uk_figs

uk_figs

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I have been flying for a while with the PM3000 intercom with XM music from the 796 injected via the entertainment input on the intercom, this input also provides the 796 audio warning such as "obstacle", "terrain" etc. I also have the SL30 and Dynon audio connected to the two PM3000 unswitched audio inputs.
When I designed the system I included a switch to allow external music input from a Ipod into the entertainment input but the problem with this setup is that you lose the 796 audio warnings when switched so this input did not get much use.
As I now have ADS-B for weather I am considering dropping my XM subscription and would like to figure out how to gang the 796 tone and warning audio with the external switched audio from my iPod or iPhone into the PM3000 entertainment input (or with the Dynon or SL30 audio as it is just mono audio and does not really need to be muted). I have the 796 bare wire cable installed (which currently has the audio connected to a 3.5 female socket as an easy replacement for the prior 496 audio).
Is it possible to gang the inputs, I thought I remembered a circuit from Vern years ago but have not found it?
 
What you are looking for is called an audio mixer. You can buy one for around $50, or build your own for around $5. (google it)
A 'poor man's' approach is to just connect multiple lines to the same input, but with a 400 ohm resistor in series with each line. This will cost you some volume so you have to currently have volume to spare, so you can turn them up to make up for the loss.
 
uk_figs said:
I have been flying for a while with the PM3000 intercom with XM music from the 796 injected via the entertainment input on the intercom, this input also provides the 796 audio warning such as "obstacle", "terrain" etc. I also have the SL30 and Dynon audio connected to the two PM3000 unswitched audio inputs.
When I designed the system I included a switch to allow external music input from a Ipod into the entertainment input but the problem with this setup is that you lose the 796 audio warnings when switched so this input did not get much use.
As I now have ADS-B for weather I am considering dropping my XM subscription and would like to figure out how to gang the 796 tone and warning audio with the external switched audio from my iPod or iPhone into the PM3000 entertainment input (or with the Dynon or SL30 audio as it is just mono audio and does not really need to be muted). I have the 796 bare wire cable installed (which currently has the audio connected to a 3.5 female socket as an easy replacement for the prior 496 audio).
Is it possible to gang the inputs, I thought I remembered a circuit from Vern years ago but have not found it?
Click to expand...

What I did with my PM3000 to mix the audio from an external source (iPod) and my Aera (stereo- which also plays MP3s) was to use a 150 ohm resistor combiner. Each source, left and right channels was combined with 150 ohm resistors in series, with the common point to the PM3000 stereo input.

But wait, there's more!

To eliminate system noise caused by the Garmin cable, the outputs of the Garmin also had 10 ohm resistors to ground. Without this termination, powering off the Garmin introduced large amounts of system noise.
 
mixer circuit

Assume the 10 ohm go from the 796 L & R lines to ground, are they placed before or after the in-line 150 ohm resistors or doesn't it matter?
 
uk_figs said:
Assume the 10 ohm go from the 796 L & R lines to ground, are they placed before or after the in-line 150 ohm resistors or doesn't it matter?
Click to expand...

These terminations are for the the 796... which I am assuming (but may be wrong) can drive speaker level outputs. Just hook them to the L&R lines to ground.

These are necessary because whenever the GPS is off (or removed), the interface cable becomes a good antenna feeding the high impedance inputs of the PM3000. There is usually an RS-232 signal in the cable next to the audio as well, happily banging away. The 10 ohm resistors effectively reduce the impedance of the interface, thus lowering the susceptibility to coupled noise.

If practical, it also helps to shorten the interface cable for the same reason.

Good Luck.
 
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