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.
Hi! Really nice post! Do you know the resistor (resistance & power) I should use to make a dummy load for a cellphone? I downloaded a signal generator to my smartphone and I want to connect my scope to the headphones jack. Thank you so much!
Best!
Hi,
Thanks for the comment.
I’m not sure I entirely understand what you want to do. The dummy load just simulates your headphones when testing an amplifier, so you just need to know the impedance of the headphones you want to use to get the resistance and the power rating of the amplifier under test. If you’re just testing the amplifier in the phone, it’ll be very low, but you would still need to check.
Cheers
Neal
Hi Enjon! Thank you for your quick response!
– I have this app installed on my cellphone (https://play.google.com/store/apps/details?id=com.luxdelux.frequencygenerator&hl=es_419&gl=US)
– I wired my oscilloscope to my cellphone headphones output so I can see the different waveforms on it (In order to do that I made a cable with a 3.5mm male jack and a piece of cord from an old broken headphone).
– The problem is that when I play a sound in the application the signal keeps going out to the internal speaker. I guess that because of the high oscilloscope impedance the cellphone doesn’t take it as an externally connected headphone.
– I thought that, in order to solve this, I could make a dummy load simulating a real headphone.
Thank you so much for your time!
Best!