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ultra fast and easy (hopefully) schematic question

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spitso

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Hi all

i have made the below schemtic which is an fm transmitter but instead of signal coming from microphone i want to use an audio source such as an ipod.
How would i conect it? I tried feeding it directly from ipod straight to the input source but ive got a feeling that the circuit was pushing into my ipod from the PCB?
 

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MikeMl

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Remove R6; just use the coupling capacitor.
 

audioguru

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The extremely simple poor quality circuit is drawn upside-down.
1) Its frequency changes when anything moves toward its antenna or moves away because the antenna is connected to its tuned circuit.
2) The frequency changes as the battery voltage runs down because it does not have a voltage regulator.
3) On an FM radio it will sound like your stereo with its treble tone control turned all the way down, because it does not have pre-emphasis (treble frequencies boost) like FM radio stations have. All FM radios have de-emphasis (treble frequencies cut) then hiss is reduced but audio sounds fine.
4) Its range is not far. Maybe across the street.
5) It is mono, not stereo.

There is a Micromitter kit in Australia that is a modern FM stereo transmitter without any of these problems.
 

spitso

Member
wow this circuit seems like it has some serious problems.
Has anyone got any schemtics of a good fm transmitter?
 

Hero999

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audioguru

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On one of these forums a few years ago, somebody complained that an FM transmitter circuit similar to the one you found did not work. It had the same problems as the one you found plus its audio preamp transistor was incorrectly biased so did not work (it was saturated) when the battery was new at 9.4V and also failed (it was cutoff) when the battery voltage dropped to 7V after being used a little.

I built the faulty circuit and confirmed the bias problem and improved the circuit with added modifications:
1) Adding an RF amplifier at the output so that things moving near the antenna did not affect the frequency and also increased the range.
2) Added a 5V low-dropout voltage regulator so that the frequency did not change as the battery voltage dropped, and so the preamp transistor kept its biasing.
3) Added pre-emphasis (treble frequencies boost) like all FM radio stations have so that the signal sounds perfect.

I built my FM transmitter compactly on Veroboard (stripboard). I tested it around my neighbourhood for only one hour because it was powerful enough to cause interference on the frequency it shared with a low power foreign language FM radio station on the other side of my city because my FM dial is full of stations.

It is mono, not stereo. If I want a stereo FM transmitter then i would make The Micromitter project that was shown in Silicon Chip magazine or build its kit.

Its range is more than 2km across a large river valley to my very sensitive home stereo and car radio. The radio station sharing its frequency reduced its range a little.
Its range is about 400m to my cheap Sony Walkman stereo radio.
Its range is across the street to a cheap FM scanning radio from The Dollar Store.
Houses and hills reduced its range.
 

spitso

Member
audioguru your circuit sounds great. Do you mind if you share a schematic.
I'm building a 120w amplifier with radio and radio transmitter for a personal project. and your schemtic sounds perfect.
 

spitso

Member
also hero999 do you have any specs on that schematic.
how far it can travel.
the two variable capacitors is for stereo transmitting correct?
how could i incorperate an audio input from an ipod,
thank-you
 

Hero999

Banned
I posted a link to audioguru's transmitter in my previous post.

EDIT:
C5 adjusts the oscillator frequency.

C13 adjusts the resonance in the class C amplifier's tank circuit, just adjust it for optimal performance.
 
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audioguru

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I have never used a "stereo microphone". My FM transmitter uses a mono microphone but an attenuator/mixer at its input will allow a stereo line-level signal instead.
 

Hero999

Banned
By the way audioguru, do you have a picture of the assembled board in PNG or JPG format?

I could only find it in GIF format which is 256 colours and poor quality for photographs.
 
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