Dummy Load Box for Headphone Amplifiers Testing

One tool I use quite often when testing amplifier is a dummy load. Because of that I have a large box of high power (50W) resistors, and a large heat-sink that is tapped for easy attachment of the resistors to it. I typically have 8 resistors of 2ohm each, connected according to the requirement of the measurement i’m doing at the moment. More often than not, they are wired as 2 independent 8ohm resistors to measure speaker amplifiers (see Fig. 1). However, when I need to measure headphone amplifiers, I typically only need lower power loads, and therefore use a couple of resistors from the spare parts box. This got frustrating over time, soldering the resistors to a TRS plug, then soldering/clipping on a couple of wires to the scope/other test instrument. Therefore I’ve decided to do a small side-project of building a simple dummy load box for headphone amplifiers testing.

Fig. 1. Dummy Load for High Power Amplifier Testing

The requirements for this were quite simple actually. First, I wanted something that will have a common connector for the input, so I went with a TRS (6.35mm) jack. Second, I wanted an easy method of connecting to the test instrument to observe the waveform (to measure distortion/observe point of clipping/etc), so I went with BNC jacks. Third, I wanted something versatile that could be used for more than a single load resistance value. I’ve opted for 3 typical values that cover most of the impedance range encountered with headphones, 32ohm,120ohm, and 300ohm. As far as power rating is concerned, I think 3W per channel would suffice, especially considering you can go significantly higher than this for a limiter amount of time (to measure clipping for instance). The schematic of this very simple circuit can be seen below.

Fig. 2. Load Box Schematic

There is a constant 300ohm resistor connected across the input, with an additional resistor added for the lower ranges (therefore power rating is somewhat higher than 3W for these ranges). Nothing really interesting about the schematic obviously, but for a total BOM cost of a few $’s (+ a cheap case) this is well worth it to make measurements easier and quicker.

Fig. 3. Assembled Dummy Load PCB

I’ve opted for the same case I’ve used for the low THD oscillator and its PS , and same as in that case, used a PCB for the front/rear panel of the case.

Fig. 4. Front (assembled) and Read Panels

When it comes to something as simple as this, there’s obviously no testing/measurement needed. needless to say, it works as expected 🙂

The Gerbers for the boards I’ve created for this post can be downloaded below:
The TRS jack can be a SN37A12B or SN37A14B. The PCB support both, and the later of the two is typically slightly cheaper despite having additional contacts.
The BNC jack are 731000105. Additional part #’s are included in the schematic.

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