Hum from PC driving me crazy!

drummer86
drummer86 Posts: 441
edited January 2014 in Going Digital
Scenario: In my classroom at work I have a computer connected to a Yamaha portable PA system (StagePas 400i) to run sound for various purposes. I have a Roland keyboard, my Taylor guitar, and the PC all hooked up to the built in mixer. Instruments using standard instrument cable (no issues here) and PC using a 50' 3.5mm to RCA from Monoprice. Whenever I use the RCA connected to the PC I get a VERY loud buzz/hum from the speakers that is not present at all with other sources.

Things I've tried:
-A different source connected through same cable: no hum
-Different cable from the same source: hum still present
-Unplugging the computer from the wall: Hum still present!
-Switching power strips/surge protectors: Hum still present
-Using both the Audio Out and Headphone jacks: Hum still present

It boggled my mind when the buzz didn't stop when I unplugged the computer from the wall.

Any other ideas or probable causes? I'm going positively mad here. The stagepas is new and works/sounds great with everything except this PC.
Display: LG 47" LCD | AVR: Marantz SR5005 | BD: Panasonic BDT-210 | CD/SACD: Oppo 980 |
Amps: Rotel RB-990bx | Marantz MA-500 | Speakers: Totem Mite : Totem Mite-C : RC60i | Sub: HSU VTF-2 MKIV

HK AVR635 | Polk R30 | Sony DVD/SACD Player
Post edited by drummer86 on

Comments

  • drummer86
    drummer86 Posts: 441
    edited January 2014
    One more thing: When I plug the PC into the small desktop stereo I have it DOES NOT hum.
    Display: LG 47" LCD | AVR: Marantz SR5005 | BD: Panasonic BDT-210 | CD/SACD: Oppo 980 |
    Amps: Rotel RB-990bx | Marantz MA-500 | Speakers: Totem Mite : Totem Mite-C : RC60i | Sub: HSU VTF-2 MKIV

    HK AVR635 | Polk R30 | Sony DVD/SACD Player
  • KurtBJC
    KurtBJC Posts: 19
    edited January 2014
    Hm. I'd figure a ground loop, which you could break up with an isolation transformer (or two if stereo), but for the unplug-the-computer bit. When you unplugged the computer, did you also unplug anything it was connected to (e.g., the monitor)? You could be completing the ground loop through the grounding pins on the monitor cable.

    Kurt
    Blue Jeans Cable
  • drummer86
    drummer86 Posts: 441
    edited January 2014
    I turned the power strip off but didn't unplug everything. I did, however, unplug each thing one at a time from the back of the tower and nothing stopped the hum. Could it be that the computer and amp are plugged into different outlets on opposite sides of the room?
    Display: LG 47" LCD | AVR: Marantz SR5005 | BD: Panasonic BDT-210 | CD/SACD: Oppo 980 |
    Amps: Rotel RB-990bx | Marantz MA-500 | Speakers: Totem Mite : Totem Mite-C : RC60i | Sub: HSU VTF-2 MKIV

    HK AVR635 | Polk R30 | Sony DVD/SACD Player
  • steveinaz
    steveinaz Posts: 19,521
    edited January 2014
    Intermixing of power sources can really complicate isolating a ground loop hum. Wish I had some ideas.

    Have you ever tried chasiss-grounding the computer?
    Source: Bluesound Node 2i - Preamp/DAC: Benchmark DAC2 DX - Amp: Parasound Halo A21 - Speakers: MartinLogan Motion 60XTi - Shop Rig: Yamaha A-S501 Integrated - Shop Spkrs: Elac Debut 2.0 B5.2
  • drummer86
    drummer86 Posts: 441
    edited January 2014
    Doesn't help that part of the building was built in 1970 and partially renovated in 2000 and that the computer isn't mine to work on. Was hoping there would be something obvious that I was overlooking. I'll keep farting around with it.
    Display: LG 47" LCD | AVR: Marantz SR5005 | BD: Panasonic BDT-210 | CD/SACD: Oppo 980 |
    Amps: Rotel RB-990bx | Marantz MA-500 | Speakers: Totem Mite : Totem Mite-C : RC60i | Sub: HSU VTF-2 MKIV

    HK AVR635 | Polk R30 | Sony DVD/SACD Player
  • Speedskater
    Speedskater Posts: 495
    edited January 2014
    What happens when you use a long AC extension cord, so that everything can be powered from the same wall AC outlet?
  • drummer86
    drummer86 Posts: 441
    edited January 2014
    What happens when you use a long AC extension cord, so that everything can be powered from the same wall AC outlet?

    That's a good idea. I'll probably try that next. Trying to avoid a bunch of cords everywhere but I'd rather trip on a cord than listen to that buzzing.
    Display: LG 47" LCD | AVR: Marantz SR5005 | BD: Panasonic BDT-210 | CD/SACD: Oppo 980 |
    Amps: Rotel RB-990bx | Marantz MA-500 | Speakers: Totem Mite : Totem Mite-C : RC60i | Sub: HSU VTF-2 MKIV

    HK AVR635 | Polk R30 | Sony DVD/SACD Player
  • KurtBJC
    KurtBJC Posts: 19
    edited January 2014
    drummer86 wrote: »
    I turned the power strip off but didn't unplug everything. I did, however, unplug each thing one at a time from the back of the tower and nothing stopped the hum. Could it be that the computer and amp are plugged into different outlets on opposite sides of the room?

    I'd unplug everything, all at the same time. Bear in mind that the ground prong isn't going to be switched either in your power strip or in your devices, so as long as there is some path to ground on that end, you're going to have a ground loop. Turning devices, or power strips, off won't help.

    If it's a ground loop, the easiest solutions are either (1) find a way to plug things in that doesn't result in the hum, or (2) use isolation transformers on the audio lines to break the ground connection.
  • recoveryone
    recoveryone Posts: 885
    edited January 2014
    Radio shack use to sale a patch cord with some type of dc isolator in line to cut out any humm from laptops. I use it when I am asked to do retirement parties and use a laptop with my external HD to play music through a old 12 channel mixing board. Before I got that patch cord you could hear the HD spinning up and a consitent hum in the system.

    Found a pic of one

    Radio.jpg
    Family Room HT 7.2/i]:Vizio Oled55h1 Pioneer Elite SC-LX502 Pioneer Elite BDP 85FD Eversolo DMP A6 Panamax M5300-EXSpeakers Fronts Fluance XF8L Center Polk Audio S35 Side Surrounds Optimus LS30's Rear Surrounds Optimus LS30's Subs SVS PB4000 x2 Living room 2ch: Crown Xli 1500 amp Teac EQ MKII FX Audio X6 Mk II DAC Squeezebox Touch Fluance Signature Tower Speakers Panamax M5100-EXOffice media room:Vizio M50Q6 50" Pioneer Elite VSX LX103 Squeezebox Touch Polkaudio R50 Towers Polkaudio CS 10 Panamax M4300 Monoprice 12" subMaster bedroom:Vizio M55Q7 Pioneer Elite VSX LX302 Pioneer Elite BDP 85FD Squeezebox Touch Polk audio RTi 6 fronts, Rears Dayton B652 Polk Audio CS10 center Monoprice 12" sub Panamax M5300-EX
  • Speedskater
    Speedskater Posts: 495
    edited January 2014
    Those little Radio Shack isolators work way better than the printed specs would lead you to believe.
  • drummer86
    drummer86 Posts: 441
    edited January 2014
    What happens when you use a long AC extension cord, so that everything can be powered from the same wall AC outlet?

    Update: I plugged the PA system into the same outlet as the computer and the humming is all but gone. Still a hint of it when I'm not playing any sound at all and really crank the volume but I'll never have it this high for playing music/videos.

    I guess I dismissed this idea when I was troubleshooting because the hum wasn't present on my keyboard or guitar. That led me to assume that the wall outlet I was using was clean enough and that the culprit was the computer. I'm no electrical engineer so can anyone explain why this happened?
    Display: LG 47" LCD | AVR: Marantz SR5005 | BD: Panasonic BDT-210 | CD/SACD: Oppo 980 |
    Amps: Rotel RB-990bx | Marantz MA-500 | Speakers: Totem Mite : Totem Mite-C : RC60i | Sub: HSU VTF-2 MKIV

    HK AVR635 | Polk R30 | Sony DVD/SACD Player
  • Glen B
    Glen B Posts: 269
    edited January 2014
    You probably had a ground loop from the PA system and ground loop plugged into different AC outlets. The two outlets could have been on circuits powered from opposite 120V legs in the electrical panel, or the two circuit grounds could just be at slightly different ground potentials. Either one would cause ground currents to flow between the two branch circuits, and result in noise.
    Main System: Denon DP-59L | Audio-Technica AT33EV | Marantz SA-11S2 | Classe DR-10 | Classe CA-300 | Classe RC-1 | PSB Stratus Gold i's | DIY Balanced AC Power Conditioner | Acoustic Zen and NeoTech cables | Oyaide and Furutech power connectors | Dedicated 20A isolated ground line.

    Home Theater: Toshiba D-VR5SU | Laptop #1 |Outlaw Audio OAW3 wireless audio system | Marantz SR-19 | Phase Linear 400 Series 2, modified | AudioSource 10.1 EQ (for subs) | Axiom M3 v3’s | Axiom VP150 | Optimus PRO-X55AVs | Dayton 12” powered subs (x2) | Belkin PureAV PF-60 line conditioner.

    Party System: Laptop #2 | Audioquest Dragonfly USB DAC | Technics SU-A6 | Acurus A-250 | Radio Shack 15-band EQ | Pioneer SR-9 reverb | Cerwin Vega DX9's | Dayton 100° x 60° horns with titanium HF/MF compression drivers.