Jim Weir: help to add car radio to aviation audio...

I'm absolutely serious, what's the issue?

Well since we're critiqing solutions...
1) I didn't like this one because you're asking the guy to take apart
his fancy audio panel (warrenty?). And there is a good chance that
resistor, if it exists, is a tiny surface mount part about the size of
a piece of pepper. Plus the solution only works for people who own the
same make/model audio panel.

2) I didn't like the emitter-follower amp because then you have to pull
(and breaker) power out to power the amps. His signal already has
enough power, its just the wrong impedance.

3) You might get away with this, but then again, as you alluded to,
that stereo may not like having its outputs connected to ground. Its
kind of a kludge to drive that 500 ohm input with an 8 (or less) ohm
output, there may be distortion because of the mismatch.

You are kidding, aren't you?

Jim

...

I'd try an impedance matching transformer. Something like a Tamura
MET-17 (available from Digikey for about $20 each and other places).
If they do what you want, put heat shrink tubing around them and call
it a done deal.

Does your stereo or intercomm have a way of controlling the pre-amp
output volume of the music? You might be a little "hot" compared your
comm radio.

It is a relatively simple problem involving nothing more complex than a high
impedance output (intended to drive 10k) and a low impedance input (510 ohm)
intended for line level inputs. If that output impedance is really meant to
deliver 4 volts (probably peak to peak, although you didn't state) into 10k,
then the voltage into a 510 ohm resistor will be something on the order of a
quarter of a volt peak to peak, or 72 millivolts RMS. If your audio panel
requires 500 mV to drive it to full volume, then I'd expect the output with
72 mV to be about 17 dB below full output ... or in lay terms, "damned low".

There are several solutions, some better than others.

1. Call Garmin? (GMA-340) and ask them if you can clip a resistor that
provides the 510 ohm load for the input. For example, on the RST-564 audio
panel, we have a 560 ohm resistor on every audio input to ground to simulate
a standard 600 ohm load. If you clip that resistor out, the input impedance
jumps up to the value of the input summing resistor (10k)
(# and download schematics for the RST-564 -- drawing
564-5803). Problem solved. The main problem is to get Garmin? to give you
that data.

2. Failing that, you can kluge together a simple little single transistor
emitter follower amplifier that will take the 10K output impedance and
translate it directly to a 510 ohm input impedance. If somebody can tell me
how to post a binary to this group (no, I'm LOUSY at ascii schematic
artwork) or some other method of posting an image in the ra* groups, I'll be
happy to do so.

3. You can use the speaker output of the Alpine (if it has one) to drive
the audio panel directly ... unless it has what is called a "bridge
amplifier" and then we have to do a little dicking around (ahem, pardon me,
heuristic engineering) to get it to play properly.

There are more, but these are the easiest...

Jim

...