another FM transmitter project

[Willen]

I think you should build it first wii..it doesn't have pre emp but it sounds clear on a good radio..sounds better than an am radio..good luck wii

OK will try.

Few month ago, I buit the Tx of audioguru and got very clear sound which amazes me! A technician friend of local community FM station also got amazed! He stopped playing song through FM tx and listened the empty signal on receiver, it was really empty. There was no hiss and other line noise just like on professional FM station!

I made other few Tx without pre-emphasis, sound was little clear but got lots of noises as a background! Mod 4 has high frequency boosted so it makes audio as a crystal clear (I got)

 

[audioguru]

How can an FM transmitter that is missing pre-emphasis sound clear?
The de-emphasis in all FM radios will reduce audio high frequencies the same as your stereo with its treble tone control turned all the way down! Very muffled.
A deaf old person might think it sounds clear.

50us de-emphasis used in most of the world cuts high audio frequencies above 3.18kHz (-3dB). Then 6.4kHz is reduced 6dB (half) and 12.8kHz is reduced 12dB (1/4).
15kHz is reduced a little more.
In North America the 75us de-emphasis cuts high audio frequencies above 2.12kHz (-3dB). Then 4.2kHz is reduced 6dB (half) and 8.5kHz is reduced 12dB (1/4). 15kHz is reduced almost 18dB (1/8).

It is the de-emphasis in the radio that cuts hiss. It also cuts the boosted high frequencies from the pre-emphasis in the transmitter down to normal.

 

[Willen]

Why we get big noise if a microphone listened its own amplified audio? What is the electronics truth? (Mic> Tx> Rx> Speaker> audio> mic= Noise)

 

[audioguru]

Why we get big noise if a microphone listened its own amplified audio? What is the electronics truth? (Mic> Tx> Rx> Speaker> audio> mic= Noise)

It is called acoustical feedback howling (positive feedback).
It happens whenever the microphone can hear its sound come from a nearby speaker. The sound goes around and around.
1) If you have a directional mic (but most electret mics are not directional because they pickup sounds all around) and a directional speaker then point them away from each other.
2) Put the mic in one room and the speaker in another room with a wall or closed door between.
3) Turn down the gain.
4) Record the mic then play it back when the mic is turned off.

 

[Willen]

Few months ago you show me preamp stage bias with pre-emphasis of Mod 4 using BC547. I am searching this post but can't find the page, and thread! It is valuable for me because 2n3904 is not available from the market. First time i bought 3 piece but now can't. Can't I make whole Mod 4 using BC547 with same quality as 3904? Because I can find lots of 547!

 

[audioguru]

A BC547B is almost the same as a 2N3904 except its pins are different.
A BC547 without a letter has a very wide range of hFE so some will be cutoff and others will be saturated.

 

[audioguru]

A BC547B is almost the same as a 2N3904 except its pins are different.
A BC547 without a letter has a very wide range of hFE so some will be cutoff and others will be saturated.

EDIT: Added the schematic

 

[Willen]

You were posted a schematic of preamp using 547.

 

[ikelectro]

This is my FM transmitter circuit. I built it on stripboard and I cut the strips with a drill bit. The strips form half of a pcb.

I'll try this in a few days!!!!!

I have began to read this thread 2 hrs ago and still reading because lots of thing are to discover for me from this thread.......

 

[Willen]

I'll try this in a few days!!!!!

I have began to read this thread 2 hrs ago and still reading because lots of thing are to discover for me from this thread.......

The thread has been as a entire website!

 

[Willen]

It was NOT designed properly. The designer tried it with only one transistor (it had low hFE) then maybe it worked. I simulated it with a "typical" transistor that has "typical" hFE and it distorts very badly because it is almost always saturated. If the hFE is even higher then it is worse.

I biased it correctly but it still distorts if the hFE is lower or higher than "typical". The pre-emphasis will work fine. I forgot to add the 100pf but it is needed in a radio transmitter.

I found this post by searching 1 hrs to see Preamp biasing on 547 by you But what happened to this website, attached image is unavailable!

I wanted to see same type peramp (with pre-emphasis) using 547 and 495 transistor with same quality as your

 

[audioguru]

I used a 2N3904 because its DC current gain (hFE) is a narrow range from 100 to 300. A BC547 has a very wide range from 110 to 800 so many of them will be saturated in my circuit and will not work. A BC547A transistor has a range of current gain from 110 to 220 so it will work in my circuit.

 

[Willen]

Radio stations compress the audio so that its average level is louder. The compression boosts sound levels that are not loud and reduces sound levels that are loud. Frequently you can hear the compressor "pumping" the level of background music with the beat of the music.

I think it is called an Audio Processor. I heard it very expensive device used for professional FM transmission. Then what is audio synthesizer? Why audio processor is so expensive? I can buy almost 8 desktop computers from the money of single audio processor (I heard)

 

[audioguru]

Radio station equipment is very expensive because it is not mass produced for consumers.

There was a TV show that talked about hearing aids:
1) The cost for a manufacturer to make a modern programmable one is about $60.00.
2) It is sold to the audio doctor for about $300.00.
3) It is sold to the hearing impaired patient for about $4000.00. The audio doctor might program it a couple of times.

 

[Willen]

Yes! Good example! I also found very expensive Hearing of almost 60 doller. When I opened the device i found a basic amplifer circuit made by 6 transistor BC547, a Mic and few capacitors and resistors; to which I can make with only ONE doller. I can buy 6 BC547 at just 0.25 doller.

 

[Willen]

I did a silly modification on oscillator schematic of your. I modified for tunning using potentiometer instead of trimmer. Will it work?

 

[audioguru]

A hearing aid is much more than just an amplifier.
It usually has a directional mic or a few mics, equalization so that high frequencies are amplified more than low frequencies, compression so that low levels have high gain and high levels have less gain and are fitted to the ear canal to avoid acoustical feedback.

The circuit uses low voltage Cmos opamps and digital processor that use very low current from a tiny 1.4V battery. New zinc-air tiny batteries work with oxygen from the air.

 

[audioguru]

I did a silly modification on oscillator schematic of your. I modified for tunning using potentiometer instead of trimmer. Will it work?

Your circuit will work fine because the varactor diode is effectively parallel with the capacitor in the tuned LC circuit.
But the frequency will change as the battery voltage runs down.

An ordinary silicon diode or rectifier or the collector-base diode of a transistor are also varactor diodes but with a small range of capacitance.

 

[Willen]

hahaha amazingly you find the shematic from where I had stolen the idea, the 2nd attachment . OK will try this.

What, any silicon diode like 1N4007 also have capacitance in reverse?? Will work instead of varactor but with small rang of capacitance?

And look at the 1st picture of oscillator, there is no capacitor parallel with inductor, is it complete?

 

[audioguru]

look at the 1st picture of oscillator, there is no capacitor parallel with inductor, is it complete?

The capacitance of the varactor diode is effectively parallel to the inductor.

Because C4 connects the positive supply to ground at radio frequencies.