Ground loop hum?

[Chris H]

Been a while since my last post but have a frustrating issue that I can't seem to resolve.

I have a Rega Planar 8 TT with MC Apheta cart into Eros II.
Prior to the Eros, I had a Rega Aria phono which gave a noticable hum, getting louder as the volume was increased.
I understood this to be a ground loop issue which is apparently common on Rega TTs when mixed with other brand gear (running into Naim Pre and Power).
Issue was never resolved but was not all that noticable when music is playing.

When I installed the Eros, prior to fitting SUTs, the hum seemed to have disappeared.
I say seemed because as it is a low output MC cart and, prior to fitting the SUTs, volume would have to go quite high to the point that tube rush became noticeable before I noticed any hum.
I fitted the SUTs (20:1) and tube rush has gone (at any volume I have ever play it) but the hum returned.
I have lived with it for a few months but it is a constant irritation I would like to resolve!

My first question: is the hum a ground loop issue as I have assumed.
The sound is the same hum I get if I remove the RCA interconnect between Eros and pre-amp at the Eros end and touch the signal part of the RCA connector.

Second question: will attaching the shielding wires of the SUTs differently make any difference.  They are connected to chassis earth (lugs 3 and 18).
Is signal ground different from chassis earth anyway as they appear to be connected between the ground post and lug 6.

Final question: If I haven't made a mistake and attaching SUT shielding doesn't make a difference, does anybody know a solution?

Additional information:
I have tried some of the suggested solutions found online involving connecting an earthing wire between phono pre and pre-amp.
I have tried from both the ground post and from the ground of the left RCA (this is where I understand Rega run their ground wire instead of having a seperate ground wire connected to the ground post).
Disconnecting the TT from the Eros makes no difference which would suggest it is not the common Rega TT hum after all..
The exact same hum is coming from boith channels.
I have tried different cables, home-made, purchased, shielded and unshielded, all to no avail.
I have also tried into different inputs of the Naim pre-amp, both RCA and DIN.

I have attached a photo of one of the SUT installs.  Can provide more photos if issue elsewhere in the Eros is suspected though I was ruling this out due to it being an issue with my previous phono stage.

Any help or suggestions appreciated, noting that replacing the Rega TT has already been suggested more than once elsewhere.
Apologies for the long post but thought it better to provide as much information as possible.

Best
Chris

[Doc B.]

SUTs are susceptible to hum induced by magnetic fields. That's why the Eros has a power transformer with a very low radiated flux. It is possible that your SUTs are being affected by some other power transformer in your system that is relatively nearby, or perhaps the AC motor in your turntable. I found that I needed to move my outboard SUTs around on the shelf to find a quiet spot in our old room. You may want to try moving the Eros around to see if you can find a spot where the hum is reduced.

[Paul Birkeland]

I just dealt with this on a Planar 8 earlier this year, and you're not going to like the answer...

Normally what we want with a tonearm is the metal tube of the tonearm to act like a shield, and for this shield to continue as a shield wrapped around the interconnects as well (braid or foil) which terminates at an earth lug on the phono preamp.  Then the signal wires come off the cartridge and run as twisted pairs all the way down to the RCA jacks. This is super useful for a moving coil cartridge especially since you can use step-up transformers and the signal wires and shield wires aren't connected.

Rega completely botched this to be honest.  Their phono interconnects aren't shielded at all which is a little alarming.  Then they take the earth drain wire from the tonearm and just send it down the signal cold of the left RCA cable.  This injects noise into the signal path, which isn't helpful. 

What I had to do on the P8 was to pop the rubber plug off the bottom of the tonearm, then it's clear where the black and blue wires are soldered together.  What you want to do is to gouge out the trace between where the blue wire mounts and where the copper earthing tang solders to the board, then move the black wire over to that connection to separate the left signal cold from the shield (the blue wire stays put).  Now that you've done this, loosen one mounting screw on the tonearm and wrap a wire around it, then tighten it down and run that wire to the earth lug on the Eros. I was prepared to also need to slip some braided shield over the RCA cables to provide some extra shielding, but everything worked nicely when I got to this point.  Be sure to pull the platter and get the ball bearing out before taking on this work, as I had to chase down a tiny ball bearing in a giant rug in a very fancy house when I did this and my customer wasn't super impressed.

This is all so ridiculous that I took a picture of the plug for future reference.  You could also switch to a moving magnet cartridge and no SUTs as well. 

[Chris H]

Think I'll try Doc's suggestion first!!! 

In all seriousness though, the hum is still there even when the P8 RCAs are disconnected from the Eros.
Wouldn't this rule out the P8 and its motor?
Although, in some ways an electrical fix might be easier than solving a location issue if pushed for space.

In any case, thank you for the quick responses - I will let you know if I have any success.

Chris

[debk]

I get no him with a Rega P6 and the Eros 2, I installed sowter SUTs in the Eros 2.  If you are using an external SUT, did you connect the ground from the SUT to the Eros?  My Bob's device SUT had a ground lift switch on it that made a difference in hum, though I don't remeber what setting I used it on

[Chris H]

I had a P6 previously with no hum but that was moving magnet. Is yours MC?
SUTs are Sowter 1990s installed in the Eros with shield wire connected to chassis earth.
The hum has been since I got the P8 with moving coil.
Previous MC phono stage had the hum.
Eros with SUTs has the hum but not when TT is disconnected which would seem to point to a siting issue with the phono stage which is a pain.
This weird grounding set up  Rega use has perhaps distracted me from the real issue but good to know how to convert P8 to standard grounding configuration if it isn't location.
Really surprised to hear the phono cable is not shielded - I did not know that!
Assume they are just relying on twisted pair to reject noise but on a low output MC cart? Would this be to keep cable capacitance low?

[Chris H]

Update - just seen you sig - Ania cart answers my question

[Paul Birkeland]

In all seriousness though, the hum is still there even when the P8 RCAs are disconnected from the Eros.
Wouldn't this rule out the P8 and its motor?

Try shorting the input RCA jacks with nothing connected to see if you still have hum.

[Doc B.]

Hum still there with the P8 not connected would rule out the P8 picking up hum through the cables and putting it into the Eros. But it would not rule out the motor's magnetic field being picked up by the SUTs. The easy way to see if that is the issue is listen to the Eros with the P8 motor on, then with it off.  Bear in mind it could also be the power transformer in some other nearby piece of gear that generates the magnetic field.

[Chris H]

It seems the humming is not as loud without the TT connected after all and may not be quite the same sound.
In moving the Eros around without the turntable connected (the phono cables are not that long), I made an interesting discovery.
With both hands on either side of the wood base and thumbs touching the top of the chassis, the hum without TT connected disappears completely.
Thumbs on top without hands gripping the side of the wood base, the hum stays.
Hand on the side without thumbs on top makes the humming louder and exactly the same as if the TT was connected. How would gripping a wooden base cause this?
With the TT connected, hum stays no matter what I do.
Both with and without TT connected, moving the Eros around (subject to limitations in length of interconnect) made no discernible difference.

I am now thinking there may a low magnetic hum AND the 50hz ground loop hum.

Before I attempt to short the RCA jacks I thought I better check what you mean.  Do you mean across signal and ground in each channel or from left to right channel?
I assume the former but thought I better check.

[Chris H]

OK, so the shorting of RCA jacks was a good shout.
A spare RCA connector in the input jack with signal and ground connected causes exact same behaviour as when TT is connected.
I guess this rules out the turntable?

I sense a lot reflowing of solder joints coming!
There's still the mystery of my hands gripping the wooden base.

[Chris H]

P8 motor on or off makes no difference.
I guess I am down to something in the Eros I have done incorrectly or something nearby interfering.

[Paul Birkeland]

I would tighten down all of the screws in your kit.

[Doc B.]

Yup, tighten everything up. You might also try running a wire from the Eros ground post to the chassis of the Naim preamp and see what happens.

[Chris H]

Have been away so just got round to suggested actions.
Earth wire between ground post and pre-amp had no effect.
Everything seemed fairly tightened down but I decided to reinstall the SUTs.
I had a nut either side of the chassis so that the toothed washer was between SUT and nut.
Now have washer between SUT and chassis, 2 nuts under the chassis.
I also moved the SUT ground wire direct to signal ground (though as far as I can tell this is connected to chassis anyway)

Anyway, hum is still there but does seem quieter. Not so intrusive in the gaps. Sowter website does say may need to experiment with different grounding locations to eliminate hum.
I am going to try switching out my Naim pre/power for my office S.E.X. amp and switching off all other components to see if other equipment is a source of noise.
If this works then the case for going full tube increases.

Thanks for the help and advice.