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DIY Speaker / Amplifier Dummy Load

 Giovanni Militano  Canadian Flag   To email Gio, type out the email address.

DIY Speaker / Amplifier Dummy Load

This is a simple DIY Speaker dummy load that I put together quickly. After constructing an AMP6 T-Amp kit, I was testing the maximum power output using a 4.7 ohm ceramic power resistor. I somehow got distracted and before I knew it, I could smell burning. The ceramic resistor had started burning through a floor mat and melting the insulation of the test leads which had fallen on the hot resistor. So rather than test with loose resistors and test leads, I figured I should make a properly enclosed dummy load.

Rather than just using a load resistor across an amp, I prefer to be able to hear what is going on during testing. The schematic below shows two dummy loads. With a speaker connected, the amplifier will see either a 4.3 or 7.4 ohm load while the attached speaker will receive less than about 1% of the output power. You can check the net load for yourself by following the basic electronics formulas. With a simple setup like this you can test at high power without risking damaging speakers, going deaf or driving other people crazy. If a speaker is not connected to the dummy load, the amplifier will see either a 4.5 or 8 ohm load.

DIY Speaker Dummy Load Schematic

Figure 1: DIY Speaker Dummy Load Schematic

The two dummy speaker loads are housed in the same type of aluminum baking pan that I used for my 50EH5 Single Ended Tube Amplifier. To help promote cooling, I mounted the 50W resistors to heat sinks that had been salvaged from dead computer power supplies. The 4.5 ohm resistor is attached to the larger heat sink. The aluminum enclosure also helps dissipate some heat from the resistors. Since the resistors are cooled somewhat, it is ok to apply more than 50W of power for a very SHORT period of time, but don't get carried away.

DIY Speaker Dummy Load

Photograph 1: Underside View of DIY Speaker Dummy Load

I used binding posts to allow for easy connection to and from the dummy load. The binding posts also make it easy to add a small capacitor or inductor if you want to try different load scenarios when testing an amplifier.

DIY Speaker Dummy Load

Photograph 2: DIY Speaker Dummy Load