the audio input jack. The green 'safety ground' is shown in green while the 0V signal ground is shown in red. Is that the way to do it with only three connectors (+15V, 0V, and -15V)? I suspect this GROUNDING SCHEME 'A' is the correct one, but I'm really not sure.

Finally, in the drawing entitled GROUNDING SCHEME 'B', the power transformer center tap is connected to the (green) 'safety ground' of the PSU chassis, which connects to the 0V connection, and the audio circuit 0V 'signal ground' is *also* connected at the audio input jack. This looks to me like it will make a ground loop, with the high ripple current in the power transformer center tap-diode bridge-reservoir capacitors connecting to both chassis and to the 0V signal ground. Or maybe I'm misunderstanding that.
 

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'A' is the better starting point.
If I were to slightly modify it, I would have a green earthing connection to the second chassis, separate from the red wiring. One possible way to do this is with an additional green wire from chassis to chassis, otherwise what is your method of having a suitable safety ground for the second chassis?

Then there is the decision of what to do with the red wiring within the second chassis. It doesn't need to be bonded to the chassis (because you have already earthed it above). This is a decision of doing one of four things:

a. Do not connect red to anything, let it float from chassis
b. Solidly bond red to chassis in second block as you have shown
c. Use a resistor (could be something from 10 ohm to 1 M) to reference red to chassis
d. Use a bunch of other possibilities willy nilly (diodes, capacitors, combination thereof) to reference red to chassis.

My general method in your case is to pick c, but it depends on what the circuit is doing, what it feeds, and how the remainder of the system is arranged. It just depends.
 
I asked a similar question recently for a tube preamp I am building with DC only in the second chassis. I was planning to only add safety ground to the chassis with the AC connection but later learned that I really need chassis ground on BOTH chassis. If you are planning to use a 3-pin XLR, just run 0, V+ and V- on the 3 wires and then safety ground on the shield pin. Build it like that and then experiment with tying 0V to earth at the earth chassis point in the PS chassis. You could also use a ground lift circuit as well. Experiment with what sounds the best.

The only other comment is I would not recommend XLR for carrying power. I have done it in the past also for low voltage circuits but there are a few concerns. First, someone could accidentally plug in an XLR signal cable and fry something. Secondly, some XLR is fine conductor microphone cable and may not be suited for power. Depending on the length you may get a voltage drop. Of course you could use the heavier DMX cables used for lighting circuits.
 
'A' is the better starting point.
If I were to slightly modify it, I would have a green earthing connection to the second chassis, separate from the red wiring. One possible way to do this is with an additional green wire from chassis to chassis, otherwise what is your method of having a suitable safety ground for the second chassis?

Then there is the decision of what to do with the red wiring within the second chassis. It doesn't need to be bonded to the chassis (because you have already earthed it above). This is a decision of doing one of four things:

a. Do not connect red to anything, let it float from chassis
b. Solidly bond red to chassis in second block as you have shown
c. Use a resistor (could be something from 10 ohm to 1 M) to reference red to chassis
d. Use a bunch of other possibilities willy nilly (diodes, capacitors, combination thereof) to reference red to chassis.

My general method in your case is to pick c, but it depends on what the circuit is doing, what it feeds, and how the remainder of the system is arranged. It just depends.

Thank you for replying.

In the attached diagram, I tried to follow your recommendation 'c'. I took GROUNDING SCHEME 'A' as the starting point and added an earth wire to connect Chassis 1 (the PSU chassis) to Chassis 2 (the audio chassis). I inserted a 1k ohm resistor between the audio circuitry and signal ground (red). I also added the signal source (turntable) and its connections.

The brown connection is the turntable 'ground wire.' There is a connection post on the audio chassis for this connection, but it goes straight to the chassis, as shown in the GROUNDING SCHEME 'C' schematic. That means the audio circuitry signal ground is lifted from the chassis by the 1k ohm resistor, but the turntable ground wire is not (it goes straight to chassis ground). Is this a guaranteed way of creating a ground loop?

Proper grounding of two chassis through an umbilical really does puzzle me.
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Using XLR connectors for power is asking for disaster - power and signal connectors should be physically incompatible to avoid expensive mistakes.


Having said that I don't know of any standard connector for +/-15V power supply connection, which seems odd.

For my tube preamp I am planning to use the TE M12 series. It was difficult for me since most do not go up to 250VDC safely. With only +/-15V, there are plenty of DIN options available by a quick search on Mouser or Digi-Key. Some even have pre-terminated cables which makes life a bit easier also.
 
Using XLR connectors for power is asking for disaster - power and signal connectors should be physically incompatible to avoid expensive mistakes.

Having said that I don't know of any standard connector for +/-15V power supply connection, which seems odd.

I'm going to give up on the XLR-3 connector idea.

I think I'll use a pair of oddball connectors I got somewhere along the way. They're 5-pin mini-XLR connectors of some kind, supposedly rated for 200VAC. The five pins will allow me to keep the signal ground and safety ground on separate wires. I'll use standard hookup wire to make my own umbilical and keep the run short (1m). The one problem with them is that the chassis-mount connector is male, so that will have to be on the audio chassis. A 1m pigtail with the female connector will have to come from the PSU chassis. Not a big deal I guess.
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I'm going to give up on the XLR-3 connector idea.

I think I'll use a pair of oddball connectors I got somewhere along the way. They're 5-pin mini-XLR connectors of some kind, supposedly rated for 200VAC. The five pins will allow me to keep the signal ground and safety ground on separate wires. I'll use standard hookup wire to make my own umbilical and keep the run short (1m). The one problem with them is that the chassis-mount connector is male, so that will have to be on the audio chassis. A 1m pigtail with the female connector will have to come from the PSU chassis. Not a big deal I guess.
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I always try to put unexposed pins on the power source so no one can get shocked or shorted so that should work well!
 
Thanks. But, now that the signal ground (0V) is directly connected to the chassis at two different locations--one at the PSU chassis AC inlet and power transformer center tap, the other at the audio chassis umbilical inlet--won't the likelihood of a ground loop problem be increased? At this point, I'm asking that as a learning opportunity, because...

It looks like the 4-wire approach is the only realistic way to proceed. Keep the safety ground separate from the signal ground, so I don't end up with ground loops. Hopefully the attached drawing shows a good grounding scheme.

- The signal ground is connected to chassis at the audio inputs *only*.

The safety ground connects at two places, one for each metal chassis, with a wire connecting the two, ultimately connecting both metal chassis directly to the ground lug on the AC plug.
- In the PSU chassis, the safety ground connects to a post in the chassis located right next to the AC mains inlet.
- In the audio chassis, the safety ground connects to a post in the chassis located right next to the umbilical inlet socket.

The basic idea is that the audio circuits have the shortest, lowest impedance path to the chassis, while the high current loop of the rectifier bridge/reservoir caps has a longer, higher impedance path to the chassis.

The safety ground merely connects both chassis to the mains earth (green wire from AC plug here in the US) to short the chassis to earth in case of an accidental short of DC voltages to the chassis. That short causes excess current to flow across the mains fuse and causes it to go open, cutting off AC.

I think that's right.
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