Op amps

At first my Sony MDR-EX81 headphones didn’t seem to get that loud. I thought maybe I needed to get an amplifier. When I built my Band Aids battery pack for the iPod, there were already people putting battery packs in Altoids tins. I soon found out that some people were also building amplifiers in Altoids tins. They have a line in from the iPod and a line out to the headphones and make the sound much louder in headphones that require more oomph than the typical ear buds (or for people who just want their music really loud). Susan even told me that her nephew made some of these Altoids amplifiers. So I started researching them around the Christmas holidays.


Most are based on a pretty simple circuit board project consisting of a few resistors, capacitors, hardware for batteries, jacks, and switches, a power LED, and a chip called an “op amp”. I had learned about op amps in college when I took a class in electrical engineering for people who aren’t electrical engineers. They make building amplifiers a lot cheaper and now just about everything with volume controls uses op amps. This project involves soldering probably 20 different parts and seemed beyond my ability to make, but people sell them on eBay. However there are all kinds of variations where people use bigger capacitors (for more bass) or separate op amp circuits for the right and left channels. They are call cmoy amps, or chu moy amps, after a guy that developed the design and helped people build their own, named Chu Moy. I saw the cmoy name long before I realized it was short for a person’s name.

On eBay a comparatively inexpensive one had a single op amp, and no volume knob (you use the volume on the source or the headphones). It usually auctions for about $22. Another one has two separate op amps and sells for $65 but the guy custom makes them and takes $5 off if you don’t want the volume knob. There are a couple of others that don’t use the cmoy design. One typically sells for around $70-80. All of those come in black plastic boxes. But there is a guy in Germany who builds his into mint tins (he uses Penguin Caffeinated mints which have several really cool tins). His typically auction for between $50 and $80. A couple of them have an extra jack for external power that will recharge the batteries inside (usually a 9-volt, but one uses 2 AA’s) or just power the amp without a battery. The eBay ads are pretty funny because they go into extensive detail about the electronic components and how they came up with the design. Some have maybe ten pages (if you printed it out) of text and pictures.

amp.jpg

I bid $21 on the cheap one several times, hoping I could take the guts out and put it in a mint tin, but I never won. People have been making these for several years and selling them on eBay, but I’m not sure why someone hasn’t figured out to mass produce them in China using the same high quality components for a fraction of the cost. You pay a lot for labor, profit, and shipping on eBay.

Lately I’ve decided my headphones are plenty loud for me and about the only use I would have for a headphone amplifier is if two people wanted to listen to the iPod at the same time. And the only time this happened was on a trip to Florida when Danny and I watched Black Hawk Down on Jeb’s laptop and the volume wasn’t that loud over the headphone splitter I had gotten. I think what these amps are really for is headphones that are usually used with home stereos instead of portables and require a much stronger signal. Still, it’s a neat project.

One thought on “Op amps

  1. My sony headphones are so loud I can use them as speakers in a quiet area. I love the mint idea for batteries and other stuff. What I REALLY would want to see is a portable hard drive built in to a mint tin. I saw a podcast where a guy ripped out his iPod’s hard drive and put it in a custome case to make a portable hard drive. Of course, it was a complete waste of money and time. The iPod acts as a portable hard drive normally, why rip out the hard drive? The only thing he accomplished is to make his iPod useless and have a $300 hard drive that can’t play music.

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