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Old 1st April 2013, 09:44 AM   #31
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by gootee View Post

If the tone-arm switches the AC on and off, make sure they didn't run just ONE of the AC wires to the switch and back. It should have BOTH wires of the AC pair, tightly twisted both there and back, going wherever either one goes! Same goes for the run to any fuse, etc............................
Gootee,
this advice is wrong.
You have stated it a few times recently and I have let is pass without comment.

The flow and return pair going to the switch works just as effectively as the single flow going to the switch and the single flow returning from the switch. Twist the single flow with it's return mate, i.e just TWO wires going to the switch.
Twisting of a flow and return pair is what reduces the loop area.
Be careful at the junction point where the switch flow& return taps off from the mains cable. There is potential for a loop area there as well.
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Old 3rd April 2013, 12:05 AM   #32
gootee is offline gootee  United States
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AndrewT,

You are correct. I should have said "untwisted", too. I guess I was only imagining certain scenarios where one wire got split off to go to a switch or fuse, or was daisychained to both a switch and a fuse, and then rejoined the other one via a different return route, so that it couldn't be twisted with itself. I have seen that a lot.

In this case, for example, maybe he'd find the AC pair at the entry point, then one wire going straight to the motor and the other going to a switch and then to the motor.

Yes, you could have the single wire that went to the switch come back to the point where it originally parted from the other wire, and twist it with itself during the excursion to the switch and back, and with the other wire the rest of the time. Or, you could run both wires wherever each one needed to go, keeping them twisted together as much as possible.
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Old 3rd April 2013, 01:47 AM   #33
Carlp is offline Carlp  United States
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Thanks, guys. I haven't had a chance to pull either deck and check things, but I hope to after tax day (April 15 or so). I'm convinced now that the buzz of the Dual and the hum of the Luxman are different issues, and it's the Luxman hum I'm most keen on solving. The other (Dual buzz) is, I think, a bad ground somewhere.

A quick look at the Luxman schematic makes it look like the tonearm switch is DC. The mains looks to be connected only to the speed strobe. Secondaries of the iron all look to be rectified and smoothed before anything else happens. I don't see the tonearm switch explicitly on the schematic, but as I said, except for the neon strobe on the trafo primary side, all else appears to be rectified and filtered.


Quote:
Originally Posted by gootee View Post
If shorting a signal wire pair just before it attaches to the cartridge does not stop the hum, or makes it worse, the problem could be that the wires are acting as a loop antenna. Shorting them would just mean that the loop is complete and current could be induced in it, by the AC fields. If they are acting as an antenna, then disconnecting them from the cartridge but NOT shorting them should greatly reduce the hum.

If it hums with the TT turned off, does the hum then stop (or reduce significantly) if you unplug the TT's AC Mains from the wall?
Shorting the wire AND leaving it open but not connected to the cartridge both eliminate most of the hum. That's why it seems to me to be the cartridge that's picking up most of the hum. I've read that the cartridge (AT440MLa) had a run of bad production wherein hum was excessive due to a grounding problem, but I think mine post-dates that period.

If I unplug the TT mains (AC) plug, there is still some hum (same level as when the TT is plugged in but not switched on) so that part is probably related to the interconnects - if I disconnect the signal cables, there's no noise (shorted or not) except for some hiss.

So it's complicated. Signal cables are picking up hum. Cartridge is picking up hum (more and more as it approaches the DD motor under the spindle). The neon TT strobe is causing hum. I'm sure I'll have to work to minimize each of these and hope that the end result is a livable level of hum.
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