The dreaded stubborn hum

[alternativeroute]

Following on from this thread: http://www.avforums.co.za/index.php/topic,4887.msg66384.html#msg66384

Basic setup is the Garrard 301 mounted with a SME 3012 arm...

Anyone who has worked with SME arms will know that the the RCA leads plug into the bottom of the tonearm with a proprietary SME connector...

Now at the end that plugs into the tonearm is an earth lead. I always assumed (from turntables I have taken apart) that this must be attached somewhere to the turntable assembly....

When I did that about 50% of the hum disappeared but it was still intrusive.... So I disconnected the lead and found that I could get rid of 96% of the hum by looping it back and connecting it to the SME baseplate     

There is still a slight hum that mostly disappears if I touch the baseplate and one of the mounting bolts on the Garrard...

Does anyone know where that earth lead is really supposed to go? Is there a possibility that the earth  wire in the tonearm is either broke or disconnected...


see attached diagram...

[ludo]

I don't know SMEs but you want the arm itself earthed and you want the metal bits of the turntable earthed, ie probably at least the platter/bearing. Have you tried connecting both arm base and the rest of the metal goodies to that ground wire? If it also finds its way to mains earth (via the amp or directly) that's even better.

[alternativeroute]

I don't know SMEs but you want the arm itself earthed and you want the metal bits of the turntable earthed, ie probably at least the platter/bearing. Have you tried connecting both arm base and the rest of the metal goodies to that ground wire? If it also finds its way to mains earth (via the amp or directly) that's even better.

I have tried the earth wire to the chassis of the turntable and that got rid of half the hum... The best result has been connecting that lead to the tonearm base. I am going to try an extra wire from the tonearm base to the chassis and see if that helps...

To give you an indication of the scale of the hum... I can only hear it if I have no record playing and the volume is turned passed 12'o'clock (which would relate to 'call the police' volumes)...

With the Thorens TD150 I have, there is an earth wire that comes from the tonearm and is attached with a nut to the chassis. Earth to the amp runs through the right channel shield. Of all my turntable set-ups this is by far the quietest. For the RCA leads I soldered on 75 Ohm Coax Cable (which apparently is a big no-no) to the terminal strip under the turntable... Using the same 'turning passed 12'o'clock' test there is no noise other than a tiny bit of static type noise (more an indication of 95db sensitive speakers and a noisy amp power supply)...

[Shonver]

The earth wire that goes to the amp/preamp must be connected to the arm. It might be necessary to earth the chassis, too, but the arm has the primary need to be earthed. If the chassis is earthed. it can just be an additional loop from the connection at the arm base.

BTW, what is the issue with using 75 ohm cable?

[mafioso]

Hi

You should only have a single earth wire from the arm to your amp or pre.

mafioso

[GearSlave]

tiny bit of static type noise (more an indication of 95db sensitive speakers and a noisy amp power supply)...

That is semiconductor noise my friend.

[alternativeroute]

BTW, what is the issue with using 75 ohm cable?

Good question. I probably read it in a forum post somewhere... Read it, and absorbed the information out of context...     

So no problem really? Does the use of 75 ohm cable not affect the cartridge loading? 

BTW. I am regretting spending high school science scratching band names into desks instead of listening to the teesher...

Hi

You should only have a single earth wire from the arm to your amp or pre.

mafioso

At the moment that is essentially the setup... There is no wire connection at all to the Garrard motor unit...

That is semiconductor noise my friend.

Ha ha - I looked up semiconductors on wikipedia.. I will have to believe you     
The noise isn't a problem... no chance it can be heard from the listening position


Some more illustrations....


source = http://www.analogue-classics.com/html/3009SeriesIIManual/Thumbnails.html

[ludo]

Is there electrical continuity between the ground wire and the arm base? There should be. If there isn't, then looping the loose ground wire back to the arm base will help, as you described. So I'm wondering if something is tarnished or loose and not connecting to the arm base/arm tube etc properly via that multipin connector. Check for resistance between the arm tube and the ground wire coming out (with the loose wire not connected to the arm base.) If you find an open circuit from the Amp side ground wire to the arm base then, the problem is somewhere in the connector or the ground wire inside the arm.

Cable impedance does not matter at audio freq. Coax is coax <ducks>

[alternativeroute]

Is there electrical continuity between the ground wire and the arm base?

This is what I suspect might be the case. I need to take the tone arm off and give it a complete service as the coupling rubbers are also shot.

I have another that I am in the process of rewiring... Once that one is done I can put it on the table and see what gives...

What I am trying to get my head around is why the motor unit should ever be earthed. Surely there is never an electrical connection between the motor board and the amplifier. So why then would an earth connection be necessary...

[Hi-Phibian]

too lazy to look it up but if the cable C is a few dozen pF per metre and the phonis terminated on the high side, say 250 pF or more, it can affect response with an mm cartridge. all depends on the pF per metre of RG6

[Shonver]

About the cable: the 75 ohm rating refers to its operation at radio frequencies. Hi-Phib's response is of course the correct one (that of caution), though I can attest to the fact that I have never had problems with RG58 and RG59 in terms of signal loss or filtering.

The short wire is indeed for connecting to your motor unit or turntable chassis and may not be required, but it is there just in case. The images you posted remind me that the SME arm is ready earthed when you use its original plugs and leads, so I would suspect a fault if you are getting hum.

BTW, what cartridge are you using? I recall a comment I read about some cartridges being reknowned for picking up hum.

[Hi-Phibian]

seems to be 54pF per metre. a little high so some caution should be exercised and the load capacitance of the phono stage considered.

[Ampdog]

That would give a theoretical -3dB point at 60kHz with the usual 47K.ohm load. But MM cartridges are inductive and depending on the value, an increase in high frequencies could be found. The head inductance is specified for the better models, or could be found from the manufacturer.

A good test with a scope and signal generator can reveal the exact electrical picture. Feed a 1k - 100kHz current (signal generator through say 100K) into the pickup lead using a low capacitance scope probe and draw the impedance-frequency graph.

[alternativeroute]

BTW, what cartridge are you using? I recall a comment I read about some cartridges being reknowned for picking up hum.

...been a bit of a cart slut with this set-up...

I have tried the following that I have at hand...

• Ortofon 2M Blue - best match so far
• Ortofon OMB 10
• Shure M85-G
• AT 8008 - I had hopes for this one... Still not a good match

[Hi-Phibian]

That would give a theoretical -3dB point at 60kHz with the usual 47K.ohm load. But MM cartridges are inductive and depending on the value, an increase in high frequencies could be found. The head inductance is specified for the better models, or could be found from the manufacturer.

A good test with a scope and signal generator can reveal the exact electrical picture. Feed a 1k - 100kHz current (signal generator through say 100K) into the pickup lead using a low capacitance scope probe and draw the impedance-frequency graph.

Is that incl the from about 100 to max around 330pF found on the input of phono stages?