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

Discussion in 'Preamps / Channel Strips' started by ymed, Sep 5, 2010.

  1. ymed

    ymed Active Member

    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: (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!
     
  2. Boswell

    Boswell Moderator Distinguished Member

    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.
     
  3. ymed

    ymed Active Member

    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?


    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 second preamp that I have tested with my mics was an external USB M-audio 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.

    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?
     
  4. Speedskater

    Speedskater Active Member

    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
     
  5. ymed

    ymed Active Member

    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...?
     
  6. Boswell

    Boswell Moderator Distinguished Member

    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.
     
  7. ymed

    ymed Active Member

    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)?
     
  8. Speedskater

    Speedskater Active Member

    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.
     
  9. ymed

    ymed Active Member

    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.
     
  10. vttom

    vttom Active Member

    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.
     
  11. ymed

    ymed Active Member

    I have connected preamp+mics to UPS battery without connecting to the wall outlet.
    Result:
    1) There is no hum from power line
    2) There is a hum when I touch mics with my hands and metal objects.

    Addition: when I touch with my hand a pin1 in XLR cable the hum also disappears.
     
  12. ymed

    ymed Active Member

    SOLVED

    Last week a service engineer from Shure service center called me and said that

    this is an attenuator Shure KSM 109 issue. He said that these mics can not work without a hum

    when you use non-grounded equipment (no grounding in wall outlet).

    Also, I can not use these mics with portable recorders!

    Attenuator works like an antenna without grounding. Servce engineer suggested to move away an attenutaor to prevent a hum.

    There is no any info in Shure's USER's MANUAL about it!

    I wonder at Shure! How they can produce the such defective mics?!
     
  13. Big K

    Big K Well-Known Member

    I was too late with my input, but I leave it in, anyhow..lol..

    M2c...
    Get an electrician to check the possibillity for a proper ground connection. Running any equipment not build for operating w/o ground is dangerous. Quite a few musicians died on stage because of improper earthing. No fast ground fault interrupter ( which should be in any house ) can protect you w/o a ground line going from the device to this interruptor. After the fuse and interruptor the ground and the neutral wire are connected to the same earthing system of the house. Getting additional earthing from an eave gutter, etc., is dangerous for the reasons speedscater already mentioned.
    In a studio you want an even better earthing by using lines connecting all at one single point, rather then daisy chained wiring, to prevent differences of electric potentials between the devices, because that is usually the hum problem #1.
    Now, if your pre-amp, etc. can be run w/o ground connection ( you see that on the plug -only 2 pins ) then try to turn the plug around. If you are lucky it takes the electric potential off the line or in line with other devices. Worked many times with my tube amps on stages...
    However, I do not know the plugs used in your country. Maybe you can't do that, at all...

    Why this hum noise exists even when you run the amp by battery and disappears when touching the ground of another device is one of the usual electrical miracles or simply a faulty amp / mic... I could think of (self) oscillating circuits that get influenced by that, but that is speculation.
     
  14. ymed

    ymed Active Member

    Hi, Big K!
    Thank you for the answer. There is no possibility to make a grounding in our house.
    See my today's post above about Shure service center answer. What do you think about their answer?
    Have you encountered the such things with mic's attenuators in your life?
    Shure produces mics that works only with grounding without a hum!
     
  15. Big K

    Big K Well-Known Member

    !!! This is partly based on European Regulations for electric installations of houses. Do not mess with Electricity, unless you are an Electrician !!!

    House grounding... Well, there is, ...must be... There is a connection to earth after the fuse box. The current is flowing back through earth to the point were it has been generated.
    If you take an isolating transformer which separates mains from the power coming out of it...
    This is called galvanic separation and is also used for locking out mains dirt and hum. You do that with a transformer which coils have a 1:1 ratio...

    If you touch a hot electric lead coming from the iso transformer there is no flow of current through your body to the ground you stand on. You must touch something connected to the neutral wire (or vice versa, of course) going back to that specific transformer.
    In the shop were I learned electronic engineering the rule was: if you work under power use only one hand...no danger of electric shock with the isolation tranformer inbetween.

    If the problem is that you have no reduntant wire for ground comming up to the room you might be able to pull another lead through the pipe. Connection should be made by an electrician. though.

    To the microphone:
    No, I have never heard of anything misconstructed like this. I would send it back as faulty, forthwith. It might be caused by the fact that this is an electret mic with a permanently embedded electric charge to the membrane in connection with a transformerless amp stage following...
    View attachment 5266


    In todays demand for mobile recording and the existence of devices like yours
    this is clearly a huge constraint to the usage of this mic. Sell it to a studio or bring it back...
     
  16. ymed

    ymed Active Member

    Hello Big K,
    Many thanks for your detailed answer!
    I have told that 1:1 transfomer can reduce hum coming from wall outlet powerline.

    1) Can you give me (www links) examples of such models?
    2) Do I need to use 1:1 (220/220v) transformer only for mic's preamp or for the whole system: PC, preamp, hi-fi amplifier etc.?

    Thank you
     
  17. Big K

    Big K Well-Known Member

    Hi ymed

    I have no specific links for ya.
    But you can search any old online electronic store for that.
    The Transformer is 1:1... means, you can put in any mains voltage or frequency and get the same out of it.
    Workshop models have only 1 power out socket for good reason, but if you don't use them for working on open devices you could connect a power strip. Mind the max power the transformer can cope with. For hum supression there are special transformers with a passive circuitry to filter out the dirt..

    Looks like that
    Trenntrafo 500 Va, ELMA TT | voelkner - direkt g√ľnstiger

    But take a model with a proper case and Socket !!
     
  18. ymed

    ymed Active Member

    Hi Big K!

    Thank you for advice!

    I think it will be interesting for you because Oktava mics have discussed there from time to time. Last week one user posted a message that his
    Oktava MK-220 mic has the same 50 Hz hum when touching an attenuator!
    His mic is not grounded too.

    I have uploaded his video on my site, see it:
    ymedvedev.ru/sound/MK-220.wmv

    I have corrected partly recently my problem with metal Scotch tape.
    I have covered a window of attenuator with a piece of this metal tape and
    hum 50 Hz disappeared.

    I had a hum even my hand was 5 mm above the attenuator. With taped attenuator
    there is no hum until I touch the tape with my hand. I think that thin layer of glue prevents firm contact with mic's body....

    Today 2 mics has the same problem:

    * Shure KSM-109
    *Oktava MK-220

    Who will report more?
     

    Attached Files:

  19. Boswell

    Boswell Moderator Distinguished Member

    I feel this has to be a problem with your pre-amp/interface and not your continuing list of microphones. Even if the proximity of your hand to a switch on a microphone induces a 50/60Hz field in the floating capsule circuit, the induced e.m.f. should be common to both signal lines and the CMR of the pre-amp should reject it. This is an issue of balancing and is independent of whether the equipment is grounded or not.

    The DMP3 is a nice bit of eye-candy, but I have never used one, and have no knowledge of how it and its power supply might behave in the absence of a mains ground. I suggest you repeat the experiment using a different (high-quality) microphone pre-amplifier integral to (or connected via) a real audio interface rather than a computer sound card.

    Incidentally, the possibility of hum injection is one of the reasons why professional recording microphones do not have switches incorporated in them, the main one being that they cannot accidentally or deliberately be switched off.
     
  20. ymed

    ymed Active Member

    Boswell, see my post above : http://recording.org/diy-pro-audio/46679-hum-pickup-noise-combination-m-audio-2.html#post356154
    where I wrote that Shure service center confirmed that this is a factory fault of all mics Shure KSM-109.
     

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