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Testing wattage output of a poweramp

Discussion in 'Amps and Cabs [BG]' started by RobOtto, Nov 5, 2002.

  1. RobOtto


    Aug 15, 2002
    Denton, TX
    Is there an easy way to test the power output of a poweramp? I bought a Yamaha p2700 not too long ago and I don't think it is putting out a full 1000 watts bridged. My Hartke 3500 (350 watts @4 ohms) is much louder than I can get my Yamaha P2700 run through a preamp into the same speaker configuration.

    Is this something a guitar/amp technician could check pretty easily or would I need to take it to an electrician of some type?

    Advice and comments are welcome.
  2. Geez, lemme see... some amps have a recessed switch in the back you need a pen-point to get to: the choices might be "stereo", "mono/bridged", or "parallel". If you're running two 8 ohm cabs, you probably want it in "mono/bridged" position. And try a couple different ways of running cable to your speakers: A jack from the amp to each cab in parallel mode, or chain 'em together in bridged/mono. I run my Crest FA901 in mono, with each output jack cabled to it's own cab. It's louder than chaining them together- don't know why...
    Also, make sure your poweramps' ohmage is working with your cabs' ohmages... and you prolly know that too low an ohmage (combined) from your cabs can blow yer amp, right!
    Wattage also doesn't necessarily mean volume. An Ampeg SVT can knock down a small building, and it's (an old one) a 350 or so watt amp!
    Now, go bring the thing t'your techie. After you make sure you've been turning on all the right buttons!
    That's what I always check!
  3. I'd imagine most technicians could probably do this.

    Some techs might have wattmeters that can directly measure power, but most would do it indirectly as follows.
    In order to do this a tech needs three things: A load resistor of the right value that can dissipate the full power of the amp, a tone generator and an oscilloscope. Resistors that can dissipate 1000W aren't that common, though...

    You connect the resistor to the outputs of the amp, then connect the test leads of the scope across it. You then apply signal to the amp and monitor the waveforms on the scope. Turn the signal level up until the waves start to flatten at the top then turn it back to the maximum level that they stay nicely curved. Read the output voltage off the scope, divide by the resistance to get the current and then multiply the voltage and current to get power. If the power you get is about +/- 10% of rated power, the amp is fine.

    You could do it at home if you had the right gear.

    But the long and short of it is that it's pretty easy to do...:D

    Is the Yamaha clipping with the preamp cranked? If it isn't you might not getting enough input signal to it. This does happen sometimes, the preamp can't put out enough voltage to drive the amp to full power. If it is clipping, make sure you have it set up right for bridge mode!
  4. Phat Ham

    Phat Ham

    Feb 13, 2000
    No kidding. Where the heck would you find one of those? And I'd imagine it would be the size of a small refrigerator.

    But then again I thought 1F caps didn't exist until my friend with a system in his car opened up his trunk to reveal a humongous 1F cap.
  5. I've got (2) Ohmite 250Watt 4 ohm resistors. They are about a foot long and 2 inches in diameter, and hollow. All you'd need would be 4 or 8 of them to make up a 1000 or 2000 Watt load of the proper impedance. They get hot as hell, and I wouldn't recommend leaving them for a long time dissipating full rated power. A fan would be a good idea.

  6. Phat Ham

    Phat Ham

    Feb 13, 2000
    Yeah I realized it would be a lot easier to just put some smaller resistors together. I was just caught up in the idea of a single resistor 4000 times the size of your typical 1/4 watt resistor:D
  7. It's simple. You'll need two people a cat and a stopwatch.

    Disconnect all speakers and crank the amp to max, then touch the cat's tongue to the speaker terminals. Have the guy with the stopwatch measure the RPM of the cat's eyeballs. Feline eyeballs rotate at 23.56 revs per watt.

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