1. Register NOW and become part of this fantastic knowledge base forum! This message will go away once you have registered.

question about odd humming in my audio recording

Discussion in 'Recording' started by Unregistered, Jan 9, 2013.

  1. Unregistered

    Unregistered Guest

    Hello,

    I am new here; just found your site. Doing some home studio recording for a podcast related to my work-- I've been out of Audio production for about 15 years (used to do small market FM radio); i've kept up with tehnology - somewhat.

    So, I have a small studio set up, I'm using a Behringer Xenyyx 802 6-channel input mixer, a (cheap) handheld vocal mike on a boom stand connected w/an xlr to 1/4 inch phono jack (yeah, not ideal at all, I know) which eventually (soon) I'll upgrade to a nicer Sterling Audio ST55 Large Diaphragm FET Condenser Mic, an Alesis L/R 1/4 inch to usb input connection to my computer, and AVS Audio editor to record my files (which I've used for 2 years on many projects and its been great!).

    When I play back the recording, I get this enormous (not overpowering) hum in the background. I was also getting it with my previous I/O connection which was L/R 1/4 inch adapters to L/R RCA conection to a 3.5mm connection into the mic input on the front of my laptop, and i thought the usb direct input would resolve that -- it did not, and thus my introduction to the forum at recording.org.

    Would someone with much more knowledge and experience in this arena please give me some pointers? I'm learning again after a long-time, and i could use some guidance.

    Thank You!
     
  2. DonnyThompson

    DonnyThompson Distinguished Member

    Hum is generally a sign of a grounding issue. Now, whether that grounding issue is within the ac outlet you are plugging your gear into, or emanating from one of the components, until you start isolating each component, or perhaps adding a ground lift to your AC outlet (this is not a fix.... just a diagnostic process), you won't know until you start removing certain things - one at a time from your chain.

    My best guess, off the cuff, is to start with the adapter you are using to convert that XLR to 1/4" phono....which brings up the question:

    Why are you even converting the mic from XLR to 1/4" when that mixer has two XLR ins on the first two channels?
     
  3. kmetal

    kmetal Kyle P. Gushue Well-Known Member

    also you don't want audio cables and power cables touching each other. this can cause hums, especially in PA systems. if you have to cross them, make sure it's @ 90 degrees.
     
  4. RemyRAD

    RemyRAD Guest

    Come on now, you are in FM radio guy and so am I. So why are you not using the XLR microphone input on your Behringer? I think that's where your biggest problem lies? Yes it's a grounding issue but you are starting off on the wrong foot with how to troubleshoot this. And the first place is that microphone input. No way you should be using anything else except an XLR-XLR microphone cable to go into the Behringer. That's the first step.

    Computers use switching power supplies. These are great lightweight but nasty little devices. In a control room wiring scenario, we would have a central ground point. In a small production and varmint like yours, that would be the console or your Behringer Xenyyx 802. So that electrical ground should be intact. The computer will likely need to be on a 3 into 2 AC adapter. Everything should plug into the same outlet strip or UPS. And you're not pulling enough amperage to need to balance the AC circuitry that you need anything more than a single outlet for. And that will generally take care of your ground hum issues.

    This wiring technique is not much different from how FM control room production facilities were wired as. It was different and more involved at the larger stations where different grounding schemes may have been utilized? Facilities where signals may have been going over hundreds if not thousands of feet of cabling such as from where I came from at NBC in Washington DC. Where all of the racks were grounded to 0.25" x 2" wide copper bars that ran throughout the building. Slightly impractical in your situation LOL. So it's basically you having to establish a proper grounding scheme that will probably require the lifting of other AC powered grounds from other pieces of equipment? Much of this equipment has unbalanced inputs and outputs so the audio wiring will carry the grounds and not the AC. The two together create a ground loop which causes hum. So you will either be ripping your cables apart or taking the chance by lifting AC grounds on everything but your mixer is connected to your mixer.

    FM 105.7/93.9/96 and the beat goes on.
    Mx. Remy Ann David
     
  5. dvdhawk

    dvdhawk Well-Known Member

    Another thing to consider: Podcast implies a proximity to a computer, and some computer monitors (especially a good old CRT) will radiate an energy that your mic may find offensive within a certain distance.
     
  6. rmburrow

    rmburrow Active Member

    RemyRAD: You are right on with grounding problems except for lifting AC grounds. The safety ground is required to protect equipment users. The safety ground (3rd pin) and the neutral (on the AC receptacle) form a ground loop because the neutral is often connected to the safety ground on the bus bar in the electrical panel. Using an isolation transformer to power the recording equipment will break this ground loop and retain the safety ground. Some isolation transformers have a Faraday shield and/or low capacitance primary to secondary coupling, good for blocking RF from nearby radio stations on AC lines. Single point grounding is essential.

    The microphone should definitely be connected to the XLR input on the mixer as you suggested. If the unbalanced mic input must be used for some reason, an input transformer (i.e. Sescom XLR low impedance mic to unbalanced 1/4 inch plug) should also work. Expect to pay $30 or more for a good input transformer. Capitol Radio (Rockville, MD) used to sell Sescom stuff but this has been years ago.
     
  7. RemyRAD

    RemyRAD Guest

    Right, my explanation was certainly oversimplified. And I do prefer having my multi-tap, electrical isolation transformer which is good for 75 kVA, a little more than 6 inches square and weighs about 100 pounds. So it's portable. At least for those that are feeling frisky? Mine just sits in my power distro box, before it goes into my panel. And where the ground is tied to neutral inside the truck. No ground connection necessary to pull power. Just two hot 120/104 bus bars, single phase or two out of three which ain't bad. So I'm always floating. Maybe it's from something I smoked? It's hard to remember?

    Hasn't gone up in flames in over 21 years.
    Mx. Remy Ann David
     

Share This Page