Audio amplifier testing with oscilloscope

Discussion in 'General Electronics Chat' started by bwilliams60, Apr 4, 2015.

  1. bwilliams60

    Thread Starter Active Member

    Nov 18, 2012
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    If one wanted to hook up a signal generator to a home audio amplifier and watch the signal using an oscilloscope, where would you find a good step by step tutorial to do such a thing? I have an audio amplifier here and although I can find my way around with a meter, I would like to learn how to use a signal generator and scope to check for a bad channel on this unit.
    I have an analog scope(phillips) and a PC based Picoscope with all the bells and whistles as well as a function generator.
     
  2. Papabravo

    Expert

    Feb 24, 2006
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    1. You set your signal generator for a very small signal like 50 mV P-P and connect it to the amplifier input
    2. Connect the oscilloscope to the output
    3. Turn the amplifier volume control all the way down
    4. Turn the amplifier on
    5. Turn the signal generator on
    6. Use the amplifier volume control to increase the gain, or the signal generator amplitude to vary the input level
     
  3. bwilliams60

    Thread Starter Active Member

    Nov 18, 2012
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    I read somewhere that you should have a "dummy" load on the outputs. Can you explain? I am assuming this is to keep the otherwise, very loud output, from blowing your eardrums out. Is it necessary?
     
  4. Papabravo

    Expert

    Feb 24, 2006
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    You can use a dummy load if you like, but then you won't be able to use your ears as a measuring instrument. I think you can hear the distortion before clipping happens. Also the dummy load may not have the same frequency response as the actual speaker system.
     
  5. bwilliams60

    Thread Starter Active Member

    Nov 18, 2012
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    Thank you for your help. Seems simple enough.I will give it a whirl and see what happens.
     
  6. KeepItSimpleStupid

    Well-Known Member

    Mar 4, 2014
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    Valve amplifiers "absolutely" require a dummy load. Solid state amps, no.

    Initial tests won't use a "dummy load", but in order to test the amplifier to deliver "power", you have to have a load.
    one of the best ways to bring up an amplifier is with a Variac or auto transformer while monitoring the voltage between the bases of the output transistors (the bias regulator), with a signal, no load and monitoring the output on a scope.

    Measuring the DC output on the speaker terminals is one of the first steps. There really shouldn't be much. 1 V is too much. Usually, its in the mV range.

    The "bias" regulator, can be monitored by looking at the voltage across the emitter resistors.

    Lots of stuff gets in the way like protection circuits etc.

    Sencore, made a nice dummy load analyzer system: http://www.antiqueradios.com/forums/viewtopic.php?f=8&t=198059

    I did professional audio servicing, part time, at home for a company. One jig that I made was a way to connect to the various kinds of output connections to an amp: binding posts, screw terminals, wire, (Speak on connectors).
    I took these to a short length of wire to a cheap connector. For the push in wire, I used a piece of solder braid.

    I then had adapters to plug in my dummy load. There was an extra connector on the dummy load for monitoring.
    e.g. connection to the scope.

    Another useful tool was two phono plugs to alligator clips made from a bottle and a dual jack.

    I rarely did any bridged amp servicing.
     
    Last edited: Apr 5, 2015
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