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Quiet As A Mouse
 
Over the years I’ve written several articles about audio noise in general aviation aircraft, mainly single engine Cessnas.  In most cases, noise problems are not solved with just a twist of the wrench; often we find that noise problems are the sum of many small problems within the aircraft.  Poor wiring, regulator pulse frequencies, poor grounding techniques often generate noise problems and the list goes on.  Newer aircraft such as a 2000 Cessna 182 seldom have noise problems.  Engineers learned from the older aircraft and designed an electrical system that for most part eliminates noise.  Modern avionics such as the Garmin GNS series radios seldom amplify noise, even when installed in older aircraft.  

Having said that, there are a lot of aircraft flying today that you can hear the beacon, autopilot servos, strobes and other noises when flying along.  Pilots get annoyed with hearing the beacon flashing for hours on end; it makes one want to beat his head against the wall!  Some modern avionics such as the Garmin portable, GPSMAP 396 will often amplify noise and transfer that noise to the intercom and then on to the headset.  Believe me, there’s nothing worse than listening to the latest Merle Haggard CD and the background being filled with alternator or flashing beacon noise!  Yes, we’ve found that many of the portable CD players on the market today will send aircraft noise to the headsets when using the cigar lighter plug located in the panel.  Even the powerful GPSMAP 396 will often pick up noise when using the cigar lighter connection in the aircraft.  Since the 396 will receive over one hundred XM stations, no one wants to listen to alternator noise while flying along.  

Here are two scenarios I often see.  The first scenario is the com’s are clear, no noise is present on the receivers, the intercom works great but as soon as a music source or some other product needing aircraft power is plugged into the cigar connection, a background noise is present.  Often this noise will change in pitch with engine RPM changes.  Another scenario we often see is a constant audio noise in the headsets such as the flashing beacon.  Shut the beacon off and the noise goes away, otherwise it’s there and drives the pilot crazy.   Changing the flashing beacon may or may not cure the problem.  

I have to admit it, during my years of running the shop, my favorite battle was fixing audio noise that other shops could not; in most cases I would slay the dragon but sometimes the dragon would win.  Recently I’ve received dozens of emails stating what could be done about the noise their new Garmin GPSMAP 396 was emitting into the aircraft audio system when using the XM music.  Aircraft owners stated the only time they heard this “alternator sounding noise” was when the 396 was using aircraft power (was OK when using the 396 battery).  This sent me on a quest to find a cure and a cheap one at that.  My goal was not to dig into the aircraft wiring unless necessary, I wanted to keep the fix simple and cheap. 

After talking to my buddy Mike Busch about the problem and having the opportunity to evaluate some filters myself, I feel “The Eliminator” made by Lonestar Aviation is the best and cheapest fix around.  The Eliminator is a great solution for alternator whine, strobe, and other noise that you may hear on through the aircraft audio system.  Using the proper test equipment I noted that this aircraft buss did have some AC ripple on the alternator but this ripple was well with tolerance of the alternator according to the alternator spec sheet.  It seemed the 396 for some reason amplified that ripple and the results was the noise the pilot often hears in the headset.  When I installed The Eliminator in the series with the cigar lighter power lead, the AC ripple coming out of the filtered side of the box was cut by 80%, thus the 396 was spewing out tunes without any noise in the headphones.  Now the XM music sounded as it should.  There was some noise present in the headsets when the engine was at idle, the reason was because the alternator had more ripple at idle than at cruise settings.  In a nutshell, audio from the GPSMAP 396 was quiet when the engine was turning over 1,600 RPM.  Prior to installing The Eliminator”, the XM music out of the 396 was so full of noise that it was useless in my opinion.  By installing a simple filter to the power line of the cigar lighter, the 396 now operated, as it should.  

The same would hold true if you have other noise in the aircraft audio system.  For example, lets say you’re hearing the flashing beacon anytime it’s turned on.  Simply install “The Eliminator” filter near the flashing beacon circuit breaker or near the flashing beacon power supply, either place seems to work fine.  In the past I’ve installed these filters in Cessna’s to reduce flashing beacon and strobe noise with a 100% success rate.  The Eliminator is FAA-PMA approved and comes with an “Approval List” that covers most make and model piston aircraft.  This approval list makes it easy to “legally” install the filter in certified aircraft.  Of course the installation must be signed off by an A&P or avionics shop, this is not an Owner Performed Maintenance job by any means.  Depending on the aircraft but in most Cessna 182’s, 210’s and Piper PA28 series, installation should take 1-2 hours to install.  The Eliminator operates up to 32Vdc and can handle 10 amps of current, which is more than ample for most portable electronics or electrical items such as the flashing beacon.  

We sell this outstanding filter for $103.95 but it can be purchased through most avionics and maintenance shops.  Got a noise that is driving you crazy or a Garmin 396 that you can’t listen to the XM music due to aircraft noise in the headset?  Discuss installing “The Eliminator” with your favorite A&P or avionics shop; it’s a lot of performance for the dollar.

Often the noise is only present in the aircraft audio when using a portable music device such as a CD player, FM receiver or the Garmin GPSMAP 396.  If you're only hearing noise when using such a device, chances are the noise is caused by a ground loop.  If that's the case, a simple ground loop filter like you see here normally will take care of the problem. The best thing about this simple filter is you just plug your music device output into the filter and the other side of the filter plugs into the same connection that your music output originally did.  I've tested three aircraft that had a whine (noise) in the headset when the Garmin 396 was used for music.  In each case, this little, easy to use ground loop filter deleted the noise from the aircraft audio system. I did note a small whine when the aircraft was at idle but once the engine RPM reached 1,600 the audio system was quiet; remove the ground loop filter and the noise was back.  The little filter worked; it's cheap and doesn't require any external power to operate.  Simply plug your portable audio system into the filter and the filter output into the aircraft stereo input jack; it's that simple.  For the flying machine that only has noise when a portable audio device is plugged in, this may be the best way to go; cheap, no installation is required, no external power needed and no FAA hassles because it's not connected in any way to the aircraft electrical system.  

For what it's worth, either of these two filters will go a long way in getting rid of those whines, buzzes and clicks that you hear in the aircraft audio system.   We sell the audio ground loop filter for $27.95, I'm sure most avionics shops carry the product also.

ELIMINATOR DC Noise Filter	$103.95
Ground Loop Filter 127SNI135	$27.95