Hum and pickup noise in combination M-audio preamp with shure KSM109 mics

[ymed]

Hi!

I have a problem that I could not resolve during a long period of time.

I have 2 Shure KSM 109 mics that are connected to M-audio DMP3 preamp.
Preamp is connected to PC sound card.

1) When I turn on a phantom power I hear a low frequency 50 Hz hum in both chanels.
2) When I touch lightly a 0dB\-15dB attenuator with my finger (when mic lies on the table) the 50 Hz hum swells (increases). We have a 50Hz power line in our country.

I have tried:

1) Checked XLR cables (1-1, 2-2, 3-3 pins) with tester. Cables are OK.
2) Installed Ebthech hum eliminator between PC and mic preamp. It did not help.
3) Checked 48 V from M-Audio preamp - OK.
4) Checked a voltage (with mics connected to preamp) between 1 and 2 pins, and between 1 and 3 pin = 30 V.
5) Checked with another preamp: M-AUDIO - MobilePre USB - USB Bus-Powered Preamp and Audio Interface (Almost the same result - hum when I touch an attenuator, no hum from power line, may be because preamp powered only via USB?).

I have found then that there is NO HUM when I run my PC and preamp from wall outlet WITH GROUNDING.

Also I have found that (when I run from wall outlet WITHOUT GROUNDING) when I touch with my one hand a microphone and with another hand I touch my Yamaha hi-fi amplifier chassis (the metal jack of head-phones connected to amplifier chassis), then the hum almost disappears.

I have found also - my problem is not related to balanced-unbalanced issue.

Now, I have a hum in this scheme: microphone->preamp. I have no any devices connected to preamp.

How I have checked a hum without connecting to PC souncard?

My M-audio dmp3 preamp has a VU meters that show an output level, calibrated to correspond to digital peak meter. I can visually see (without sound) if I have any hum from mics without any devices (such as PC) connected directly to mic preamp.

Please, see my very short videos related to this issue:

1.YouTube - 113 (without PC connected)

2. YouTube - 50 Hz HUM when I touch a Shure KSM 109 microphone attenuator (with PC connected)

3. YouTube - 50 Hz Hum when I touch a Shure KSM 109 microphone attenuator (without PC connected)

On this videos I only touch an attenuator with my finger and VU meters show immediatelly that my hand produces a noise pickup (hum) in mics.

I have checked mic-preamp combination running from UPS battery (without wall outlet, the result is the same like running from USB powered preamp, see above). And I can conclude - this is not a power line issue.

Should mic and preamp work good without earth grounding?

I guess they must do, because there is portable recorders that do not have any grounding.

Thank you very much for any help!

[Boswell]

I think you have two problems. The first comes from running the balanced output of the DMP3 pre-amp into the unbalanced inputs of a PC soundcard. The second is due to the lack of ground continuity through the external power supply of the DMP3. You could fudge the present system or you could tackle the problem more radically.

You could help the first problem by using an attenuator between the pre-amp output and the soundcard inputs. The hum level would be reduced by roughly the amount of attenuation. A 12dB balanced attenuator could be used with unbalanced outputs and give you a better signal than what you have now.

For the second problem, I would take a separate earth line from the pre-amp ground to a good mains earth. Any ground-loop hum created by this will be much less than you are getting now.

The proper solution is to use an external audio interface instead of your PC soundcard. You should look for an interface such as the RME FireFace400 that has genuine balanced line-level inputs in addition to any microphone-level inputs. This is so that when you continue to use the DMP3, its output is not coloured by being attenuated and put through the microphone channels of the interface. You would not need the attenuator, and the whole system should not have any issues with mains hum even without the separate earth line.

[ymed]

I think you have two problems. The first comes from running the balanced output of the DMP3 pre-amp into the unbalanced inputs of a PC soundcard.

Thank you very much, Boswell, for the answer.
Regarding balanced-unbalanced issue, I have noticed in my previous post that I have tested combination of only two devices "preamp-mic" without PC connected to preamp. The hum is present. I see it on VU meters:
YouTube - 50 Hz Hum when I touch a Shure KSM 109 microphone attenuator
Did I prove that the problem is only in "preamp-mic" combination? Or this is a not correct test?

For the second problem, I would take a separate earth line from the pre-amp ground to a good mains earth. Any ground-loop hum created by this will be much less than you are getting now.

I live in the house without grounding in wall outlet. There is no possibility to install a good grounding for my flat. Also I have read that sometime earth has noise and some preamps have a button "ground lift" to break this noisy earth grounding.

Also I have wrote that I have connected "preamp-mic" scheme to UPS battery without wall outlet connection. This is imitation of portable recorder. Even in this scheme I had a noise pickup from my hands.

The proper solution is to use an external audio interface instead of your PC soundcard. You should look for an interface such as the RME FireFace400 that has genuine balanced line-level inputs in addition to any microphone-level inputs. This is so that when you continue to use the DMP3, its output is not coloured by being attenuated and put through the microphone channels of the interface. You would not need the attenuator, and the whole system should not have any issues with mains hum even without the separate earth line.

The second preamp that I have tested with my mics was an external USB M-audio audio interface:M-AUDIO - MobilePre USB - USB Bus-Powered Preamp and Audio Interface
It has no external power supply at all. The power goes only via USB. Even with this device I had a noise pickup from my hand and metal objects.

The second is due to the lack of ground continuity through the external power supply of the DMP3. You could fudge the present system or you could tackle the problem more radically.

I think - this is the main issue to consider.
In Shure KSM 109 mics the pin1 of XLR cable is connected with external metal case of mic. When I touch a head-phone metal jack that is inserted to hi-fi amplifier (Yamaha amp not a M-audio DMP3) I connect external body of mic with amplifier chassis - hum decreases (I add additional chassis grounding to pin 1 and external body of mic). With computer grounding connected to pin1 and therefore to mic's external body - the hum fully disappears.
I think that pin1 of XLR input in preamp is not connected to its chassis.
What do you think about this?

[Speedskater]

With computer grounding connected to pin1 and therefore to mic's external body - the hum fully disappears.
I think that pin1 of XLR input in preamp is not connected to its chassis.
What do you think about this?

It won't be the first time that commercial equipment had pin #1 problems.
I was just looking at one of Jim Brown's many papers (Power Point to pdf) and he pictures some bad units.
Start at the last page and work backwards.

http://www.audiosystemsgroup.com/AES-RFI-SF08.pdf

More Jim Brown papers:

http://www.audiosystemsgroup.com/publish.htm

[ymed]

It won't be the first time that commercial equipment had pin #1 problems.
I was just looking at one of Jim Brown's many papers (Power Point to pdf) and he pictures some bad units.
Start at the last page and work backwards.

http://www.audiosystemsgroup.com/AES-RFI-SF08.pdf

More Jim Brown papers:

Audio Systems Group, Inc. Publications

Thank you for info. I have found recently in this source: Sound System Interconnection

the next words:

"A common solution to these noisy hum and buzz problems involves disconnecting one end of the shield, even though one can not buy off-the-shelf cables with the shield disconnected at one end. The best end to disconnect is the receiving end. If one end of the shield is disconnected, the noisy hum current stops flowing and away goes the hum -- but only at low frequencies."

I will try to disconnect shield (pin1) in microphone cable (from the side of mic's preamplifier). But I do not know how it will work? The phantom voltage is measured between 1 and 2, 1 and 3 pins...?

[Boswell]

For the phantom power to return correctly, a condenser microphone like the KSM109 needs continuity from pin 1 of the XLR through the screen of the microphone cable to the pre-amp. It will not work without this.

If your mains outlets really have no ground, you should try to bring in a ground connection via a wire from a metal pipe buried in the earth outside. It's not surprising you have hum problems if your whole rig is floating with respect to earth.

[ymed]

For the phantom power to return correctly, a condenser microphone like the KSM109 needs continuity from pin 1 of the XLR through the screen of the microphone cable to the pre-amp. It will not work without this.

If your mains outlets really have no ground, you should try to bring in a ground connection via a wire from a metal pipe buried in the earth outside. It's not surprising you have hum problems if your whole rig is floating with respect to earth.

Yes, it was my mistake - to disconnect shield from on side.
I have found in this book: Practical recording techniques - Google

that it's not recommended to disconnect 1 pin between mic and preamp. It can be done between another sound equipment.

Again, I return back to idea that M-audio preamps had no connection between 1 pin and chassis. Therefore when I touch a mic with my right hand and the metal jack of head-phones inserted into hi-fi amplifier with left hand - noise pickup disappears.

But how to achieve this in practice to solve the problem?

Why I have a noise pickups from my hand and metal objects even in case of powering "mic+preamp" from UPS battery (without other devices, such as PC, connected to preamp)?

[Speedskater]

Lots of audio systems work very well without any connections to Mother Earth! However sometimes different parts of the audio system have different noise voltage referenced to M.E. so adding a ground connection may reduce this noise voltage.

Adding a separate ground rod to a system that is already connected to M.E. is Dangerous if lightning should strike anywhere nearby! Thousands of volts can be developed between your ground points.

Are the PC and the pre-amp connected to the same AC outlet?
Is the PC connected to the outside world via cable modem or telephone line?

Sometimes a jumper wire from the PC case to the mic-pre case helps.

[ymed]

Are the PC and the pre-amp connected to the same AC outlet?

Is the PC connected to the outside world via cable modem or telephone line?

1. The same AC outlet.

2. As I wrote before the problem lies only in scheme "mics+preamp". There is a hum even if only preamp is connected to AC power outlet. There is no other devices connected to preamp (except mic), such as a computer and etc.

[vttom]

A thought just occurred to me....

What do you have for lighting in this room? Have you tried turning the lights off?

I've gotten coupling from CFL (compact fluorescent lighting) transformers into my mics before.