Wireless Boom Hiss - Sennheiser - Need Help

[Thoomp]

Hi everyone,

 

Nice to meet you all. As I’m an all-purpose production guy and am admittedly not a dedicated sound recordist, I’ve lurked on this group for a bit and I’m amazed by the wealth of knowledge there is here. I finally do have a question (which I’ve cross-posted to some other industry forums); maybe someone might be able to help?

 

Here's the scoop. I'm having a terrible time trying eliminate really bad hiss (I mean noise, not just background hiss per a noise floor from poor quality preamps) per a wireless boom setup that I’m assembling. What I have right now theoretically should work, but there's a hiss that 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 SKP100 wireless microphone XLR transmitter —(WIRELESS CONNECTION)--->
Sennheiser EW100 G3 wireless receiver —(AF Out via 3.5” male jack to XLR)--->
Sony FS7 (line in)

Again, what I hear is clearly a hiss and not a hum or a rumble. There is no AC power involved in the system, let alone an earth ground anywhere. Yes, I'd love a used SKP 300 or a Sound Devices MixPre-D (or the like), but per my budget neither is an option right now. I totally blew my budget getting the used AT4053b to at least try and capture quality indoor voice (which is apparently rare as hen's teeth to find use), and was lucky to find a used and affordable MZA14 to power it.
 

So after a bunch of testing, 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.

 

 

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

Per the Sennheiser EW100, I have played with all the levels - squelch, sensitivity, changing channels, and even dropping the preamps on the FS7. No luck. 

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.
 

*********

Finally, I found two older threads that may put me on the right track – but after reading those thoroughly – I'm still stuck. The threads are:

 

http://jwsoundgroup.net/index.php?/topic/15767-wireless-boom-rig-need-help/&tab=comments#comment-178369

 

http://jwsoundgroup.net/index.php?/topic/15768-sennheiser-mza14-48v/&tab=comments#comment-178393

 

 

Any thoughts? Thanks in advance!
 

- Seth

 

[daniel]

Check your frequencies.

Sorry if this sound s condescending (i think my first attempt at a reply did so i edited it) but check if you have have phantom power on the input from the RX and make sure you have optimised the gain structure across the system.

Don't buy your cables on amazon, go to to someone who makes cables for production sound - its the cheapest part of a system yet scrimping on this can be extremely detrimental.

It sounds like a problem between the phantom box and the TX - it's the cables or the boxes

[Rory Reshovsky]

I have had this exact same issue with an MZA14 going into an SKP100. This probably isn't the "proper" solution, but I got my setup working acceptably well by putting a transformer between the MZA14 and the SKP100. 

[Thoomp]

Rory - What exact transformer did the trick for you? Maybe there is hope here! Can't wait to hear details!

 

Daniel - I changed frequencies and gain structure checked. Signal from RX was line level into FS7. Cables - I will invest in good ones, albeit as short as possible.

 

[Masaki Hatsui]

I don't have an experience with the gear you use but just a guess.

Sennheiser's plug on transmitter uses its body as an antenna, like Lectro's HM TX. This may cause an RFI to certain material. Disconnect or connect Pin 1 to shell might be a solution, that I think it's worth a try but depends on how it is designed. It is possible that Sennheiser's Phantom power supply does not well designed RFI proof. If that doesn't work, I'd rather consider purchasing SKP 300 which has its PH48 integrated.

 

[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. Thanks, Masaki!

 

Would Rory's 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? I'm just not sure how the phantom power would jump the gap. (How are you doing that, Rory?)

 

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?

 

@Masaki - I would love-love-love an SKP300 but I just can't afford it right now. I blew all my money on the AT4053b.That makes perfect sense that the SPK 100's body is an antenna, as I can modify the hiss by touching/wrapping my hand around the unit.

 

 

 

 

[chrismedr]

6 hours ago, Thoomp said:

Any thoughts? What's the next step?

 

Use a cabled boom or if that is not possible save up for a proper wireless system for wireless boom?

 

sorry if that sounds harsh, but compromising your primary source of audio with unreliable and low quality components just seems penny wise and pound foolish to me. unless you're doing this as a hobby of course, then anything is fair game  : )

 

 

[Rory Reshovsky]

51 minutes ago, chrismedr said:

 

Use a cabled boom or if that is not possible save up for a proper wireless system for wireless boom?

 

sorry if that sounds harsh, but compromising your primary source of audio with unreliable and low quality components just seems penny wise and pound foolish to me. unless you're doing this as a hobby of course, then anything is fair game  : )

 

 

 

I agree with this. Even with my transformer workaround making this system function, I'd never trust it for anything where I cared about the end result. I'm actually planning on selling my MZA14 and SKP100 in favor of an SD MP-1 going into an SK100, or an SKP300.

[bilagaana]

A wire can eliminate this problem.  Try Canare Star Quad.

 

[Thoomp]

Hey guys,

 

@chrismedr - If all else fails, you can bet that I'll be taking your advice. I expected at least someone to recommend a cable! The key thing is that given that we usually film in restaurant kitchens (with oil-slick rubber floor mats that don't take well to gaffer tape) with crazy busy traffic where we can't afford a chef falling down -- we'd really hope for a wireless solution here. If I found a used SPK300 for about $250 - that would certainly change things but it'll have to wait.

 

@Rory - How did you get a transformer to work as I don't believe that it passes phantom (+48V) power? Again, might I ask what exactly you used? BTW - the MP-1 looks wicked! But that's pushing $500 (which is SKP300 territory). Right now, I'll take a passable transformer solution as a solution - but it would be admittedly temporary.

 

@bilagaana - Thanks! I was thinking of picking up Canare Star Quad wire yesterday; glad you suggested it as I had been wondering if it might help.

 

Any other thoughts? Again - thanks for all the wonderful advice, everyone! 

[Rory Reshovsky]

I put the transformer between the SKP100 and the MZA14. According to the manual, the SKP100 is actually an unbalanced input, so by using the transformer like how you'd use a direct box I'm taking the balanced out from the MZA14 into the unbalanced input of the SKP100. This isolates the two devices, and seems to have solved my hiss problem.

 

The noise floor is still noticeable higher than with a cable, though. 

[Thoomp]

Rory - What exact model transformer are you using and exactly how did you set things? Forgive my lack of knowledge here as I'm admittedly more of a novice per sound work. Again, I'd love to know at least this setup in detail - as although it is clearly not the cleanest option; in the very least bit it might get me by until I do a major upgrade here (maybe in the spring?). Again, I'd be grateful for whatever you'd be willing to share.

[Rory Reshovsky]

It's just a cheap transformer from an old RadioShack Hi-Z to Lo-Z adapter (basically a really cheap DI box). Check the SKP100 manual for the pinout on the unbalanced side.