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Motor hum? Electromagnetic Interference ? Ground issue?

cmcguire

cmcguire

Active Member
I'm at a total loss. I have two identical turntables (Mission 775s) with Shure cartridges. They use slightly different grounding schemes and BOTH have the same issue: when the cart reaches the centre of the record there is a low, but audible hum. These tables sound fantastic and solving this issue would make them darn near perfect for my system. Any help would be much obliged!
 
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I'll try a thicker mat- though I'm not sure that will make an audible difference as this table has a 13lb platter! Here's hoping though. I should also add the following,

TT off: slight hum near spindle
TT on: greater hum near SPINDLE
TT unplugged: no hum whatsoever
 
No I meant - is there a power supply built into the turntable (supplying a DC motor) or is the motor direct mains powered?.
 
mhardy6647 said:
sounds like an unshielded cartridge. Grados, e.g., have this problem with some tts.
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Certainly could be the case but it' a Shure M95ed. Never had a problem with one before. I'd read somewhere that the motor on these is the same as on the early Linn models that had a tendency to be a little noisy.
 
I believe I am correct in saying that these turntables have an aluminium sub-platter. It's a long shot, but I wonder if the motor is inducing hum into the sub-platter and that's what the cartridges are picking up as they approach the centre of the turntable. It might be worth grounding the centre bearing to a good earth - I would not suggest using the phono ground wire though. It might just work.
 
gadget73 said:
Does the M95ED have a ground strap from one of the ground pins to the cartridge body? It may need one.
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I'll try it tonight. I'd wager it's the cartridge because I'm experiencing the exact same phenomenon with the same cart on my old td160!
 
Many turntables have step-down transformers in them to convert AC to a low voltage, then DC rectify it. The location of the transformer is sometimes troublesome. I've owned a couple of tables that had the transformer right under the cartridges when it was parked. Sounded awful when it wasn't playing.
 
Powertech said:
I believe I am correct in saying that these turntables have an aluminium sub-platter. It's a long shot, but I wonder if the motor is inducing hum into the sub-platter and that's what the cartridges are picking up as they approach the centre of the turntable. It might be worth grounding the centre bearing to a good earth - I would not suggest using the phono ground wire though. It might just work.
Click to expand...
Aluminum is nonferrous.
 
Powertech said:
I believe I am correct in saying that these turntables have an aluminium sub-platter. It's a long shot, but I wonder if the motor is inducing hum into the sub-platter and that's what the cartridges are picking up as they approach the centre of the turntable. It might be worth grounding the centre bearing to a good earth - I would not suggest using the phono ground wire though. It might just work.
Click to expand...
No, the sub platter is not ferrous. In fact, it's not even metal, it looks like some kind of phenolic. The center spindle is grounded to the sub-plinth.
 
Aluminium is non ferrous, but you can still induce an electric field into it, and that field can then induce a magnetic flux into anything that will magnetise.. Ever seen strip lamp tubes lit up next to overhead HV power lines - these are quite often aluminium cables.
 
Powertech said:
Aluminium is non ferrous, but you can still induce an electric field into it, and that field can then induce a magnetic flux into anything that will magnetise.. Ever seen strip lamp tubes lit up next to overhead HV power lines - these are quite often aluminium cables.
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something else you can do with aluminum. spin it in a magnetic field, and it will also become partially magnetic. That's how the old speedometers worked
 
I always thought that worked by some sort of electrical induction. The magnet spun into what amounted to a shorted turn and that was what drug the needle along.
 
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