Audio Professionals Forum
Interference or bad amp?
Interference or bad amp?
by Adam Bernardi on Sep 5, 2011 at 6:44:09 am

Hi all,
I'm an editor and I've got a pair of Event ALP5 powered monitors that I use with an iMac when I need to bring a project home. I've got a weird problem. One speaker crackles and pops when connected to the iMac. The second speaker also picks up some of it up but at a much lower volume, like crosstalk. It sounds like static or EM interference. I've tried to narrow down the problem and I'm getting confusing results.

Here's the initial setup:

iMac FW out to Presonus Inspire to speakers via balanced XLR. Here's what I've tried:

Rerouting attempts:

iMac FW to Inspire to speakers via unbalanced RCA
iMac 1/8" headphone out to y-cable w/RCA ends to Inspire to Speakers, via XLR and RCA
iMac 1/8" headphone out to y-cable w/RCA directly to speakers, bypassing Inspire all together.

Trying to isolate the problem:

I swapped out cables.
I moved the speakers away from the iMac (shielding).
I swapped speaker positions.
I wrapped the offending speaker in tin foil (makeshift Faraday).
I plugged the speakers into a separate circuit.

So now I'm thinking it's a bad amplifier. Here's what's confusing, the static only happens when it's hooked up to this iMac, either directly or through the Inspire. It's quiet with other sources (I tried my ipod and blackberry).

When I disconnect from the iMac, it's quiet.

So it has to be the iMac, right? Not so fastl If I hook up the good speaker to the iMac by itself on either channel it stays quiet! And when I put the "bad" speaker back into the loop, the static is back, again mostly on the "bad" speaker and slightly on the "good" speaker. Even more confusing, if I turn off or unplug the "bad" speaker and keep it connected, the "good" speaker still has the static, but at the same low level.

Another note, sometimes the static/interference lowers the volume dramatically when I do something on the mac, open a window, a program, etc. But then shortly thereafter it gets loud again. There doesn't seem to be a pattern other than I do something on the mac and it changes. Which leads me back to EM interference or a Mac related issue. But why should it affect only one speaker and not the other?

So I'm stumped. The only thing I can think of is that one of the speakers is more sensitive than the other to EM or something else? But then why would the "good" speaker still exhibit the slight static when the "bad" speaker is powered down, and still connected? Confused? I am!

If' you've made it this far can you make any sense of this? Is there something I'm overlooking?

Any ideas? THANKS!

Re: Interference or bad amp?
by Eric Toline on Sep 5, 2011 at 7:01:21 pm

Regardless of all the connection permutations you've tried, you'e answered your own question. You said that you get clean sound from your Ipod & Blackberry and I bet from any other source also.

You've got connection and or grounding problems somewhere in your computer chain, maybe just a bad computer audio circuit. If you plug headphones in your computer HP jack is the problem still there?


Re: Interference or bad amp?
by Adam Bernardi on Sep 6, 2011 at 5:27:09 am

Eric, I meant to mention that. I did try headphones from the imac and it sounded fine, no noise.

Re: Interference or bad amp?
by Jordan Wolf on Sep 6, 2011 at 12:18:46 am


Upon looking at images of your audio interface, I don't see any balanced outputs. I see XLR inputs on the front and RCA outputs on the back. What do you mean when you say in your original post, "iMac FW out to Presonus Inspire to speakers via balanced XLR"?

Are you using an RCA-XLR cable? If so, it is not a balanced connection. However, as long as the cable run is under 15', you should be okay using RCAs either way. This leads me to think the issue maye be elsewhere, since you already tried going RCA into the speakers directly.

Okay, first I'll offer some advice(not to be snarky, just informative): when troubleshooting, it is important to try devices that are both similar and dissimilar to the DUT (Device Under Test). It is also important to only change one variable at a time.

So far, you have discovered that your computer (connected to the AC power from the wall) causes hum/buzz. You have also discovered that devices NOT connected to AC power exhibit a hum-/buz-free response.

If you haven't already, run your audio interface via firewire and not using the AC adapter (if you are even using that). If this has already been done and the problem remains, try running an extension cord from another outlet and plugging your devices into that.

You ideally want all deviced that are going to be interconnected on the same circuit so that the voltage potential to ground between each device is kept low (that "potential" or voltage is what you hear as 60Hz and the odd-order harmonics thereof).

As another idea, if you have access to a bus-powered audio interface with balanced outputs, try that to see if it will help - my guess will be that it helps somewhat, but not entirely, if it's a power issue.

Please keep us up-to-date on your findings.


Re: Interference or bad amp?
by Adam Bernardi on Sep 6, 2011 at 5:52:00 am

Upon looking at images of your audio interface, I don't see any balanced outputs.

Jordan, whoops, you're right. I was looking at the front of the interface when I was writing my post and and assumed there were XLR outs. This has been a long running problem that I've been trying to ignore and all the testing I mentioned above I did some time ago. I knew I had tried everything, so I assumed there were balanced outs, my bad.

As to changing one variable at a time, I did.

I'll try your suggestions, I think I tried the interface w/o the AC, but I don't remember, I'll try again.

I've always suspected something with the AC. I only have it hooked up to a UPS, but I think my AC in this neighborhood is pretty dirty, my lights dim fairly often. In fact, first thing I should do is get a power conditioner....

But I still don't know how to explain the hum/buzz/spitting/static following the one specific speaker and not the other, regardless of what channel it's plugged into or position....

Re: Interference or bad amp?
by Adam Bernardi on Sep 6, 2011 at 6:34:22 am

OK Jordan, I tried the FW interface with and w/o AC and it makes no difference.

I'm not sure I've described the sound accurately, so I've uploaded a clip. I recorded it on the built-in mic so there's room noise, but I think you'll get the idea.

Re: Interference or bad amp?
by Geoff Tirrell on Sep 7, 2011 at 5:27:24 am

Hmm, Ok new guy is going to give this one a crack. From what I gather it is both monitors getting the hum. You have tried a humbuckler, ground lift etc so it is likely not coming up the ground from the wall. The reason I mention this is that I can think of two situations in 10 years that fit this scenario. The first was a shoot where a dp had plugged a consumer grade monitor in to his camera out and the wall. The power leaked (for lack of a better term) Up the ground wire through the camera and back into the mix. Which we realized later is why we only heard it when tape was rolling. Point is start checking the weird stuff I would take a ground lift and start with your monitor and start hitting every peripheral attached to that station until you find it.

Ps. An engineer friend later told me that while most broadcast and industry equipment has shielding against this type of thing it is not always the case with cheap or consumer grade stuff.

Re: Interference or bad amp?
by Adam Bernardi on Sep 8, 2011 at 5:00:50 am

Thanks Geoff,
I think you may be onto something, I have a lot of consumer electronics on the same circuit.
A clarification: It seems to be only one speaker that gets the static, and when it does, it leaks a little into the other speaker.

Re: Interference or bad amp?
by Chuck Purnell on Sep 10, 2011 at 12:43:20 pm

I am currently having a very similar problem with my Macbook Pro and Behringer Truth Monitors..See my thread I posted yesterday. I am still troubleshooting the problem and have not narrowed it down yet. I am wondering if there could be an issue with using Apple computers but then again I know plenty of DJ's that use Apple computers while Djing and their music sounds clean with no static or other noise so I am not exactly sure what it is going on. I was hoping you found a solution since our issues are similar! LOL!

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