USA-850

Moderators: Cameron Shoffner, Christian Cook, Kirk Fyvie, Martin Barbour, Gary Evans_QSC, QSC-Dale Sandberg

Thu Jul 02, 2015 9:06 am

  • Hi chaps,

    I've been using my old QSC USA-850 for a while now. I know its very very out of warranty, so I have a few questions...

    - Is it worth replacing the power supply caps? Are there any other electrolytics that might be worth replacing? - according to the service manual, the switch-on delay and some other bits are controlled by electrolytics.

    - Is there any info available about THD vs power output? - I've been using it as a HiFi amp, but am wondering if its worth getting something a little more modern if there's improvements to be had.

    I'll also be re-wiring the input jack sockets so that tip is positive and ring is negative. Since I'll have the thing in bits, I might see if there's space for XLR inputs and SpeakOn outputs. I'll post up some step-by-step pictures, too, if anyone's interested.

    Chris
    chris661
    The high rail
     
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Thu Jul 02, 2015 11:42 am

  • If you already have the amp apart and there are no significant issues with it, then replacing the electrolytics in the power supply is a worthy effort, especially the big reservoir caps. They undergo the greatest thermal stress.

    Assuming the bias is set correctly and nothing else is malfunctioning, the THD will be extremely low—below audibility—until the point where the amp channel clips, and the output voltage starts to flat-top. Even then, it's often surprising how much transient peaks can clip before the distortion starts becoming detectable by human listeners.
    Bob Lee
    Technical Communications Developer
    QSC, LLC
    Fellow, Audio Engineering Society
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    "If it sounds good, it is good." —Duke Ellington
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    Bob Lee
    QSC Audio Products
     
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Fri Jul 03, 2015 5:19 am

  • Cheers, Bob.
    The amp works just fine, but having re-capped another old PA amp (C-Audio RA-3001) I think it might be worth doing this one too.

    Given its running a pair of 4ohm studio monitors (~88dB@1w), I won't be pushing it to clipping. That's why I like this amp, though - for domestic use, there's just oodles of clean power.

    Chris
    chris661
    The high rail
     
    Posts: 132
    Joined: Tue Apr 21, 2015 8:37 am
    Location: Sheffield, UK



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