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Transmitting audio over FM - help needed

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boylesg

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If I have this per-amplifier as a kit from Jaycar and I wanted to transmit the audio via FM to a standard radio then which of the the attached FM transmitter schematics would work best?

101590_3lo.jpg


I was thinking I could modify the FM microphone (first attachment) schematic and use the output stage only, swapping the BC548 for a BC337 to generate a bit more power.

But I am not so sure how to bias the BC337 to properly interface with the output of the of the per-amplifier with it.

Or alternatively just build the FM stereo transmitter (second attachment) and connect my per-amplifier directly to its input.

Which is likely to work best?


This is to solve a specific problem by the way.
 

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  • FMStereoTransmitter.jpg
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  • FMMicrophone.jpg
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FvM

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The post leaves me helpless.

No FM transmitter in the first schematic. Rohm BA1404 might be O.K., but the chip is obsolete, I think.
 

boylesg

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The post leaves me helpless.

No FM transmitter in the first schematic. Rohm BA1404 might be O.K., but the chip is obsolete, I think.

Actually I since found this in Short Circuits 2. Ready made design for purpose.
Basically I want to use it as a sort of intercom for my Dad who has Parkinsons and has difficulty making himself heard from another room.
 

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BigBoss

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Actually I since found this in Short Circuits 2. Ready made design for purpose.
Basically I want to use it as a sort of intercom for my Dad who has Parkinsons and has difficulty making himself heard from another room.

If you're able to write a bit code, I recommend this IC.
https://www.silabs.com/products/audio-and-radio/fm-radios/si4710-11-radio-transmitters
Otherwise, a simple VCO that works around 100MHz with a simple AF amplifier ( LM358 ) will work well.
 

boylesg

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Audioguru

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It looks like cheap junk at a high price. It might have (or has) these problems:
1) Does its radio frequency change if something moves towards or away from its antenna?
2) Does its radio frequency change and does the transmitted audio become faint or distorted when its battery runs down?
3) Does it have pre-emphasis to sound clear and if it has, which type of pre-emphasis is used in Israel?

Its circuit is obsolete. Years ago Rohm made an FM transmitter IC with 4 radio frequencies that are accurate and are selected with a switch. This junk transmitter has a trimpot to set its radio frequency with difficulty. Does its radio frequency change due to your hand being near it when setting it?
 

Aussie Susan

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The mention of Jaycar makes me think that you are in Australian (or New Zealand). These comments will probably apply to most other countries as well.
The kit as sold will be designed to use a very low power because of the possibility of interference with other radios. If you try to boost the power output then the risk of interference gets a whole lot worse.
If a neighbour picks up the signal then they can report you to ACMA who have every right to prosecute you for illegal transmissions.
If you are just trying to make a 'sort of intercom' for your father, then Jaycar have units that you can use for this that do not run this risk (and use either a wire of the mains to connect them). These have the advantage that you can talk back as well!
Susan
 

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