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.
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
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
The modern electronics is game, though. Most radios
will produce SOME output in an aviation headset. Some
actually do pretty well.
OK, all the previous is basically engineering
gobbledygook. What does it really mean,
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
Many folks that buy these radios and adaptors intend the
radio for emergency use. They'd be fine for that, in
most GA aircraft.
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
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.
This is probably the easiest way to handle the problem:
Replace the speakers in your headset with 8 ohm speakers.
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.
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
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
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.
OK, let's look at some ways one can build the
Here's the easiest.... putting a panel-mount jack and
the transformer inside a small "Project" box.
Radio Shack and Fry's sells small boxes like this, but
just a random plastic box sitting around will work as
This is one I was real eager to try: Use a
mold and casting resin to make a single-piece
adaptor. If you use the smaller XICON
transformer, it can all be sized to fit in a pill
Here's the plug and an in-line adaptor
"Siamese-Twinned" so that they overlap each other
by a considerable extent. A single wire is
wrapped around the body of the in-line jack (which
is electrically connected to the sleeve),