PIREP: Garmin GMA340 impedance-matching audio cable

[Peter R.]

Back in April there was a thread in rec.aviation.ifr from a Garmin GMA340
audio panel owner who was questioning why the audio level of a plugged in
device was so low:

http://tinyurl.com/lvaap

Tom Borchert replied with a post that explained about the impedance-
mismatch and included instructions (from a fellow Cirrus pilot) on how to
build an impedance-matching cable from Radio Shack parts.

At the time I was able to overcome my GMA340's volume problem by using the
volume control on my MP3 player and satellite radio receiver so I really
didn't give much thought to buying or building a cable.

Just recently, however, I attempted to plug in a mobile DVD player's volume
cable and discovered that the highest volume setting still was not high
enough to be audible over the intercom. Initially I gave thought to
purchasing a cable from here:

http://www.fremarllc.com/accessories/index.html

but the principle of paying as much for shipping, handling, and delivery
insurance as the price of the cable made me decide to try to build my own
using Tom's instructions. Here are the details:

6 foot stereo cable with male mini-plugs on both ends: US $6.00 at Target
2 Radio Shack audio transformers (8 ohm/1000 ohm): $2.99/each or $6.00/2
1 Project box (small plastic box to hold the transformers): $2.29

Total was about $16.00 with sales tax included. Total time to modify the
standard stereo cable by clipping the cable and soldering the audio
transformers properly took about 15 minutes.

What a difference the cable makes. If you are at all handy and have a
Garmin GMA340 audio panel and would like to use the external audio plug, I
strongly recommend building this cable. The improvement in audio level is
very dramatic.

Tom B, a belated thank you for posting the instructions.


--
Peter

[Thomas Borchert]

Peter,

> Tom B, a belated thank you for posting the instructions.
>

You're most welcome. Glad I kept that post with the instructions back
then. Works great, doesn't it? The only problem I have is that I still
haven't gotten around to marking the cable ends, so that I end up
connecting it the wrong way often, which decreases the volume even
more.

FWIW, my project box is an old 35-mm film container (remember film?).

--
Thomas Borchert (EDDH)

[Jay Beckman]

"Thomas Borchert" > wrote in message
...
> Peter,
>
>> Tom B, a belated thank you for posting the instructions.
>>
>
> You're most welcome. Glad I kept that post with the instructions back
> then. Works great, doesn't it? The only problem I have is that I still
> haven't gotten around to marking the cable ends, so that I end up
> connecting it the wrong way often, which decreases the volume even
> more.
>
> FWIW, my project box is an old 35-mm film container (remember film?).
>
> --
> Thomas Borchert (EDDH)

The usually black canister with a gray lid have been the enclosure of choice
for years in the US TV industry for making up all kinds of adapters, etc...

Very handy little devices...doomed to extinction in the age of digital
photography.

Jay B

[Peter R.]

Thomas Borchert > wrote:

> The only problem I have is that I still
> haven't gotten around to marking the cable ends, so that I end up
> connecting it the wrong way often, which decreases the volume even
> more.

I purposely placed the box at one end, leaving perhaps seven inches of
cable protruding. The shorter side plugs into the audio jack of the audio
panel, which conveniently allows the box to rest inside my Bonanza's
mini-glove box (inside which, the previous owner had the GMA340 external
audio jack installed).

Had the audio jack been on the outside of the panel somewhere, I would have
probably placed the box in the center of the cable and color-coded each
side using tape around the cable to indicate which is which.

--
Peter

[Thomas Borchert]

Peter,

> I purposely placed the box at one end, leaving perhaps seven inches of
> cable protruding.
>

I did the same, only I can't remember where there short end goes.... ;-)

--
Thomas Borchert (EDDH)

[Ray Andraka]

Jay Beckman wrote:

>
> The usually black canister with a gray lid have been the enclosure of choice
> for years in the US TV industry for making up all kinds of adapters, etc...
>
> Very handy little devices...doomed to extinction in the age of digital
> photography.
>
> Jay B
>
>
Even better are the old aluminum 35mm film cannisters with the screw
on top. I've got several old projects in those, which make for a
durable shielded project box. Most of them I had were natural aluminum
on the can and kodak yellow painted tops. Good luck finding any of
those these days though.