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Thread: Question About Ohms and Speakers And Power Amp?

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    Question About Ohms and Speakers And Power Amp?

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    I don't know why I can never remember this, but can someone clue me in on what this means.

    My QSC Power Amp (USA850) says in the manual in the specs table "under output power (bridged mono)" - next to “4 ohms” - 1100 watts, and next to 8 ohms, it says 850 watts.

    Earlier in the manual it says it produces 425 watts per channel at 4 ohms.

    Now here is where I get confused. This venue has 2 unpowered speakers (Peavey PV 112). The Peavey site says these are 8 ohm speakers with power rating of 200 watts continuous and 400 watts program i the Q&A section (PV(R) 112 | Peavey). But the actual speaker specs say:

    - 2-way PA speaker
    - Power rating is 400W
    - Peak power capacity 800W

    So if these are 8 ohm speakers, how does that match up with the QSC amp? I'm confused by all the different power numbers and what happens based on the impedance. bottom line - will this amp work well with these speakers?

    Thanks!

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    Bottom line, you could blow your speakers if you're not careful.

    The QSC is in bridge mode, which ties its two amps together making one big amp (super simplified term). With your amp bridged, you have no choice but to run your speakers in parallel. A parallel connection would make the two speakers into one big speaker (super simplified term, again ) and would cut the impedance in half. So each speaker is 8 ohms, but together, in parallel, they are 4 ohms. The amp only 'sees' 4 ohms of impedance.

    A bridged amp is 1100 watts when going into 4 ohms. Your speakers are 400w each x2 = 800 watts. You could potentially send more power to the speakers than they can handle. (Note: You don't want to use the peak power as a reference. It's more marketing hype than anything.)

    I am pretty sure the attenuator knobs on the QSC are linear. If you set them to half, then you are reducing the max output power to half, or 550w.

    If I'm wrong on any of this, someone will come by and correct me.

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    Don't run in bridge mode, run it in 2 channel mode, Now if you plug in one speaker to each channel it will run at 8 ohms, at 8ohms the amp will produce less than 270 watts according to the spec sheet, link. Exactly what you need for a 200 watt speaker with a 400 watt peak.

    Alan.

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    What Alan said ^^

    Your amp is fine to drive those speakers with no major issue.

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    In addition to the good advice above, there's more to it that's generally not given proper attention. Three things that are worth understanding are 1) desired volume, 2) amp gain and 3) speaker sensitivity. But since most likely you'll be safe with that setup I won't get into those details beyond suggesting you study up when you have a chance.

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    Thanks everybody! I'm pretty sure I'm not in bridge mode. I've used the amp many times over the years with different speakers. I just didn't know if I was "doing it right." I don't even know how to make it be in bridged mode. How could I check/tell? Oh also - should the QSC attenuator knobs be at max? I have always (ignorantly) used them at sort of half as if they were gain knobs on a mixer or something. I'm guessing I shouldn't do that?
    Last edited by Ryan Teasnob; 03-05-2019 at 06:33. Reason: addig a question

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    Quote Originally Posted by Ryan Teasnob View Post
    Thanks everybody! I'm pretty sure I'm not in bridge mode. I've used the amp many times over the years with different speakers. I just didn't know if I was "doing it right." I don't even know how to make it be in bridged mode. How could I check/tell? Oh also - should the QSC attenuator knobs be at max? I have always (ignorantly) used them at sort of half as if they were gain knobs on a mixer or something. I'm guessing I shouldn't do that?
    I always run all my power amps with the volume full up, and control the overall volume from the mixer

    Alan

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    Sorry, not sure why I assumed you were in Bridge mode. It was late, I guess.

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    Quote Originally Posted by Ryan Teasnob View Post
    Thanks everybody! I'm pretty sure I'm not in bridge mode. I've used the amp many times over the years with different speakers. I just didn't know if I was "doing it right." I don't even know how to make it be in bridged mode. How could I check/tell? Oh also - should the QSC attenuator knobs be at max? I have always (ignorantly) used them at sort of half as if they were gain knobs on a mixer or something. I'm guessing I shouldn't do that?
    That depends on your desired volume and the amp's gain, and whether or not you want to trim it with the mixer's master fader.

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    I generally have Amps (or Speakers) at max, and control levels from the mixer.

    The exception to that is when doing that is too loud for the venue, i.e. the faders need to be at the bottom of their range so there is very little travel available to control levels. Under those circumstances I knock the amp levels back so that I can use full fader travel on the mixer.

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