Mackie SR24.4 Humm with Radio???

[Anonymous067]

Last night myself and a senior sound technician were running sound for a play rehearsal in a church gym. (NO the acoustics DO NOT suck).

We're running two combined snakes, 24 channels, 4 returns, probably 250 feet of snake. We're getting inherited noise from the cable length I assume, because we have a lot of noise when channels are gained too high, we're having to drive the instrument and mic outputs hot so we aren't gaining to hot at the board.

Problem--two of our three wireless mics are running very very strong hum and noise. Worse than any channel, and the cable runs are actually the shortest on these two channels. We've tried ground lift/ground switch (yeah..everything's on the same ac circuit). My suspicion is the receivers are located near the first electric (fact), and whenever those lights go up, the hum gets much worse (fact), I just have yet to determine if that's whats causing the whole hum problem.

My other problem is I'm getting local radio through miscellaneous wired microphone channels (ie, not through the wireless microphones).

I cannot get rid of it, no matter what I do.

 

[FMEng]

Let's take the radio frequency interference (RFI) first. In my experience, Mackie consoles are great about rejecting strong VHF signals from FM and TV broadcasts. Most audio equipment is mediocre about rejecting strong AM radio, so that's what I think is the culprit here.

The reason AM broadcast can be such a pain is that the transmitter towers tend to be close by populated areas, and the frequencies involved are very close to audio and op-amp based circuits readily amplify them. Few manufacturers do a good job of designing equipment to survive AM, because of the cost.

The best way I know to get rid of AM broadcast RFI is to use ferrite toroid donuts of the correct material (type J in this case) for the frequencies we want to get rid of. I suggest buying donuts that are big enough for an XLR connector to pass through along with some cable wraps. Wrap each mic line several times around the donut just before the connectors that plug into the console. The more turns, the more attenuation.

Take a look at the FT-140A-J donuts
https://www.amidoncorp.com/items/21

I have also encountered a few condenser mics that demodulate the RFI, so if some of them are still bad after treatment at the console, put a toroid at the mic end of the cable, too. In really severe cases, you might have to wrap all cables going in/out of the console, including the power cord, but that is unlikely.

I do suggest that before you spend money, identify the radio station you are hearing. Start listening carefully about five minutes before the top of the hour. They have to do a station identification within a few minutes. If it is not an AM station post back here and I will offer some different ideas before you spend money on a possible fix.

As for the other hums and buzzes, I suggest shortening the cable lengths and re-routing them. It's hard to imagine a scenario where 250' of snake is really necessary. Move the wireless mic receivers to a location away from other electronic systems and wiring. Also, don't connect cables to un-used console inputs. The mic preamps may also be able to do a better job with the 60 Hz stuff if they are not getting hammered with the RFI, too.

I have heard that Mackie tech support has a design for RF filtering for mic inputs. I suspect building 24 of those would be more work and expense than the toroids.

Good luck and let us know how this works out.

 

[Eboy87]

Unplug everything and start plugging back in until you hear the buzz. If you crank the gain wide open on a lot of desks they'll make noise, so that's not surprising. The lighting noise you describe almost sounds like (no pun intended) your sound is on the same AC circuit as your lighting, so you're getting dimmer noise. The wireless, are you using balanced out from the receivers to the board?

Just thinking out loud.

 

[Anonymous067]

We can actually understand the radio station it is, without needing station identification...that's how noticeable it is.
To temporarily solve the problem, we used an outboard mic preamp on stage for the three mics grabbing the AM radio, and then ran line levels back to the board so we'd have a stronger signal. This fixed that problem, for the night.

Last night we went through all our channels, and isolated bad connectors, a bad microphone, and a cable routed side by side with a power cable (idk who pulled this one). We eliminated most of our hums.

As far of the 250 snake run...it ISN'T necessary, but our head sound tech doesn't want to run a snake through the seats, and would rather prefer to have the snake routed into the ceiling...like I said, wasn't my decision. And when we disconnected extra unused channels, our hum was low enough that it wasn't an issue anymore.

Wireless mic receivers, still a terrible hum on one of them. We moved it circuits to match the same circuit as the board and minimized the problem.

HOWEVER, we did have a different problem last night. Half hour into rehearsal, we discovered our mains weren't very loud. Our head sound guy told me to make something feed back....and nothing.
As he started checking outputs on the board (we determined all routing was correct and all outputs were live), we found our answer, best described as our head sound tech saying "leave it to mackie to put a volume control for the mono out on the back of the board where the patch is".

 

[avkid]

"leave it to mackie to put a volume control for the mono out on the back of the board where the patch is".

That and the "tape break" switch are what ruined Mackie for many people.

 

[silvrwolf]

That and the "tape break" switch are what ruined Mackie for many people.

I hate that **** switch !!