audio hum issue

M

masnart

Active member
hello everyone,

i'm having an audio issue that i'm wondering if anyone else has experienced. i have 2 af100's and i am experiencing the same issue with both. My audio is crystal clear when using my rode shotgun mics, or my sennheiser wireless lavs. But i'm noticing a prominent low-end hum when using my countryman wired lavs. I have two countryman lavs that i've had no issues with up to now.

I've been trying to isolate the issue and here is what i've come up with so far.

The hum is present regardless of camera channel. It's present in both cameras. I've changed mic cables, power options (AC or DC), it doesn't seem like a grounding issue because it is NOT present in the shotgun mics or the wireless lavs. It is only present when the internal phantom power is in use.

I'm down to my last test tomorrow which is using an external mic pre-amp and by-passing the internal phantom power supply and go via line-level. I hope to be able to determine whether it's the mics or the internal phantom power. I'm definitely not a technical guru, so i thought i'd throw it out to others to see if i was missing something.

It seems many people commenting on the forum use mic pre-amps when recording, and i have no issue with that, just never needed one before. just found it curious that i currently have an issue with either both mics or both cameras internal phantom power.

thank you in advance for your input.
glenn
 
If you are powering the receiver with AC or DC power why are you using phantom? It may be a case of having both the mics power and the phantom power. I have never had this issue with my Sennheiser e100s or Lectrosonic lavs and my AF100.
 
I'm confused - the Countryman's presumably are the ones with the XLR designed for phantom power, and not the usual radio pack style that someone has simply terminated in an XLR?
 
paulears: the B3 countryman lavs are factory hardwired mics with XLR connectors and designed for phantom power. they have worked great for a year with the AF100 and a couple years before that with my other panasonic cameras.

OliverRush: i'm not powering the mics separately. the lavs are being phantom powered by the AF100 only. i've tried running the camera on house power or battery power and the hum is the same regardless.(that was the reference to AC/DC) the lavs and cameras have worked flawlessly and silently up to this point. i'm just trying to figure out what now has changed.

thanks
 
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Just a thought, but has anything else been added close to the camera/mics? Lights with noisy ballasts, low budget battery charger, external recorder, Delorean with a flux capacitor, that sort of thing? The cables are very thin on the B3 and may be more susceptible to external fields than regular XLR cables on your shotgun.
 
Yes, for what its worth to you I experienced the low level hiss using the countryman wired lav as well. B3 I believe. I tried every which way to eliminate it ( unsuccesfully). I only discovered this hiss several weeks after I took delivery of the mic from B & H. I couldn't return it as it was too late. I never thought there would be any problem with these popular and recommended mics - another newbie lesson. Sigh.
 
StephenM: Thanks for your input. No low budget gadgets or anything added to the mix that I don't always use.

I thought I'd pass along my results.

I was able to eliminate the hum problem temporarily for a shoot yesterday by using an external phantom power supply with my countryman lavs. After the shoot, I opened up the XLR connector on each lav and cut the soldered wire contact that was grounding the mic wiring to the XLR housing. Both lavs now are quiet as usual and work perfectly with the built-in phantom power of AF100.

Thanks to everyone for their input.
g.
 
How were you powering the AF100? Cutting the ground on the XLR is a brute-force way of accomplishing
a "ground lift".

Can you detail how the AF100 was powered and any other peripherals that may have been connected to the AF100?

I'm trying to figure out if you might have a "ground loop" problem.

thanks
jeff
 
jmarkham: i determined it was a ground loop problem of some sort. The AF was powered via AC into a 6 input box. The only other peripheral attached to the AF was a SmallHD monitor also on AC power and a set of headphones. I thought it could be the 6 input box or something else attached to it, but the hum existed whether it was powered by AC or battery. That was the confusing part. On the second camera, which was also transmitting the hum, it had no peripherals at all attached to it other than the headphones, and again switching between AC and battery made no difference. On the other hand, the hum was not an issue for either camera when using shotgun mics with their own internal power or wireless lav mics on either camera regardless of power supply. (Scratch head now)

"Snipping" the grounding wire on the XLRs was a really simple way of accomplishing the task. It doesn't effect any other equipment, i didn't have to purchase anything new (lavs or external phantom power supply), it doesn't have any effect on the lavs themselves and it prohibits the lav hum issue from being a problem in the future. Simple, easy, and my head doesn't hurt anymore from trying to figure out what the heck was going on.

I'm not an audio expert by any stretch, but this is how it was explained to me. In a balanced audio connection (3-wire audio where pin 1 is ground, pin 2 & 3 carry the positive & negative analog audio), some companies also connect the ground wire to the connector shell. This is a very bad practice because it introduces the possibilities of a "ground loop". A ground loop occurs when your grounding signal (on pin 1) passes to ground through the circuitry but also, simultaneously is being grounded through some other means, such as the metal chassis of a device. Because there exists the possibility of the grounded circuit seeing different amount of electrical resistance when it is being grounded through 2 different electrical paths at the same time, you end up with hum.

Equipment manufacturers, in order to comply with electrical safety regulations, often have to ground everything in multiple ways. So they wire the equipment chassis to ground, and they wire their circuit board to ground, and they wire their power supply to ground. With all those different sources getting grounded through different electrical paths, you are highly likely to end up with a ground loop, especially if that device will then be connected to another audio device, who is getting grounded in its own set of circuits.

So then, along comes the cable manufacturer, and he has to comply with these safety standards, and so he jumpers the ground of pin 1 to the cable's connector housing, and then that housing touches the metal of the camera, which is grounded separately from the audio circuits ground, and bingo!!, you've got a nice little ground loop. No audio cable should ever have that jumper connected. Or, if you really want to connect it, you can have it connected on just one end of the cable (the outgoing end), but not on the other end (the receiving end). That way, you won't be sharing chassis grounds between your two audio devices.

I'm not suggesting everyone start snipping ground wires on mic connectors, although it doesn't really sound like a bad idea. I'm just relaying the simple way I fixed the issue for those that were trying to help diagnose the problem with me.:smile:

g.
 
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Ok .. from your description, I can envision how the ground loop happened. The root cause is likely that the AF100's phantom
power is not isolated. No surprise .. it requires an inductor and that adds weight to the unit. That's why the sources that
were isolated ( Wireless or self-powered ) didn't have the hum and your AF100's that supplied the phantom did. The second
AF with no external connections is mystery though .. because it should have been isolated by definition. That suggests the
environment may have been "noisy" elecro-statically.
The reason I don't recommend cutting the ground pin on XLR mic cables is that's the connection that goes to the braid of
of the cable thus shielding (the phase/anti-phase audio) signals from external electro-magnetic sources.
Yet another reason why I don't record audio to the AF100 and use external audio paths.
It's funny .. we never ran into this stuff with our 300's or 200's .. but we did all the time with the AF100 .. and I suspect a
cost-cutting decision was made regarding isolating the audio path on this camera (particularly the phantom power).

jeff
 
jmarkham said:
The reason I don't recommend cutting the ground pin on XLR mic cables is that's the connection that goes to the braid of
of the cable thus shielding (the phase/anti-phase audio) signals from external electro-magnetic sources.
Click to expand...

I don't think he's talking about cutting the connection to pin 1 of the plug. That would leave you with an essentially unshielded cable (although I did read a book once that said it should be left open at the microphone end). A lot of cables have an additional jumper between pin 1 and the outer metal connector shell (for which there is an additional solder lug inside the connector), and he's talking about snipping that one.

From the Wikipedia article "XLR connector":

Although covered in AES48-2005 and in AES54-3-xxxx, there is still some disagreement on the best way to handle the usage of pin 1 at both ends of a cable, particularly with respect to the cable shield, the connector's shell, signal ground, and a third cable conductor connected to pin 1, which may (or may not) be connected to the shield. The main controversy is whether the shell of the connector should be connected to pin 1 or the shield, or left floating. AES standards mentioned above recommend that shells of cable-mounted connectors should never be connected to pin 1 or the shield, because inadvertent contact of the shell with another grounded surface while in use can create unwanted current paths for fault current, potentially causing hum and other noise. On the other hand, equipment containing active circuitry should always have pin 1 connected to the conductive enclosure of the equipment as close as possible to the point where the signal enters the enclosure. The argument centers around the radio frequency shielding provided by the shell of the connector, which may be reduced if it is left floating. An alternative solution is to connect the shell to pin 1 and the shield through a small value capacitor, providing RF shielding but allowing very little audio-frequency current to flow. This capability can be built into a fixed jack or a cable terminated with XLR connectors.
Click to expand...

- Greg
 
You're right .. he didn't cut pin 1, he cut the ground to the XLR connector housing .. which is an OK thing do to. The XLR housing frequently touches
the mount clip, which is attached to the housing .. and can cause grounding issues.

Cutting pin 1 (which I see all the time) is bad.

jeff
 
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