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FM Modulator

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adamthole

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I am wanting to get high quality sound from an aux input in my truck without buying a new radio. (My radio has no aux input). I used the ipod iTrip for a while, but that really sucks in sound quality. While looking at XM, I found a thing called an FM modulator. I think it hooks directly to the antenna or something? The one I say cost about $30.00. Is it possible to make one? Does anyone know anything else about them? https://www.amazon.com/exec/obidos/...81692-8120957?v=glance&s=electronics&n=172282
 
Yes, you could make one but at $30 I'd recommend purchasing one if your goal is just to have the modulator. If learning is what you want then by all means - do it. Lots of good circuits out there. FM transmitter would be a place to begin. Just be aware that simple is just that - and the simpler ones aren't stable, sound isn't good, etc. I do know folks who have purchased units such as you describe and they are happy with them. Not perfect but very good sound. They run the MP3 player music thru to FM radio in rental cars.

As a different twist you might look to Ramsey or other kit suppliers. Some have high quality, stable transmitters - for not a lot more than $30. You can learn a lot, there is less struggle, etc. Just a thought. As far as I know, most or all are compliant with regulations - low power, etc.
 
Most newer FM modulators I've seen use the Stereo Micromitter (search on Google) project design. The nicely finished product at full list price, about $30.00, costs about half the cost of its parts and about 1/4 the cost of the kit. :lol:
 
okay, thanks. I'll buy one for now, and perhaps build one later. I searched google, but couldn't find any plans for the micromitter, only kits. Know of anywhere the plans are?

Also, is an "FM Modulator" just an FM transmitter with the antenna of the antenna hooked to the antenna of the receiver?
 
If you search Google for Stereo Micromitter, the first link is to Silicon Chip magazine's project with schematic, parts list, pcb and tips on how to make one. Kits for it are also available. It has an attenuator on the output to keep it within Australia's RF power limit which is quite low. The $30.00 "FM Modulator" transmitters you can buy also have an attenuator since they are used for short range between an MP3 player and a nearby car radio. You select their crystal controlled FM frequency with a 4 or 8 position DIP switch.

You don't join the antenna of a transmitter to the antenna of a receiver, the transmitter transmits through the air like a small radio station. Without the attenuator, its range is pretty far. :lol:
 
The kit sais it has a range of about 30 meters, with the built in attenuator. If I did not install the resistors in the attenuator, would it still work porperly? Any estimates on the range I would be able to get, or the W it would produce? anyone know the legal limit in U.S.?

Also, I am still confused on the difference between an FM modulator and an FM transmitter. Is their a difference?

Any good ideas on where to buy those parts? I checked all of my normal places (Mouser, Digikey, Jameco) for the IC and only one of them had it, with a minimum order of 2,000 of them.
 
An FM modulator is a low power transmitter that radio station techies call "an exciter" that is followed by an RF amplifier.

The manufacturer of the FM Stereo Micromitter transmitter IC and Silicon Chip don't say what is the output power without an attenuator on its output. Judging its power supply voltage and current, it isn't any more than good (like mine) simple ones. It shouldn't cause a visit from the RF cops unless it is transmitting on the same frequency as a broadcast station causing interference, or its harmonics are causing TV interference and a complaint to the RF cops. It is illegal only if you get caught!

One guy built my mod4 FM transmitter and reported seeing the RF cops in their truck with antennae on top slowly going down his street the day after he tried it. I told him that they were looking for me. :lol:
 
So if I am looking for the best quality in my truck, would I be better off going with your mod4 design, or the micromitter design?

How close is the quality to a direct connection?

What is the punishment by RF cops?
 
adamthole said:
So if I am looking for the best quality in my truck, would I be better off going with your mod4 design, or the micromitter design?

The Micromitter is stereo, mod4 is only mono (as are most such simple transmitters) - so there's not much choice really?.
 
Hi Nigel,
I was in a truck once, a long, long time ago. It didn't have stereo.
Its mono AM radio sounded awful! He, he. :lol: :lol:

My FM transmitter isn't crystal controlled so its tuned frequency drifts a little. The crystal controlled tuning in the Stereo Micromitter probably has better frequency stability than Adam's truck radio.

For those of us who can still hear well, the audio pre-emphasis response on my Mod4 FM transmitter is flat to 15kHz after de-emphasis by a radio, while the Stereo Micromitter's response begins dropping above 6kHz and is down 5 dB at 13kHz and much more loss at 15kHz. :lol:
 

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