Patch cable problem?

[Chuck H]

Definitely what I would call hum and buzz. The buzz would be a harmonic of the hum and probably from the power supplies in the individual systems. Notice when you switch to lower gain preamp modes the noise goes down but the ratio of hum to buzz increases. Have you tried different stacking arrangements in the rack? I think it's probable that there's an EMF induced 60Hz hum between units and the buzz is harmonics of that. Have you tried disassembling the rack and connecting everything unstacked? I understand that would mean a lot of cables but the test would answer for this possibility. If it is induced power supply hum then you'll have to find the quietest arrangement for how things are stacked. It may also be possible to do additional shielding between units as well with grounded iron sheets. I might try removing individual units one at a time and reconnecting them outside the rack to see if any one unit is predominantly responsible. Start with the power amp since it likely draws the most current through a transformer. If any of the units use a wart style power supply try to keep those away from the rack rather than mounted inside it.

[Helmholtz]

It could be direct interference between units as Chuck described.

But I suspect a ground loop problem. That's always possible when safety grounded (class 1) units are interconnected via standard (unbalanced) signal cables.

BTW, it seems you taped off the safety contact of the mains plug in the power strip. That would be very dangerous.

[Andrex88]

no the earth has not been removed from the power strip, I'm adding a photo I'm trying to understand what that GND screw behind the samson is for, you can connect a lug. The topmost backup Jmp1 and Jfx 1 are completely disconnected, I could try to connect them and disconnect those closest to the 9200, I would also try to power the rear power strip separately to the rack where all the power supplies are connected.

[Chuck H]

I can't even begin to tackle a ground loop in that system. Obviously all devices should have a chassis earth/safety ground but there should only be one signal/0V ground. Since some devices don't adequately isolate the two this is even more problematic.

Signal ground loops with multiple devices can sometimes be solved by isolating interconnecting cable shields to one end or the other. That is, only one end of the cable would have the shield connection. But which device should that shield connected end be plugged into??? And likely not all cables will need this isolation. In that maze of connections negotiating the nearest thing to a single 0V signal ground is daunting. And I'm certain I won't be able to do it remotely based on trial and symptom descriptions.

[Helmholtz]

I was referring to the picture of post #13. What is the white stuff around the power plug bottom left to your rack?

Does moving cables at the rear of your rack influence your hum/buzz?

[Chuck H]

And there's this plug (for something) at the lower right. It's hard to tell but I don't see a ground pin.

[Helmholtz]

Signal ground loops with multiple devices can sometimes be solved by isolating interconnecting cable shields to one end or the other.

I don't think you can do that with (single) interconnect cables between separate units.
Where/how would the signal return current flow?
You don't want any signal currents to use the power ground wiring.

[Chuck H]

I don't think you can do that with (single) interconnect cables between separate units.
Where/how would the signal return current flow?
You don't want any signal currents to use the power ground wiring.

You're right. For some reason I was thinking of the shield as being separate. I've been away from my bench too long. I must have made a hundred cables but it's been many years since then.

[Chuck H]

I remember trouble shooting a system ground loop for one of my paint job customers. Connecting his powered woofer unit caused a bad hum in the whole system. As I recall I used a ground lift adapter at the AC mains. Then I took off the ground lift adapter and told him it wasn't ideal from a safety perspective to lift the AC ground. I handed him the ground lift adapter and told him I couldn't be responsible and it was up to him what happened next. He put the ground lift back on and plugged it in while I was still standing there.

[Helmholtz]

The electrical safety of each class 1 equipment depends on its low resistance safety ground connection being able to stand a short-circuit current until the house circuit breaker triggers.

Lifting the ground makes safety depend on the unreliable interconnect cable and jack contacts. Even worse before the cable is plugged in.
It is dangerous and against safety regulations.
In case of an accident insurance won't pay.

With ground loop problems it often helps to minimize the loop area.
Requires to identify the relevant loop first.

[Chuck H]

The electrical safety of each class 1 equipment depends on its low resistance safety ground connection being able to stand a short-circuit current until the house circuit breaker triggers.

Lifting the ground makes safety depend on the unreliable interconnect cable and jack contacts. Even worse before the cable is plugged in.
It is dangerous and against safety regulations.
In case of an accident insurance won't pay.

And that is why I described it here and expressed my unwillingness to perform it rather than actually suggesting it.

[Chuck H]

To further express my unwillingness to actually isolate AC grounds for the reasons expressed by Helmholtz above I'll mention that I've played many bars with creative and questionable wiring. And for outdoor shows running off generators AC safety circumstances can also be questionable and with power surges. If anything goes wrong you wouldn't want the only current path to ground to be through your signal cable shields.

[Andrex88]

And there's this plug (for something) at the lower right. It's hard to tell but I don't see a ground pin.

ahh sorry I didn't understand, the earth and lateral plugs and schuko sockets don't have the central pin, I forgot to add that I'm in Italy and there is 230v 50hz

[Andrex88]

I will never remove the grounding for both my personal safety and that of the devices, clearly it is not easy some speak of star grounding but I did not understand what it is, but even if I wanted to create a copper bar with all the grounds connected to it would become an impossible job, I would have to somehow extract the ground wire from every single power cable that goes to each appliance. As I mentioned earlier, I had once connected the guitar directly to the JMP1 for testing, the latter to the JFX and the 9200 made a slight tolerable hiss, but not such an infernal hum, really difficult especially for my knowledge on the subject which is not so thorough I limited myself to assembling it, trying to order and build the cables in the best possible way, I would never have imagined these problems would be so complicated to solve, also because concert equipment of this type, it would be necessary to understand if they could already have a minimum of noise, or they were totally silent, but I think it is unlikely that they are silent. There are few photo or video troubleshooting guide of racks

[Tom Phillips]

... I'm adding a photo I'm trying to understand what that GND screw behind the samson is for, you can connect a lug....

I suspect that ground screw is to allow you to ground the chassis to a point of your choosing. To be effective, the chassis must not be electrically connected to the rack via the mounting screws. Your photo in post #13 looks like you may be using non-conducting mounting screws on all the equipment but I can't tell for sure. If any of the chassis are electrically connected to the rack rails then you loose control over some of the ground loop tracks.