TOPIC:

Hello, 
I tested a different amp on my mid-bass and I got a hum (pleasures of rca vs xlr).
Tried several things that didn't work out.
So I wonder if I could use the unit ground, but not sure how to do it.What I have in mind is to cut an old power cord and wire the earth to the flex ground.
Is it ok?
Or should I link the amp and the flex ground in some way?

Thx!

Pursuing my quest here... So I did few tests:
#1 flex ground to outlet ground (called earth?), and ground to amp > nothing changed, same noise. 
#2 amp plugged in a different room outlet > lower noise, manageable with lower gain

So I’m a bit lost here, when #2 tells me it might just be the amp SNR/gain, while #1 tells me it's also an interaction with the flex.
Am I wrong here? 

I was advised to test a linear power supply, what do you guys think?
Otherwise I’m thinking about getting a dedicated preamp/buffer symmetrizer to go back to xlr here, so I could lower the amp gain.

Grounding issues are difficult to track down I'm afraid. But a word on changing the PSU - the standard SMPS that is provided with the Flex (and similar ones with their other products) has a "floating" output - its ground side is not connected to mains ground. If you replace the PSU with another (linear or SMPS) that has its ground side connected to mains earth you may inadvertently create a ground loop which will make the hum worse. I did some experiments with my miniDSP DDRC-24 a while back following an online discussion about the benefits of replacing SMPS with linear supplies (spoiler alert - I didn't find any benefit!) and discovered that this was a potential issue. I also discovered that ground loops can arise from indirect routes - at one point, running an analogue source with a linear supply into the miniDSP gave me hum issues that went away when I switched to an optical source, even when the analogue source was still connected; I eventually traced the problemto the HDMI cable that connected my laptop to my desktop monitor, and the hum was the result of a loop created via the monitor's ground, the USB connection to the miniDSP and the ground of the LPSU for the analogue source...somehow the internal input switching of the miniDSP must have removed the problem when the optical connection was in use.

Suffice it to say, there may not be a simple answer. If you have another amp that doesn't cause a problem, why not use it?

Great answer thank you Tony_J,
More and more it appears as a combo of issues, the amp magnifying what was already there with its gain.
I’ve read about the hdmi potential impact but didn't think it could apply to me, I really didn't think it could go back through the usb! But now I reconsider, will check that.
My home setup is almost like a desktop setup, macmini/monitor etc 

Yes current amp is fine, in fact I did replace 3 amps recently because of a hiss, from hypex to iota, and in an effort to rebalance power usage of the whole active system.
Saved quite some money in the process, with lower noise floor at the end.
So I wanted to repeat it on mid-bass, but it might not be possible as here a good SNR amp with high power will always be on the expensive side.

Will check the hdmi, maybe the monitor outlet first before trying linear PSU (knowing it might bring some other issues).
 

Hdmi, USB, monitor out, or even everything else than dsp and this amp in fact > no effect on the noise.
I'm left with trying another psu, or/and add a buffer.

Also, the house is pretty old and I still have to renovate half of its electric system.
The outlet I use now is one of the best ones, but most are just bad, leaky or still w/o ground.
I might pull a line for hifi only in the same time, to be at least 100% ok on this side.
 

Hi grosso 

Hey Andy! 

Good wrote:

Hdmi, USB, monitor out, or even everything else than dsp and this amp in fact > no effect on the noise.
I'm left with trying another psu, or/and add a buffer.

Also, the house is pretty old and I still have to renovate half of its electric system.
The outlet I use now is one of the best ones, but most are just bad, leaky or still w/o ground.
I might pull a line for hifi only in the same time, to be at least 100% ok on this side.

 

Good luck!

Thx!
Some feedback, it was not a ground loop but just elevated noise floor. 
I tried an rca-xlr buffer/symmetrizer that boosted the signal by 8db, and allowed to use the low gain setting on the amp + manually reduce the gain.
Noise was definitely much lower and acceptable (silent from listening point).
Funny but the box itself had a little noise too, high pitch from the psu I guess, but silent once hidden in the rack.

In the mean time I measured voltage outputs for each amps with sines, and found out I just need much less power (/4).
So I decided to put the xls back on subs only, and am looking around for lower power amp.