Reducing Headphone Noise from the HyperMac Battery
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.
Matthew McCullough Said,
February 8, 2010 @ 8:53 am
As a new owner of a HyperMac, I salute you. You are like a liason from a foreign society (hardware) bringing hand waving and words of harmony to the noise-suffering masses in the software community. Thanks. A toroid it shall be.
Scott Davis Said,
February 8, 2010 @ 10:02 am
Hey Everyone, look at the brain on Tim! Strong work, Friend. Strong work.
Ben Said,
February 16, 2010 @ 12:32 pm
As a new owner to a Hypermac Battery (150) and apple power books in general I thought I was going crazy or my battery was just a dud. This at least makes some kind of sense as to what could be causing the issue. With my Bose QC2 the noise is like a hairdryer being plugged into a wall socket and the TV going fuzzy.
I will buy the toroid and let yall know if it makes using the battery on the plane is bearable. It is rather unfortunate that we have to do this with something that costs soo much to begin with…
Thanks for the idea
Also, my battery makes noises. This is the first I have ever experienced this phenomenon. The hypermac staff mentioned that this way natural. I was wondering if anyone else was experiencing this as well?
-Ben
tlberglund Said,
February 16, 2010 @ 12:43 pm
Yes, my battery squeals also. This is another dead giveaway that there is a switching power converter in there. It’s not unusual for switchers to whine occasionally (especially under conditions of very low or very high load), but I can’t comment on whether it’s indicative of any kind of problem. As it stands, I think of it as giving my HyperMac some personality.
If somebody who actually knows switchmode power conversion happens upon this post, your comments are welcome.
Brian Said,
February 16, 2010 @ 7:04 pm
My also makes a whine with headphones on and using hypermac. I wasn’t able to fit the power cord into toroid, I guess I bought a smaller size, so what I ended up doing was put the headphones cord through toroid instead. With music on the background static hiss noise wasn’t noticeable. Thanks for this solution, works for the moment. Hypermac should really address this issue or maybe hire the person who came up with this temporary fix, since their engineers can’t figure it out.
tlberglund Said,
February 16, 2010 @ 7:13 pm
Heh. Well, thanks, Brian, but when it comes to actual hardware design, it’s best done by people who give their full time to that profession. I can hack a bit and interact with hardware guys, but ultimately I trust HyperMac to fix this properly in the next rev.
One caution about putting the filter where you’ve got it: you’re killing the hiss, but you’re also filtering out valuable audio signal. I’m sure it still sounds better with the toroid in place, but it will be knocking down high-frequency audio energy that should be allowed to make it all the way to your headphones. My post was definitely not clear enough, but the part you want is DigiKey 732-1534-ND. That guy is big enough to fit the MagSafe connector through with six windings of the cord like I show in the picture.
Regardless, I’m glad this helped! Thanks for contributing to the post.
PoPPaP Said,
December 11, 2010 @ 3:38 pm
I do not have the toroid yet but I’ve found that if you put the HyperMac directly under the MacBook Pro or on top of the keyboard the noise becomes bearable so that’s my temporary fix for now…
PoPPaP Said,
December 14, 2010 @ 1:08 pm
I’ve found a removable ferrite core that came with one of my USB cable and use it with HyperMac. It works perfectly.
Image: http://j.mp/gAWLMP
tlberglund Said,
December 14, 2010 @ 1:46 pm
Excellent! Much more elegant than the solution I found. Plus, my ferrite torroid was brittle, and eventually broke under the stress of travel.