How to Build a Switchable Dummy Load

Written by larry rivers
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How to Build a Switchable Dummy Load
Switchable dummy loads can facilitate quicker amplifier repairs or modifications. (Comstock/Comstock/Getty Images)

Dummy loads are an essential tool for repairing or modifying many types of audio amplifiers. Each amplifier design requires a resistance load, usually a speaker system, to operate safely. The value of this resistance, known as impedance, can vary between amplifier designs, requiring dummy load resistances to be matched to the amplifier's specifications. A switchable dummy load device can be built to more easily facilitate amplifier repair and modifications.

Skill level:
Moderately Challenging

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Things you need

  • Metal project enclosure
  • 1/4-inch tip sleeve chassis mounted audio jack
  • 4 ohm aluminium body power resistor
  • 8 ohm aluminium body power resistor
  • 16 ohm aluminium body power resistor
  • Three position high voltage rotary switch
  • Marker
  • Drill
  • Drill bits
  • Crescent wrench
  • Screwdriver
  • Low wattage soldering iron
  • Electrical solder
  • High voltage hookup wire
  • Wire strippers
  • Wire clippers
  • Multimeter

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    Preparing the Enclosure

  1. 1

    Layout the locations of the 1/4-inch tip sleeve chassis mounted audio jack, three position high voltage double pole rotary switch and the 4 ohm, 8 ohm and 16 ohm aluminium body power resistors inside the metal electronics enclosure. Space the power resistors apart across the longest surface of the enclosure to ensure proper airflow and heat dissipation. Mark the mounting locations of each component on the enclosure with the marker.

  2. 2

    Label each component location and function with the marker. Label the audio jack "speaker" and the three positions on the rotary switch "4 ohm," "8 ohm" and "16 ohm."

  3. 3

    Drill each mounting hole in the enclosure using the drill and drill bits. Ensure proper sizing for each hole and its corresponding components.

  4. 4

    Mount the audio jack, rotary switch and three power resistors into the enclosure using the screwdriver and crescent wrench. Inspect each resistor's mount, ensuring a tight, flush installation against the enclosure.

    Wiring the Device

  1. 1

    Connect the audio jack's tip lug with the rotary switch's common lug using the high voltage hookup wire, low wattage soldering iron, wire strippers, wire clippers and electrical solder. Inspect each solder joint for cold solders or any breaks.

  2. 2

    Solder the rotary switch's first positions lug with the first lug on the 4 ohm power resistor, the second position's lug to the 8 ohm power resistor and the third position's lug to the 16 ohm resistor.

  3. 3

    Solder all three resistor's remaining lugs to the audio jack's ring lug. Inspect each joint for cold solders or any breaks.

  4. 4

    Test the device using the multimeter. Set the rotary switch to "4 ohms" and take a resistance reading at the audio jack's tip and sleeve lugs. The multimeter should read 4 ohms. Test the remaining two positions to confirm their resistance.

  5. 5

    Close the project enclosure using the included screws and screwdriver.

Tips and warnings

  • When selecting the power resistors, ensure their wattage rating is double that of your amplifier's maximum output.
  • Never allow an amplifier or dummy load device to overheat. Should either device become too hot to touch, unplug the amplifier and allow it to cool before resuming operation.

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