F5 Ground Loop hum help please

Hi, my F5 has a weird 60Hz hum that I cant cure.
Here's the situation.
With no inputs connected dead silent
With 1 input connected dead silent
With 2 inputs connected HUMMMMM
So is this a "cross channel" Ground Loop ?

So far I have tried a Hum Buster No Change
Lifted the ground via cheater plug No Change
Went from a non shielded i/c to a shielded i/c a little better but still not good

The amp is connected to a Battery Powered Preamp, don't know if that makes a difference ?

Could someone here give me some advice?
I have searched the F5 builder thread but didn't really see anything that would help
 

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Joined 2011
Yes, if you are using shielded twisted pair, instead insert the 10R in series with each "ground" lead
(instead of the shield). I'd guess that is the white wire? Use a separate resistor for each RCA,
even if only one resistor removes the hum.

The far end of the shield can remain connected where it is. The RCA end of the shield should remain unconnected.
 
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Joined 2011
Some amplifiers do use them as standard. Some don't need them, like monoblocks.
Depends on the layout, construction, and circuit, and DIY varies all over the map.
A stereo amplifier has an inherent susceptibility to ground loop hum.
 
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Joined 2019
Paid Member
If this cures the hum, why isn't it standard on the build ?
Is there a trade off ?

It sort of is standard. There is a "ground loop breaker" in the form of an NTC thermistor in the PSU schematic between the PSU (Audio GND) and chassis GND. I don't see that implemented in your build, but I may have missed it in the photos.

Separate topic / different thermistors - You should have your thermistors touching the body of the output devices (preferably) on the plastic body vs. hovering over your bolts. Contact, so they work to their best effect. On the body vs. the washer or bolt b/c if they fail and short to chassis ground, it may not end well. The latter part may be being a bit too on the edge of erring toward safety, but I've seen that practice referenced quite a bit. I used a different type of thermistor, so YMMV.
 
Interesting article , however I wouldn't know how to implement it on my F5.

I did put the 10r resistors at the rca jacks and it did help but did not totally remove the hum.

Is this not the "ground break" thermistor in the picture
 

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