A dummy load simulates your headphones by presenting the amplifier with an impedance as close to that of your headphones as possible, using high wattage resistors. There are a number of reasons you may wish to use this – but primarily you are able to protect your headphones and your hearing. You can use higher voltages than normal, test new amplifiers without fear of damaging your headphones and you don’t have to listen to test tones at high volume. The addition of output connectors allow easy connectivity to measuring equipment. I drew inspiration from posts by Tangensoft and Abzza, which allowed me to come up with my own variation. I drew a quick schematic of it because sometimes it is easier to follow something visually than from written text.
A cable from the headphone socket of the headphone amplifier connects to a jack on the dummy load (I soldered another headphone jack to make a double-ended cable). The left and right channels are directed through the respective poles of a DPDT switch allows me to choose one of two sets of simulated headphone loads. BNC connectors allow the output to be connected to an oscilloscope for measurement (these could be replaced with banana plugs for a multimeter).
I used 330ohm 20W and 25ohm 30W resistors as these most closely represent my headphones (Sennheiser HD600/650 and Sennheiser HD569 respectively) in values that I could obtain. The resistors are TO-220 and TO-126 packages and can be bolted directly to the chassis for heatsinking purposes. The enclosure was one of a number I bought at less than half price when Maplins ceased trading.
In the image below you can see the way I have set up my dummy load. I’m using my Akitika 1kHz sine wave generator to send a signal to the Objective 2 amp via phono cables. The headphone cable from the O2 goes to the dummy load and the switch on the top of the box selects 25 or 330 Ohms impedance. This signal can be measured by connecting an oscilloscope to the dummy load BNC connectors.
For my first tests I was measuring the output power of the O2. To do this you need to know the voltage (RMS) and the resistance (impedance) and use Ohm’s Law (W = V2 / R). To get the voltage, increase the volume until just before the signal begins to clip on the scope and check the RMS value. Square this and divide the answer by the resistor value (or use an online calculator), this gives the output power.
With the Akitika signal level set to full and 25 ohms impedance on the dummy load, the 02 (in low gain mode) is clipping at just over 3 volts, so I turn the volume down and get a clean signal at 3.06V RMS. Using the calculations above we get W= 9.3636V / 25Ohms =0.3745W or 374.5mW. This is comparable with the official measurements of 337 mW into 15 Ohms, so not bad at all.