Purpose of individual channel GND in NCxxxMP modules?

[morethinkingcapacityneede]

Hi,

Basically, as per the title. How do you use the CH1 GND of a NC500MP module. I have XLR socket on a custom board, where pin 1 is shorted to chassis. I am planning to use screened cable between the custom board and the module for the balanced audio input. Should I connect that internal screen to the CH1 GND? Where should I connect it on the custom board's side? GND or chassis? Any suggestions appreciated, thanks.

 

[tmuikku]

If i understand correctly that you talk about balanced input on the amp board, and that pin 1 there should be connected as xlr pin 1, a shield. But you have another board which already connects pin 1 to chassis so you are bot sure where to connect amp board pin 1?

I recommend reading Henry W Ott book Noise reduction techniques in electronic systems and you'll be confident with this stuff rest of your life

I'd say do not connect pin 1 at the amp end or you likely get a ground loop through chassis. But because I have no idea how the PCB:s are wired, nor full picture of your assembly I could also be wrong. You can try connecting it and not, and measure which ever has least noise, you might even hear it. But it's not all, I suppose the chassis is connected to mains safety ground and all the chassis panels have proper conenction to each other. If not, then it might give less noise if pin1 went also through the amp, but it would be better make sure the chassis is properly grounded. Remember to test lid closed as well, stray magnetic fields inside the chassis might change lid open/closed. Have fun!

Model number of the amplifier hints there are high power around, so be careful, know what you are doing/poking, don't hurt yourself!

 

[Speedskater]

Books by Henry Ott (RIP) are great, but very deep, only for serious engineers.
Papers by Bill Whitlock and Jim Brown are more audiophile orientated.
* * * * * * * * * *
XLR pin 1 is a shield, but not a ground. It should be attached to the chassis at the connector.
Audio circuit commons are often attached to the chassis near the input connectors.

 

[Davide]

Hi,

Basically, as per the title. How do you use the CH1 GND of a NC500MP module. I have XLR socket on a custom board, where pin 1 is shorted to chassis. I am planning to use screened cable between the custom board and the module for the balanced audio input. Should I connect that internal screen to the CH1 GND? Where should I connect it on the custom board's side? GND or chassis? Any suggestions appreciated, thanks.

Connect the shield of balanced cable to GND pin of the module and then the shield to Pin 1 of chassis connector, in turn to be connected to the chassis with the shortest possible path.
Obviously there must be continuity in the connection to the chassis. And of course all the chassis panels must have continuity between them.
All this is not always taken for granted ... indeed ... with painted panels it is more likely that there is no continuity.
The side effect is the lowest EMI immunity.

 

[tmuikku]

Yep, if chassis panels have continuity, as they should for proper shielding, then if the amp pin 1 connects to chassis somewhere, and also the input connector on the other pcb connects to chassis there forms a ground loop if pin1 are connected both ends on the interconnecting cable Connecting the intercpnnect cablenonly from either PCB would be enough to keepnsignalnshielded, and not make ground loop.

If chassis panels are not connected properly, like the back panel where the connector board is, it is possible the first PCB pin 1 does not ground the shield, so so also no ground loop forms. Now the interconnect cable should be connected both ends, as it would ground cable shield trough the amp PCB.

Had to mention connections of the chassis, all modushop enclosures I have, and in fact any case I have, did not have proper electrical continuity due to plating on the aluminium or paint on steel, which I have sanded off around screws to make continuity. Depending how these are, guide for pin 1 connection might vary.

 

[Speedskater]

Connect the shield of balanced cable to GND pin of the module and then the shield to Pin 1 of chassis connector, in turn to be connected to the chassis with the shortest possible path.

Just the opposite.
With Audio Engineering Society standard AES 48.
The cable shield is first attached with pin #1 to the chassis at the connector.

 

[Davide]

Just the opposite.
With Audio Engineering Society standard AES 48.
The cable shield is first attached with pin #1 to the chassis at the connector.

That's what I meant.
Just use an image for clarity and avoid misunderstandings due to the way of expressing myself.

EDIT. The image does not answer the question of the op. But I did the same question to Hypex. Here their answer:

 

[Davide]

Had to mention connections of the chassis, all modushop enclosures I have, and in fact any case I have, did not have proper electrical continuity due to plating on the aluminium or paint on steel, which I have sanded off around screws to make continuity.

I did the exact same thing ..

 

[Salt]

Most Neutrik plugs do have an extra pin that should connect to the case, not?

 

[morethinkingcapacityneede]

If i understand correctly that you talk about balanced input on the amp board, and that pin 1 there should be connected as xlr pin 1, a shield. But you have another board which already connects pin 1 to chassis so you are bot sure where to connect amp board pin 1?

I recommend reading Henry W Ott book Noise reduction techniques in electronic systems and you'll be confident with this stuff rest of your life

I'd say do not connect pin 1 at the amp end or you likely get a ground loop through chassis. But because I have no idea how the PCB:s are wired, nor full picture of your assembly I could also be wrong. You can try connecting it and not, and measure which ever has least noise, you might even hear it. But it's not all, I suppose the chassis is connected to mains safety ground and all the chassis panels have proper conenction to each other. If not, then it might give less noise if pin1 went also through the amp, but it would be better make sure the chassis is properly grounded. Remember to test lid closed as well, stray magnetic fields inside the chassis might change lid open/closed. Have fun!

Model number of the amplifier hints there are high power around, so be careful, know what you are doing/poking, don't hurt yourself!

Thanks for the reminder for the case surface treatment. My case is indeed treated, and I'll make sure there's continuity.

Maybe I can describe the situation in some more detail. I'll provide a system diagram at some point, since those are much better than writing than lost of text. Pin 1 of the XLR connector is connected to chassis with shortest possible physical connection, as per AES48 standard. The chassis is connected to earth safety ground. The balanced signal pair from the XLR connector goes through a relay (for input switching), then to a 16P box header connector similar to the one on the amp module. A screened cable, similar to this one https://ghentaudio.com/products/sig-12 but with Hirose connectors on both ends, carries the signal pair from the box connector on my board to the box header on the amp module. The question is what to do with shield of the internal cable.

On the amp side of the internal cable, I could leave the shield unconnected or connect it to pin3 of amp connector J4 labelled CH1 GND: https://www.hypex.nl/media/27/e5/d9/1708610783/NC500MP_04xx_R11.pdf.

On the custom board side, I could leave it unconnected, connect it to chassis or connect it to my board's GND. My board uses the amp module standby 5V power supply, so the board's GND connects to pin 4 (labelled GND) of the smaller 10P box header connector (J6) at a single point.

Connect the shield of balanced cable to GND pin of the module and then the shield to Pin 1 of chassis connector, in turn to be connected to the chassis with the shortest possible path.

EDIT. The image does not answer the question of the op. But I did the same question to Hypex. Here their answer:

If I understand Hypex's answer from @Davide's post, they suggest connecting the internal shield to chassis on my board's side, and then to CH1 GND on the amp module side. I can see that potentially introduces a ground loop through the amp module, but hopefully not through its audio circuit.

Does that make sense?

edit: added another quotation

 

[antcollinet]

To answer the OP main (subject) question - separately grounding each channel back to the chassis ground point will minimise crosstalk.

A shared ground connection would allow ground currents from one channel to influence the other, and vice versa.

 

[Speedskater]

The question is what to do with shield of the internal cable.

I would only attach it to the chassis at the connector end (and float the other end).
* * * * * * * * * * * *
On the other hand, if it has a braided shield, you could use it to attach the audio circuit common to the chassis, rather than running a wire.

 

[morethinkingcapacityneede]

I would only attach it to the chassis at the connector end (and float the other end).
* * * * * * * * * * * *
On the other hand, if it has a braided shield, you could use it to attach the audio circuit common to the chassis, rather than running a wire.

Sorry, couple of questions.

I guess your first point is about trying to avoid ground loops? hypex themselves seemed not to be worried about that.

I'm not sure I understand your second point. When would I connect amp module gnd to chassis with a wire? I was only taking xlr pin 1 to chassis, probably via the pcb onto which the xlr connector is soldered onto.

 

[Speedskater]

XLR connectors should first be attached to the metal chassis.
Audio circuit commons should be attached to the chassis near the XLR input connectors.
How all this happens, depends.

 

[Salt]

XLR connectors should first be attached to the metal chassis.
Audio circuit commons should be attached to the chassis near the XLR input connectors.
How all this happens, depends.

If you screw it all, input, output and circuit ground to one together, then there are ground loops, not?

 

[Speedskater]

Just the opposite.
Ground Loops have length & area (& resistance) between the different paths.

 

[antcollinet]

If you screw it all, input, output and circuit ground to one together, then there are ground loops, not?

Not.

Essentially that is what you need to do. Just using the chassis as the star point. Every circuit individually earthed to the chassis.

Start daisy chaining earths, and you get a mess of cross contamination.

 

[Rick Sykora]

Hoping you have your answer, but thought some context might help. The posted response from Hypex support is correct but is given relative to Hypex parts. Since is not clear which interface board you are using, here is what the conditions are in play with the amp module using the Hypex interface board.

1. The ground on the NCx500 module is a separate board ground and has no integrated connection to the chassis ground.
2. The board ground often gets connected to chassis ground via pin 1 of the XLR connector (as it is connected to chassis ground). I think the drawings are done to emphasize the chassis grounding as otherwise some just see a wire going to pin 1 and may not see the chassis ground connection.
3. No other chassis ground connection is (or should be) made to the amp module or the interface board.

As @Speedskater alludes, there can be more than one right way to connect the board ground to chassis ground. My Hypex kit had bare case holes, bare contact areas and wave washers to ensure the back panel makes a good ground connection to the bottom and other panels. The wave washers are a pain and, if you forget them, you risk a loose one causing a short. Otoh, not one for hoping that screws make good grounds, I tend to run (short) ground wires when a chassis ground is critical. Ofc, I also measure the result to be sure the grounding is optimal.

The ground loop aspect has been covered so will not further emphasize.

 

[morethinkingcapacityneede]

So, I'm not sure things are fully clear, but I think I understand things a little bit better. I'll try to summarise below.

• The GND of the amp module should be connected to chassis. I was assuming up until now that is achieved via the heatsink and mounting bolts. In other words, J4.3 doesn't need to be connected to chassis in order to connect the board GND to chassis. I tested the resistance between J4.3 and the mounting plate/bolts, and it's 100k (probably a C in parallel in there as well). I think this is a good confirmation. This was discussed in this thread, https://www.audiosciencereview.com/...ds/balanced-interconnection-and-ground.51570/, and I think that was indeed the conclusion. @Davide, since the other thread is yours, please feel free to comment on the conclusion I drew.
• For best shielding performance, the screen of the internal signal cable should be connected at both ends. On the signal input side, it should be connected to chassis.

Now, what is not clear to me is the trade-off between leaving the internal shield only connected at one end vs breaking the star pattern of the amp module grounding bypassing the RC connection between amp module GND and chassis. Connecting the internal shield to J4.3 introduces a second connection between the amp module GND and the chassis, and this connection is a short, rather than using RC.

 

[morethinkingcapacityneede]

As @Speedskater alludes, there can be more than one right way to connect the board ground to chassis ground. My Hypex kit had bare case holes, bare contact areas and wave washers to ensure the back panel makes a good ground connection to the bottom and other panels. The wave washers are a pain and, if you forget them, you risk a loose one causing a short. Otoh, not one for hoping that screws make good grounds, I tend to run (short) ground wires when a chassis ground is critical. Ofc, I also measure the result to be sure the grounding is optimal.

Indeed, as @Speedskater says, with such things, there can be many acceptable solutions. Are you saying you run multiple wires from the board GND to each case panel? Again bypassing the RC connection via the heatsink? edit: You probably short wires between the panels?

1. The ground on the NCx500 module is a separate board ground and has no integrated connection to the chassis ground.
2. The board ground often gets connected to chassis ground via pin 1 of the XLR connector (as it is connected to chassis ground). I think the drawings are done to emphasize the chassis grounding as otherwise some just see a wire going to pin 1 and may not see the chassis ground connection.
3. No other chassis ground connection is (or should be) made to the amp module or the interface board.

My module is a NC500MP rather than NCx500, so not sure how much applies. In any case, my board uses the GND provided by J6.4 (the connector with the 5V standby power supply), and it does not connect it to chassis. J4.3 and J6.4 are shorted together on the NC500MP, I just checked this. With regards to points 2+3, connecting the amp to chassis via pin 1 already introduces a second connection between module GND and chassis, I guess what matters is that there aren't two or more shorts between amp module GND and chassis. I think this may go against what I was understanding earlier (above paragraph) about you using multiple ground wires.