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



 
 
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  #1  
Old April 18th 05, 05:29 PM
Brian DeFord
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Default Jim Weir: help to add car radio to aviation audio panel music input

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).

I'd be happy to supply more info is required. This is probably a
simple question, but I'm stumped!

Regards,
Brian DeFord
  #2  
Old April 18th 05, 07:08 PM
RST Engineering
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Default

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).



  #3  
Old April 19th 05, 11:44 PM
external usenet poster
 
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Default

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.

  #4  
Old April 20th 05, 05:04 AM
RST Engineering
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Default

You are kidding, aren't you?

Jim






wrote in message
ups.com...
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.



  #5  
Old April 20th 05, 06:04 AM
external usenet poster
 
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Default

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.


RST Engineering wrote:
You are kidding, aren't you?

Jim






wrote in message
ups.com...
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.


  #6  
Old April 20th 05, 06:30 AM
RST Engineering
external usenet poster
 
Posts: n/a
Default


wrote in message
oups.com...
I'm absolutely serious, what's the issue?

Well since we're critiqing solutions...


Thankful we aren't crititquing spelling...


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.



I'm not asking the guy to do anything. I'm simply pointing out how the
problem can be solved. You predicate surface mount, and even if it is, the
part is about the size of a matchhead, not a piece of pepper. So what if
the solution only is valid for that particular make/model of audio panel?
The general solution is valid for ALL audio panels, not just this
make/model. What don't you understand about generic solutions to problems?



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.


Didn't spend too much time in engineering school, didja? Do me a favor. Do
an Ohm's law calculation for the volts into 10K versus the volts into 500
ohms as a function of power. It isn't the wrong impedance, dunderhead, it
is not enough power to drive the requisite load. Your stupid suggestion of
a transformer indicates that you have not a clue about power transfer.




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.



Oh, bull****. Don't post here with this sort of stuff unless you've done it
before.


Jim






  #7  
Old April 20th 05, 08:58 AM
Scott
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Posts: n/a
Default

Didn't spend too much time in engineering school, didja?

Oh here we go. Somebody dares to question the great Jim Weir and is going
to catch a rash of it. His next post will tell us again how many years he's
been doing electronics and how anybody not wanting to use their real name on
usenet is a coward.

Asking you for help on usenet is like calling into a radio talk show
psychologist. While the psychologist is usually right, the caller soon
becomes very sorry he asked.


  #8  
Old April 20th 05, 12:40 PM
Darrel Toepfer
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Posts: n/a
Default

Scott wrote:

Didn't spend too much time in engineering school, didja?


Oh here we go. Somebody dares to question the great Jim Weir and is going
to catch a rash of it. His next post will tell us again how many years he's
been doing electronics and how anybody not wanting to use their real name on
usenet is a coward.

Asking you for help on usenet is like calling into a radio talk show
psychologist. While the psychologist is usually right, the caller soon
becomes very sorry he asked.


Yeah, I really hated it in school when the teachers made you think...

Shame they didn't do it more often...
  #9  
Old April 20th 05, 04:23 PM
RST Engineering
external usenet poster
 
Posts: n/a
Default

I answered the person asking for help straight up with several answers that
solved his problem. What is your beef with that?

If you are talking about the guy that answered without doing the homework,
what is your beef with that?


Jim


Asking you for help on usenet is like calling into a radio talk show
psychologist. While the psychologist is usually right, the caller soon
becomes very sorry he asked.




  #10  
Old April 20th 05, 05:21 PM
external usenet poster
 
Posts: n/a
Default

Without all the name calling here's what I took about a minute to do:

Stereo preamp out: 4V, into 10K impedance
Audio panel input : 500mV, into 510ohm impedance.

Using formula :P=V^2/R I get

Stereo preamp outputs 1.6mW=2dBm
Audio panel plays full volume at .5mW= -3dBm

So we're in the ball park. You've got a little more power (about 5dB)
coming out of the preamp, but that's a a nice place to be because its
easy to lose power, hard to add it (you need to do crazy stuff like put
in amplifiers ;^) ).

Regarding formal engineering education, unless you're Dr. Weir, I win
that contest which really shouldn't be a requirement to participate in
a forum like this anyway.

 




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