Presonus Firepod output hum

B

borriffick

New member
I'm using the PreSonus FP10 for live recording. So everything goes in the inputs, and then from the outputs on the back to the PA.
All of the inputs seem to work just fine (everything records perfectly). The problem is between the outputs on the back of the unit to the PA. There is a low humming sound that comes from the PA as soon as I plug something in back there. I've tried all different combos of cables, two different PAs in two different venues as well. Does anyone know if there are any known issues like this? I can't find much about the output hum, only some people reporting a hum with the inputs which isn't my issue.
Thanks so much!
 
You have a ground loop, this could take a bit of working out. Is the Presounus plugged into the same outlet as the PA? Then again how big is the PA and is it using multiple outlets? The presounus is plugged in to a computer, so the computer could be causing the ground loop or multiple loops, is the computer a laptop? If the computer is a laptop, have you tried running the computer on battery with the power supply completely unplugged? Does the hum stop then?

Alan.
 
Thank you so much for your reply, I have some stuff to try now. Hopefully it's as simple as plugging things in different outlets. It's a macbook, so I can try running on battery and see if that helps. The DAW needs to be plugged into the wall to work... I'll be able to try it out this weekend.
 
One standard hum loop "trick" is to make up a pair of leads (I am assuming TRS to TRS jack?) with the screen, sleeve connection disconnected* at one end but if you do this make sure the cables are marked up as specials!
You could use a transformer isolator box such as the Art Clean Box range but it is an additional expense and something else to cart/forget. Also, transformers can themselves pickup hum, likely to happen in a variety of venues,

*There is slight chance that this procedure will cause the pickup of RFIinterference but this is rare and there is a fix!

Lastly: No matter WHAT anyone else tells you DO NOT LIFT ANY MAINS SAFETY EARTHS!

Dave.
 
There is a device you can get at most hardware stores that you plug into an AC outlet to test how it's wired. It will show if the hot and neutral are reversed and if the earth ground is present or not. Either of these situations can cause issues with digital equipment and can also cause the hum you are experiencing.

Also, the more things you have plugged in, the more chances you are going to have some kind of interference, or ground hum. It's best to always use balanced and shielded cables as well as correctly wired and grounded AC outlets. Using the trick in the above post could help, too, as it can sometimes cancel out ground loops. Especially older buildings can give you headaches that you might not have in your home studio, where you have taken all precautions and tweaked things just right. They might not have updated their wiring to current wiring code standards.

Always remember, everything needs to be grounded. And when you are talking about power amps, this is even more important. Safety first!
 
Tried recording in a different venue. The only thing I did differently is plug the Firepod into an outlet about 40 feet away presumably on a different circuit. Laptop and PA and everything else plugged into the same area near the stage. Everything worked perfectly. No hum from the PA this time. Recording sounds flawless. I'm going to do it this way from now on...if I run into problems again I'll report back, but hopefully it's that simple of a fix. Thanks so much!
 
borriffick said:
Tried recording in a different venue. The only thing I did differently is plug the Firepod into an outlet about 40 feet away presumably on a different circuit. Laptop and PA and everything else plugged into the same area near the stage. Everything worked perfectly. No hum from the PA this time. Recording sounds flawless. I'm going to do it this way from now on...if I run into problems again I'll report back, but hopefully it's that simple of a fix. Thanks so much!
Click to expand...

The funny thing is that the way you set this up usually causes more problems. Maybe if there was an extension lead for the power it caused a different earth resistance and it lowered the noise? or If the Presonus it was plugged into the PA via a DI was the earth lift on the DI switch on this time?

Oh well if it works do it, I know in my studio when I chase new earth loops (when gear is moved or added) logic sometimes disappears LOL.

Alan.
 
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