Using handheld radios as the primary communication device in light aircraft is pretty common.  You can buy a handheld transceiver for $200 or so, and the cheapest bolt-in aviation radio will be about $800.  Plus, if you have something other than a homebuilt, you have to go through the whole rigamarole regarding installing any sort of device in your panel.

So folks buy ICOMs, YAESUs, etc, buy the headset adaptors, and plug in their Flightcoms or David Clarks and launch into the blue.

Some of them aren't completely happy, though.  If they fly in a noisy airplane, the radio may just not be loud enough.  Maybe some brands of headsets won't work at all.  They usually blame the handheld...after all, the headset works just fine when it's plugged into to their Narco or Garmin.

And, in truth, it *is* the handheld radio's fault.  But the good news is, it can be easily fixed.  For $5, if you've got a homebuilt that you can just modify, for ~$15 if you build a simple adaptor, or $30 if you want to adapt your current headset, or ~$150 if you want a brand-new headset perfect for your handheld radio.

I've written a lot on how to build one's own headset.  This web page is different:  I'll tell you how to get your existing headset to "play nice" with any handheld radio.

It's all a matter of amplitude.

Table of Contents

• Sound Science  (How to define sound level)
• A Short History of Aviation Headsets
• The Problem with Handhelds
• How to Eliminate the Mismatch
• Replacing the Speakers in a Headset
• Using a Transformer as an Impedance Matcher
• Converting a Consumer-Grade Headphone
• Using Ear Buds
  
• Building Adaptors:  A Practical Guide
• Parts
• Tools
  
• Schematics
• Implementation
• Using a Component Box
  
• A one-piece, molded adaptor
• An in-line adaptor
• Buy, Not Build
• Feedback Squeal
  

Sound Science

So...when we're talking sound, what IS amplitude?  Sound consists of alternate waves of (slightly) compressed air and (slightly) rarefied air.  The amplitude of the sound is the difference in pressure between the compressed and rarefied potion of the cycles.

We measure sound energy in "Decibels" (dB).  The term is named after Alexander Graham Bell, who declared that "One Bell" represented the difference between a reference sound and one that seemed twice as loud.  That's a fairly coarse unit, so engineers and scientists split it into tenths for Decibels.  Decibels aren't a "level" themselves; they are a ratio between a level and a given reference.

Two things to remember:  Decibels are logarithmic measurements of sound energy.  Each "Bell" (ten decibels) represents TEN TIMES THE ENERGY.  If a sound changes from 80 dB to 110 dB, there's a thousand times more energy involved (10^3 power). 

It gets important even at lower differences.  If you see a given sound rise 3dB, there's actually twice as much sound energy in the new signal.

This is important later when we compare the performance of some of our adaptors.

However, this brings us to the second point.  This sound energy increase is not directly correlatable as to how the sound is perceived.  That 3 dB difference might be barely noticeable to the human ear.  But because its logarithmic, every step higher means a disproportionate increase in energy.  You might not notice 3dB, but you're much more likely to notice 4 db, even more likely to notice 5 dB, and so on.

So when we look what's happening between your headset and your handheld, we'll be talking dBs.

History

Next, let's look at how headsets and handheld transceivers got developed.

When people are trying to develop a new product, do they invent everything from scratch, or do they try to build on existing hardware and technology?

The latter, of course.  Companies scan the catalogs and pick standard connectors, and other common electrical components, and use them as much as they can to save from having to design and manufacture them themselves.

The history of aircraft communications radios is no different.  The companies trying to develop them in the '20s and '30s didn't want to invent the plugs, the microphones, and the headphones.  They went for parts that were commonly-used, and were already being produced in the hundreds of thousands.

And who was making microphones, headphones and plugs/jacks by the millions in the 1920s?  Ma Bell, of course:  The telephone industry.

Yes, the standards used for General Aviation headsets today came from the telephone systems of the 1920s.  You can probably take one of today's headsets and plug it right in to those old switchboards. 

The headset the young lass is wearing in the above photo contains two small speakers pressed against her ears.  The two speakers in the headset have an impedance of about 300 ohms and are connected in series.  The characteristic impedance of the phone system back then was 600 ohms, so the headsets had to match.  Larger speakers for home radios might have a lower impedance, but the telephone speakers had to stay higher.

("Impedance" is a fancy name for resistance.  It's actually NOT the same, since Impedance varies with the frequency of the input, but just think of it as "Resistance" to understand what's happening.)

GA first started going to radios in the 1920s, when there weren't many consumer electronic devices.  Radios used headphones, and music lovers listened to their Victrolas, which didn't HAVE any electronics.

But as the 20s and 30s rolled on, consumer electronics skyrocketed.  More and more devices used speakers, not headphones.  And it's more efficient if the speakers have 8-ohm impedance.  By the 1940s, 8-ohm was the defacto standard.  It STILL is, from car stereos to IPhones.

The military eventually bit the bullet and converted to the new 8 ohm standard.

General Aviation, to this day, hasn't changed.  General Aviation radios still expect to have 300 ohm speakers attached to them.  Plug in an 8-ohm speaker, and it's like shorting the output.  We are the last holdouts for 300-ohm speakers and, for that matter, the slightly-smaller 0.21" jacks used for microphones.

The Problem with Handhelds

Aviation handhelds have been around for a while, but were pretty pricy.  In the past 30 years or so, though, a number of companies appeared that made small transceivers for Hams, or police, or the military.  With a slight modification to change the frequency range, the modern aviation handheld was born.

One problem:  Most retained the 8-ohm speakers/headphones that the rest of their production line used. 

The result:  While it's easy to build and adaptor to make the electrical hookups to an aviation headset, the aviation headset produces an impedance mismatch.  You have a circuit that expects 8 ohms, but is confronted with 150 ohms (modern headset speakers are connected in parallel).  It's like when you hook a standard house light bulb to a 6 volt battery instead of the 120 volts it expects.

The modern electronics is game, though.  Most radios will produce SOME output in an aviation headset.  Some actually do pretty well.

How Does it Affect Performance?

OK, all the previous is basically engineering gobbledygook.  What does it really mean, performance-wise?

I took a stab at measuring this.  I set the output of an ICOM handheld to 100 dB, as measured at an 8-ohm speaker using a Radio Shack sound level meter.  Then connected three different aviation headsets to it and measured the sound level at the headset cups.  Here's the setup:

And, here's the results.  Remember, the sound level should have been 100 dB:

Headset #1:  89 dB
Headset #2:  94 dB
Headset #3:  92 dB

All at least 6 dB quieter than the reference sound level.  That's four times less sound energy!  Note that #1 was down 11 dB... a complete Bell, which meant the perceived volume was less than half.

Any way you look at it, this is a serious decrease in volume.  Flying in a nice quiet closed-cockpit airplane?  Probably automatically just crank the volume up a bit.  But if you're in an antique, or open-cockpit homebuilt, or an ultralight?  It might make the difference between a readable and a non-readable radio.

But I'm Using ICOM's (YAESU's etc.) Headset Adaptor!

I can't speak for every headset adaptor from every company, but the one I bought from ICOM only provides the physical and electrical connection from the miniature plug that goes into the radio to the two jacks where the headset plugs in.  It does not do anything about the impedance.

Many folks that buy these radios and adaptors intend the radio for emergency use.  They'd be fine for that, in most GA aircraft. 

Eliminating the Mismatch

Indeed, that 6 dB difference may not matter, if you're flying a nice quiet Cirrus or Bonanza, or even a 172.  But us antique, classic, and open-cockpit guys need as much as we can get.  At cruise, my Fly Baby measures 105dB at chest height.  That's louder than a circular saw, and it's running continuously.  Even with a good headset with passive attenuation, I need as much power to my ears as I can get.

To fix things, we need match the impedance between the headset and the radio.  There are three different ways to do this:  Modify the headset so it's compatible with the radio, insert a device in the circuit that handles the impedance matching, or convert a consumer-grade headset.

Replacing the Speakers in a Headset

This is probably the easiest way to handle the problem:  Replace the speakers in your headset with 8 ohm speakers.

Rugged Radios sells replacement speakers for their headsets in both 300 and 8 ohm.  There's a small chance they'll fit your existing headset, but Rugged sells a brand new aviation headset for $100.  Buy their headset, and replace the speakers ($15 each).   Replacement is just a matter of removing the old speakers and soldering the wires for the new ones. 

However, this modification makes the headset unusable with a standard aviation radio.  You can do an adaptor, as I'll describe in a bit, or buy a second headset exclusively for using in handheld-equipped aircraft.

Since my Fly Baby has a handheld permanently installed as a comm radio, I went this latter route.  The Rugged Radios RA200 Aviation Headset has good passive attenuation and appears to be a nice, sturdy unit.  The modification works just fine in the air...good strong volume, compared to what I was getting before.

One caution:  If you have an ANL headset, do NOT replace the speakers with 8 ohm units.  There's electronics on the headset itself, and the speaker change will mess it up.

Using a Transformer as an Impedance Matcher

You're probably familiar with transformers, that can take 120 volts and drop it down to a lower voltage.  Transformers also work as impedance matchers, too.  You'll need one that has one side with an 8 ohm impedance, and the other with 150 ohm impedance.  These are usually referred to as "audio output" transformers, since they're designed to drive 8-ohm speakers. 

Transformers have two sets of windings, called the "Primary" and the "Secondary".  The Primary is usually the side power is coming from, and the Secondary is usually the output side.  Primary and Secondary are just labels, though...good thing, because we're basically reversing the way the transformer is normally hooked up.  Again, since Primary and Secondary are just labels, it doesn't matter.

Circuit Schematic tradition always draws the secondary on the right, so the illustration above shows how the 8 ohm input is connected to the 150 ohm output.  This would typically be addressed at as a ratio:  150:8

Since consumer electronics often need to output to 8 ohm speakers, it's easy finding transformers with 8-ohm side.   It's a bit tougher finding 150 ohms on the other side.  Fortunately, the actual impedance is not that critical.

I tested three transformers... one at 200:8, the second at 250:8, and the third at 1000:8.  This last transformer had a center tap on the 1000 side.  I had thought that gave an effective ratio of 500:8, but it actually gives 250:8.  Even better.

How'd they work?  Nicely.  With two of the headsets and the same 100 dB reference sound level, all three brought the signal level up to 98 dB or higher.  You aren't going to notice that 1-2 dB.  On one of the transformers, on one of the headsets, the level only got up to 96 dB.  Might be a bit noticeable, but the same transformer worked fine on the other headsets.

Note that this is MINIMUM of a 6 dB increase.  That's four times the sound energy hitting your ears.

One of the transformers I tried cost about $20, but the other two were $4 or less.  These were the Radio Shack Miniature Audio Output Transformer (#273-1380) and the XICON 42TL004.  I've used the Radio Shack transformer for years, since they're available at most stores and online.  The XICON transformer can be ordered from Mouser (www.mouser.com), Digikey (www.digikey.com) or any other electronic parts house.  The Radio Shack unit is probably easier to get (just go to your local store) but the XICON transformer is about half the size if you're trying to keep your adaptor really small.

Installation is pretty straight-forward...the figure below shows the connections that must be made.  Options for Installation:
1.  If you've got a jack on the panel, you can install the transformer between the output of the radio and the panel-mounted jack
2.  In an adaptor box that your headset plugs into and then is plugged into the radio's output jack.  More detail on this option below.
3.  Inside the headset itself (however, this does mean the headset can't be used with a conventional aircraft radio any more)

Making the adaptor is pretty easy.  You can buy jacks, plugs, and a small plastic box from Radio Shack.  Or, Radio Shack makes a 1/4" Y-Adaptor that comes with plug and jacks.  Cut off one leg, splice the transformer between the remaining jack and plug.

Converting a Consumer Headphone

One option would be to NOT fiddle with the aviation headset, and use a standard set of consumer headphones in the airplane.  I basically do this now, in the summer, by using Ear Buds.  You have to come up with a microphone, boom, and holder, but I've got instructions for that.

However, one bright idea I had:  Consumer-grade headphones often include Automatic Noise Limiters, and they're often fairly cheap.  Why not do an over-the-ear type consumer headset!

Bright idea...shot down by reality.

You see, General Aviation aircraft are much noisier than living rooms, dorm rooms, bus terminals, or even airliner cabins.  We handle it by having our headsets include noise-absorbing material to reduce the outside energy that reaches the ear, and seals that minimize leaking.  This is called Passive Attenuation.  Consumer grade headphones have little passive attenuation; most just sit atop your ear and don't try to seal out external noise.  They may have active noise limiting, true, but ANL works best if the majority of the noise it kept out before it tries to cancel it.

The exception to this is ear buds, especially buds like the Plugfone, which includes standard foam earplugs with passive attenuation.  I fly my open-cockpit airplane with Plugfones in the summer.  But when fall comes, I like the additional warmth that an over-the-ear headset provides.  Either adapting a headset or building an adaptor is a better way to go.

Using Ear Buds

One type of Consumer headphone which is QUITE practical with a handheld are ear buds...a pair of in-the-ear speakers.  I use these in the summer, along with a homemade microphone boom, and the volume is very good.   They've got the standard 8-ohm impedance.

I use a product called "Plugphones"; these are conventional foam earplugs with the speakers buried inside.  A set costs $25 or so, I usually buy two at a time with some spare tips to get free shipping.

The advantage to these, vs. conventional ear plugs, is that they provide significant passive noise attenuation.  Like the consumer headphones, consumer ear buds may not do as well for reducing the background noise.

However, there are ear buds sold with Active Noise Limiting (ANL).  These are a quite attractive option, although the price is higher.

The only drawback to using ear buds is comfort.  I, personally, don't like having things stuck in my ear.  I tolerate it in the summer, since it's great to get rid of the "cans" over the ears.  But for me, it gets irritating to have them stuck in my ears for more than an hour or so.

Also, for my fellow open-cockpit aviators, when cool weather comes it's kind of nice to have a big headset over the ears to keep them warm.

You could wear the ear buds under a conventional headset, of course (not plugging the headset into the audio).  Or you could build a stand-alone microphone system and free yourself entirely from the conventional headset world.

Building Adaptors:  A Practical Guide

Using Ear Buds is a pretty good option, but for most of us, the easiest way to do this is to build a small adaptor.  It plugs into the headphone jack on the radio, and the headset plugs into it.  It lets you use your aviation headset on either type of radio setup (assuming you remember to bring the adaptor!).

As part of my experimentation, I've bought a lot of parts...so I'm experimenting with different types of adaptors.  This section will be updated periodically to note my progress.

Parts

There are three major components you'll need:  A transformer to match the impedance (see above), a 1/4" phone plug, and a 1/4" phone jack, either a panel-mount or an inline jack.

The plug is a common Radio Shack part.  You can buy 274-1544, the monaural plug, or 274-1545, the stereo plug.  Here's a diagram: