I first bought my 100WH HyperMac battery on the recommendation of Scott Davis. It’s a great way to survive a long flight with a layover or spend some time in a cafe without worrying about being near a plug. Scott had warned me about the hiss it caused in the headphones (which didn’t stop me from temporarily convincing myself my headphones, then my MacBook audio output, were broken, before I remembered his advice), so I theoretically knew what I was getting into.

The hiss is actually pretty bad. On my machine, it quickly got to the point where I couldn’t enjoy listening to music while using the battery. Since the my two primary use cases both recommend headphone usage, this was a major bummer.

I have a theory of what caused the hiss. I haven’t been able to test this with my spectrum analzyer yet—yet!—but I think the problem has to do with the power converter in the HyperMac. The unit isn’t just a dumb lithium ion batter and a charger; it’s actually got a DC/DC converter onboard, presumably because the specified voltage on the MagSafe connector was not easy to achieve with lithium ion cells. And switching power converters mean noise.

I’ll skip some tedious details, but you can read more about switching power supplies on your own if you’d like. Basically, despite the best efforts of the power supply designer, a switcher will couple noise into its output at the switching frequency of the converter (plus its harmonics). Linear power supplies, distant cousins to switchers, avoid this, but at the expense of size, weight, and efficiency. This is why your stereo receiver is big, heavy, and hot—but sounds great.

So the HyperMac battery contains a switching power converter, and this switcher is busily coupling its noise into your MacBook when you’re plugged into it. My theory is that some of that noise is getting coupled through the power input and into the audio output circuitry, causing the hiss. Pretty straightforward, and suggestive of a simple fix.

But wait! Before I “fix” this problem, you’re dying to point out a flaw in my argument: Apple’s plug-in power adaptor (what power supply designers call an “off-line” supply, because it draws power off the line voltage) is a switcher too! Why doesn’t it hiss? Well, this is what I’d have to take some measurements to confirm, but I’m guessing the switching frequency of the Apple product is matched to noise rejection circuits in the power input or our audio output circuitry of a MacBook. The switcher in the HyperMac battery either switches at a different frequency that bypasses the MacBook’s filtering, or its noise is just louder. I could be wrong about this, and I don’t mean to cast aspersions on a great product. However, the headphone hiss is certainly real, and this is a decent hypothesis, easily strengthened with some simple measurements.

Which I hope to do soon. In the mean time, it’s easy to knock this noise down below audible levels. Enter the HyperMac user’s best friend, the ferrite toroid:

Ferrite toroids are a beloved hack of noise-hating hardware designers everywhere. They don’t deal with the noise problem at its source, but they do stop it from spreading. With a 19mm internal diameter toroid from DigiKey, I was able to wrap the HyperMac’s power cord through it several times.

With this in place, I am no longer able to hear a hiss in the headphones. Your mileage may vary, but this hack has no choice at least to help a little bit, plus it looks kind of cool. With this hack in place, you have no excuse not to have a HyperMac battery. Let me know how it works for you!

UPDATE: a reader asked offline if the toroid was a concern near non-solid-state hard disks. The answer is no, since it’s not a permanent magnet by itself. It’s a ferromagnetic material with high magnetic permeability, but on its own can’t harm your hard disk or any of those 8½” floppies you’ve got lying around.