When testing audio equipment - especially power amplifiers - it pays to be careful. An ESD strap to prevent frying your components. A bulb tester to prevent frying yourself. And a dummy load to prevent your speakers/eardrums from exploding.

An audio dummy load is typically a high-powered resistor (or series of resistors) that presents an amplifier with an impedance similar to that of a speaker. See it as a kind of "silent speaker". Instead of hooking up actual speakers or headphones to the output of your DIY amplifier, you connect the dummy load to perform tests and see what the amp is capable of.

Step 1: Why Use a Dummy Load?

1. Safety

Amplifiers mean power. And power means danger. Even puny amplifiers typically drive enough current to stop the average heart... but more importantly, a dodgy connection (or miscalculation) could send your precious headphones to hifi heaven!

So it's probably not a good idea to give your DIY amplifier its maiden flight by plugging in your fancy Sennheiser HD650s, those HifiMan 400 cans or (God forbid) the Audeze LCD-Xes you mortgaged your house for. A dummy load allows you to test without consequences to your expensive hifi equipment.

2. Measurement

A dummy load can be built with quick and easy connections for your DMM, oscilloscope or spectrum analyser, allowing you to get an objective and analytical look at the performance of your amp that actual speakers can't provide.

3. Blissful silence

Nothing's worse than exposing yourself to non-stop 1kHz sine waves at full volume for extended periods of time. A dummy load is dead silent, meaning you can crank up your amp up and keep your eardrums intact. And by selecting beefy resistors, you can test your amp's power pushing ability far beyond what your actual speakers might handle... giving you plenty of headroom to play with.

4. Consistency

Having a standard set of speaker or headphone dummy loads keeps your measurements consistent and comparable. And a non-inductive design helps you test the raw current handling capability of your amp by eliminating a big test variable - the induction you'll get in actual magnetically-driven speakers. More on this later!

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In this Instructable I'll detail how to make a dummy load to test out headphone amplifiers - from DIY monstrosities to the output of your humble MP3 player or phone.

Step 2: Design

Here's the idea: the output from a headphone amplifier gets fed into a 5 way, 2 pole rotary switch. The switch then sends both left and right channels to a choice of dummy load resistors of varying impedances. At the same time, two BNC connectors allow easy access for connecting a scope or other measurement tool across the chosen pair of resistors. A switch swaps the left and right channels as a small quality-of-life enhancement (no plugging/unplugging required).

Like my DIY External Volume Control, the design is entirely up to your specific needs. You could replace the BNCs with banana jacks if you prefer. Or include another 3.5mm jack as an audio passthrough. Or ditch the switches and only have a single set of resistors. Or do it in mono. The choice is yours, and it depends how you want to test your equipment.

Step 3: Parts / BOM!

1 x aluminium enclosure (Hammond 1590BSLG)

4 x rubber feet

1 x 5W2P rotary switch (link)

1 x 3.5mm stereo jack (these ones are top-notch)

2 x BNC panel-mount connectors

1 x DPDT switch

5 x matched pairs of resistors! (See below)

Mounting hardware (bolts, nuts, washers)

Thermal paste

[Optional] Strip board

[Optional] Dupont connectors/wires

Step 4: Picking the Resistors

Like full sized dummy speaker loads, there are a couple of things to consider when choosing resistors to test headphone amplifiers. That being said, because headphones require far less power than speakers, your resistor choices here are more forgiving.

No reason not to do this right, however :)

Considerations: Resistance, power rating, precision, and inductance (or lack thereof).

Resistance...

...or in this context, impedance. While speakers typically come in 4, 6 and 8 ohm varieties, headphone impedances vary wildly from brand to brand and model to model. Most "iPod era" headphones are an efficient 20-36Ω, while the hardest-to-drive studio cans might be as high as 300Ω or 600Ω. Yowza. However, a big misconception is that higher impedances are "better" - I highly, highly recommend reading this post by the legendary (and mysterious) NwAvGuy.

What I would suggest is at very least picking a pair of resistors (one for each channel) that closely match your favourite/reference headphones, and then a bunch of other resistances that compare similarly to typical higher and lower impedance headphones on the market.

Decent headphone amplifier manufacturers will rate their amps against varying impedances like this - check out the specifications page for the Schiit Magni for example (they take 16, 32, 50, 300 and 600Ω measurements).

Power rating

An iPhone's headphone jack (remember those?) outputs at most ~60mW of power into your cans, while a small USB powered headphone amp can output a couple of hundred mW. So 1/8w resistors might be cutting it a bit fine - expect a puddle of hot goo if you expose them to significant power while testing.

At the same time, you really don't need to go overboard - beefy 300W power resistors are best saved for full sized speaker loads.

You want a good middle ground between puny and beefy, particularly if you're designing your own headphone amplifier and want it to be able to drive almost any headphone you throw at it. The Schiit Magni is capable of a monstrous 2W per channel at 32Ω, but even that is modest by crazed audiophile standards.

As a rule of thumb, resistors capable of juggling around 5W or more should give you plenty of headroom, while conserving physical space and cost. Bonus if they're in a package where they can be easily attached to a heatsink.

Precision

No point hunting down the perfect resistor only to have its actual resistance vary by 20% or whatever. If possible, aim for a tolerance of 1% to ensure you get a rating that's bang on the money.

Inductance

This is where everyone has an opinion. Of course, speakers themselves are inductive. Much like inertia resists acceleration or deceleration of movement, inductance generated by the wire coils in a speaker resist changes to the flow of current (and therefore the movement of the speaker cone).

Some claim that choosing non-inductive resistors in a dummy load is a waste of time, as speakers themselves are inductive. Others argue that because we're aiming to accurately measure performance of an amplifier, we need the load to be as consistent and stable as possible, without inductive effects on the outcome.

I'm on team non-inductive - although they're more expensive, I prefer having a more consistent playing field to test devices against.

My choices?

My current beloved headphones - B&O H6 (phwoar) - are in the region of 30Ω, so a couple of 30Ω resistors will be perfect. I then picked the following resistances to give me a broad spectrum of measurements, to see how a given amplifier would stack up against different kinds of headphones:

15Ω

30Ω

120Ω

300Ω

600Ω (I used two 300Ω in series)

I decided on Caddock MP915 series resistors - non-inductive, 15W rated, 1% precision TO-126 package resistors. These are a little pricey, but offer massive power headroom, great precision and are in tiny heatsinkable packages. Nice!

Step 5: Wiring the Resistors

The first decision was to use the case as a heatsink for the resistors - meaning the design had to ensure that their thermal pads made contact with the raw aluminium itself. There are no shared contacts on the back of the resistor packages, so no micas/insulators would be necessary.

I used a piece of stripboard to act as a common ground "rail" for the resistors. I prepared it by cutting two short lengths (to avoid the annoying recessed Hammond logo in the middle of the case) and drilled some mounting holes. The screws and washers mounting the boards give them an electrical connection to the inside of the case. And as the case itself will be used as common ground, my stripboard ground rails are good to go as soon as they're screwed in without any other wiring needed.

I rotated the "positive" leg of each resistor up vertically, closer to the lid-mounted rotary switch to keep cabling short and tidy. Also, the legs of TO-126 and T-220 packages make a good fit for Dupont connectors, meaning my resistors can be attached and detached without solder.

Step 6: Mounting the Parts

Using my completed stripboard/resistor circuits as a template, I marked all the holes that would need drilling - both the stripboard mounting holes and the resistor packages themselves. I centre punched them to prevent the bit from drifting, and then attacked the enclosure with a drill press.

After that, I drilled the holes for the stereo jack, the BNC connectors, the throw switch and the rotary switch, dry-fitting everything before the next batch of soldering. When the fit was good, I removed the resistors, applied thermal paste to their pads and seated them in place, tightening the screws.

Step 7: Wiring the Chassis Components

I wanted the resistors to be detachable from the rotary switch if I ever needed to tinker or make adjustments, without having to desolder them.

So, I made up some thicker-gauge wires terminated with Dupont connectors, which can connect to the free legs of the resistor packages.

Aside: In theory I shouldn't have had to use thicker wire. 28 AWG wire, which is standard for breadboard jumper wires, can handle around 1.4A of current in short runs. Even in my very very worst case, assuming I somehow produced 15 watts of power into my lowest impedance (15Ω) resistor, I'd be driving...

I = sqrt(P/R)

I = sqrt(15/15)

I = 1A

...at most 1A of current. BUT, just to be extra safe and put less pressure on my wires, I used a slightly beefier 26 AWG stranded wire which can handle ~2.2A.

The wires were colour coded in pairs to match the resistors, tinned and soldered to the rotary switch in sequence.

From there, I soldered the stereo jack's left and right channels to the common terminals of the rotary switch. For the BNC connectors, I wired them to the DPDT switch to swap the outputs if needed, and connected the common terminals of the DPDT to the common terminals of the rotary, to scope/measure whichever set of resistors are being driven.

Step 8: Finishing Touches

It's probably a good idea to give the unit some feet if you have exposed screw heads on the bottom - I added some rubber standoffs to lift it off the table. Finally I finished it off with a knob of choice (tee hee).

Step 9: Test It!

The easiest way to check if everything is wired properly is to use your multimeter (specifically, the ohmmeter) to test the resistances at the BNC connectors. I tested both the left and right channel for each of the resistance settings - looks good!

Step 10: Testing an Amplifier

To use the dummy load, simply treat it as a pair of headphones and attach it to a phone/MP3 player/headphone amplifier. Then scope one or both of the channels while playing something - standard practice for testing is to use a pure 1kHz sinewave and measure the RMS voltage to determine the power.

Turn the volume up as high as you can without the waveform clipping - if it does, lower the volume just enough to get a clean sine wave and use that point as your voltage reference (here's where my volume control really comes in handy).

As a brief refresher:

P = V²/R

...where R is the impedance of the load. My phone spat out around 0.8V RMS into the 30Ω load, so:

(P RMS) = (V² RMS) / R

= 0.8²/30

= 0.0213

= 21.3mW RMS per channel

Here are the clean measurements I made using a dedicated headphone amplifier - the (awesome) Fiio E10K - to drive the dummy load at each of the impedances:

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(Low gain / High gain):

15Ω:

78mW / 160mW*

30Ω:

42mW / 218mW

120Ω:

12mW / 58mW

300Ω:

5mW / 23mW

600Ω:

3mW / 12mW

* 15Ω clipped on high gain, volume reduced until clean.

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These numbers are very close to what Fiio specifies - 200mW into 32Ω on high gain. Nice.

Firstly, you can really see the power advantage of even a low/USB-powered headphone amp. Secondly, the benefit of a selectable gain switch when it comes to pushing tough-to-drive headphones - at low gain you'd only get around 5mW into a pair of say, Sennheiser HD650s (300Ω), but high gain brings you right up to a much more comfortable 23mW of clean output per channel.

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There you have it! Watch out for my full-sized speaker dummy load Instructable, as well as my process for testing audio equipment with an Analog Discovery 2. Cheers!

This post from the website tangentsoft was a huge inspiration for this project. Thanks!