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Amp noise caused by iMac plugged in on same circuit

sadwinkey

sadwinkey

Aug 12, 2017
32
32
#1
Hey everyone! Sorry if this isn't the right forum to post this.
So I recently purchased a new iMac, and it works great. But, it seems that having this new computer plugged into the same circuit as my PF-50T is causing the amp to pick up some sort of interference from the computer. I took a video to demonstrate:



You'll notice that as i decrease the brightness of the screen, the noise gets louder and changes. It also seems to change depending on what I'm doing on the computer. If I open up a new tab or even click, the amp seems to buzz accordingly.

As of now, I have the computer plugged into a 1500VA battery backup and the amp plugged into an Isolating power strip. I get the same results when I plug them both straight into the wall; It seems that as long as they're on the same circuit, I get this noise.

I also have some powered monitors that are plugged into the same circuit as well, but they don't have this issue. My question is, is this a computer issue, an amp issue, or just something that happens sometimes and I have to deal with? This is all new equipment, so if it is related to hardware, I would like to get it figured out so I can get the faulty gear replaced :)

Thanks in advance!
 
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#3
ddnidd1 said:
Try physically moving the iMac away from the amp.
Click to expand...
Tried that already! At first, I was under the impression that it was some sort of wireless signal interference, but moving it didn’t make any difference. I also tried unplugging the battery backup from the wall, allowing the Mac to run off of battery (to take it off of the circuit) and the noise went away.
 
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#4
Not sure how to edit my original post! But I tried plugging a different amp into the same circuit, with the same result. Seems as if this makes the computer unusable for studio purposes.
 
#5
Just as a data point, does the noise go away when the amp is plugged into a different circuit (meaning controlled by a different circuit breaker)?
My suggestion is to try this with both devices straight into the wall outlet without any battery backup or isolation device.
 
#6
Are there audio connections between the amp and computer?

How are these connections made?

Does the noise go away without the audio connections?
 
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#8
My experience is the circuit is not grounded properly, it might not be the computer, especially if the amp is not wireless and you have the WiFi off. You would need to have a licensed electrician look at the breaker panel.

To edit a post, just click Edit at the bottom of the post.
 
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#9
Not the same equipment, but similar results, ( noise ) when I hooked up my Bass Buddy to my PC, by using the Headphone out from the PC to the Aux In on the BB. It went away when I used the Headphone Out of the powered JBL speakers to the Aux In of the BB.
 
#10
The amp could be picking up noise from a power supply or a transmitter (got a phone or router nearby, note the ticking sound). Put some more distance between them and see if it helps. Also, try rotating the amp ninety degrees.

Does this happen without the instrument plugged in?

Also, unplug the battery backup unit from the mains. Does it make the same noise? Then try the computer directly into the wall. Keep the battery backup unplugged from the wall.
 
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#11
Hold on a minute, there are a lot of guesses going on here that are barking up the wrong tree.

If there is a direct connection between a grounded audio device that's connected to the computer, and the computer which is also grounded, it's likely that there is a simple ground loop between audio ground and the protective ground.

The noise made is different due to the ground loop containing different harmonics from the switchmode power supply and the steered currents.

Once we confirm how the connections are being made, we will know what kind of solutions would be practical (and helpful)

I deal with this sort of problem all the time.
 
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#12
Your UPS more than likely has a switching dc power supply to charge the batteries and batteries are pretty forgiving with how smooth the DC current is. In other words the switching power supply may not be very well filtered and can generate RFI.

As already mentioned try the iMac without the UPS/BBU and see what happens.
 
#14
agedhorse said:
Hold on a minute, there are a lot of guesses going on here that are barking up the wrong tree.

If there is a direct connection between a grounded audio device that's connected to the computer, and the computer which is also grounded, it's likely that there is a simple ground loop between audio ground and the protective ground.

The noise made is different due to the ground loop containing different harmonics from the switchmode power supply and the steered currents.

Once we confirm how the connections are being made, we will know what kind of solutions would be practical (and helpful)

I deal with this sort of problem all the time.
Click to expand...
Thanks for the reply!

So there is no connection between the amp and the computer. And there is no bass plugged into the amp.
I also tried taking the amp into a different room (which is on a different breaker) and the noise is still there whenever the computer is on, and it goes away when the computer is turned off.
 
#15
i can't speculate on your particular problem, but I experienced a similar problem with noise on my Mac. It seems that my basses were noisy when hooked up to my UA Apollo interface, but only the front pickup of every bass regardless of whether it was a humbucker or not. I isolated it to the power outlet, which was broken. Replaced the outlet and problem solved. Since then I noticed that quite a few of my outlets in this house are broken.
 
#17
sadwinkey said:
Thanks for the reply!

So there is no connection between the amp and the computer. And there is no bass plugged into the amp.
I also tried taking the amp into a different room (which is on a different breaker) and the noise is still there whenever the computer is on, and it goes away when the computer is turned off.
Click to expand...

Ok, just the computer plugged into the grounded AC power and the amp plugged into the grounded AC outlet, and just a speaker plugged into the speaker out, nothing else? No pedals, no external interfaces, etc?
 
#18
agedhorse said:
Ok, just the computer plugged into the grounded AC power and the amp plugged into the grounded AC outlet, and just a speaker plugged into the speaker out, nothing else? No pedals, no external interfaces, etc?
Click to expand...
Right. Computer on one circuit, amp on another(different room), cab plugged into amp. No other interfaces plugged into computer. No bass or cable plugged into amp.
 
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#19
First, as mentioned above, the UPS could also be injecting noise of its own.

It also may be that the Mac power supply is defective and injecting noise into ground or something? They're noisy on full charge, but usually it's not that much of a problem. Do you know someone who would loan you a different one?

Check the house wiring? How bad is your house ground?

If nothing is plugged into the amp, it's definitely not a problem with ground noise (unless there's a very badly designed input circuit), and it's not a ground loop.

From the description, it sounds like a combination of two things:
  1. The computer power supply (or UPS?) is injecting digital noise into the power lines (no surprise there
  2. Your amp has inadequate filtering.
If the amp is older, it may not have good RF filtering on the power supply. I've run into this with some older gear, and I did see an Ampeg logo. You might want to try a tripp-lite or similar RF-filtering power strip, and plugging the mac into it--it should filter noise in both directions...but of course it also might not be enough to make a big difference. So, that's like a $50+ experiment; good line fliters are not cheap.
 
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