Making a Headphone Dummy Load

So you've got a headphone amp, and you want to run some tests on it. It's often best to have the amp running under load while you do the tests, but you might not want to use real headphones. Perhaps you need to test at higher voltages than your headphones will tolerate. Perhaps you don't want to listen to test tones. Or perhaps you want to be able to collaborate on tests with others who don't have the same headphones as you.

This article shows you how to build a "dummy load" that can be plugged into a headphone amp like a pair of headphones. It's very durable, it can be run to much higher currents and voltages than real headphones, and it ought to be silent. :)

If you build it as shown in the pictures, it'll run you about $35. You can cut it down to about $20 if you don't want to be able to switch between two different load values.

Parts

The enclosure is Bud part CU 123 (Digi-Key 377-1107-ND). A nearly identical enclosure is Hammond part 1590A (Digi-Key HM150-ND). The Bud box is cheaper and it's got a nicer finish. (They're both ugly, but the Bud box is slightly less so.) The Hammond box's extra cost is worth it if you don't have a tap set: its screw holes are tapped out already so you don't risk stripping the screws out when closing it up the first time. A bit of time with some fine sandpaper would fix the Hammond's finish problem.

If you don't want a switchable dummy load, both Hammond and Bud have much smaller boxes in the same line as the ones I've recommended above. They've got one that's roughly a 1.25" cube that would work wonderfully.

The resistors are Ohmite TCH35P series, though lower-wattage ones like the TBH25 series would also work. They're pretty expensive, but I like them because the TO-220 package easily bolts to the side of the case. You could use cheap cement wirewound types instead. The values are 33 Ω and 330 Ω, to be similar to typical headphones. The box has room for a third pair of resistors, though I'm not sure you could find the DP3T toggle switch you'd need, and a rotary would probably be too big.

The headphone cable is good old Canare Star Quad (L-4E6S) with a connector I found locally. You could use lots of other things here.

Beyond that, you need some #4-40 bolts and nuts, a DPDT switch, some wire, a strain relief for the cable, and basic casework tools.

Assembly

Since all of the resistors have one leg going to ground, and the headphone cable also has a ground, I decided to make the enclosure ground. I just had to run a wire from one leg of each resistor to the mounting tab of the resistor and trap the stripped end under the mounting nut. It would have saved me some time if I had done this bit of wiring before mounting the resistors. (Hint.) You also have to add a bolt for connecting the headphone cable's ground shield to the case.

The common lugs of the switch go to the headphone cable's signal wires. The remaining switch lugs connect to the ungrounded end of each resistor. Pretty simple.