P20, Chord 3005 amp, Rel S812 - hum from high level sub input

Looking for some help / advice from the PS Audio experts on resolving a hum issue with my Rel S12 which seems to be impacted by my P20.
(I’m in the UK.)

P20 driving 2 Chord monoblock amps.
Both mono blocks connected to a Rel sub via high level input.
One of the subs is a S812, (class D amplification).
Both subs are on the same zone as their associated mono block amps to provide a common ground for amp and sub.

When the amp and S812 sub are both on I get a hum from the sub,
This hum is linked to the high level input and increases and decreases as the high level volume control on the back of the sub is raised and lowered.

I’ve located the source of the hum by removing plugs from my P20 and the culprit is my Chord 3005, a 5 channel amp for my home theatre speakers. when it’s plugged into the P20 the hum is there, when it’s removed from the P20 the hum disappears.

Also if the Chord 3005 amp is subsequently powered from an extension block from a wall socket the hum is gone as well.
Power cords checked for any faulty wiring, none found.

I’ve spent a lot of money on the P20 to power all of my equipment and I’d prefer not to have to run my Chord 3005 from an extension block.

So why does the Chord 3005 cause the S812 sub to hum when both are powered via the P20?

One other consideration, when only the P20 is running I hear a very low level hum , I presume coming from the P20? I am considering buying a IsoTek EVO Syncro Uni,( DC blocker), to see if that cures the hum.
Obviously once anything is playing the hum can no longer be heard.

When I switched my system on last night, expecting no hum, the hum was back.
I experienced the following :-

Even having the 3005 amp plugged into the extension block is very socket specific.

Yesterday when I tried it in the block, no hum.
But after further testing I ended up plugging the 3005 plug into a different socket on the block, just because it was further away from the back of the sub.
(the extension block sits directly behind the sub.)

When I tested it last night the hum was back.

This morning with the 6000 and the sub on I tried removing plugs from the back of the P20.
Pretty much removed them all and the hum was still there.
I then moved the 3005 plug into the same socket on the extension block as I used yesterday and the hum disappeared.

I now also have all the plugs back of the P20 and the hum is still gone.

I’ve a working system now but I’d be happier if I could find the source of the problem, fix it, so that I can re-plug my 3005 amp into the P20.

I’m really hoping some of the PS Audio experts will respond to this thread.

See my other thread about adding a Isotek EVO3 Syncro Uni in front of my P20.
(Purchased to see if it could resolve my issue)

The unit seems to have had a positive effect on my hum but not cleared it.
The sub high level volume control now needs to be raised to the 12 o’clock position before it can be heard.
I have also been able to move the Chord 3005 power cord from the extension block back into a socket on the P20.
Previuosly as soon as the UK earth pin, (the longest prong), touched any socket on the back of the P20 the loud hum started.

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I can only speculate, but I wonder if removing the ground pit will eliminate that. Short of that putting something like the iFi ground loop defender on the end of the power cord into your sub might do the trick.

One last thought, and forgive me if you said you tried it already, but have you tried bypassing the P20 and plugging the REL directly into the wall?

I completely sympathize with your situation I myself am battening some hum and hiss that is currently winning.

Thanks for the helpful suggestions.
I’ll try plugging the Rel sub into an extension socket, (no spare wall sockets).
If that doesn’t work I’ll try the Ifi ground loop defender, (is that the same as the Ifi dc blocker?)

Tried the Rel power cord removed from the P20 and into an extension block, hum was worse.

Checked on Amazon, the Ifi ground loop defender and dc blocker are different products.
I’ll try both of them to see if either work.

Good luck, Let me know if either help….

This is a really easy thing to solve without investing in any additional products though iFi does make one that will work. Your ground issue as you may have suspected is due to the Class D amp in your S812. The simple solution is to take a single piece of wire of whatever gauge you have around and run it from one of the screws on the Class D amp to a chassis ground point on another component or better yet a grounding lug.

Expecting the Ifi DC Blocker later today.
I’ll try out the suggestion of the earth wire going from my Chord amp, (it has a ground terminal) to the Rel S812. I’ll need to check the back plate of the Rel to see if any suitable screws.

I don’t think they are that easy to use for grounding. I remember I had to have a Rel rep come to my house to tighten something on the back of the Rel. (It wouldn’t tighten for me).
Checking with Rel to see if they suggest any suitable screw, quite a few on the back late.

Any of the 12 screws along the perimeter of the amp should work fine. The least visible would be along the bottom edge. I’ve always just used one of the pairs from old “zip cord” speaker cable.

Yes, that seems to have worked well.
I used the middle screw along the closest side, I might well move it to a lower bolt now that I know it works.
I’ve had this issue for a couple of months now, it’s great to get a solution.

Additionally this allows me to run my Chord 5 channel amp again from my P20, thus getting the benefits of the P20 again.
(I did have to run the amp from an extension block while this issue was ongoing.)

Thank you again for your very helpful suggestion.

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Glad it worked out. That is the easy solution for either REL subs with Class D amps or Class D amps and REL subs with AB amps. With these combos one or the other is not providing a true ground through their negative speaker terminal and requires an alternative ground source.

I can definitely confirm the earth wire between my chord amp and sub has cleared the hum issue.
Watched a movie for the first last night since the fix applied and no hum from the sub, even with the sub high level volume increased past the 12 o’clock position.

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I spoke to the Rel engineer today regarding the bolt on the back plate.

He stated that there is a bolt near the power inlet that has a ground symbol beside it and people have, in error, been using this for a ground connection.
This earth connection on the inside of the backplate has all the grounds from the amplifier boards connected to it so dislodging this bolt can cause problems.
Better to use the other OUTER bolts along the edge of the backplate.
(I used the middle one on the left hand side of the backplate).
You can fully remove these bolts to allow the addition of a washer on the bolt which will give a much better connection for the earth wire rather that trying to wrap it around the bolt.
I’ve just done this today, with no issues. The bolt was actually much longer than I had expected.

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