Garmin Intercom (GMA 340)

Any suggestions for a portable CD player that works well with the Garmin 340 Intercom? I have had a problem with the volume being too low. Garmin tech support suggests increasing the gain on the intercom, but this may cause excessive noise. Hopefully, a different CD player may help.

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Thanks for the offer. We do not own a Cirrus, but we know that Cirrus come equipped with the Garmin intercom. Any suggestions on how to solve this problem would be appreciated.

In reply to:

Any suggestions on how to solve this problem would be appreciated.


The problem is that the GMA340 is designed for a 600 ohm input impedance (standard in aviation), while consumer electronics devices outputs are more like 8 ohms.

There are three ways I know of to get around this.

1) Have the mod done to the 340;

2) Buy a "Headphone Amplifier"

3) Build a small device to match impedances.

Solution (1) is quite expensive, and subjects you to other noise.

Solution (2) works, is easy to implement (device is available from Radio Shack), but you sacrifice quality – it’s a “brute force” approach, so you’ll introduce some distortion. However, some who have used this don’t find it objectionable.

I like Solution (3) best. You need a pair of small audio matching transformers - these used to be available at Radio Shack, but now you might have to order through their catalog or online. You want the 8 ohm - 1000 ohm device. But building one of these requires at least a passing aptitude with soldering and handling light electronics.

  • Mike.

Do you think the following headphone amplifier would work?

I built one of Sir Mike’s impedance matching devices this weekend. First of all, hats off to Mike, it works! I now run my MP3 player at 20 instead of 29 (full volume is 30). The music seems to be of higher quality when I’m not at the high end of the volume knob (but that’s subjective).

I made some slight changes to the design that I felt was worth passing on…
1 - I used a small Radio Shack project box instead of a film canister.
2 - I used two female 1/8" jacks, cleaner looking solution.
3 - I used crazy glue to attach the two transformers directly to the housing (no PC boards).

Construction time was under 30 minutes, and that included teaching a 3 year old how transformers work, and why they have the same name as his cars that turn into robots.

 - Marc -

In reply to:

Do you think the following headphone amplifier would work?


It looks as though it might, but…

  1. I don’t know that for sure (whereas I DO know that the one Radio Shack sells will)

  2. This one looks clunky-big, vs. the more elegant RS one

  3. It’s about 4 times the price

Again, any amplifier is actually fixing the wrong problem, but it might work in practice (like steering your car with the horn).

  • Mike.