Let me understand, you are running 30' of ICs BEFORE the signal gets to the phono stage? Remember the cartridge generates a VERY low level signal compared to a CD player, tuner, etc. If I had to place my tables that far away I would buy a cheap phono stage to put next to them; this very long cable run may be the source of your hum; in any case it is a big no-no.
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Just not reasonable to expect to run thirty feet of interconnect from any TT.
IF you must have them away from the equipment, you need to get some separate inexpensive phono preamps and have them next to the TTs. Then you can run a long wire from the phono boxes to your other equipment.
Also, the ground wire from the TT has to go tothe same place as the interconnects. That ground wire can be any small gauge wire of nearly any type.
Solid, stranded, as small as 30 gauge, or as bigas 10 gauge. a normal 18gauge lampcord wire will do to.
The mani thing as I mentioned it has to ge from the TT TO the same box as the interconnects.
First hum...the ground wire may not be the cause of your hum it could be the interconnect between the pre amp and turntable. This can occur if the interconnect has been placed too close to a power cord of any kind.
Note placing the power cord or the interconnects at right angles to each other may not solve your hum problem. Only distance between the power cord and interconnect will make a difference.
Replace the 30 foot section of ground wire with a single contiguous piece of 20 to 24 gauge wire.
When your cartriage output is only 3 to 4 mv, 30 feet is a long way for the signal to travel without picking up noise. Can you use a step up transformer for the cartriage to boost the signal over this distance?
In addition to the good comments above, another reason that a 30 foot run from turntable to phono stage is unreasonable is cable capacitance. With a moving magnet or other high output cartridge, sonics will be significantly degraded unless you use a cable having extremely low capacitance per unit length AND the cartridge is specified to work into high load capacitances. With a low output moving coil cartridge that may or may not be an issue, but the likelihood of noise problems will increase significantly.
Get a separate phono stage, as Stan and Elizabeth suggested.
I've had signal degradation with 12 foot cables from the turntable. You may not realize it unless you could compare the shorter length first. Upper end treble loss, no air, and it was just plain boring sounding. If I didn't know how it sounded before the longer cables, I may have blamed a lot of other stuff first, and wasted money.
Running a 18 awg for the ground temporarily may help hum.
In the long run, move them closer, or get a phono stage as mentioned above.
Thanks, all, for your suggestions. The length is more like 18-20' not the 30' I mentioned originally. My setup and physical layout of the room and the audio rack preclude my getting the two TTs any closer than they are currently. I'm going to distill the advice you've given me and try to solve this issue. I've had a busy schedule recently but must try removing the sub woofers from the equation, temporarily to see if that helps.
Getting 20-24 gauge grounding wire might be another step but I'm still not sure what kind of wire I should use. Hmmmm.
The extension phono cables could also be an issue. Perhaps a heavier gauge of female to male RCA phono cables could help things. Thoughts?
Thanks all. I'll check back for further responses soon.
18-20 feet is still mighty mighty long for a cable transmitting such a miniscule signal. Above all other cables I would try to keep the phono cable as short as possible (and make sure it was shielded). Without shorter phono cables (or maybe balanced cables) I would be surprised if you are able to get the hum out of the vinyl setup.
The gauge of the rca extension cables won't make any difference. What is important, in addition to shielding effectiveness (which is hard to predict) is (as I mentioned earlier) keeping cable capacitance as low as possible, particularly if you will be using moving magnet or other high output cartridges. I suggest Blue Jeans LC-1, which provides very low capacitance per unit length at a relatively low price.
For purposes of minimizing hum, the kind of grounding wire you use is not critical, as Elizabeth indicated above. I suggest initially experimenting with whatever is handy, or whatever you can get cheaply and conveniently at Home Depot, Radio Shack, etc.
If and when you eliminate the hum, however, you may then find that you are picking up radio signals or other interference. To address that kind of problem, the best type of ground wire would be a "braided ground strap." Google that term for further info, if necessary.
Continued thanks to all for your suggestions. I'll let you know how I make out once my schedule slows down in a few weeks.
Since I cannot reduce the distance the between the two turntables and the respective pre-amp and receiver, Elizabeth and other suggest getting inexpensive phono preamps for each turntable. I'm looking at some that are about $50 each. Does that sound right? Any brands anyone suggests, in particular? Does jumping straight to getting phono preamps (I may even have one in the attic now that I think of it!) sound like a good starting place (before changing the extension phono cords to better shielded ones, before replacing ground wires, before disconnecting cables to my two (separate system) sub-woofers? Thanks folks!