Eliminating hiss in a wireless boom mic setup?

[thoomp]

Hey everyone,

Here's the scoop. I'm having a terrible time trying eliminate really bad hiss (a truly unacceptable noise floor) per a wireless boom setup that I’m assembling. What I have right now theoretically should work, but the hiss renders the sound capture totally unusable. Maybe it’s a simple fix that just involves adding an inline RF filter somewhere?

The signal chain is as following:

Audio Technica AT4053b (boom for indoor voice) ***OR*** Audio Technica AT875b (short shotgun for outdoors) ---(8” XLR patch cable)--->
Sennheiser MZA14 48 (supplies +48V phantom power to either mic) ---(8” XLR patch cable)--->
Sennheiser EW100 G3 wireless microphone XLR transmitter —(WIRELESS CONNECTION)--->
Sennheiser EW100 G3 wireless receiver —(3.5” headphone jack to XLR)--->
Sony FS7 (line in)

Again, what I hear is clearly a hiss and not a hum or a rumble.
There is is no AC power involved in the system, let alone an earth ground anywhere.
In some cases, I can attenuate the hiss by holding the MZA14 and the EW100 transmitter boxes differently (I guess I’m the ground in that case?).
But then, once I have the noise quieted a bit, sometimes I hear music in the background - which sounds like pop music that you wouldn’t typically hear on AM radio these days.
When I let go, or change the position, the hiss comes back in full force.

Per the Sennheiser EW100, I have played with the squelch, sensitivity, changing channels, and even dropping the preamps on the FS7. No luck.
Again, it seems to have something to do with there not being an earth ground involved.

Per testing individual components in order to rule things out, the signal is totally quiet when:

* Either mic is directly connected to the FS7 just via a simple XLR cable (with +48V provided by the FS7
* Either mic is directly connected to the FS7 via an XLR cable and just the MZA14 (with +48V provided by the MZA14)
* The EW100 system is used without the MZA14 in the chain (such as with an MD46 mic that doesn’t require +48V or an EW100 G3 wireless lav transmitter)

Per my research, I thought a “ground lift” (eliminating ground pin 1) might eliminate the hiss - but that seems to be for ground loop issues - meaning hums and not hisses. I haven't tried it, but I'm guessing it won't work.

Any thoughts? Thanks in advance!

[Noiz2]

My first guess would be a bad connection, either dirty or broken wire. A broken ground could act like a big antenna and pick up his and the odd radio station.

So I would make sure all the setscrews on the XLR connectors are tight. Then I would look at the pins and see if they are bright or oxidized. If they are dirty or oxidized clean them till they are bright. I would also turn off if possible the "plugin-power" from the transmitter, that voltage may be screwing with things. If you can't turn it off look for an adapter cable made to hook it up to a dynamic mic., that should block the voltage.
If none of that works I would swap out the cables and see if that makes a difference, it's faster than opening each one up.

Good luck.

[Alex H.]

I would also turn off if possible the "plugin-power" from the transmitter, that voltage may be screwing with things. If you can't turn it off look for an adapter cable made to hook it up to a dynamic mic., that should block the voltage.

Just so this doesn't get confusing, this probably should be clarified:

Sennheiser EW100 G3 wireless microphone XLR transmitter —(WIRELESS CONNECTION)--->

Does this mean the XLR plug-on transmitter? If so, is it the SKP100? That one is for dynamic mics only and does not provide any kind of power. You may be better off spending a bit to get the SKP300, which is the XLR plug-on transmitter that does provide phantom power, thus eliminating the need for the inline phantom power suply.

[paulears]

If the transmitter is the non-48V version that plugs onto a dynamic mic, then your Sennheiser phantom power supply should be fine. You mustn't use a ground lift because that will of course stop the 48V phantom working.

Interference products and especially AM decoding is usually down to dry joints, and faults that allow the preamp or whatever to work as a detector.

First thing is to confirm that the mic works into the camera with 48V applied - I believe you confirmed it does, so just checking.
Next, plug in a dynamic mic to the XLR transmitter, and test the system works with the camera receiving good audio. To start, select 0dB attenuation on the transmitter, and a mid way level on the receiver. Does this work? If it does, remove the working mic, and then plug that into the phantom unit, connecting that to the transmitter. Now check again - you know the mic works, you know the transmit and receive works, so if it now plays up - the fault is only in the new components to the chain - the phantom unit and the XLR cable. If this suggests the phantom unit is working, then turn OFF phantom on the camera, and connect the phantom power unit and the known working dynamic. If this works, swap the dynamic for one of your condensers. If it doesn't work, then the phantom unit or cable is faulty - swap the cable. Still duff, then its the phantom unit. You just need to be logical to identify where the problem is. I suspect it will just be a cable.

[thoomp]

Thanks for the updates so far, everyone! I’ve been away for the past week and away from my gear; I’ll do some more testing now that I’m back. Here’s the update:


@Noiz2 – Connections seem fine, but I’ll recheck. One of the XLR cables is brand new and the other is “like-new.” The SKP100 transmitter doesn’t offer any “plugin-power,” as the function of the Sennheiser MZA14 is to do just that.

@AlexH – Apologies if I wasn’t clear. The XLR plug-on transmitter is indeed an SKP100. I’m doing this as a bare-bones setup (I got the MZA14 very inexpensively second-hand, knowing I already had the SKP100 in our kit). I’d LOVE to upgrade it to the SKP300, but the AT4053b destroyed our budget for now and we’re trying to make do with what we already have.

@paulears – Gosh…I hope it’s just a cable. Again, it is the standard SKP100, and not the SKP300 (+48V) version. Thanks for helping me avoid testing with a ground lift. I have confirmed that mics are quiet when plugged into the FS7 without any transmitter (with +48V provided by either the FS7 directly or by the MZA14.) I’ll try and troubleshoot further now that I’m back.

**************

Here’s an update after a bunch of testing this afternoon, which will hopefully help rule things out. It looks like I have two separate problems here:


(1) RF SPRAY FROM THE SKP100 TRANSMITTER?: Check out the following picture of my setup. When I hold the SKP100 just like this, with my three fingers fully wrapped around the XLR connection and with the palm around the body of the SKP100 transmitter, almost all the noise goes away. Can I put something in there (ferrite RF chokes?) to shut down what may be RF Spray?

I’ve pulled the mics themselves out of the equation; the noise happens regardless of whether a mic is connected to the chain. Stripped down, it happens when the MZA14 is directly connected to the SKP100.


(2) FM RADIO INTERFERENCE: Once I hold the grip that way, most of the noise is gone. But in certain circumstances I’m now picking up variably slight broadcast radio noise. What’s weird is that I am in fact picking up FM (not AM) signals. At its worst, the sound is a couple of FM stations playing all at once. The clearest of the radio station signals is WBMX 104.1 FM, which is a powerful FM radio station (21,000 watts) broadcasting from the Prudential Tower exactly two miles away.


By the way, as far as cable quality goes, I’m planning to return the brand new 8” one (shiny cable ends) to Amazon -- but neither cable seems to do anything to significantly solve the main hiss problem. I think the coiled black cable (Pearstone, with Neutrik ends) picks up less of the lower FM signal, but there’s higher handling noise when the coil moves around – so I’ll likely have to replace both cables for this application.

Thanks again in advance!


IMG_1084.jpg

[Noiz2]

This REALLY sounds like a bad ground. Since the transmitter is the older piece it may be there. With cables a telltale sign is if the working part (pins or female side) have any wiggle. There is a set screw that can loosen up and then you can get a weak or intermittent ground with the case, which turns them into radio receivers.
You are probably not too excited at opening up the transmitter so if it's coming down to that being the issue it's probably time for a visit to the shop.

[Noiz2]

” The SKP100 transmitter doesn’t offer any “plugin-power,” as the function of the Sennheiser MZA14 is to do just that.

Actually the MZA14 is giving you "phantom power" "Plugin Power" is different and what a lot of 1/8" mics use. I mistakenly assumed you were using the belt-pack transmitter.

[thoomp]

@Noiz2 - Thanks! The MZA14 is almost definitely the older piece here; I got it used and it has seen plenty of wear (why I could afford it in the first place). I believe that the SKP100 is just a few years old. Again, when the SKP100 is used with the MD46 mic (my original setup), there is next to zero noise with that. Sorry for any confusion per "phantom power" as that's definitely why I bought the MZA14 in the first place.

BTW - Someone said on a different forum that they have the same exact problem. They're getting acceptable (albeit not perfect results) by putting a transformer in between the MZA14 and the SKP100. Any thoughts on this?

[thoomp]

I'm tentatively optimistic here! With a test of effectively disconnecting Pin 1 (by jumping the Pin 2 and Pin 3 pairs respectively -- but not Pin 1) between the MZA14 and the SKP100, I think I created a ground lift - which has quieted things down. As the mic works, the +48 phantom power is in fact being passed through.

Would a solution of using a transformer instead of a ground lift do the same thing here? Would that be a quieter solution than a ground lift?

Assuming I just go pick up a ground lift (as a permanent solution to the first problem here), I now need to get rid of the rest of the hiss and FM signals that I'm picking up. Any thoughts? What's the next step?