Jim Weir: help to add car radio to aviation audio panel music input

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)
(www.rstengineering.com 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



"Brian DeFord" wrote in message
news:XhR8e.35465$A31.28773@fed1read03...
Jim,

I've read your columns regarding aircraft electronics for years and I just
can't seem to get an answer for a problem I have. I'm trying to connect a
Alpine AM/FM/CD player in my plane to the music input of my audio panel (a
GMA340) The problem is that I can't hear the music
because the volume is so low. I believe it has something to do with
the input and output level of the two devices, but not being an
electronics guru I don't know how I can make them work together. Do you
have any suggestions? The output of the radio (pre-amplified) is 4V / 10K
ohms max voltage and the audio panel music input is 510 ohm impedance and
500 mV RMS for full output typical, 1V AC RMS MAX (3V peak to peak).