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

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


Thankful we aren't crititquing spelling...
What don't you understand about generic solutions to problems?
Didn't spend too much time in engineering school, didja?
It isn't the wrong impedance, dunderhead
Your stupid suggestion of a transformer indicates that you have not a clue
Oh, bull****.


Do you listen to yourself ? You're a jerk, that's the problem.

"Scott"
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.


Ya, like when you're out on a ledge and they ask you if you can hold on
through the break?


Montblack

Stereo preamp out is almost always given as volts peak-to-peak. The person
giving us the question stated that the audio panel input voltage was rms.
You will note that I converted into like units in my original post. If
Alpine DID give their output in RMS, then you will note that they are trying
to swing within less than a volt to the 12 volt rail and ground -- and I
can't conceive of any quality audio manufacturer trying to do that.

We can go either way, but the easy way is to convert the p-p to rms. 4V pp
= 1.4 volts RMS. 1.4^2 / 10E3 = 200 uW.

Audio panel plays full volume with 0.5^2 / 510 = 500 uW (in round numbers).

Seems to me we are just a bit short here. No transformer is 100% efficient,
so the problem gets worse as we go along. We start off life being down (10
log (500 uW/200uW)) (about -4 dB) and any passive solution simply adds to
the problem.

Let's give you the benefit of the doubt. Let's say that Alpine has their
act together and gave the output voltage as RMS. The equation then tells us
(as you noted) that we have 1.6 mW output and need 500 uW input, or an
excess of 5 dB. Let's see if we can practically do the solution you
propose.

You have to design for people who don't have access to their own transformer
factory as well as those in East Undershirt who only have access via mail
order, so let's examine the usual suspects.

Rat Shack has nothing of the kind.

Mouser has a few, the 42TU011 being the best of the bunch. However, if you
examine the curves of the transformer, you find that the frequency response
down in the bass is particularly terrible. The 3 dB points are given as 300
Hz and 3.4 kHz. respectively, so this great little Alpine tuner has turned
itself into a tin horn. I find that solution unacceptable.

Digi-Key is no joy, as is LKG/Philmore, Jameco, and half a dozen other
lesser lights.

One possible jury-rig solution may be to use a filament transformer as an
audio transformer to get the low frequency end, but if you have ever tried
this, you find that the high end falls off WELL before a couple of
kilohertz...lots of bass, but dearly lacking in treble. Not a solution.
Not only is it not a solution, but with a hunk of iron that will get down
into the tens of hertz range, you start picking up a fair amount of weight.
Remember, this is an AVIATION as well as an ELECTRONIC problem.

As to power, the fellow already needs to pick off power for both the tuner
and the audio panel. Stuffing a tiny single stage transistor impedance
converter inside either of these boxes and tying into the existing power
coming into the box isn't a great problem. NPN transistors can be had by
the bucketful at your local Hobby Shack store, as can a couple of resistors
and a couple of capacitors. Weight? A couple of ounces with enough left
over to stuff a flea's navel.

As to the Dr. Weir, no, I dropped out of the doctoral program when I
realized that what was being taught was roughly five to seven years behind
the stuff that I had already done. That wasn't what I wanted to waste my
time on ... and my own company wasn't going to pay me one cent more for a
doctorate. I'm the "dumb" one of the family; both my brothers have their
PhD.

I find your solution, while theoretically possible, to be difficult to
impossible to do practically.

Jim







wrote in message
oups.com...
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.

No, I expect and get answers like this from my students. I don't expect it
from a person who claims to know how to solve a problem without the briefest
nod towards a practical solution.

Jim



"Scott" wrote in message
...

Do you listen to yourself ? You're a jerk, that's the problem.

I thought it was pretty practical. I gave him a part number and
distributor.

When I read the tone of your response I thought must have said
something completely crazy so I went back and checked my numbers. If
the stereo voltage was RMS (the orginal post didn't state) then still
you're talking about .5mW vs. .2mW, and I assume a fancy audio panel
such as the Garmin one has some kind of level control to adjust the
user preference on music vs comms volume. You probably want the comm
radio to be louder than the music for safety sake.

The transformer I'd suggested is less than a half inch on a side, and
is +-2dB (pretty good for a headset in a GA airplane) from 300Hz out to
100KHz. The loss on those is really pretty good, can't be much more
than a dB.

Tacking together EF amps may be easy for someone like yourself (and
will totally work), but the layperson wants something he can take out
of the package and hook up (new RST product?).

wrote in message
oups.com...
I thought it was pretty practical. I gave him a part number and
distributor.

And a $20 price tag? You've gotta be kidding.



When I read the tone of your response I thought must have said
something completely crazy so I went back and checked my numbers. If
the stereo voltage was RMS (the orginal post didn't state) then still
you're talking about .5mW vs. .2mW,

If you go through my explanation, you will see that the most probable case
is that the stereo is peak-peak, not RMS. 4 volts RMS means that they are
operating with less than a half of a volt head-tail room on a 12 volt
battery bus. I really don't think Alpine would do that. Given p-p voltage,
you have a margin deficit of 4 dB.




and I assume a fancy audio panel
such as the Garmin one has some kind of level control to adjust the
user preference on music vs comms volume. You probably want the comm
radio to be louder than the music for safety sake.

I'd bet cash money that it does not.



The transformer I'd suggested is less than a half inch on a side, and
is +-2dB

If you find a transformer with a power gain of 2 dB, you are in line for the
Nobel this year.


(pretty good for a headset in a GA airplane) from 300Hz out to
100KHz. The loss on those is really pretty good, can't be much more
than a dB.

Falling off at 300 Hz. will make that sucker sound like a telephone. This
is a MUSIC application. And $20 for a transformer? Not in my bag of
tricks.



Tacking together EF amps may be easy for someone like yourself (and
will totally work), but the layperson wants something he can take out
of the package and hook up (new RST product?).

In the first place, if I can get a two-week freshman engineering student to
be able to cobble one together in less than an hour, I think I can get
somebody with a little experience to do it in about the same amount of time.
In the second place, every (EVERY)thing that RST makes is in kit form, so
whether the guy buys the parts himself and nails them together or buys the
parts from me in a bag, the assembly process is identical. It isn't in my
vocabulary to put a two cent transistor, a couple of half-cent resistors,
and a couple of nickel capacitors in a bag and then have to charge $7 to
ship the sucker to him.

Jim

RST Engineering wrote:

As to the Dr. Weir, no, I dropped out of the doctoral program when I
realized that what was being taught was roughly five to seven years behind
the stuff that I had already done. That wasn't what I wanted to waste my
time on ... and my own company wasn't going to pay me one cent more for a
doctorate. I'm the "dumb" one of the family; both my brothers have their
PhD.

Dumb isn't lacking a Ph.D., it is having your own company. :-)

Matt

Jim,

I originally posted this, so I'll try to answer some of the
speculation. This is a copy of the Alpine CD spec from their web site -
same as the booklet I got with the radio:

GENERAL
Power Requirement 14.4 V DC
(11-16 V allowable)
Maximum Power Output 60 W =D7 4 (CDA-9815/
CDA-9813)
50W =D7 4 (CDA-9811)
Maximum Pre-Output Voltage 4 V/10 k ohms
Weight CDA-9815/CDA-9813
1=2E7 kg (3 lbs. 12 oz)

So as you can see they don't say wether the 4V is rms or not. The
Garmin unit did, so that's why I quoted the specs as I did.

As far as the circuit goes, I looked on the web for some help and came
across a site that appears to show the circuit you are talking about.
The site is:
http://hyperphysics.phy-astr.gsu.edu.../npncc.html#c3

and shows a diagram of the circuit. Is this the right one? If so, I
still need help (I'm not in the league one of the freshmen students you
refer to - sorry!) in determining the actual value of the resistors and
capacitors and transistor parts needed. I can make my way to Radio
Shack or Frys Electronics to buy the stuff and assemble it, but I just
don't have the background to determine the values. Thanks for your
help!
Brian

Well guys, I started off annoyed that folks were arguing in a harsh
manner. However, after reading all of the posts I realized, you can
learn a lot from two engineers arguing. Keep up the good fight, just
put more smileys in there!

RST Engineering wrote:

jury-rig solution

Okay, this is way off topic but I have to know the history of this
expression. It is my understanding that the correct term is
"Jerry-rigged" or "Jerryrigged" which is based on the pejorative term
"Jerry" for Japanese, used during WWII. The way it was told to me is
that near the end of WWII Jerry (the Japanese) were badly beaten and
much of their equipment was non-operational. However the Japanese
were very resourceful at doing whatever it took to get things working.
So people started talking about "Jerry-rigging" things.

Obviously the term "Jerry" is racially insensitive which probably led
to the alteration of the term. However, isn't "Jerryrigged" a
complimentary term when you consider it implies resourcefulness?
Lemmonaide from lemmons?

So, when I read Jim's post I did a quick Google search and found a
different explanation:

Most sources claim that the origin of the word jerryrig is unknown,
but William and Mary Morris, in Dictionary of Word and Phrase Origins
(see my bibliography), indicate that the term is likely a corrupted
form of juryrig, which referred to temporary rigging on a ship. Jury
as used in juryrig (which arose in the 17th century) likely comes from
Old French ajurie `help, relief.' The 'temporarily repair' sense
remained with the word juryrig, while its nautical roots faded away.
The vulgar expression to which you refer is likely patterned after
juryrig and jerryrig.

To say that something is "jerryrigged" is to mix idioms a bit,
because the proper term is "jerrybuilt." A "jerrybuilder," a term
dating to 19th-century England, was originally a house builder who
constructed flimsy homes from inferior materials. The "jerry" in the
term may have been a real person known for the practice, or may be a
mangled form of "jury," as in "jury-rigged." I tend to think that
"jerrybuilt" arose separately from "jury-rig" simply because their
senses are slightly different. Something that is "jury-rigged" is
concocted on the spur of the moment to meet an emergency, but
something "jerrybuilt" is deliberately constructed of inferior
materials to turn a quick buck.


So, what do folks out there think?

Carl.

PS- Isn't it strange what catches your interest sometimes?