Measuring Amp Characteristics

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This is a short description to characterize an amp to ensure proper performance over the operating range.

  • First disconnect speaker and connect dummy load. Adjust the dummy load to match the amplifier's output impedance for maximum power transfer. For example, if the amplifier has an 8 ohm output impedance, set the dummy load to present and 8 ohm load.
  • Disconnect any inputs and connect the signal generator to the amplifier's input.
  • Connect the multimeter (set to measure RMS voltage) and connect the distortion analyzer to the amplifier's output.
  • Set the amplifier to Channel 1 or the channel of interest. Turn off channels such as reverb and tremolo and set all of the equalization controls to max to remove the filtering out of the circuit as much as possible.
  • Turn on the amplifier, check that the supply current is within range, and wait 5 minutes for warmup.
  • Set the signal generator to 200 mV @ 1kHz and adjust the amplifier's volume control until the output is at the amplifer specification, 15 W in this example. Check to make sure the distortion is under 5%.
  • Sweep the frequency of the amplifier from 20 Hz to 20 kHz with steps at 1, 1.2 and 1.5 of each decade. For example, 20, 50, 100, 200, 500, 1kHz...
  • Plot the data.
    • The distortion should be plotted with respect to frequency.
    • The RMS voltage should be converted to power using the formula P = Vrms*Vrmw/8
    • Plot Power in terms of decibels using the following formula. Power (dB) = 20*log(Power Measured / 15)
  • Check that frequency response is greater than -3dB at the frequency limits of the amplifier (20Hz and 20kHz).

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