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Old 09-09-2005, 20:16   #1
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Lightbulb Mixing Signals to External Speaker

I want to combine the output from two separate devices and input both to one speaker. I have stereo speakers in the cockpit now, and want to hook one of them up to the external speaker jack on my VHF radio (it's about 2" away).

My idea was to use 4 diodes, one on each of the 4 wires coming to my speaker (2 from the VHF and 2 from the CD player). I thought they would work as "check valves" to ensure that no power from the CD player's output would make its way into the speaker jack on the VHF and fry it. A friend of mine who knows a lot more about electronics and me told me that won't work. But he didn't think a very simple circuit with one transistor would work. He said what I needed to find/make was essentially a very small "amplifier" (but with no gain). It would work is a solid-state mixer to combine the signals.

Then if I'm listening to music a radio transmission comes in, I can hear it mixed in over top of the music. If it's important and I want to reply, I'll turn off the CD player. No extra holes required in the fiberglass, and no 3rd speaker to install.

Has anyone done this? Can anyone point me in the direction of a circuit diagram? I don't mind building it, if I have plans.

Thanks for any help,

Craig
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Old 09-09-2005, 21:41   #2
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Your friend is right, diodes won't work for several reasons. My first inclination is to just hook the outputs together in parallel to one speaker. I think it should work fine.

I do remember seeing a device a few years ago that would sense an audio signal on the VHF output and automatically switch it on and mute the stereo output. You might try calling some Ham Radio stores and asking what they have.
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Old 10-09-2005, 00:40   #3
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No the diodes won't work. Nor will hooking the two outputs together. An amplifier has a "dampning factor" in it's design. Without getting to technical, it is the ability to control the speaker cone. This is seen as a very low impedance at the amplifier, so it would represent a short tot he other amplifier. How ever, it is not a physical short, but an eletrical impedance in the output transistors and they just may not like it up em if ya get my drift.
There are some solutions, but nothing I can easily explain here. But as exposure suggested, a Radio shack, tricky dicky place should have something. What you want is an electronic switch that operates when it senses the VHF output and switches the CD off and the VHF on to the speaker.
Oh and a 1:1 amplifier probably won't easily work either as you are dealing with higher signal levels than most of those will handle. It's possible, but maybe not off the shelf.
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Old 10-09-2005, 14:39   #4
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High level mixer

One solution is called a high level mixer. A low level mixer is very common as it accepts multiple low level signals (like from pre-amp outputs, CD outputs to an amplifier, etc.). The low level mixer allows you to vary the relative input levels before combining them into a variable output delivered to a power amplifier.

Because you don't have a low level output from your VHF radio, only a high level amp output, you need a high level mixer. This device is sometimes configured as an "L-pad" or a "T-pad". The L-pad only requires two ganged resistors rated to handle the total VHF amplifier output power. The T-pad is more expensive and difficult to find as it requires three ganged resistors rated to handle the output power of the amp it is connected to. The T-pad has the advantage of allowing impedance matching to both the amp as well as to the speaker. You problably don't care about that refinement as it is not "HI-FI".

So, the easiest solution is two L-pads, one to each source, the outputs are wired together and to a speaker. You have to set each individual level to get the desired mixing of the two sources.
You would probably need 4 Ohm pads rated for about 4 Watts each.
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Old 17-09-2005, 20:57   #5
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Great Advice!

Wow. Thanks guys for all the great advice. I never would have guessed the solution would as complex, nor that others would have it all figured out.

Once I have a fully working system, I'll post the details.

Thanks again.

Craig
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Old 17-09-2005, 22:47   #6
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Something else you might consider. I used to work for a Honda dealership years ago. There were aftermarket radio equipment places where we could order things just like that. (For the Goldwings)

The motorcycles had multiple devices wired to their speakers. Stero, CB, Intercom, etc. and needed impedance matching devices with multiple valume controls. Sorry I wish I could remember the names of the companies that made that. The newer bikes already have all that from the factory. But who knows you still be able to find an old timer in that industry that knows where to get what you need.

Another place to check might be the cell phone stores. They hook cell phone mounts into car speakers all the time. My second thought there would be that the outputs you are working with would have a greater output than what they would have can handle.

Good Luck,
Bill
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Old 17-09-2005, 23:04   #7
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THe Radio Shack suggestio will get you on your way. THey should have a switch setup that can be pugged in cheap and fast. If they, in their move away from real electronics, have stopped carrying it, you local electronics place like Frys or Circuit City will have what you need. Should be about $20.
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Old 18-09-2005, 06:26   #8
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another speaker

This seems to be an exercise in futility. What is wrong with the speaker in the VHF. Why not simply install another speaker if the 2" in the radio is not sufficient?
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Old 18-09-2005, 17:37   #9
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For Daddy's Dream

Wherever did your daughter get the general idea that hot water freezes faster than cold? (I can't wait to hear this one)
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Old 18-09-2005, 19:25   #10
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Dr. Magan, Physics Dept. at West Virginia State College, (University now), explained it to her at one of her summer camps for gifted kids when she was 7 yrs old.

Hot water does freeze faster, but she never asked him if ice water boils faster. I can't explaine it so I won't even try, but it has something to do with hot water being expanded and forming the crust faster than cold water because of the expansion.

She loves stumping her teachers.

The sad part is she's had 5 science teachers and 5 gifted teachers since then and all except one told her they had no idea and didn't know how to find out. The one that did answer told her that cold water does boil faster. Then went on to tell her that the time differance wouldn't make that much differance.
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Old 18-09-2005, 19:53   #11
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Now I understand the context, Daddy's Dream!

Here's the deal, although from a thermodynamic standpoint it takes more energy to freeze hot water than it does cold (ultimately the hot water has to pass through the same temperatrue as does the cold on its way towards the same "end point temperature", the dynamics of freezing can be different.

It is well known that if one carefully controls the vibration of the experiment that water can remain in liquid form a few degrees below the freezing point. If one then merely "taps" the container, thereby causing a pertibation in the liquid, that the shock wave passing through the container causes sites of crystalization which rapidly results in the whole container becoming ice. If the water is impure then freezing commences at the sites of the impurities due to assymetrical pressures (non-uniform) and catalyzes the formation of ice.]

If a container having water several degrees above freezing is rapidly cooled then the liquid will not have time to pass the temperature uniformly throughout the system and the colder spots become sites for the crystalization and, therefore, a container having a warming starting point standing next to one with a colder one might freeze first.

Similarly, if two containers are rapidly warmed (one having colder water than the other) then the dynamics are greater within the colder container than the warmer one and individual sites can be observed to begin boiling in the colder one than the warmer one. Now let us understand that the temperature differences between the two cannot be great else the phenomenon will not occur and the warmer one WILL begin boiling first.

As an example of observing one of these dynamic effects haven't you noticed that a clear container having a surface defect like a ding or casting defect on the bottom will evidence the bubbles of boiling from that site when heating water before many othe sites of bubbling occur?

Rick
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Old 18-09-2005, 20:06   #12
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I heard about it watching "Bill Nye THe Science Guy" on TV with my son. SInce I do not have a lab, I have never had optimal conditions to research the validity of this claim. Bill Nye also says that water will not conduct electricity, only salt water. Never put the multimeter to it, but Iam sure someone knows. Not sure abotu the boiling point.
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Old 18-09-2005, 20:52   #13
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pure water insulation

Pure water is a fairly good insulator yet it is difficult to keep pure. Pure water is used to cool some types of nulcear cores in reactors. The last impurity that cannot be kept out is sodium and it causes the water to glow a beautiful blue right near the core where the radioactivity is the greatest causing escallation of the energies of the outer electrons of the sodium ions in the water releasing light energy as they reduce their energies and "fall" back to lower energy levels.

Pure water was used in some of the old radio transmitters for cooling that I worked on when I was a young man. The high plate voltage used in the final amplifiers (about 10 kV) had to be insulated by the water which was used to cool the anodes of the big tubes. If the water got contaminated by any salts then anyone touching the heat exchangers would be in trouble. As a result there were water purity detectors, fairly simple in that all one had to do was place a high voltage across some of the water and measure the current flow to give indication of the purity.

A DVM will show ordinary good quality filtered tap water to be on the order of 1 million Ohms per inch of probe separation but there is not sufficient compiance voltage in a DVM to properly make a resistivity measurement with pure water.
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Old 18-09-2005, 21:10   #14
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So in other words, only salt water conducts electricity, but almost all water is salt water to some degree. OK
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Old 19-09-2005, 01:08   #15
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Brilliant explanation to the water freezing theory Rick. I have carried out this experiment and to say boiling water freezes faster than cold is actually in the reality of everyday normality, a myth, or, misinterpretation of the facts. Rick has explained it perfectly.
Yes also as Rick said, pure water is non-conductive. We used pure NZ spring water to cool a Radio transmitter dummy load. Now spring water isn't exactly pure, but it worked in this case and we are talking 50Kw transmitter.
It's actually the minerals and impurities that cause the water to contuct.
Ummm, though Rick, isn't the water around the reactor heavey water?? That isn't real "water" as such. I don't think the Reactor ever comes in contact with the actual cooling water which is pumped around the reactor and is what drives the turbines etc.
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