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Old 17th March 2013, 02:43 AM   #21
Carlp is offline Carlp  United States
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Thanks agdr. My cartridge (AT440MLa) is a moving magnet, not moving coil, and in any case I don't feel comfortable monkeying with that. And I have no fluorescent lights in the room or anywhere near the hifi gear. But your wiring ideas sound reasonable. I've partially replaced the wiring on my Thorens TD-165 with shielded cable (single inner conductor though). I'll be interested to hear how it sounds when I finish it.

I'm particularly interested in the two-conductor coax style wiring and will research that more. The links you provided will be quite helpful. I'll post back with any reports.

I do expect to get some hum even after addressing most of the problems, but I can live with a little hum. It's the big hum I'm getting now that drives me crazy.

OT - Canned about a gallon of syrup today, more to come! PM me and I can send you a small sample. It's a little smokey b/c I don't have a fancy rig, just a very DIY boiler.
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Old 17th March 2013, 01:02 PM   #22
agdr is offline agdr  United States
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One of the keys is not connecting the other end of the shields so there are no ground loops. Note in that single conductor diagram the shields turn back into the signal ground, so they stay isolated at the other end. Then the rest of the shields are single-point grounded. But then there is always what happens through the third-wire power plug! Grounding gets tricky.

If all goes well you should be able to short the connections for one channel together right at the back of the cartridge and get no hum, same as shorting the amp inputs at the RCA connector. If that works, but you still get hum with the contacts unshorted, then like AndrewT says your cartridge itself is picking up some hum.

I might take you up on that syrup sample! Thanks for the offer.
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Old 17th March 2013, 03:59 PM   #23
Carlp is offline Carlp  United States
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Thanks, agdr. I know about connecting one end of shielding to avoid ground loops and will be sure to do that when the time comes. I've read connecting at the source end is preferrable (not sure why) but would that be true for a TT? Grounding is a PAIN, yet SO very important. Conceptualizing the loops and then finding them hurts my head.

Is there some kind of shielding technique for cartridges you can point me to? I'll look into it myself, but suggestions are certainly welcome. I'll also try shorting the pins on the cartridge. I assume you mean shorting each channel, + to -. If I still get hum, the wiring is picking up the hum (or there's a ground loop), while if not, as you say, it's the cartridge coils.

On another related thread about extra hum I was getting from the turntable just before I built the PS (Phono hum from turntable?) someone asked about whether the tonearm tube were grounded (it is) as that would provide some shielding for the tonearm wiring. I'm thinking at this point there's some kind of ground loop associated with the TT wiring, so will look into that carefully.

I guess in a way it's good to find out the issue is not the amp or PS, but rather the TT/Cart/leads. But I can't imagine if I were boosting by 60dB for an MC cartridge...

PM me your address if you want to try some syrup.
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Old 17th March 2013, 04:32 PM   #24
AndrewT is offline AndrewT  Scotland
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All the TT metalwork should be connected to the shielding wire and then back to the Amplifier Earth connection.
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Old 18th March 2013, 10:21 PM   #25
Carlp is offline Carlp  United States
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Here's the latest report. I've now hooked up a second TT that has no ground wire (and whose tonearm is NOT connected to the negative of the RCAs, so is not grounded at all). It's a Dual 1219. This turntable has less noise, though it does have a buzz (sounds like typical ground loop frequency, but more scratchy). This noise is quieter but more annoying. Arggghhh.

On my Luxman, I shorted the blue and white (left channel) pins with tonearm wires attached as agdr suggested and got seriously increased hum. Quite loud.

At this point, I'm likely to try some of the grounding schemes from agdr's links as well as AndrewT's suggestion once I finish wiring up my Thorens. But first I will check to see that all the metal is actually grounded on the Luxman. Yesterday I read up on the Luxman and found that some consider it quite noisy, in particular with hum. If not for the hum, the Luxman sounds quite nice, so I feel like it's worth working on.

Well, despite my frustrations with this, I greatly appreciate all the help, and I haven't given up yet!
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Old 19th March 2013, 04:44 AM   #26
agdr is offline agdr  United States
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If you don't get it resolved, post a sketch of exactly how your wiring is set up, similar to how it is done in those links. Show which sections are coax, which are unshielded wire, and most importantly exactly what points along the way are grounded to what. Be sure to include any 3rd wire power plug grounds and where they connect. Also show tonearm ground, and if the turntable itself is floating on springs how that is grounded. Photos would be good too - someone here might have an "aha" moment on seeing a photo.

Just as a thought experiment, since you could short the amp RCAs and get no hum, you should be able to plug a length of coax cable into those, short the end, and still get no hum. So something has to be happening along the way going from the amp RCAs to the cartridge.
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Old 26th March 2013, 02:12 PM   #27
Carlp is offline Carlp  United States
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Thanks, agdr. I'll try to sketch out the wiring and setup soon, along with a photo. I'll also try the coax cable connection to see if it's picking up in the wiring. Not entirely sure, though, what wiring the TTs use (are they always coax?).

Finally, I've posted some audio clips of the two noises I'm getting. The hum from a Luxman PD-277 and the buzz from a Dual 1219. Recording directly from the jfet amp resulted in signal level that's too low (I can only hear through headphones on my computer output), so I "normalized" them in Audacity. Result is the other noise floor (hiss, etc.) from the amp is much higher than it sounds on the system. Try to ignore that. The clips are here (with notes about what's going on below the link):

Luxman Hum Normalized
0-5 sec - TT connected, not on. (there is a low hum)
5-11 sec - TT power switch on, platter not turning
11-20 sec - tonearm moved over platter, platter spinning

Dual Buzz Normalized
0-4 sec - amp hiss with no TT motor running and cables shut down (Dual 1219 switches off the interconnects until the arm is down)
11-18 sec - moving the arm across the face of the platter (same noise whether moving by hand or by wooden stick)
18-25 sec - tonearm stationary over the platter, not touched

I realize there may be two different problems going on with these 2 turntables.
Thanks again,
Carl
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Old 27th March 2013, 05:24 PM   #28
Carlp is offline Carlp  United States
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Quote:
Originally Posted by agdr View Post
Just as a thought experiment, since you could short the amp RCAs and get no hum, you should be able to plug a length of coax cable into those, short the end, and still get no hum. So something has to be happening along the way going from the amp RCAs to the cartridge.
All is very quiet if I do this. No hum from just the cables, so I'm increasingly convinced it's the cartridge. Note the hum gets louder as the cart/arm move toward the spindle. Motor is underneath the platter in the center (it's direct drive). Arm is grounded so should shield the arm wiring fairly well.

After doing some other reading, I found that the neon strobe for the platter may cause hum in some cases. Also read where there was a bad batch of AT440MLa cartridges that were susceptible to hum due to bad shielding. I'll try pulling that and installing another cart. The TT came with an MA 2002 cart (ceramic/piezo) and it sounds like they are very resistant to hum, which may be why it was installed.
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Old 29th March 2013, 04:28 PM   #29
agdr is offline agdr  United States
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Quote:
Originally Posted by Carlp View Post
All is very quiet if I do this. No hum from just the cables, so I'm increasingly convinced it's the cartridge. Note the hum gets louder as the cart/arm move toward the spindle. Motor is underneath the platter in the center (it's direct drive). Arm is grounded so should shield the arm wiring fairly well.
That really does seem like a cartridge problem. AndrewT nailed it. I agree, swapping out the cartridge would be a good test, especially with the ceramic type.
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Old 1st April 2013, 02:13 AM   #30
gootee is offline gootee  United States
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Quote:
Originally Posted by Carlp View Post
I rebuilt this PS switching to full bridge rectification (giving me 34v to start) into the first two caps of the original, and adding a LM317 set to pass 24v or so. Final cap was 22uF followed by 5R then into 3300uF final cap. Note that the jfet amp board has some PS decoupling on-board.

Hum is a bit reduced but still there. Then I went back to battery supply (two 9v in series), and lo and behold, the hum is still there at pretty close to the same level. So at this point the PS isn't adding any hum, but I still have hum!

The phono stage using the 9v has no AC, but the TT of course does. (Again, the amp is pretty quiet). Could the TT be passing AC on to the phono stage? Even without the TT turned on? The TT is Luxman PD-277, which has a power switch and a tonearm switch. Platter starts to turn when tonearm is moved to platter. Hum exists even when power to the TT is turned off, but it's louder when turned on, and even louder still as the tonearm moves toward the center and the platter motor.

Tomorrow I'll try disconnecting the TT leads, and if I can rig something up, try shorting RCA plugs on the inputs to see what that does.

I'm guessing this hum problem is bigger than just the PS...
The hum is apparently radiated from the AC in the TT, and picked up by the signal wiring. There is apparently enclosed geometic area in loops formed by the conductor pairs, in the AC or signal wire pairs, or both, which makes them into good antennas (transmitting and receiving). (See Faraday's Law.)

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?

If so, then proceed with mitigations, per below. (But I would check them, either way.)

Is the AC wiring in the TT all tightly twisted together, everywhere? If the AC mains pair's wires become physically separated from each other, at all, anywhere, then the enclosed loop area forms a much better transmitting antenna. The platter and tone-arm metal will not shield the time-varying magnetic field, much at all.

If there is a transformer, etc, the same rules apply to all of the secondary wire pairs, and to any rectifier-to-caps conductor pairs.

If the wire pairs in the tone-arm are not tightly twisted, ALL the way to each end, they will be a good receiving antenna for the time-varying magnetic field from the AC wiring, or the motor itself.

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.

Edit: "Tightly twisted" is more difficult for larger wire gauges. But try for at least 4 turns per inch. Tiny wires probably need more turns per inch, just for "physical integrity" (and because you can, and more twists per inch is better).

Last edited by gootee; 1st April 2013 at 02:28 AM.
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