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Old 13th October 2014, 05:28 AM   #1
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Default turntable hum

After posting a regrettable thread about the impressibility of having a no noise valve pre amp (mea culpa mea culpa), it seems that the hum is in the turntable. i have done some experiments and used different ss and valve preamps, turntable etc and it is the turntable. Its not the cart, so i am strongly thinking of rewiring the arm. i have messed with the ""outs"from the arm a lot and maybe i have detached a ground?
So, I want to rewire the arm.
How hard is it?
were can i get suitable wire?
I dont want to use some ridiculously expensive wire, but i want to make sure its going to work. The turntable is a technics 1200 that i put outboard power in. Mostly because it was easier than the step up as its 110v and i am now 240v.
All this started when i went with a MC cartridge, so i thought it was the preamp/SUT but i know believe there was always some hum in there but now its been amplified by the added gain in the SUT. This is not what i do for a living, so if you see some problems in my logic tell me
mick
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Old 13th October 2014, 05:55 PM   #2
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If you take the case off some Cat 5 wire, you will get 24 ga twisted pair. This would seem suitable to me.
Is your arm metal? is the metal grounded to the case of your preamp? Is there a green wire from the turntable to the preamp with a fork terminal on it? This was the standard way to eliminate hum the last time I bought a turntable, in 1981. I had to unscrew a case screw on my PAS2 preamp to fix the turntable ground to it, but it worked. If your arm and headshell are not grounded metal, you will need to buy shielded twisted pair cable and stuff it in the arm somehow. I've got some surplus Belden cable from 40 years ago, so I'm not the one to tell you where to buy it.
The BIC940 turntable from 1981 (changer actually, a 1.5 g one) still works BTW, I'm now reaching 1000 LP's since the price of used ones is down to $.33 or $.25. Some are damaged, out to the trash bin they go.
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Last edited by indianajo; 13th October 2014 at 05:59 PM.
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Old 14th October 2014, 04:45 PM   #3
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Before going to all the trouble of a rewire, I would make sure that:
1. Headshell and cartridge contacts are squeaky clean - those are a common trouble spot.
2. The cartridge does not connect the L / R returns (some like Grados apparently do, creating a ground loop). In case of RCA wiring, there must not be a connection of L / R returns anywhere before the amp.
3. Connecting the aforementioned ground wire has the desired effect (otherwise it may have come loose internally - it connects to the tonearm so that one can act as a shield).
4. The step-up transformer is kept well away from any mains transformers.

You can do a check of resistance from cartridge contacts to RCA outputs with a multimeter. You should be getting about an ohm or thereabouts.
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Old 15th October 2014, 06:05 AM   #4
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Thank you
I will check this.
I have tried different TT (a lot less hum but still some)
Battery preamp (no difference)
moved changed shook and hit everything. Its not a big hum but I can hear it.
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Old 15th October 2014, 06:49 AM   #5
Zero D is offline Zero D  United Kingdom
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As well as the above good suggestions, some carts have a little earth strap from the cart case to one of the carts ground terminals, if the case is metal. CAREFULLY removing it usually works.

Also if you have a multimeter, use the Low Ohms setting to trace for continuity, or not, with the phono leads disconnected from ANY preamp etc. If you get a reading between either/both of the phono ground leads & the earth lead and/or metalwork, that's where a problem lies.
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