RFI issues with phantom power, Mackie board

[RoyJeeBiv]

hey all. i ran sound for a college choir performance last week and experienced some RFI issues -- wondered if anyone could help me find some answers.

the problem only presented itself with phantom power engaged (Mackie 2404 board), using three Shure MX202 hanging mics; solo mic channels were fine (58s). i seldom use condensers with this board, but never experienced the problem before, so i'm inclined to think it's the hanging mics (which the conductor insisted on using).

i also tried two separate snakes, both are quite old but have been usable in the past. RFI came and went on certain inputs with either snake at rehearsals and sound checks, so it's unclear whether this was part of the problem or not.

the performance was fine, EQ'd what i could and rode the faders...but any ideas what might be causing the interference?? like i said, i think it was the mics, but perhaps it's a combo of the board, snake, cables, etc...this summer i'm finishing inventory and will likely test everything, but if anyone can offer any assistance, or what i should be looking out for, it would be appreciated.

thanks.

rjb

[John Brook]

It cannot possibly be the Mackie. A while back, I mentioned that I had some RFI issues with a Mackie and was roundly chastised. It apparently was a figment of my imagination (even though the radio station was plenty clear in my recordings). Must be the cables or bad solder joints on the cable connectors.

Anyway, you're not alone and good luck solving the problem. I ended up moving to discrete preamps and using the same mics and cables, the problem disappeared.

[DCtoDaylight]

The advice I got here on a similar question last year was just right and very helpful in tracking down my problem: be very methodical.

Start with the most minimal possible setup (like one hanging mic plugged into a portable mixer) and then add or change one thing at a time until you can isolate the trouble spot. I prepared for this by drawing up a little step-by-step procedure that I could follow, took notes at each step, and also made a recording of each combination that I could refer back to later. You could swap mics, cables, channels, mixers, etc., and if none of that works, also experiment with different positions and orientations for the hanging mics - again, changing only one thing at a time is essential.

The MX202 mics have separate capsules and preamp units; be sure you include both pieces and the interconnects in your experiment. The mics I was having trouble with also had this sort of setup, and the problem turned out to be bad solder joints in one preamp/electronics module.

It sounds like a lot of work, and on some level it is, but I can't describe how satisfying it was when I finally isolated my RF problem. Here's the thread in case you want to refer to it: RF issues at live venue – attenuators?

Good luck!

[RoyJeeBiv]

thanks for the replies, guys -- i assumed it would come down to testing everything this summer, so i'll have to sit down one week with a couple pots of coffee (or cases of beer) and isolate the problem. i appreciate the assistance.

[nosebleedaudio]

I would doubt very seriously if it's the phantom...
Have seen RF come from 3 sources: Mic, mic pre, speakers/amp.
Mic cables are at the bottom of the list, using good cables of course...Mogami ect..
My point is I have yet to see better cables correct an RF problem, and NO doubt I have NOT seen EVERYTHING Yet...

[EV676]

I'm surprised no one mentioned to check the AC power. I'd look at how the power is routed to your mix position. And it wouldn't hurt to invest in a power filter like a Furman or Monster.

[Mulmany]

I have what sounds like an interference problem when using Phantom on some channels at a facility that I installed. I'm at the point were I need to just go and pull all the stage boxes and re solder everything.

I first noticed it on one countryman DI then the other week I spot mic'ed a violin, applied phantom and there was the noise. I switched channels and the noise went away. Now on the DI, if I power the DI from the internal battery I'm fine, no noise.

This is why testers are sometime a joke. My testers say the channels are fine because there is continuity, but it is not enough to supply the correct Phantom power to the DI and the condenser.

If they always used dynamic I would have never known that there was a problem.

Anyone else find problems like this?

[John Brook]

Roy, I hope you caught my sarcasm. I think it's the Mackie. I believe they use a pseudo balanced input circuitry, which may be the culprit, or it may be the PS, as mentioned above.

In one location, I got RFI with nothing attached to the board!

After one serious bout of RFI, I broke down and invested in pre-amps and that ended the RFI problems.

[RoyJeeBiv]

John -- i read too quickly to note the sarcasm. well done.

EV -- i hadn't thought about the power. i think i'll check that first, thanks.

[JEGG]

Originally Posted by John Brook

Roy, I hope you caught my sarcasm. I think it's the Mackie.

I believe they use a pseudo balanced input circuitry, which may be the culprit, or it may be the PS, as mentioned above.

Your belief is not correct, and you have some fundamental misunderstandings of inputs.

The term "pseudo balanced" does not at all refer to inputs nor outputs. Rather it is a connection (wiring) protocol for connecting unbalanced gear to balanced gear, to substantially reduce noise by a mere wiring convention. (Beyond this discussion, more or less the same protocol can be used to connect two unbalanced devices for similar benefits.)

There is no such thing as a "pseudo balanced input"- or output.

If you are referring to "impedance balancing" an input-that as well is impossible. The input has to be summed (with some exceptions such as "through balanced" devices-Millennia, one example).

You can only "impedance balance" an output, and there is nothing wrong with that. There is an equal amount of common mode rejection when such a device is "received" by a balanced input. Much high quality gear has single ended outputs-most Neumann mic's, for example.

Originally Posted by John Brook

In one location, I got RFI with nothing attached to the board!

I thought this comment only came from people complaining about their powered monitors! (Sorry.)

This in itself doesn't indicate anything is wrong-at all. If you had RF when the input was bridged with shorting wires, test resistors, a microphone, or a microphone test body/capsule, THAT would indicate a problem. (Note: do not apply phantom power with all the pins shorted.)

BTW, you shouldn't leave channels open/un-muted (especially mic inputs) if you're not using them.

If by PS, you and/or the OP are indicating phantom power supply, this is also extremely unlikely in a Mackie board.

Noise introduced by phantom power supplies is typically found in bottom of the barrel "pro" gear. Cheap flash recorders with XLR inputs, for example.

Noise in the phantom power is cancelled (or attenuated) by the same mechanism that differentially sums the input. Consequently, manufacturers "allow" this in some low end products. Not very good design, but as a user you would trouble identifying this problem. The noise floor might be raised a little, but with low end gear you'd probably not notice.

More importantly, this sort of problem would not manifest itself as an RF problem.

Originally Posted by John Brook

After one serious bout of RFI, I broke down and invested in pre-amps and that ended the RFI problems.

I seriously doubt it was the Mackie, if the Mackie is at all recent. They are relatively well designed. And mine have been bullet proof in situations where very august equipment was struggling with RF.

It's nice that you have new preamps, but unless there was a fault, the problem probably could have been solved with a little investigation. With the new preamp came slightly different circumstances.

The OP needs to do-a step by step investigation until the cause is found. There is nothing in the OP's post that would lead me to conclude this is a phantom power issue. Nor a non existent "pseudo balanced input."

[JEGG]

Originally Posted by Mulmany

I have what sounds like an interference problem when using Phantom on some channels at a facility that I installed. I'm at the point were I need to just go and pull all the stage boxes and re solder everything.

Your methods would fix a wiring problem, not a problem with the phantom power supply. But if it works-and I hope it does, why argue semantics?

[John Brook]

JEGG, it's been 40+ years since I attended EE classes, so I'm not going to try to tilt windmills with you.

If your Mackie has no RFI issues, great. Please don't be so quick to dismiss the possibility that others are not so fortunate.

[RoyJeeBiv]

quick update -- i haven't had time to do any testing, but apparently the auditorium experienced similar issues 20-25 yrs ago...so i imagine it's the outlet i was using at the back of the auditorium...?? i'll try to remember to report back this summer when i've verified the source of the problem. in the meantime, i'll look into a power filter. everyone's comments have helped a lot in narrowing this down, so thank you all.

[fifthcircle]

The worst (and some of the only) RF issues I've ever had have been with Mackie consoles and locations with questionable grounding.

There is obviously a lot that can cause this, but in the end, most of the time it comes to grounding and the way that the devices in your chain deal with it.

Been a long time since I've had issues, but I'm now also completely self-contained and almost never use house gear when I'm on location. All my cables do not have the shield connected through the connector (the ground/shield is just that- no connection to the XLR at all). I bring all my own power conditioning and I've even been known to bring an isolation transformer to reestablish ground for my rig. Feeds from the house all go through isolation transformers, etc...

It is a lot of work to isolate yourself from those issues, but it is worth it in the end.

--Ben

[DannyL]

Mackie desks, especially the VLZ versions are VERY prone to RFI. Phantom power should not have an effect either way. The length of the mic cable absolutely will. I have found that cable runs exceeding 10 meters are prone to RFI with Mackie's VLZ inputs. The effect becomes increasingly more pronounced with every extra meter. If you can place mic pre's close to the mics and send the signals to the Mackie at line level your RFI problems should disappear.

Danny

[bishopthomas]

Originally Posted by DannyL

If you can place mic pre's close to the mics and send the signals to the Mackie at line level your RFI problems should disappear.

Because Grace 801 pres into an SR32 seems like a good match...