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Old 23rd May 2011, 01:07 PM   #1
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Default phono preamp grounding/hum long story

So, I'm upgrading a Mobile Fidelity LP2 phono stage for a friend in exactly the same way I upgraded my own LP2. Replaced all electrolitic caps, output poly caps, rca jacks, internal wire, power umbilicals (phono stage is 3 cans, power supply can is transformer only with rest of power supply on the boards of the 2 phono circuit cans. I replaced the stock umbilical by hard wiring at the power supply and adding a hospital grade locking jack and plug on each of the phono cans.) All finished, hook it up, hum in one channel. All day yesterday checking solder connections, staring at the thing. Looking at the new Cardas rca jacks that are not isolated from the metal plate that is the rear of the cans, as the old ones were, because the plastic rings do not fit the holes. Removed them from the plate and still have hum with them in free air. Put old isolated rcas back in - hum. Then I'm looking at the power jack which is metal as opposed to the old one which is plastic. Removed it from the metal can back and hooked up - no hum! Put it back in it's hole with the intention of touching a ground wire from chassis ground to it's second grounding pin, but now I can't make it hum no matter how I tighten it or wiggle it. So obviously I'm either a dummy, or just confused. Three tabs on the power jack, hot pin and 2 ground pins. Should the third pin be connected to chassis ground? Why is the stupid hum gone? Do rca's need to be isolated with the plastic washers? By the way, the metal where all the jacks mount is powder-coated so I doubt things mounted to it are grounded to it. I'm sort of assuming I need to enlarge the holes for the rca jacks and use the isolation washers.
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Old 23rd May 2011, 01:46 PM   #2
h_a is offline h_a  Europe
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Hum can become very easily a problem with phono stages, even more so for MC-headamps.

If you don't know the exact grounding concept of the phono pre, I can only recommend to do everything as it was originally, even the routing of the wiring can easily introduce hum if it passes the power supply section.

Hannes
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Old 23rd May 2011, 02:26 PM   #3
DF96 is offline DF96  England
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When a hole is cut in powder-coated metal the edges of the hole can be bare metal, so can add an unwanted ground connection - except when you want a ground connection! Grounding is very important, especially for a phono stage. It is not just the fact of grounding, but the exact route and point of connection which matters. As h_a says, copy the original unless you know enough to attempt an improvement.
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Old 23rd May 2011, 08:40 PM   #4
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Thanks h_a and DF96. The hum is back, so I don't know what happened when I thought it was gone. I took your good advice and put the old rca's and their leads and power jack and it's leads back in just to test. Hum is still there. Now it must be something on the board.....maybe a defective part that I didn't even touch. The caps and rectifiers that I installed are all oriented correctly and with good solder joints. I just bought this phono stage, played it for half a song to make sure it functioned, and then went to work on it. It may have had the hum then and I didn't notice. I do remember one channel took 3 or 4 minutes to come to life.....that may have been a bad sign.
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Old 24th May 2011, 04:34 AM   #5
Bonsai is offline Bonsai  Taiwan
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You need to make sure that there are NO connections between any power, input or output socket grounds to your chassis. The ground connection should only go from 1 point on your board to the chassis - usually the centre 0V where the filter caps join is a good place. I personally would not trust powder coat to provide insulation . . . use the insulating washers.
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Old 24th May 2011, 06:27 AM   #6
h_a is offline h_a  Europe
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Quote:
one channel took 3 or 4 minutes to come to life
You're right - that's certainly no good sign.

Is that the humming channel? If so, replacing the power supply elcos might help.

Hannes
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Old 24th May 2011, 09:04 AM   #7
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I have done the following: Disconnect MC jack wires from the board. Disconnect MM jack wires from the board. Power jack removed from it's hole and hooked up in free air. MC jack in free air. Reinstalled old MC rca. Reinstalled old MM rca. reinstalled old plastic power jack. While the old jack was installed, I was using a different power tap on the power supply can. Swapped in "my" power supply can (which definately has no problem). Drilled bigger holes for the Cardas rca's and now using the insulating rings. Hooked a wire on chassis ground lug and touched to ground on power jack and both rca's. None of this worked. All I did on the board itself is swap out caps, rectifiers, and rewire. At this point I have completely reversed the rewire. There is proper continuity on all jacks and the ground lug. Thanks Bonsai and h_a. I think at this point I have tried most everything with rca and power jacks, and the hum is likely being caused by a short or bad component in the board itself. Solder joints all look good. Nothing bridging positive and negative traces. Now I'm over my head unless I start putting old caps back in. How can I test for a short on the board? It IS confusing that for an instant, with the power jack in free air, I didn't have hum. That must have been imaginary, cause I cannot recreate it.
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Old 24th May 2011, 10:08 AM   #8
DF96 is offline DF96  England
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Possibly dry joint, or other intermittent connection?

When 'upgrading' a circuit you should really change one thing at a time, and test. Otherwise, even if the 'upgrade' is a real improvement you don't know which of the changes is responsible. When you introduce a fault, you don't know which change caused it. You can never be certain you have fully removed a change either, as the circuit will never be exactly as it was before you started.

Checking for continuity on grounds is necessary, but not sufficient. Everything which should be grounded, should be grounded, but the route and manner of grounding matters too.
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Old 24th May 2011, 10:19 AM   #9
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I hear you DF96, but there were 30 caps replaced and that would be a lot of testing. Too late anyway. I have been soldering crossovers for 20 years, but a bad joint can happen, I'm sure. If I bought an LCR meter, do you guys think I might discover a bad component by comparing readings from the right channel board to readings from the left channel board?
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