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Is there a way to test an amps output power?

Discussion in 'Amps and Cabs [BG]' started by cauda, Apr 1, 2004.


  1. cauda

    cauda

    Apr 1, 2004
    Boone, NC
    Hi, I just ran across this forum today and was looking through some of the posts. There seem to be some really intelligent people in here. Anyway, I may be having a problem with my Yorkville XM200 amp. The thing is less than two months old and it seems to be losing volume. Its rated at 200 watts and when I first got it, a volume setting of about 3 was plenty enough when I was practicing with my band. Now I need to turn it up to about 7 just to be heard and if I go any louder I start getting distortion. Could I just be going deaf or can an amp get gradually quieter? Is there anyway to test the output power of the amp? I work as a two way radio tech, so I have a good understanding of electronics and have access to all sorts of test equipment. Any advice is apperciated.
     
  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
    You'll need an audio sine wave generator, a load resistor, an RMS voltmeter, and either an oscilloscope or a distortion analyzer.

    The load resistor must be of a value that corresponds to the load impedance specified in the amp's power rating, like 8 or 4 ohms. And it must be rated for at least the power level you're testing for--in your case, 200 watts.

    Set the sine wave generator for 1 kHz and turn it all the way down.

    Connect the load resistor, voltmeter, and the distortion analyzer or oscilloscope to the amp output and the sine wave generator to the amp input.

    Turn the amp on. Gradually turn up the sine generator and the amp's input gain control. Then gradually turn up the master volume. Make sure the amp isn't clipping (very high distortion or visible flattening of the tops and bottoms of the sine wave.

    Adjust the sine generator, input gain, and master volume in whatever combination gives you the highest but still clean output voltage. When you do this, make sure you leave some room for upward adjustment of the master volume.

    Now start turning up the master volume control until you either reach the distortion level cited in the amp's power rating (like 1%, 0.1%, 0.5%, or whatever) or you just start to see the sine wave tops and bottoms flatten out. This is the onset of clipping.

    Measure the RMS voltage.

    Calculate power: POWER = (VOLTAGE^2) / (RESISTANCE)

    For example, if you measure 30 volts RMS into a 4-ohm load:

    30^2 = 900

    900 / 4 = 225 watts
     
  3. Passinwind

    Passinwind I Know Nothing Supporting Member

    The simple version, fellow techs may berate me for lack of detail:

    --snipped basically what Bob said--

    Oops...by the time I posted, Bob Lee had it covered. :cool:
     
  4. ihixulu

    ihixulu Supporting Member

    Mar 31, 2000
    getting warmer
    And thank god they are the one's who replied. :hyper: Keep in mind that I am not one of them as you read on.

    As to the amp seeming to lose volume, hearing damage might be a part of it but a couple of other things to keep in mind:

    - The battery in your bass, if it is equipped with one, may be running down

    - Have your eq settings changed? Boosting bass at the expense of mids can reduce perceived loudness

    - I've had cases where the strings themselves lose output as they get played out

    - Has the placement of the amp changed? Where you put you amp in the room can have a pretty big impact on how loud it sounds due to room acoustics issues.
     
  5. 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
    Hey, you should've left it up there, man! What you wrote was really good. And if someone read what I posted and didn't quite get the concept, maybe yours would've done the trick; sometimes reading different explanations of the same thing helps comprehension.
     
  6. cauda

    cauda

    Apr 1, 2004
    Boone, NC
    Thanks for the info Bob, I think I can follow your instructions with the test equipment that I have. I don't have a load resistor capable of handling 200 watts, can I just leave the speaker hooked up in the amp and use it as my load resistor. I could place the scope probe across the speaker, right?
    In response to ihixulu, I will try replacing my battery before anything else. The settings on my amp haven't changed since I first set them. I did take the amp out on one gig and had to crank the volume up quite a bit, but nothing else has changed. I haven't changed strings in about 6 months, can old strings have a drastic effect on the output of my bass?
     
  7. Passinwind

    Passinwind I Know Nothing Supporting Member

    I don't have a load resistor capable of handling 200 watts, can I just leave the speaker hooked up in the amp and use it as my load resistor. I could place the scope probe across the speaker, right?

    Don't do dat! First, you will be deaf after that, and second, the speaker will likely cook in short order. A sine wave is a pretty tough signal for speakers, especially at high power.

    You can make a dummy load with a couple of welding rods in a salt water bath. Just adjust the distance between your electrodes and the salinity of the solution until you hit 4 or 8 ohms. I'm sure you can google up a tutorial on that. You can buy 50 or 100 watt resistors pretty easily too, and do a series-parallel configuration to get the load you need. If you have a tech background, it'll be worth it to own a set of loads. I got mine for $3 at a surplus joint.

    edit: you can buy dummy loads here: http://www.partsexpress.com
     
  8. Nightbass

    Nightbass

    May 1, 2001
    Seattle, WA
    The PartsExpress dummy loads look kinda cheap to me, though they would probably work.

    If you have the coin, I'd recommend the industry standard, the Dale NH-250 250W non-inductive resistors. About $40 each from Mouser. Two 8 ohm models would give you 250 W at 8 ohms, and then you parallel them for 500 W at 4 ohms. If you mount them on a massive heatsink or block of aluminum, using thermal grease, and fan cool them, they will handle twice the power. If you water-cool them, up to ten times the power.

    http://www.mouser.com/index.cfm?han...ductid=373040&e_categoryid=15&e_pcodeid=71013

    Nightbass
     
  9. Passinwind

    Passinwind I Know Nothing Supporting Member

    Or use an oil bath, and whip up an artery clogging meal while you burn in your amp...[​IMG]

    edit: You're right, the PE loads look pretty cheesy these days. They used to sell some nicer ones. Still, you could put together one from 20 watters from there for under 10 bucks , if this is a one-shot deal.