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Nothing but loud Humming sound coming from MDD

MatthewLTL

MatthewLTL

macrumors 68000
Original poster
Jan 22, 2015
1,684
18
Rochester, MN
For awhile now I have had a loud humming coming from my MDD Last night it got to the point it wont go away (Makes using my amp next to impossible).

The hum comes out both the speaker out (3.5mm) jack on the back as well as the headphone jack on the front.

What could be causing this? Could the fact I have one of the rear CPU Fans grounded to the CPU Heatsink be a factor?
 
MatthewLTL said:
For awhile now I have had a loud humming coming from my MDD Last night it got to the point it wont go away (Makes using my amp next to impossible).

The hum comes out both the speaker out (3.5mm) jack on the back as well as the headphone jack on the front.

What could be causing this? Could the fact I have one of the rear CPU Fans grounded to the CPU Heatsink be a factor?
Click to expand...

I'm not sure if that's what is causing your specific issue.

Grounding a CPU heat sink doesn't sound like a smart idea in my mind. Really of all things why the heat sink?

Think about it- if you had electricity flow through through the ground wire, the heatsink, made out of aluminum & maybe some copper (both of which conduct electricity well), would become electrified. Next, there is a layer of thermal paste between your CPU and heatsink. Thermal paste *typically* ins't a good conductor of electricity but if it may contain elements that can conduct electricity i.e. silver. So then you could potentially have an errant electric current hitting your CPU and your logic board.

I think grounding to metal in the case would be a much safer plan. I don't play around with CPU's much or grounding additional items but it seems to me like a very poor and potentially catostrophic idea.
 
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Why would you ground something to the heatsink? What good is that? Unless the heatsink is also connected to ground, it is useless. All it would do is conduct electricity to the CPU and cause problems. Probably this problem. Just connect the fan to the real ground.
 
poiihy said:
Why would you ground something to the heatsink? What good is that? Unless the heatsink is also connected to ground, it is useless. All it would do is conduct electricity to the CPU and cause problems. Probably this problem. Just connect the fan to the real ground.
Click to expand...

The +12V wire is hooked to the 12V line of the PSU the - side of the fan (black Wire) is what is connected to the heatsink (one of the heatsink screws is what the wire is wrapped around) since the Heatsink is grounded by the screws holding it to the case it grounds the fan.
 
A.Goldberg said:
I'm not sure if that's what is causing your specific issue.

Grounding a CPU heat sink doesn't sound like a smart idea in my mind. Really of all things why the heat sink?

Think about it- if you had electricity flow through through the ground wire, the heatsink, made out of aluminum & maybe some copper (both of which conduct electricity well), would become electrified. Next, there is a layer of thermal paste between your CPU and heatsink. Thermal paste *typically* ins't a good conductor of electricity but if it may contain elements that can conduct electricity i.e. silver. So then you could potentially have an errant electric current hitting your CPU and your logic board.

I think grounding to metal in the case would be a much safer plan. I don't play around with CPU's much or grounding additional items but it seems to me like a very poor and potentially catostrophic idea.
Click to expand...

The CPU Heatsink is grounded anyway the screws that hold it on are grounded by the PSU when screwed into the studs
 
The MDD heatsink is connected to EARTH ground, not ground(0V) out of the PSU.

That's probably your problem-you're bringing the noise inherent in earth ground into the rest of the PSU.
 
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bunnspecial said:
The MDD heatsink is connected to EARTH ground, not ground(0V) out of the PSU.

That's probably your problem-you're bringing the noise inherent in earth ground into the rest of the PSU.
Click to expand...

Earth ground is from the PSU is it not? the 3rd pin on PSUs go right to the case of the PSU than the screws for the PSU going into the metal of the case ground the case. You can also Ground HDDs in this manner (i have in the past). I'm not a expert on PSUs so I am not trying to argue. Just questioning is all. I figured myself the the fan was probably doing it, however I assumed since the fan spins that no harm was done. I wonder if that is causing my constant lockups and KPs or perhaps it's my SATA to IDE adaptor doing that.
 
Just hook it up right and see if the humming goes away. I'm guessing it's most likely because you are inadvertently bringing in earth ground.
 
bunnspecial said:
Just hook it up right and see if the humming goes away. I'm guessing it's most likely because you are inadvertently bringing in earth ground.
Click to expand...

Again, I didn't think there was a difference because I figured the Ground on the Molex plugs were the same ground... oh well I'm no expert. I'll hook the ground of the fan up to the fan ground pin on the fan connector by the CPU
 
Just connect it as its supposed to be connected then see if it goes away, then start trying new "configurations" after that
 
MatthewLTL said:
The CPU Heatsink is grounded anyway the screws that hold it on are grounded by the PSU when screwed into the studs
Click to expand...

That may be true but you wouldn't want extraneous electricity to be flowing through the CPU. Even though the CPU may be grounded, the electricity would still flow through CPU which could cause disruptions in its normal functioning (or perhaps even damage it).

I guess the question I don't believe anyone has asked is did this humming start after you installed the fan ground to the heatsink? Did you have the same arrangement with the old logic board? Did it start as soon as you put in the new logic board?

Did you rewire and get any improvements?
 
I'll re-ask my questions because I'm curious.
1. Did this humming occur with your old logic board: Y / N
2. Did your old logic board have the fan grounded to the heatsink: Y /N
3. Was there humming start after you recently installed the new logic board:Y / N
4. If NO to #3, did it start when you grounded the fan Y/ N
5. If you disconnect the fan, does the humming stop: Y / N

All you have to do is unplug the fan and that would be your answer.
 
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A.Goldberg said:
I'll re-ask my questions because I'm curious.
1. Did this humming occur with your old logic board: Y / N
2. Did your old logic board have the fan grounded to the heatsink: Y /N
3. Was there humming start after you recently installed the new logic board:Y / N
4. If NO to #3, did it start when you grounded the fan Y/ N
5. If you disconnect the fan, does the humming stop: Y / N

All you have to do is unplug the fan and that would be your answer.
Click to expand...

1 yes
2 yes
3 no. the humming was there on the old board 2
4 can't remember
5 no. It still hums when the door is open but stops when the door is closed... just like when the fan was grounded to the case

----------
fuchsdh said:
Isn't audio hum a typical issue with MDD Macs, alongside the Windtunnel fans? IIRC Apple said the issue was mostly with pro audio equipment causing interference.

https://support.apple.com/kb/TA46196?locale=en_US&viewlocale=en_US
Click to expand...

I don't have apple pro speakers. I use a Stereo amp with a 3.5mm to RCA cable. Once I find where i stashed my Compaq 19" Wide monitor with built in speakers ill give that a shot see if the hum goes away
 
MatthewLTL said:
For awhile now I have had a loud humming coming from my MDD Last night it got to the point it wont go away (Makes using my amp next to impossible).

The hum comes out both the speaker out (3.5mm) jack on the back as well as the headphone jack on the front.

What could be causing this? Could the fact I have one of the rear CPU Fans grounded to the CPU Heatsink be a factor?
Click to expand...

Probably because your MDD is not grounded like I said in another thread. Get a wire, attach one end to the casing of the MDD, and attach another end to a grounded object like a water faucet, drain pipe, or your furnace which you know is grounded.
Then see if you have the humming.
 
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