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Old 1st March 2013, 02:09 PM   #1
Carlp is offline Carlp  United States
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Default Phono hum from turntable?

I have a hum from my current analog system. Turntable is Luxman PD-277 with a power switch that turns on power to a Neon (I think) strobe light and possibly to the motor (haven't looked at that, but the platter doesn't spin until the tonearm is moved to the LP). I'm running two phono stages, a Boozhound jfet and a Yaqin MS-12B that uses shielded 12AX7s for the phono stage. And the hum affects both.

It seems like the hum is related to the turntable itself, and not some interaction (since it's equal in the two very different phono stages), but I can't figure it out. The turntable is grounded to the phono stage, and the jfet phono stage chassis is grounded only at the turntable input ground. Power supply to the jfet is two 9v batteries in series.

If anyone has troubleshooting ideas, I'd love to hear them.
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Old 2nd March 2013, 04:18 AM   #2
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Make sure the turntable arm and headshell are grounded to the green wire. This is the wire that you ground to the preamp metal case. Check it with an ohmmeter. If not, clean connections. My PAS2 preamp metal case is not grounded through the power cord, so I run a separate wire from the case to the 3rd pin of the power strip (safety ground) using a separate 3 pin power plug. The power amp is grounded to the preamp through the RCA plug rings and cable shield, not the power plug. This is a tree ground system. This has been banned by the product liability lawyers, but a tree ground system provides the lowest hum IMHO.
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Old 2nd March 2013, 02:37 PM   #3
Carlp is offline Carlp  United States
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Thanks for the reply. I'll check that a little later today.
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Old 3rd March 2013, 07:44 PM   #4
Carlp is offline Carlp  United States
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OK, checked the green lead and ground lug and they are connected as you say. I get continuity between the green pin on the cartridge and the ground wire/lug on the turntable wires (blue pin is also grounded there). I was pretty sure that was the case.

I noticed yesterday that the ground lug on the front of the preamp was not isolated from the ground connection to the chassis at the rear at the power inlet, so I figured there was maybe some ground current through the chassis (2 "ground" points). BTW, this is a Boozhound phono stage in a metal case with inputs to the front, outputs on the back along with the power inlet, which is a typical DC power plug connection (like from wall warts). Only I'm using batteries for the supply while I work on a linear PS for it. It's my construction.

Today I isolated the ground lug from the chassis, but that did nothing to help the hum.

Note that the PCB for the Boozhound phono stage has a ground plane and the ground lug and the negative for the power supply are both connected to the ground plane.

Hmmmm. (or maybe hummmmm?). Any other ideas?

Thanks!
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Old 4th March 2013, 01:28 AM   #5
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I asked about the ground wire being connected to the head shell and the arm. The turntables I've owned, the arm was a metal tube and the headshell was an aluminum casting the cartridge screwed to. This makes a flux shield around the twisted pair wire inside the arm. The cartridges I've owned have been made of plastic, and the wiring (coils) inside was totally isolated from the screws, headshell, or arm. If they are making the arm out of plastic now, they would have to buy shielded two twisted pair cable to insert inside of it to achieve the same hum shielding. The shield, whether metal arm or aluminum wrap inside the cable, should be connected to the wire that goes to the preamp metal case. My twisted pair on one turntable had a green wire, and it was one of the isolated twisted pair wires. The two wires of one twisted pair go to the coil for the right pickup, and the two wires of the other twisted pair go to the coil for the left pickup. At the outside end of the one of the twisted pairs is a coax cable with an RCA plug on the end. Same with the other. Red RCA plug for right, I think. Hope this helps, your turntable wiring doesn't sound right at all.
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Old 4th March 2013, 02:23 AM   #6
Carlp is offline Carlp  United States
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Ah, I see now, my mistake. I misread your post. I'll check that as soon as I can. Thanks.
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Old 16th March 2013, 08:40 PM   #7
Carlp is offline Carlp  United States
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I've tested the ground scheme and found the following: Tonearm tube IS connected to the ground wire. R and L cartridge negative terminals (green and blue) are connected to the ground/negative side of the RCAs, but not to the ground wire itself. It is, however, connected at the ground plane of the amp PCB.

TT ground wire is connected to the chassis of the preamp at one point (as in a star ground). Preamp chassis is sometimes connected to the PS ground - I sometimes am running it on two 9v batteries in series, and the case IS connected to the negative of the battery (meaning the negative of the DC power jack is the one place the amp ground and TT ground are connected to the chassis). I've also breadboarded a linear PS and tried it both ways, floating/not connected to mains ground and connected to mains ground. Hum is the same in all cases.

I asked a related question about my PS, as it started out with an extra hum (louder than when running batteries). That thread has turned into a troubleshooting thread for the general PS hum, which I've found is coming from the input (shoring the inputs or just removing TT wires removes all hum). You can see my latest discoveries there: Help needed for 18v phono stage PS hum. Maybe we should move the rest of this discussion there as they are starting to merge?

Thanks.
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Old 20th March 2013, 11:48 AM   #8
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Paragraph 1 sounds correct. The only other suggestion I have is if the wiring is picking up hum even if correct, perhaps there is a motor or transformer too close to the coax cables, or the shield coverage of those cables (the ones with RCA plugs) is less than ideal.
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Old 20th March 2013, 12:04 PM   #9
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by Carlp View Post
...........I get continuity between the green pin on the cartridge and the ground wire/lug on the turntable wires (blue pin is also grounded there). I was pretty sure that was the case.
That Signal Wire connection to Ground is almost certainly creating a hum loop.
You must disconnect the 4 output pins of the cartridge from the Grounding scheme.

The Signal Ground MUST be connected to Chassis at ONE point and that point will come much later in the amp string.
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Last edited by AndrewT; 20th March 2013 at 12:11 PM.
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Old 26th March 2013, 02:23 PM   #10
Carlp is offline Carlp  United States
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I remain stumped on this hum issue. I've posted some files to show the hum on the following thread, which I'll shift to for further discussion:
Help needed for 18v phono stage PS hum

(Oh, and AndrewT, I found that I was wrong about the connection between ground wire and the green pin. I was measuring it while the RCAs and ground wire were attached to the preamp - where the grounds are ultimately connected at the ground plane.)
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