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TT ground issue

M

matt j

pfm Member
I should really sort this now as it is getting rather boring.

Thorens TD-125 Mk2 and SME 3009 S2 improved fixed headshell.

I get quite an audible hum in the background and static pops when lifting/placing an LP on the platter, all other aspects of replay are deathly quiet. It was also quiet with the stock arm.

I've tried all manner of earthing setups but I can't get rid of it, I have had silent TT replay here before so it has to be something about this deck and arm and how it's wired/earthed.

I'm thinking of trying a different arm because I can't see what else it can be.

Any other suggestions? It ruins what is my preferred source and has resulted in me not using it much of late.
 
Does the hum/pops go away after you’ve placed and queued the record, and are no longer touching anything?

Does this happen mostly during the dry spells of winter? If yes, then it sounds like you are the source of static discharge.

Do you have a grounding strap that you could try wearing? If that works, placing an anti-static mat on the floor in front of your table may help.
 
The hum is constant. The pops are just when I go near the platter with an LP, it didn't do either of these with the stock arm fitted, it's only since I changed the arm this started.
 
You don't detail the current wiring arrangements. That makes it hard to spot any likely potential causes.

But if the hum, etc, appeared when you changed the arm, the obvious implication is a change to the arm's ground wiring, etc. Details?...
 
These are always difficult to diagnose at a distance but from what you say my guess is that the TT is not earthed properly. The 'pops' are likely due to the build up of static electricity and that points to bad earthing. Your original arm was providing the earth to the TT but your new arm isn't hence hum and static build up.

Cheers,

DV
 
Have a look below deck to determine whether the tonearm to turntable chassis ground wire is attached to the sub-chassis of the TD-125. I've seen a few SME wiring harnesses where this has been snipped off, possibly from when the arm was previously fitted to a solid wood plinth deck.

Also, the clip/socket that makes the ground connection at the tonearm end of the arm cable can become a loose fit. Try removing the plug assembly and pinching the ground clip/socket tighter before refitting. It wouldn't hurt to clean the electrical contacts here as well.

SME2pge8.jpg
 
Sorry for the delay, been away with work with no net access.

Thanks for the replies in the meantime, I'll work through them.

The arm doesn't have the standard SME cable, it goes to one of those RCA socket conversion kits mounted at the arm base and then a standard RCA arm lead is used from there.

So far I've tried-

1. Arm earth connection on arm base -> connected to arm lead earth -> ground lug on phono stage.

2. As above plus with a chassis earth cable back to mains earth via a 13 amp plug top.

3. Just the chassis earth to mains earth

4. None of the above.

I notice there is no TT earth just a live and neutral, the best arrangement from the above is No. 2, but with that I get loud pops when touching the arm or arm lift.
 
Make sure that somehow the turntable’s chassis and main bearing (i.e. the spindle that touches the record) is grounded to mains earth. I don’t know enough about the TD-125 to know how this is implemented, but it is very easily achieved with the 124. This gives any static charge in the record a path to ground other than via the cartridge and is the secret (along with properly clean vinyl) of quiet replay IMHO. My TD-124, like the 125, has a wooden armboard, so I treat the turntable as one thing grounded to mains earth, and the arm (in my case a 3009) another which is grounded via its earth terminal to the preamp or phono stage. I have no electrical connection between the arm and deck.
 
Thanks Tony.

I'll get the multimeter out and see where I'm getting continuity. In theory should grounding the TT chassis also earth the bearing by default?

It will probably be the weekend before I get time to pull it all apart again so I'll update when I get chance.
 
matt j said:
In theory should grounding the TT chassis also earth the bearing by default?
Click to expand...

Assuming a metal chassis and bearing, yes. I test by seeing if I have continuity between the centre spindle and the earth pin of the mains plug. If so I know I’ll have no issues with static.

Hum is a whole different thing and can be a PITA to really sort.
 
I think you are nearly there wrt solving the hum problem; the turntable chassis ground should be connected to the amplifier chassis along with the tonearm ground wire (preferrably via a single wire 'Y' connected at the tonearm base) else you create a lesser ground loop by connecting the tonearm ground to amp and the turntable chassis to mains earth. If that improves, but doesn't eliminate the hum then add an earth wire from the amp's ground post to mains earth.

The static pops are likely to remain until such time as humidity rises. I long ago invested in a humidifier upgrade on the central heating and haven't had a pop, crackle and/or fizz since.
 
I think I only get the pops when there's no chassis earth connected.

I'll try as you say and run an earth from the chassis to the phono amp post with the arm earth, along with some of the other suggestions.
 
well hum hum. I had hum problems with my Roksan Xerxes and Rega RB300 into my Tag Mclaren preamp. config was cartridge earth to arm earth & that to preamp "earth"/case. Preamp was a floating earth design so had no genuine earth. That was ok for a MC cartridge but when I moved to a MM Ortofon 2M got the hum - partly solved by removing the cartridge to arm link. Still some hum, in the end then ran a real earth from the preamp case to a dummy mains plug ( with only the earth pin in) I made up solve the issue.
 
You've not mentioned what cartridge you are using. Some cartridges are unshielded, such as Grados. I had a real saga sorting out hum when I first used a Grado, but it turned out that the most important thing was keeping the cart away from the mains sockets (my rack had the mains sockets directly behind them at the time). Keeping the cart away from the motor was also crucial, but I knew that. It was the proximity to mains issue that took me ages to hit on.
 
Grado cartridges are shielded, just not very well.

'Keeping the cart away from the motor was also crucial'

Yes, and not playing the last one or two tracks of every record side has the added benefit of eliminating end of side distortion. :)
 
Aside from eliminating hum, I've found making sure earths are correct can have a significant effect on sound quality. It's quite possible to have no hum, but a compromised performance.
 
Elephantears said:
You've not mentioned what cartridge you are using. Some cartridges are unshielded, such as Grados. I had a real saga sorting out hum when I first used a Grado, but it turned out that the most important thing was keeping the cart away from the mains sockets (my rack had the mains sockets directly behind them at the time). Keeping the cart away from the motor was also crucial, but I knew that. It was the proximity to mains issue that took me ages to hit on.
Click to expand...

I've ruled out the cartridge seeing as it worked fine on the same TT in the stock arm, the cart variable has stayed the same.
 
Yes, good call, a multimeter should show continuity between the tonearm metal and amplifier chassis, as well as between the turntable chassis and amplifier chassis. If you've summed both the turntable chassis and the tonearm grounds at the tonearm end and aren't seeing continuity between tonearm metal and amp, but are seeing continuity between the turntable chassis and amp then you have a ground fault within the tonearm.

At this point, I am curious to see some high resolution 'upskirt' pics of this RCA socket conversion, if that is possible.
 
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