New lightweight 'digital' heads and line hum?

Discussion in 'Amps and Cabs [BG]' started by macmrkt, Jan 24, 2006.

  1. macmrkt

    macmrkt Banned

    Dec 4, 2002
    I have a few of the newer Markbass products including the LMII with Traveler speakers and the Mini CMD 121P combo. These are 'digital' power supply head designs. I noticed that in some rooms there's a fairly loud line hum coming from the amp/speaker. This occurs on a variety of bass guitars, but is only audible with the active electronics engaged and the gain is close to wide open (but this is the way I use my basses). It happens when I'm not touching any metal (strings, controls). When my hands are on the strings, there's virtually no hum. This reminds me of ground loop stuff you could eliminate easily on amps I had years ago, but there's no polarity switch on these new amps and the plug is grounded, so no hope of reversing it in the outlet.

    At this point, I've isolated the problem to...power strips! I've used the gear in a number of clubs and practice rooms. When I plug directly into the wall or through an extension cord, no hum. When I'm in a power strip with other devices plugged in as well, hum. By the way, passive pickups or preamps don't exhibit the problem. Are there some issues with digital or switch-mode power supplies and power strips that any of you are aware of?
  2. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    That's normal. If you've still got hum when you are touching the strings you've got a problem.
  3. Mcrelly


    Jun 16, 2003
    Minnesota, USA
    sounds like a shielding issue too! if it was ONLY electrical, meaning the power, plugs, strips etc then I don't think touching the strings would lower the hum?

    maybe I'm wrong.

    Help! shielding experts!!!

    BTW I had my bass HEAVILY shielded recently and it reduced my suceptability to onstage hums, but also changed my tone...:meh:
  4. I've never noticed any difference in hum, sound, AC impact, etc. between my digital heads (Focus, WW) and my analog heads. Sounds like a possible Marbass issue vs. a general switching power supply issue (I think the Marbass LM series is switching power supply with an analog amp, vs. the switching power supply/class D amps of the WW and Focus).
  5. kasbrey


    Aug 23, 2002
    Northampton UK
    I've been using my LM2 for a few months now and have never had a problem.
    I don't think that it's an inherent problem with Markbass amps
  6. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    Isn't the mains there balanced +110/-110/ground? That factor alone eliminates 90% of AC sourced noise/RFI/EMI.