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60 cycle humm but only one power connection, I'm stumped

Discussion in 'Electronic Repair Modifications DIY' started by Stuckshutter, Dec 7, 2014.

  1. Stuckshutter

    Stuckshutter Active Member

    Hi everyone,
    I'm not an audio engineer or even sure what one is but I have a problem with a humm. It happens when ever I have some audio device connected to the wall outlet. Let me explain further and see if one of you gurus sees how to fix it or at least track down the problem:

    I have a Sony EX-3 video camera that can use batteries or a power connection via it's battery charger. On battery, no problem. Plug in the battery/charger-power for the camera and instantly, the hum appears.

    I just bought a PreSonus Blue DP V2 audio amplifier for use with some mics. It uses a wall wart DC power unit 12v 1amp for power. I use that into a Azden field mixer that runs on AA batteries. It goes into the EX-3. Using the Azden alone into the EX-3 when all are on battery, no problem.

    Connect the wall power to either unit and presto, the hum is back with a vengeance.

    More info:

    • All cables are XLR Canare or Mogami, no computer hookups anywhere near.
    • No power lines are crossing or near anything.
    • There is a light dimmer on the circuit but it's off if that's a clue.
    • I ran a power cable to an entirely different circuit and the problem was there.
    • I use a Steinberg audio interface to the computer and I've used the same setup with the PreSonus amplifier and field mixer into the Steinberg and there is no hum.
    • I run into the problem with only one input into the field mixer and that's from an AKG microphone.
    • The hum also occurs with just the field mixer connected to the EX-3 and no PreSonus connection.
    I surmise it is the power connection and it's feeding through the DC power supplies and causing the problem ... except that using ad different circuit entirely does not change the problem.

    I'm stumped about how to track down the problem and what to use or do to get rid of it. At the moment, I'm thinking about an exorcist...

    Admittedly, it doesn't take a big problem to stump me... But I am hoping some of you experts can give me the knowledge to diagnose and fix the underlying problem.

    Many thanks for you help.
     
  2. paulears

    paulears Well-Known Member

    Switch mode power supplies are often the causes of audio hums - Dell laptop power supplies frequently do this. You can try a nice big chunky toroidal choke with the DC lead to the camera passed through it several times. Loudspeaker magnets are pretty good for this. Wrap ten or more turns of the DC cable through it and give it a go?
     
  3. DonnyThompson

    DonnyThompson Distinguished Member

    Yes, but the separate circuit is still in the same house, running off the same power source, right?

    This could be a ground fault somewhere in your main circuit...

    Have you tried this exact same rig at other locations, using different power sources?
     
  4. Boswell

    Boswell Moderator Distinguished Member

    I'd be surprised if this were due to a problem with the mains wiring at your property; it's more likely to be a problem with the design of the power supply units for the Sony and Presonus devices. My guess is that they are 2-pin, i.e. no earth, and I'm assuming that the hum is no different whichever way round they are plugged in.

    Is it possible you could borrow a better-quality (or at least different type) power supply for the Presonus box and see if that makes any difference, better or worse? Make sure that the voltage rating is the same as the one you are replacing and that the current rating is at least as much, and also that the polarity of the inner and outer of the barrel connector is the same on the two devices.

    The Presonus web site gives the power inlet spec for the Blue DP V2 as "+12 VDC, 1A, center pin positive". The Sony PMW-EX3 also requires 12V d.c., but I can find no specification for the current needed or the connector polarity. I would not recommend that you try a different power supply for this unit. It looks a lovely camera, by the way.

    Have you tried grounding the shell of the mixer output connectors when you are getting the hum? It would be interesting to see if that made any difference.
     
    kmetal likes this.
  5. Kurt Foster

    Kurt Foster Distinguished Member

    this sounds to me like a ground loop. everything in the studio should be plugged into the same electrical circuit and if possible into the same outlet. .
     
  6. paulears

    paulears Well-Known Member

    Almost certainly NOT a ground loop as the power supply will not have a ground for any circulating currents to be induced to. Switch mode PSUs are very well know for doing this - and as the ground/chassis is not linked at all to the safety ground, extra earthing often makes the problem worse. The snag is the induced noise simply travels down the low voltage power cable. As I said, Dell computer PSUs are extremely well know for sticking hum into PA systems, and running on batteries noise free is the clue. People have tried transformers to isolate the audio from the PA, but very often these don't work either. Look at practically every low voltage PSU you have - the big plastic knobbly bits in the cable are filters to attempt to stop this, but the ferrite cores are small and while they are a bit effective - and actually required to ensure passing the EU regulations on RFI we have here, they're just not enough for really nasty PSUs - radio hams have had this kind of problem for years and they use this kind of filter to stop similar problems. Ground loops make a similar noise, but PSU interference is usually independent of ground circulating currents.
     
  7. dvdhawk

    dvdhawk Well-Known Member

    Also a clue.

    I'd like to see the details on what kind of audio cables the OP is using to tie all those devices together.

    The EX3 is a serious camera, the Azden should be passable quality. Maybe an XLR ground-lift adapter would be in order.
     
  8. Stuckshutter

    Stuckshutter Active Member

    Thanks everyone, lots of good advice and things to try. I'll go dig around and try your suggestions and report back. I appreciate your thoughtful replies and help a lot.
     
  9. MrEase

    MrEase Active Member

    The ferrite cores you see on PSU leads (and also on many other "digital" leads such as VGA monitors) will do nothing for 50/60 Hz hum problems. They are there to get conducted RF problems within the EC and FCC regulations. As "Boswell" says the best bet is to try an alternative PSU on the PreSonus. As the ground is not usually used on these adapters I also doubt if a ground lift will help as it could remove the signals return path resulting in no signal and possibly huge hum! It is well worth testing different things as suggested by Boswell.

    For anyone confused by ground loops etc. here's a link to some good information - http://www.douglas-self.com/ampins/groundloops/grndloop.htm although it doesn't specifically cover none grounded supplies
     
  10. Stuckshutter

    Stuckshutter Active Member

    Thanks to all who suggested a solution. I did try the ferrite core and it did help some but the hum was still there. I solved the problem temporarily at least by creating a battery connector which, not surprisingly, got rid of the hum completely.
    PreSonus recommends a "Line Conditioner" but didn't provide any specs to look for. Since I have this same hum coming from the transformer power for the EX-3, if the line conditioner were a solution, it would get rid of the issue on multiple pieces of gear.
    APC, Tripp and Panamax all make such devices and there are probably others...
    I hate to spend the bucks to get on that doesn't do the job because there is some special spec it needs to have or worse, it simply doesn't fix the hum.
    Since this forum is loaded with experts, I'm hoping for some sage guidance and what to look for if the line conditioner is the solution.
    Many thanks in advance to all those who help!
     
  11. kmetal

    kmetal Kyle P. Gushue Well-Known Member

    Id try the one less expensive options first. Although I have seen 500w isolation transformers for like $90 on fleabay, it's still a crapshoot without more things crossed off the potential culprit list. If running on batteries solved the problem then the issue is with your equipment and they're interconnections not the actual electricity it's being supplied with. But I may have missed something.
     
  12. Kurt Foster

    Kurt Foster Distinguished Member

    try a new power supply. get a universal power supply. they run around $30 for a decent one.
     
  13. Stuckshutter

    Stuckshutter Active Member

    Thanks Kurt and kmetal, appreciate your response. I figure the following may help run this to the dirt:
    The 60hz hum appear on just the camera when plugged into the wall.
    The hum appears when only the power amplifier is plugged in.
    The hum appears on my EX-3 when it is plugged in.
    The hum disappears not surprisingly, with both units attached and the preamp plugged in (the field mixer runs only on batteries at this point).
    The hum disappears when I run the field mixer and preamp on batteries. It also disappears when I plug it into my APC UPS in the office. No hum there.
    For the one person who wanted to know how the units are connected, they are connected with Canare quadstar xlr balanced cables.
    I can get a line conditioner from WalMart for about $40 so that may be the next step here unless someone can offer another solution.
    If the line conditioner is the next step, any suggestions for what to look for beyond KVA ratings?
    Thanks for all the help everyone, it's great to have "pros who knows" to turn to when you're lost in the weeds!
     
  14. rmburrow

    rmburrow Active Member

    If you try another power supply, first make sure you aren't going to void any new equipment warranties. You could try battery power in all but one of your "boxes"...until you eliminate the offending one when operated on AC. Check the power supply output with a voltmeter (under load) to insure the voltage supplied is correct. If the output voltage is OK, and you can get to the +/- DC supply leads, try connecting a 1000 uF capacitor across the DC supply (Pay attention to polarity!) and note any change/improvement.

    Have you tried the "Hum-X" (available from B&H in NYC for around $65 give or take)? I have three, and they are lifesavers on remotes with dirty power. Real bad cases may require a isolation transformer. Video isolation units are available in BNC type "barrel" fittings. Are the XLR connections balanced? Which component(s) are unbalanced?

    Did you attempt to look at the offending hum with a scope? I assume the wall wart AC supplies are two prong, which assumes ungrounded on the secondary (DC?) rail. Try a clip lead ground from the offending equipment to a real ground or the wall plate screw and note any changes. This problem appears to be a balance or missing ground situation; maybe a ground loop if a power amplifier is involved. Ground lifting may help isolate offending connections or equipment, but in any permanent installation, all the equipment (especially AC powered!) should be grounded for safety reasons.
     

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