December 8, 1997 ANR Retrofit: Making an Old Friend Better

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by	Carl Marbach

About the Author ... Carl Marbach is a co-founder of AVweb and its Publisher. Carl is a 4,000-hour pilot, and commutes between homes in Boca Raton, Florida, and Aspen, Colorado, in his 1978 Aerostar 601P. Carl was the founder and CEO of Professional Press which published five computer magazines including DEC Professional. After Professional Press was acquired by a venture capital firm, Carl founded Internetwork Publishing Corporation and now devotes himself to the emerging field of electronic publishing via the Internet. A lifelong resident of Philadelphia until 1994, Carl and his wife Helen now live in Boca Raton, Florida.

Although nowadays I usually fly with my Bose ANR headset, I still have the first aviation headset I ever bought: a venerable David Clark H10-40 that I bought back in the 1970s, now covered with scratches earned during many years of faithful service in the cockpit. It has new gel ear seals, but has been relegated to back-up and passenger use since I switched to noise-canceling headsets in the front office.

Recently, I became intrigued by the $169 active noise cancellation retrofit modules offered by Headsets, Inc., because it seemed as if it might be nice to be able to offer my passengers some (if not all) of the advantages I have with my expensive ANR headsets in the front seats. I also liked the thought of taking something that has done its job well for many years and making it new again.

In an earlier life, I built a lot of electronic kits purchased from (now-defunct) Heathkit of Benton Harbor, Michigan, including two color televisions, several radios, amplifiers and some test equipment. I also built two portable intercoms from kits obtained from RST in Grass Valley, California. So although it had been awhile since I used a soldering iron, I felt reasonably confident in my ability to install the ANR conversion kit myself (rather than paying an extra $50 to have it factory-installed). In addition to a soldering pencil, my toolbox also includes a digital volt-ohmmeter, which turned out to be invaluable during subsequent troubleshooting. If you find electronics intimidating, you might want to send your headset to Headsets, Inc., and have them install the kit for you.

Figuring out what to do

When my kit arrived, I opened the carton and spread all the components out on the table. Compared to my first Heathkit, this looked easy. The instructions, however, were complicated and required a couple of passes before they made complete sense to me. The problem is that Headsets, Inc. offers their retrofit for a very wide range of headset models (all David Clark headsets, plus those from Telex, Peltor, Flightcom, SoftCom, Sigtronics, Pilot Avionics, RST, Hush-a-Com, AV-Comm, etc.), and the instruction sheets try to cover all of them. Some of these headsets have dual volume control, some have a single control, and others have no volume control at all. Some are wired for stereo, some aren't. Different manufacturers use different wiring schemes and color codes. In attempting to cover all possible cases, the instructions are more difficult to follow than they should be.

But after some study, it became clear what had to be done. Each of the two ANR modules in the kit must have three wires connected to it: audio, power and ground. Once you understand this, and determine which of the various alternative wiring schemes your particular headset uses, it's easy to figure out how to hook up the kit. The important thing is to take plenty of time to look over the directions and think everything through before you start cutting, drilling or soldering anything.

Most headsets have a cord that goes to one earcup, and a crossover cable that connects to the opposite earcup. The ANR kit requires that the crossover cable have at least three wires (for audio, power and ground). Stereo headsets and those with dual volume controls require more than three wires. As it happened, my David Clark had only one volume control and was not wired for stereo. Its crossover cable consisted of two wires plus a braided shield, and that turned out to be sufficient. For headsets that don't have enough conductors, the Headsets, Inc. kit includes a new crossover cable. Count on using it if your headset has dual volume controls and/or stereo.

Installing the kit

The first step in the actual installation process is to remove the old speakers and some of the sound-absorbing foam from the earcups. Discard the speakers, but retain the foam and be sure to put back as much as you can so that the passive attenuation characteristics of the headset are not adversely affected by the conversion.

Next, you must drill a quarter-inch hole in one earcup (the one with the cord) so you can add the power cable required by the ANR modules (plus a rubber grommet to protect it from chafing). The new power cable will be tie-wrapped to the existing audio cord, and its other end hooked to the battery pack supplied with the kit.

Now comes the installation of the two new speakers with their attached noise canceling electronics. You'll have to solder three wires to each module (that's right: audio, power and ground). To do this, you'll probably have to do a little splicing of wires so that the three can also be routed to the opposite earcup via the crossover cable. There are more wires to worry about in dual volume control and stereo headsets, but each speaker module still gets only three connections: audio, power and ground.

My best-laid plans...

I carefully installed everything according to the instructions. I then installed fresh batteries, donned the headset, and flipped on the power switch. Uh oh! No active cancellation on the left side. (You should hear "the sound being sucked out of your head" when you power up the cancellation circuitry. In my headset, I heard it on one side only.)

I phoned Headsets, Inc., and my call was routed to a technician who led me through some troubleshooting steps with my volt-ohmmeter to verify that there was continuity through the crossover cable (there was) and that both modules were receiving power from the 9-volt battery pack (they were). The inescapable conclusion was that one of the two noise-canceling modules was bad. "I checked those myself before I shipped them out," he told me. "I'll send you a replacement ASAP."

Two days later, the replacement module arrived. I soldered it in place and repeated my test. This time, the noise canceling worked perfectly.

Flight testing

After flying with Bose ANR headsets for years now, the passive noise attenuation of the David Clarks seem fair at best. Even with DC's latest gel earseals, the headset lets a lot of low-frequency noise through — so much that in my Aerostar or my friend's Aztec, I found the sound level on takeoff to be downright uncomfortable.

But with the Headsets, Inc. kit installed and the active cancellation turned on, the Aerostar and Aztec both became very quiet. Frankly, the difference in sound level between the modified David Clarks and my $1,000 Bose headsets seemed hardly noticeable. I was quite surprised.

The company claims the retrofit kit provides 15 to 17 dB of active noise cancellation. Bose claims 20 dB or better, and 3 to 5 dB is a big difference. Why didn't I notice it? Probably because a David Clark headset has substantially better low-frequency passive noise attenuation than does the Bose. (Frankly, most designed-for-ANR headsets are pretty noisy with the ANR turned off.) The difference in passive attenuation would explain why the modified DC seems very nearly as quiet as my Bose. Keep in mind that I don't have any fancy decibel-measuring test equipment...I'm just telling you what my ears and brain told me.

Another friend flies a Cessna 310 and uses David Clark's new lightweight H10-13.4 headset. I let him try out my modified old H10-40 set. He was extremely skeptical before the flight, but afterwards told me that he'd decided to order a Headsets, Inc. kit to install on his H10-13.4. I'd guess that the 13.4 with the ANR kit would be provide an excellent combination of comfort and noise cancellation. I'm looking forward to trying it once he installs the mod.

Quiet isn't everything

There are three important factors in selecting a headset. One is noise reduction (via passive attenuation and/or active cancellation). On this score, my modified David Clark headset stacks up quite well. If it isn't as quiet as my Bose, it certainly seems 80% to 90% as quiet.

A second factor is fit, feel and comfort. These are very personal and subjective items. The Headset, Inc. kit does nothing in this area. If you consider your passive headset to be uncomfortable, it will be just as uncomfortable after you install the mod. If you like how it feels, you'll like it just as much after the modification. If the headset you are thinking of modifying is one you've used for some time, you have a pretty good idea how it fits.

To decide if a headset fits well and feels comfortable requires that you wear the headset for a long period of time. I have found that I can wear mine for 3-4 hours before I start to think about when it will come off. On the other hand, my wife can wear a David Clark headset for all of about 45 seconds before ripping it off her head and announcing that she could never wear this thing...and besides, look what it does to her hairdo! She insists on wearing a featherweight Plantronics earset that puts a tube in one ear and an earplug in the other. The Plantronics offers next to no noise attenuation at all, but it weighs next to nothing and you hardly know you have it on. Like I said, this is a very personal and subjective area.

Finally you need to consider sound quality. My $1,000 Bose headset has the best audio fidelity I have heard in any aviation headset. Frankly, it's probably a lot better than it needs to be. Unless you spend a lot of time listening to classical music CDs while flying, the sound quality of the Bose is probably overkill.

A "stock" David Clark headset has a good deal less audio fidelity than the Bose, and my modified David Clark (with the Headsets, Inc. kit installed) has even less bass response than a stock DC set. It has the sharp, somewhat "tinny" sound that seems to be a trademark of many noise-canceled headsets I've tried (but not the Bose). Some time back, I tried a Telex ANR headset that had a very similar "no bass, all treble" sound quality to it. Whether or not this is a problem depends on what you expect from a headset. The lack of bass does not interfere with speech intelligibility...if anything, it makes ATC easier to understand. On the other hand, if you listen to music in-flight, you probably won't be too pleased with the fidelity. Compared to the Bose, I would rate the sound quality of the Headsets, Inc. modification as fair.

Recommendations

Based on my experience modifying and flying with my old David Clark headset, my conclusion is that if you have a DC or other passive headset that you like, it's definitely worth the time and money to do the Headsets, Inc. conversion. While the result won't be as comfortable as a new ANR headset, it'll be a big improvement in noise reduction over what you're used to and the additional cost is quite reasonable for what you get. If you're like me, you'll be quite pleased with the result.

On the other hand, if you aren't happy with the comfort of your current passive headset, the modification won't make it any more comfortable. Or if listening to music is important to you, you may not be satisfied with the audio response of the Headsets, Inc. speaker modules. In this case (or if you need one or more additional headsets), I'd recommend the $1,000 Bose or the $439 LightSPEED 20K (see Mike Busch's review article).

At $169.00, the price of the Headsets, Inc. kit is right, and it does everything they advertise it will do. I was more than satisfied with my results, and I feel as if I have turned a 20-year-old headset into a modern noise-canceling instrument.