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NAV audio hum - simple question

 
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mitch(at)skybound.com
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PostPosted: Tue Feb 13, 2007 1:18 pm    Post subject: NAV audio hum - simple question Reply with quote
I've verified the hum coming out of the Garmin 430's NAV 1 audio hi pin
(on connector 4006), it has peaks at 500 and 1500 Hz, and is 60
millivolts when measured with an oscilloscope. Many people won't care
about this because they have audio panels and the NAV audio is typically
turned off all the time unless you're identifying a VOR. I care,
because I don't have an audio panel and my NAV 1 audio is always present
(mixed by Bob's Iso Amp with COM audio, marker beacon, and traffic
warnings). I can knock down the level of the hum a little bit with a
T-Pass filter of sorts: two 500 ohm resisters with a 100 pF cap going to
ground in the middle. I have hi-buck Bose-X noise canceling headphones
and stereo music, and here is the million dollar question:

Will a 60 millivolt hum into an intercom be audible when the engine is
running?

If so, my choices are
1) return the unit to Garmin for testing/fixing (hopefully), might have
a bad capacitor on board or something, or it might be considered
"normal" by them, in which case I've just wasted two weeks.
2) put the nav 1 audio on an on/off switch on the instrument panel,
which I would like to avoid.

Thanks for any educated responses!
Mitch Faatz RV-6A Never Ending Finish Kit Auburn, CA

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Hopperdhh(at)aol.com
Guest
PostPosted: Tue Feb 13, 2007 1:44 pm    Post subject: NAV audio hum - simple question Reply with quote
Mitch,

I don't have a simple answer for you, but forget the T-Pass filter. The reactance of 100 pF at 1500 Hz is about 1 megohm. In other words, you have inserted 500 + 500 ohms in series and will just knock down all audio frequencies the same. On the other hand maybe this is what you need to do.

Sixty millivolts is 24 dB below a 1 volt signal. One volt at 500 ohms is 2 milliwatts. Sixty millivolts is 7.2 microwatts. It seems to me like you may not have a problem. The question is, what is the audio level out of the 430 when you tune in a VOR station? Is the hum small compared to that level? Small would be say 30 dB below that level (about 3 percent). So if your signal level is more than 2 volts, then don't worry about it. If it causes too much noise, you can use a resistive pad to lower the level from the 430 into the intercom and still have enough VOR audio. I don't know the schematic of your intercom, but maybe you could reduce the 430 audio level there.

Dan Hopper
RV-7A
Retired EE



In a message dated 2/13/2007 4:20:36 PM Eastern Standard Time, mitch(at)skybound.com writes:
Quote:
--> RV-List message posted by: Mitchell Faatz <mitch(at)skybound.com>

I've verified the hum coming out of the Garmin 430's NAV 1 audio hi pin
(on connector 4006), it has peaks at 500 and 1500 Hz, and is 60
millivolts when measured with an oscilloscope. Many people won't care
about this because they have audio panels and the NAV audio is typically
turned off all the time unless you're identifying a VOR. I care,
because I don't have an audio panel and my NAV 1 audio is always present
(mixed by Bob's Iso Amp with COM audio, marker beacon, and traffic
warnings). I can knock down the level of the hum a little bit with a
T-Pass filter of sorts: two 500 ohm resisters with a 100 pF cap going to
ground in the middle. I have hi-buck Bose-X noise canceling headphones
and stereo music, and here is the million dollar question:

Will a 60 millivolt hum into an intercom be audible when the engine is
running?

If so, my choices are
1) return the unit to Garmin for testing/fixing (hopefully), might have
a bad capacitor on board or something, or it might be considered
"normal" by them, in which case I've just wasted two weeks.
2) put the nav 1 audio on an on/off switch on the instrument panel,
which I would like to avoid.

Thanks for any educated responses!
Mitch Faatz RV-6A&nbsp======================


[quote][b]

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