Strange Audio Hum Issue with HPX250

[Larry E McPherson]

This is my first posting on any online forum. I have learned quite a bit of helpful information on the DVXUSER site, particularly from Barry Green, so I will give it a go. I have reported this problem to Panasonic ProVideo—they took careful notes, but I have not gotten a response yet.

Audio with Sennheiser EW100 G3 Wireless Receiver plugged into HPX250 XLR 1 Input:
Scenario 1:
Sennheiser Receiver power turned On
The Phantom power switch on the Input 1 of HPX250 turned to Off position
Result: The recorded clip plays back in NLE with same good quality I heard while monitoring the camera audio with headphones during the recording. This is as I would expect.

Audio with Sennheiser EW100 G3 Wireless Receiver plugged into HPX250 XLR 1 Input:
Scenario 2:
Sennheiser Receiver power turned Off
The Phantom power switch on the Input 1 of HPX250 turned to Off position
Result: There is a hum in the recorded clip just like I heard while monitoring the camera audio with headphones during the recording. This is not what I would expect.

Audio with Sennheiser EW100 G3 Wireless Receiver plugged into HPX250 XLR 1 Input:
Scenario 3:
The Sennheiser Receiver power turned Off
The Phantom power switch on the Input 1 of HPX250 turned to On position
Result: There is not a hum in the recorded clip just like I didn't hear a hum while monitoring the camera audio with headphones during the recording. This is not what I would expect. I do not want to do this, but did it to see if it can provide a clue as to what is happening.

In summary:
If I hook up the Sennheiser EW100 G3 Wireless Receiver to the HPX250 XLR Input 1 and the Receiver is turned On, everything works well as I would expect.

All I have to do is turn off the Receiver and a hum enters the audio recording system of the camera. I wouldn't expect this! Turning off one audio input is something I want to be able to do without it creating a hum in the audio. A work around is to unplug the Receiver XLR connector, then there is no hum. But of course that is an inconvenience I would rather not have. I tried the identical tests with my HMC150. As my previous experience with that camera suggested I am able to turn Off the Receiver when it is hooked up to the HMC150 without it creating a hum in the audio.

Please! What is going on here?

[Donald Gregory]

Do you have any of the auto settings on for the audio? Is the hum present in all the channels recorded?

[Larry E McPherson]

None of the auto audio settings are turned on, all manual switches and menu switches for auto audio are turned off! The hum appears only in the audio channel from Input 1 that the XLR of the Sennheiser Receiver is plugged into.

[Barry_Green]

I can't verify your results. Now, granted, I don't have a Sennheiser receiver here, but I used my old Audio Technica one to test. I tried in XLR 1 and in XLR 2, with it set to line or mic, with phantom on and off, with the receiver turned on and off. Every combination of all those variables.

No hum. At all.

I hate to say it, but maybe there's an issue with your particular camera?

Oooh, wait, one last thing - was your camera on battery power or AC?

[Larry E McPherson]

Thanks Barry!

The Camera is on battery power, as is the Sennheiser Receiver, everything!

An Audio technician at Trew Audio wondered if it has to do with the fact that the Sennheiser XLR cable is not balanced. I do not know the answer to that, but as I say the identical tests with the HMC150 resulted in no hum under any circumstances using the same Sennheiser Receiver and XLR cable.

I tried the same test with the HPX250 using an Electro Voice RE50N/D-B Dynamic Mic. Result: There is no hum when it is connected (and of course the Phantom power is Off) and working. But also of course there is no way to have it connected, but turned off, no on/off switch on the mic.

None of the auto audio settings are turned on, all manual switches and menu switches for auto audio are turned off! The hum appears only in the audio channel from Input 1 that the XLR of the Sennheiser Receiver is plugged into.

[Larry E McPherson]

Barry and all, here are additional bits of information I have come up with:

By the way, I am using 2 fairly new and pristine sets of Sennheiser EW100 G3 Wireless Receiver & Transmitter. I get the same result using either wireless Receiver or both at the same time on Input 1 and 2. If I unscrew the Sennheiser XLR cable from the Receiver, disconnect it at the end that connects to the Receiver, but leave the XLR cable plugged into the camera I get the hum. I wouldn't expect a cable plugged, but not actually connecting to anything to create a problem hum.

Plugging the same Sennheiser cables, just the cables, nothing attached, connecting nothing, into the HMC150 causes no hum as I would expect.

[Barry_Green]

I would really advise that you try to recreate the issue on another HPX250. I have a strong hunch that you won't be able to, which would point to some sort of wiring issue inside your camera.

[Larry E McPherson]

Thank you Barry for all your effort on this!

I live in Memphis, TN and know of no local place that would have an HPX250 to test. I need to find someplace that has BOTH an HPX250 and a Sennheiser EW100 G3 Wireless Receiver with XLR connecting cable, and is willing to try to duplicate the test.

I would appreciate it if there is someone out there that has the ingredients to duplicate the test, and is willing to try it? I bought the camera and Sennheiser from B&H, perhaps they will be willing to make the test.

[Donald Gregory]

I can try to duplicate your problem, but all of our HPX250s are out, so this wouldn't be until the end of next week.

[Terry_Martin]

I just did a quick test since I have the exact same gear. (except I'm not going to phantom power the receiver).
Result: HPX250 plus Sennheiser EW100 G3, no hum, with or without Sennheiser power. If there is no hum without anything plugged into the HPX250, I would suspect the Sennheiser cable or receiver. BTW, one of the Lectrosonics receivers is famous for not tolerating phantom power at all... it burns out the output stage.

[Larry E McPherson]

I do appreciate all of you folk's effort and feedback on this issue!

Here is my thinking at this point:
1) Actually I hope Barry's hypothesis that my HPX250 was wired wrong at the factory is correct, because if all HPX250s work like mine does that would mean the audio design is actually less functional than the HMC150.

2) Terry Martin, thanks for your test:
I think that furthers the case my HPX250 is just wired wrong at the factory.
However, I do not see where you arrive at there is something wrong with the Sennheiser cable or receiver. There could be something incompatible about the designs of the cable, receiver and camera, but it seems pretty clear there is nothing faulty about the cable and receiver, because they work perfectly with the HPX250 when the receiver is turned On, and they work perfectly with the HMC150 at all times, with Receiver power On or Off. Also I have two sets of cable and Receiver in pristine condition and both perform identically in all tests.

3) For those wishing to duplicate my test, and I hope there are more out there wishing to and report the results, here it is in its simplest and basic form:
1) Plug in a pair of monitor headphones and set the headphone audio volume on the camera to the loudest setting (I can hear the problem hum on my unit anywhere from 1/3 volume to max, but just to be sure you don't miss it set it at max).
2) Set the audio settings on the HPX250 so whichever input you use, XLR Input 1 or 2, is turned to mic, no phantom power.
3) Take the Sennheiser CL 100 XLR cable by itself unconnected to the Receiver or anything and plug it into the camera XLR connector
4) That's all there is to the test. On my HPX250 there will be a hum. On my HMC150 there is not a hum. Apparently, with Terry Martins HPX250 there is not a hum.

Just so you know exactly what equipment I am using:
Here is a link to the exact model of my 2 sets of Sennheiser EW100 G3 Wireless Receiver & Transmitter:
http://www.bhphotovideo.com/c/product/618739-REG/Sennheiser_EW_112P_G3_A_EW112_p_G3_Camera_Mount.ht ml

Here is a link to the CL 100 XLR unbalanced output cable that comes in the set at B&H:
http://www.bhphotovideo.com/c/product/233893-REG/Sennheiser_CL100_CL_100_1_8_M_Mini_to.html

4) I would like to be more knowledgeable about audio electronics so that I could understand what could be creating the hum in the HPX250 audio when either the unbalanced XLR cable alone, or the cable and Receiver in Off create the hum, yet it works fine without hum with the same cable and Receiver is connected in On. And further understand enough to explain why the same cable and receiver never create a hum with the HMC150.

Thank you all!

[Terry_Martin]

Larry,

I did another test which may explain the hpx250 vs hmc150 difference. First try to normalize both cameras with mic gain up and headphone volume up. Now (with phantom power off) just plug in the sennheiser cable with no receiver attached to the other end. Listen for hum first, with the 1/8" plug isolated, and then second, with your finger on the tip of the 1/8" plug.

In my test, the HPX250 produced a low hum with the isolated plug, but the HMC150 was quiet. Now for the human antenna trick, the HMC150 responded with a loud hum when the connector was touched (expected result), but the HPX250 didn't respond to the finger touch (unexpected result).

There are a lot of ways to design mike input circuits, and it seems clear to me that the HMC150 and HPX250 are simply different designs, just like any other camera you might pick up.

I would suggest that you try the bare cable test to see if you get the same hum as before.

I might speculate that applying phantom power to the sennheiser receiver may have blown some filter capacitor which might produce the naked cable effect when power is off. All speculation because I would think they would design a receiver that could take the occasional 48V.

Let us know if your hum compares to the naked cable test.

Good luck,

Terry

[Larry E McPherson]

Thank you Terry, for your further testing and post. You have stimulated me to do further testing and organizing the results, as you can see below.

However, if I understand you correctly you reported some different results for the same test in your first and second post. Additionally, some of my test results differ from yours. Please review all of my results and let me know how they compare with yours.

Thank you!

Test comparing the Panasonic HMC150 & HPX250 Audio systems response using a Sennheiser EW100 G3 Wireless Receiver plugged into the respective cameras XLR 1 Input:
(Standard procedure for all tests except where noted:
1) Plug in a pair of monitor headphones and set the headphone audio volume on the camera to the loudest setting (I can hear the problem hum on my unit anywhere from 1/3 volume to max, but just to be sure you don't miss it set it at Max).
2) Set the audio settings on the camera so whichever setting you use, XLR Input 1 or 2, is turned to mic, no phantom power.
3) Turn that channel's manual level control to Max.)

Test with Panasonic HMC150:
1) Wireless Receiver On and supplied Sennheiser CL 100 XLR unbalanced cable connected to both camera and Receiver:
No Hum

2) Wireless Receiver Off and supplied Sennheiser CL 100 XLR unbalanced cable connected to both camera and Receiver:
No Hum

3) Sennheiser CL 100 XLR unbalanced cable connected to camera, disconnected from Receiver, 1/8" plug isolated:
No Hum

4) Above test with phantom power on:
Increases the signal or noise level, but I wouldn't call it a hum

5) Sennheiser CL 100 XLR unbalanced cable connected to camera, disconnected from Receiver, tip of 1/8" plug being touched with finger:
Loud Hum

6) Above test with phantom power on:
Increases the signal level, but doesn't change the tone of Hum

Test with Panasonic HPX250:
1) Wireless Receiver On and supplied Sennheiser CL 100 XLR unbalanced cable connected to both camera and Receiver:
No Hum

2) Wireless Receiver Off and supplied Sennheiser CL 100 XLR unbalanced cable connected to both camera and Receiver:
Low Hum with cyclical rhythm

3) Sennheiser CL 100 XLR unbalanced cable connected to camera, disconnected from Receiver, 1/8" plug isolated:
Low Hum with cyclical rhythm, but slightly louder than when connected to receiver in OFF

4) Above test with phantom power on:
Increases the volume, but doesn't change the Hum quality
5) Sennheiser CL 100 XLR unbalanced cable connected to camera, disconnected from Receiver, tip of 1/8" plug being touched with finger:
Very Loud Scream

6) Above test with phantom power on:
Increases the volume, but doesn't change the tone of Loud Scream

I am tired of this testing already! Nevertheless, it is providing some practical understanding on what I can expect from the equipment and how I have to use it.

Why does all this matter? Because generally I want 2 wireless receivers mounted on and plugged into the camera at all times. Further, I want to be able to use either one, both, or neither receiver and control this at the camera simply by turning any given receiver's power on or off. I can do this with the HMC150, with the HPX250 I have to also plug in or unplug the appropriate receiver to avoid a hum in the headphone and recording.

Further question that has come up:
Can you damage a Sennheiser EW100 G3 Wireless Receiver by turning on the 48V Phantom Power of the XLR Input it is connected to?
(Not something I want to find out by experimenting!)
a) When the Receiver power is On?
or b) When the Receiver power is Off

Barry Green, do you have anything to add at this point?

Thanks all!

[Terry_Martin]

Normally I lurk around here to learn more about video from Barry and the gang, but I jumped into this thread because I have some experience with audio and electronics.

From my tests and your info, I conclude that your HPX250 is working normally, and the hum is the result of an effective antenna (unpowered sennheiser), plugged into the camera. (Also assuming you get no hum with nothing plugged into the HPX250).

I would suggest you contact sennheiser about the effect of applying 48V to it's output.

The best way to build a balanced mike preamp is to use a transformer input stage, but audio transformers are about the size of a 35mm film can, and only show up in high end mixing boards. So cameras, like a lot of other equipment, use various electronic circuits with varying techniques of hum rejection. It is clear to me that the HMC and HPX cameras have different circuits, and the HPX impressed me more in the area of hum rejection even though there is a low level hum on a naked cable. I have more experience with the HMC150, and have noted hot input signals can still distort in the input stage, even when the levels are not clipping.

Since I shoot a lot of music video, I record audio into a sound devices 788T, which is a recorder that records only 8 audio tracks and costs the same as an HPX250. The 788T mike pres are electronic, but much better designed than any camera. The point is that there is a wide variety of mike pre design and cost options.

In general I would recommend:

1) Never apply power to an output stage unless it is expecting it (like a condenser mike). Lectrosonics SR receivers will fail after application of 48V, but other models seem to survive, ... all depends on design.

2) Check your gain staging. If your sennheiser produces a minimal signal, then you would have to turn up camera gain, which would also increase the hum. If you think of camera gain on a 0 to 10 scale, I suggest recording in the 3 to 6 range, and if not, figure out why not. Also, we haven't mentioned the mike gain setting in the menu, which should be -50db in this scenario. Hope you were running the gain at max only for testing.

Good luck

[Larry E McPherson]

Thanks much, Terry, for your last post. Here is my response to sections of it:

"From my tests and your info, I conclude that your HPX250 is working normally, and the hum is the result of an effective antenna (unpowered sennheiser), plugged into the camera. (Also assuming you get no hum with nothing plugged into the HPX250)."
Yes, this makes sense to me, and I get no hum with nothing plugged into the HPX250 XLR audio inputs.

"In general I would recommend:

1) Never apply power to an output stage unless it is expecting it (like a condenser mike). Lectrosonics SR receivers will fail after application of 48V, but other models seem to survive, ... all depends on design."
Yes, I wouldn't normally apply phantom power to anything that doesn't call for it. Early in my testing, out of desperation to gain empirical information, I turned on the phantom power to the connected Sennheiser Receiver when it was Off, because that is when the hum occurred. And surprisingly, I believe the hum went away when then. Nevertheless, I wouldn't predict that would happen from subsequent tests when the Receiver wasn't connected.

Thus my question: Specifically with the Sennheiser Receiver can you harm it by applying power to it, particularly when it is Off? I am pretty certain that my unit was not harmed by that early test. Nevertheless, until I get knowledgeable information about that specific unit I am not willing to try it again. Turning on the phantom power to reduce the hum that occurs when the Receiver is turned Off wouldn't be a very good solution in the field anyway.

2) Check your gain staging. If your sennheiser produces a minimal signal, then you would have to turn up camera gain, which would also increase the hum. If you think of camera gain on a 0 to 10 scale, I suggest recording in the 3 to 6 range, and if not, figure out why not. Also, we haven't mentioned the mike gain setting in the menu, which should be -50db in this scenario. Hope you were running the gain at max only for testing.
Yes, my gain settings in the menu are at -50db, and max gain with the manual control was just for the testing. Normally I set the manual gain in the middle (5 on your scale) and fine tune the audio level by adjusting the Receiver's AF Out Level.

In summary: It seems my HPX250 is functioning as designed, there is nothing different or wrong with my unit. It works great with the Sennheiser Receiver when the Receiver is On. Nevertheless, it would be nice to be able to turn Off the Receiver without having to disconnect it.

???Anyone know of a cable or switch that would make that possible???

???Or a practical solution to being able to simply have the Wireless Receiver On or Off???

Thanks again!
Larry

[Larry E McPherson]

Eureka Folks!!!

I have stumbled upon the solution to the audio hum with the HPX250 when the Sennheiser EW100 G3 Wireless Receiver is plugged into the camera and the Receiver power turned Off!

If the Phantom Power to that input of the camera is turned Off you will get a Hum....

If the Phantom Power to that input of the camera is turned On you will not get a Hum....

The best way to use the Sennheiser EW100 G3 Wireless Receiver on an HPX250 is to turn the cameras Phantom Power ON!!! and leave it ON!!!

You get perfect audio when the Receiver is On and no hum when it is turned Off!

I thank Rich Topham, the owner of Professional Sound Services in NYC (www.pro-sound.com), for the keys to this solution! In an in-depth discussion with him it is clear this man knows audio! He said having the Phantom power on when the Sennheiser receiver is ON or Off will not damage it in any way!

I will call Rich the next time I need an audio problem to solve!

Many thanks to you all!
Larry

[Larry E McPherson]

My last post was made too quickly because I wanted it to be true. After carefully testing again I get different results. Here they are:

Panasonic HPX250 and Sennheiser EW100 G3 Wireless Receiver and Transmitter used for all tests:

Test 1
Wireless Receiver On
Transmitter On
Phantom Power On
Result: In the headphones while recording I hear a low cyclical hum like a motor running, In NLE after breakout of channels I can see and hear the hum in that channel

Test 2
Wireless Receiver Off
Transmitter On
Phantom Power On
Result: No hum heard in headphones during recording, In NLE after breakout of channels I can see and hear the hum in that channel

Test 3
Wireless Receiver Off
Transmitter On
Phantom Power Off
Result: Hum and cyclical beep heard in headphones during recording, In NLE after breakout of channels very noisy hum and cyclical beep heard in that channel

Test 4
Wireless Receiver On
Transmitter On
Phantom Power Off
Result: No hum heard in headphones during recording, In NLE after breakout of channels I can see and hear the hum in that channel

Did the following tests to eliminate the possibility of the voice and other sounds covering up the hum.

Test 5
Wireless Receiver On
Transmitter Off
Phantom Power Off
Result: No hum heard in headphones during recording, In NLE after breakout of channels I could see and hear absolutely no hum in that channel

Test 6
Wireless Receiver On
Transmitter Off
Phantom Power On
Result: Low cyclical hum heard in headphones during recording, In NLE after breakout of channels I could definitely see and hear the hum in that channel

Summary:
Test 1 and 6 confirms that when the receiver is On and the Phantom Power is On you definitely get a hum.

Test 2 and 3 confirms that when the Receiver is Off and still connected to the camera you get a hum whether Phantom Power is On or Off.

Test 4 and 5 confirms that when the Receiver is On and the Phantom Power is Off you get no hum.

Conclusion:
The only way to avoid the hum when you use the wireless receiver is to have the Receiver On and the Phantom Power Off. The only way to avoid hum when you do not want to use the wireless receiver is to disconnect the receiver XLR plug from the camera. This seems the only way to use the camera and receiver as they come from the factory.

I would like to be able to use either of 2 wireless receivers mounted on the camera, both at the same time, or none and control this at the camera simply by turning any given receiver's power on or off. I can do this with the Panasonic HMC150. With the HPX250 I also have to unplug any receiver I turn off to avoid a hum in the headphone and subsequent recording. I am still looking for a solution.

[Terry_Martin]

Larry,

One more thought.
Why not switch your camera from mike to line input when you don't want to use the receiver?
Still strongly advise no phantom power, but going to line input should significantly reduce the gain and reduce the hum.

[timbook2]

I do not understand the problem at all. if you turn off the reveiver it means you dont want to record audio right? so why bother if you record a hum? you dont want hum? turn down the volume in the humming audio track in your nle, delete the track do whatever you want. but the basic problem seems a NON issue to me ( unless I totally misunderstood everything written here).
its as if your are saying: if I film with the protective cap on my lens I see grain......

[Larry E McPherson]

Thank you Terry Martin, you made the suggestion to flip the Input from Mic to Line when you turn off the Receiver to eliminate the hum you would normally get if you left it connected. I tested that and it seems to work quite well. Also it seems flipping the Input from Mic to Line is a practical way from the camera to Mute a Receiver that is On when you just want to temporarily concentrate on the other Channel, and will soon flip it back to Mic.

I was talking with a Sennhieser support person today and among the things he said that really surprised me was that the Wireless Receiver is intended to be used on Line not Mic setting. In all my previous discussion everyone talked as if you use it on Mic. Nevertheless, I did a comparison setting the different components levels to get what I thought was good audio for each way. Below are the settings I used to for the different ways:

Test 1
Settings Using the Sennheiser Receiver with Camera Input set to Mic
Receiver 1 On
Receiver 1 AF Out -30db
Transmitter 1 On
Transmitter 1 Sensitivity -12
Input 1 Mic
Input 1 Phantom Off
Camera Manual Input 1 Gain Middle
Camera Menu Mic 1 Gain -50db

Test 2
Settings Using the Sennheiser Receiver with Camera Input set to Line
Receiver 1 On
Receiver 1 AF Out +12db
Transmitter 1 On
Transmitter 1 Sensitivity -12
Input 1 Line
Input 1 Phantom Off
Camera Manual Input 1 Gain Middle
Camera Menu Mic 1 Gain -50db

It seems when using Line there isn't as much room to adjust for different sound levels if the Receiver AF Out is set to +12db. When I was using Mic setting I had the Menu Mic Gain at -50db with the option to go to either -40 db or -60 db if needed. The Mic Gain setting options apply to Input set to Mic, not input set to Line, so there is no wiggle room using Line.

Let me know folks, how is a Sennheiser Wireless Receiver normally set up on a camera using Mic or Line?

Thanks timbook, for your perspective! I see your point. Perhaps I was so startled by the appearance of the hum with the HPX250 compared with using the HMC150 that doesn't have it. I wanted to know where it was coming from, I didn't want to be distracted by it when I was trying to listen to other things or spend time adjusting it out if I didn't have to, and I would rather record so I don't have to take it out in the NLE.

Thanks all!