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Power amp question

Discussion in 'Amps and Cabs [BG]' started by Tugboat, Jan 13, 2013.

  1. Tugboat

    Tugboat Supporting Member

    Nov 7, 2003
    NY, County of Orange
    I apologize in advance for my ignorance...and I did search and couldn't find this answer. I've always used "all in one" heads in the past. But now I have a nice preamp that I'd like to get a seperate poweramp to run, e.g. Crest, Peavey, Crown, whatever. Here is my questions/s:

    If the power amp has 2 channels....if I plug my pre into input channel A, only the output for channel A will carry what's coming out of my pre, correct? So I could NOT have two 8ohm cabs running powered by channel A, and also have one 4ohm cab on channel B at the same time?

    Also, and this is "master of the obvious", but just figured since we're here....when the rating on the power amp says each channel is rated for 200watts at 8ohm, 400watts at 4 ohms, is it just like what I've always used and I can hook two 8ohm cabs to one channel and get 400watts.

    Thanks in advance guys, just trying to educate myself before I buy the wrong power amp, and/or blow something up. :D
  2. craig.p


    Sep 28, 2008
    New Hampshire
    Your best option is to download the user manuals for all the power amps you're considering. They will explain everything.

    However, here are the short answers to your questions:

    What you input into A will come out only A. What you input into B will come out only B. Consider A and B two separate mono power amps in the same box. (Fellow experts, don't even think of dinging me on that slightly inaccurate statement. I'm just trying to help the new guy conceptualize this.)

    Now it gets murky, because you can "Y" one preamp's output into both power amp inputs (its A and B inputs) so that one preamp signal can drive both power amp channels. You can use a Y cable, or some amps will let you accomplish it internally with a switch. This is where the user manuals come in.

    Second question about the 200/400 watts -- the answer is yes. Two 8 ohm cabs will present a 4 ohm load to that amp channel and let it provide 400 watts to the pair of cabs, 200 watts per cab of course.

    You could also "Y" the single preamp output into both the A and B sides, as I said above, and run one cab off the A side and the second off the B side. The amp would provide the same 200 watts to each cab, but each side would run cooler than one side would run at 4 ohms.

    One other item which you didn't ask about but I'll address anyway: It's a common misconception among folks new to this that a power amp's gain controls are limiters. No, the power amp will give you full output at "whatever" gain setting, as long as you're not overdriving its input(s) with your preamp. You should be able to run your power amp's gain controls wfo (that's a motocross term for wide open and I have the gimpy knees to prove it) unless your preamp is so noisy that backing those gain controls down helps mitigate the problem.

    Hope this helps.
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