Bzzzzzzzzzz.... and some humming

Discussion in 'iMac' started by f00f, Jan 15, 2010.

  1. f00f macrumors 65816


    Feb 18, 2009
    New Yawk
    I have an iMac9,1 (24") with horrific hum (my hearing is particularly sensitive so it might not be that bad) -- the kind we get from the display brightness being lowered, as well as a constant high-pitched whine, both of which appear to be common given the number of topics on the subject. Are the power inverters in these things crapware or what? My woman's 20" iMac from 2006 has similar hummage; so far I've written this off as an iMac feature and am not convinced that if I were to Apple Care this thing that the replacement would not scream as well.

    How about anyone else? Have you had this inverter issue, had it taken care of and eliminated? Or is it just an iMac feature that I live with by drowning it out w/ loud music? :D
  2. micven55 macrumors newbie

    Jan 19, 2010
    This appears to be a common issue with the imac's. The backlighting is fluorescent and the electronic ballast runs at about 40 to 50 kHz. In order to dim the display the backlighting is actually shut off for a brief period of time. As it turns out the frequency of the dimming is 2000 to 3000 Hz, right in the hearing range. With proper attention during the design the power to the backlighting could have been properly shielded and the "noise" could have been kept inside.

    Not sure if the new imacs with the LED backlighting do the same thing. The dimming method is the same but the voltage to the LEDs are much lower than they are to the fluorescent tubes reducing the radiated energy.
  3. Ecoh macrumors 6502a


    Oct 30, 2009
    I have an i7 27" and it is very quiet, no matter what level of brightness.

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