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Power amp question (bridging)

Discussion in 'Amps and Cabs [BG]' started by Juniorkimbrough, Jul 3, 2005.

  1. Juniorkimbrough


    Mar 22, 2005
    Mississippi / Memphis, TN
    Endorsing Artist: Lakland Basses
    I recently bought a QSC RMX850 power amp. When i first got it I was confused with the DIP switch settings on the back of the amp and I accidentally set channel 2 and it's filter to the "on" position, and I was bridging the amp.
    Then I used it for the first time at practice and it was clipping like crazy at low volumes. Then I realized the problem and fixed the settings and its working great now.

    My question is, Why or how does having the ch. 2 filter on when running in bridged mode effect it?

    Every since this has happened I've been wondering what makes it do this, I feel more comfortable knowing "why" it does this rather than "just because it does".

    on a side note, I gigged the power amp for the first time last night with my Eden 410 cab at an outdoor gig and it did WONDERFUL!! I was a little worried at first because of the back of the stage being open so I ran through the PA as well, at setbreak i talked with the guy running the sound for us and he said he turned me all the way down on the PA because the stage volume was plenty loud. :)
  2. Bob Lee (QSC)

    Bob Lee (QSC) In case you missed it, I work for QSC Audio! Gold Supporting Member Commercial User

    Jul 3, 2001
    Costa Mesa, Calif.
    Technical Communications Developer, QSC Audio
    It happens because the low-frequency filters are just a couple pairs of resistors and capacitors that lead into the input of the op amp that drives the output section. In bridged mono, the feed from channel 1 goes into the same op amp input on channel 2 as the filter does. If you bypass the filter on channel 2, then it won't affect the signal from channel 1 and all will be well, but with the filter engaged, it'll affect the really low frequencies on channel 2, and as a result the two channels' outputs won't be mirror images of each other. There is a protection circuit, active only in bridged mono, that checks continuously that the channel outputs are mirror images, and collapses the supply rails to the op amps if they are not (this is to protect the amp and loudspeaker if one channel's output was shorted to ground or developed a fault). With the imbalanced caused by the filter, this protection circuit could get triggered intermittantly as you play.

    This is only a problem with the RMX amps. The PLX amps have more circuitry in the audio path, and the bridged mono feed from channel 1 to channel 2 and the filter circuitry are completely isolated from each other.
  3. Juniorkimbrough


    Mar 22, 2005
    Mississippi / Memphis, TN
    Endorsing Artist: Lakland Basses
    Thanks Bob!!