Connect with us

Dummy load testing of amps query

Discussion in 'Electronic Repair' started by N Cook, Mar 28, 2007.

Scroll to continue with content
  1. N Cook

    N Cook Guest

    What is a reasonable testing regime for each of the following cases, in
    terms of driving with continuous sine wave, what frequency and what
    percentage of power in the load to represent in service use ie rather than
    continuous sine, for

    Bass amps
    200 Watt rating, %, f
    600W, %,f

    Guitar Amps
    200W rating,%,f

    And overall duration of such testing ?

    I use 100Hz for bass and 400 Hz for guitar amps and 1 hour duration but I'm
    concerned about what sine to in-use equivalent percentage I'm using
    a/ to accurately represent periodic in-use signal rather than the more
    stressing continuous signal .
    b/ under or overstretching the amp
  2. Ron(UK)

    Ron(UK) Guest

    Not sure what you are trying to achieve here. Personally, I don't think
    soak testing using a sine wave is very useful or desirable. A sine wave
    is rarely very representative of normal duty.
    I use a sine wave for setting up bias, checking for output power before
    clipping etc. but for prolonged bench testing I prefer to use a mixed
    program signal such as a cd player or FM Radio. I would generally run an
    amp up to just below clipping and let it run for an hour or so -
    monitoring the heatsink temperatures in an SS amp.

    You should be aware that a lot of budget SS amps wont run continuously
    at full power without letting the magic blue smoke escape.

  3. N Cook

    N Cook Guest

    I've no justification, from theory, for the following, just not coming a
    cropper in practise.
    Assuming the quoted amp rating, X watt, seems consistent with transformer
    size and number and rating of output devices and not clipping then I feed a
    contnuous sine in that gives X/3 as measured via true RMS ac over the dummy
    As I seem to be dealing with higher wattage stuff these days I would like to
    monitor and record heatsink and fan outlet air temperatures but that only
    makes sense using some standard input form. Does anyone use ,as a source,
    gated sine, eg 1 : 4 sine signal to zero signal, for higher peak amplitudes
    but (ballpark) same average power .
  4. Eeyore

    Eeyore Guest

    What do you hope to achieve with a sine wave ?

    And the short answer to the above is that it varies hugely from one amp to
    another depending on how conservatively (or not) it was designed.

  5. Eeyore

    Eeyore Guest

    1/3 power operation actually gives the highest dissipation in the output devices
    for any sine wave test signal.

  6. Eeyore

    Eeyore Guest

    I use a rock or rave CD with very little dynamic range and drive the amp until
    it's 'just' clipping on some peaks. With amps that have inbuilt 'clip limiters'
    I actually often deliberately 10dB overdrive them too. This is torture for the
    power transformer too but my amp designs will handle this.

    You're being unnecessarily technical about this.

    Another method is to use white or pink noise.

  7. Rich Koerner

    Rich Koerner Guest

    Wow, there is a lot to this, and no one method is going to fit all cases.

    The average amp tech just needs to confirm factory specs through final bench and play testing.

    Now, STRESS testing is a totally different story.

    The types and methods of stress testing varies due to the specific goals such testing is to

    You can subject an amplifier to a multitude of all manner of stress testing. Vibration, electrical,
    thermal, environmental, physical pressure, etc... to find the most likely trigger for any future
    possible failure event.

    If the major amp manufacturers did 1/10th of what I had just mention here, with some freaking
    integrity behind it, you would NOT have as many Polished Turds on the market.

    Yes, you could put improved reliability, service life, and quality in cheap products without them
    being Polished Turds.

    THAT, can be done. But isn't.

    Because, there is no money in it if they last too long.

    It's no longer a cheap short term disposable product.

    <after thought>

    I'm old school. That was the way I was raised.

    If the speedo on the dash board says 120 mph, I expect the car to do it.

    So, I head out to the highway late at night, and I put my foot on the floor.

    Yes, a 1962 Corvette has a 160 mph speedo, and I put my foot on the floor.

    That's when I found the body rises up, and gets light at about 115 mph.

    Which, forced me to back it down. The 65 Stingray next to me didn't have that problem, and could
    safely reach higher speeds by its non air plane wing body design.

    Now, how would I know that the 62 body design functions as an airplane wing at 115 mph if I had not
    tested, and found that to happen.

    The only way to find out what is really in front of you, is to test for the LIMITS, and find out
    first hand what they are.

    I test EVERYTHING unknown to me THAT way.

    Including vacuum tubes, and amplifiers.

    From EL-84's to KT-88's..... beat 'em till they quit. Then, you know what ya can or can't depend on
    at different levels of operation.

    Then, knowing the limits of the tubes placed in an amp to be stress tested, you KNOW you are
    starting out with tubes that can take a beating, and are NOT likely to trigger a failure even on
    their own.

    When it comes to tires, I get the best performance tire available.

    With vacuum tubes, it's the same way.

    All quality high performance items have their price.

    For those outside of that life style, polished turds are fine.

    Especially, when you never have to put your foot on the floor.


    Rich Koerner,
    Time Electronics.

    Specialists in Live Sound FOH Engineering,
    Music & Studio Production,
    Vintage Instruments, and Tube Amplifiers
  8. RonSonic

    RonSonic Guest

    I usually just run everything at 400 for the bulk of testing with a few sweeps
    to see if anything stupid happens at the extremes. Bass amps will spend most of
    their time at 200Hz. I'll also do square waves, do a nice job of verifying tone
    control, onboard FX or EQ function for gear you don't want to have to listen to.

    For solid state stuff I do the traditional 1/3 power power warmup for about a
    half hour - hour and then measure my max power, recheck bias and whatever else
    might be in question. Anything more than that is just beating it up. I'll run a
    brief full power while beating on the thing and watch for glitches.

    Tube gear gets a similar treatment, but 1/3 power isn't a magic number in the
    burn in phase. They tend to get up to heat and stabilize quickly. For combos
    I'll run a low frequency with speakers and see if anything starts to shake apart
    at 70 - 200 Hz.

    While a steady sine at half power isn't really a simulation of actual use, it's
    still a fair test. If you want a reality check, plug in a CD you like.


    Effect pedal demo's up at
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day