another FM transmitter project

[Willen]

I found little difference between his schematis and PCB layout in pre-emphasis. Shematic has 1nf capacitor paralled with 47K but PCB layout has 1nf capacitor paralled with 4.7K. I cannot understand this parallel system and pre-emphasis, please.

 

[audioguru]

When a resistor has a parallel capacitor in an audio path then the resistor is an attenuator for low frequencies and the capacitor boosts high frequencies.

Where is the link for the new stereo encoder you are talking about today?
It might have an error on the pcb layout:

 

[Willen]

When a resistor has a parallel capacitor in an audio path then the resistor is an attenuator for low frequencies and the capacitor boosts high frequencies.

Where is the link for the new stereo encoder you are talking about today?
It might have an error on the pcb layout:

It is not different encoder. It is Harry's own PCB layout but valus has been modified by me as you said before.
Ok I want to show you another schematic of biasing. Is it correct or not? I think his PCB layout has this type of biasing wires. If it is also incorrect, then I will say Harry's PCB layout has a problem and I will post his original PCB layout to show you.

 

[audioguru]

Your new schematic is biased like any other opamp circuit.
Today's schematic provides pretty good pre-emphasis for Europe and Australia. I don't know how much pre-emphasis is used in your country. Its gain is 0.9 at low frequencies (47k/(47k + 4.7k) and has a maximum high frequency boost of only boost of +20dB at high frequencies.

The audio source must have a very low output impedance.

 

[Willen]

-Ok then all errors has been improved!!! There was my fault, not in Harry's PCB. I have modified pre-emphasis as u said and I will add TL072 as additive mixer later (look at 1st attachement). My country is newly introduced with FM so I think here is used pre-emphasis like Europe and Australia. Thank you!


-I understood that 'connecting 1nf capacitor paralled with 47K resistor in audio line is a pre-emphasis' for Europe and Australia. Am I right? Look at 2nd attachment, is it pretty good too? Can I connect this simple pre-emphasis to all general FM oscillator? (in 2nd attachment:- I connected this simple pre-emphasis to your oscillator)

 

[audioguru]

-I understood that 'connecting 1nf capacitor paralled with 47K resistor in audio line is a pre-emphasis' for Europe and Australia. Am I right? Look at 2nd attachment, is it pretty good too? Can I connect this simple pre-emphasis to all general FM oscillator? (in 2nd attachment:- I connected this simple pre-emphasis to your oscillator)

The 1nF capacitor with 47k resistor is supposed to feed the extremely low input resistance of an inverting opamp. But the transistor oscillator circuit has an input impedance of 22.9k so there will not be enough pre-emphasis.

See my attachements for explanations.

 

[Willen]

Hi, audioguru,
Are there any detail described (article) file of Mod-4 in PDF or Doc with you? Or any web page?

 

[audioguru]

Hi, audioguru,
Are there any detail described (article) file of Mod-4 in PDF or Doc with you? Or any web page?

No.
I did not design it to be a project.

Somebody posted a simple FM transmitter that did not work:

1) Its preamp transistor was not biased properly so I fixed it as Mod1.
2) Its radio frequency changed when the battery voltage ran down so I added a low dropout 5V regulator as Mod2.
3) Its radio frequency also changed when something moved toward or away from its antenna so I added the RF amplifier output transistor to isolate the tuned circuit from the antenna as Mod3.
4) It sounded muffled so I added pre-emphasis as Mod4.

Since my FM band is full then I tested it on the same frequency used by a weak foreign language station on the other side of my city so I was causing illegal interference. I tested its range for only 1 hour and never again.

 

[Willen]

I like to listen everything form experienced like you so. Hope you also found some amazing circuits or facts during your past days,
because it is so interesting field!

 

[Willen]

Can your Mod-4 able to drive Class C amplifiers? You said once- Mod 4 has high impedance, so which type of Class C biasing is perfect (in figure)? Few trials are here...

 

[audioguru]

Can your Mod-4 able to drive Class C amplifiers? You said once- Mod 4 has high impedance, so which type of Class C biasing is perfect (in figure)? Few trials are here...

My Mod4 FM transmitter has an output power of only 23mW into a 75 ohm antenna.
The 2N2219 class-C amplifier has a power gain of about 10 times so the output power is about 230mW.

Since the output of the Mod4 transmitter is tuned then the simple A amplifier circuit should work fine. The B amplifier has additional tuning at its input that is not needed.
You can try increasing the value of the 22pf coupling capacitor feeding the A amplifier to see if the power increases.

 

[Willen]

* In pre-emphasis, if I use 10uF instead of 330nF, does it harmful for high frequency? Is it critical value in pre-emphasis?
* There are 160k and 30k at base biasing, is it critical too in their values? Can I bias in different way like 150k and 27k? (I think it is related to base voltage)

 

[audioguru]

* In pre-emphasis, if I use 10uF instead of 330nF, does it harmful for high frequency? Is it critical value in pre-emphasis?
* There are 160k and 30k at base biasing, is it critical too in their values? Can I bias in different way like 150k and 27k? (I think it is related to base voltage)

The 330nf coupling capacitors in my FM transmitter pass ALL audio frequencies the same. The 100nF or 150nF capacitor C4 that bypasses the emitter resistor is used to boost high audio frequencies for pre-emphasis.

I simulated the preamp transistor using 160k and 30k then using 150k and 27k resistors and the biasing changes only a small un-noticeable amount.

 

[Willen]

Do you mean that 330nf and 10uF capacitor will perform the same and can be replace to each another in this pre-emphasis? (I think Yes, because I have seen this 10uF coupling capacitor on Harry Lythall's pre-emphasis too.)

 

[audioguru]

The 330nF input coupling capacitor and the 330nF output coupling capacitor on my FM transmitter's preamp have nothing to do with pre-emphasis, they simply pass all audio frequencies at the same level. If you replace them with 10uF then extremely low frequencies (rumble from traffic, jet airplanes or wind) will also be passed at the same level as audio frequencies and the 10uF capacitors will take a long time to charge when the power supply is turned on.

C4 on my FM transmitter provides pre-emphasis.
I do not not see a 10uF coupling capacitor on any of Harry's transmitters and it will not provide pre-emphasis anyway. Please post his schematic.

 

[Willen]

It was harry lythall's 222 stereo coder about which i asked to you before.

 

[audioguru]

Do you mean that 330nf and 10uF capacitor will perform the same and can be replace to each another in this pre-emphasis? (I think Yes, because I have seen this 10uF coupling capacitor on Harry Lythall's pre-emphasis too.) It was harry lythall's 222 stereo coder.

Harry's stereo encoder input stage has a 10uF input coupling capacitor feeding 4.7k plus 47k at low frequencies so its cutoff frequency is only 0.3Hz which is much too low. Use a 150nF coupling capacitor for a cutoff frequency of 21Hz or 100nF for a cutoff frequency of 31Hz.

The 330nF input coupling capacitor in my FM transmitter preamp produces a cutoff frequency of 21Hz.

EDIT: Harry's transmitter #7 or #7b has a frequency response up to 76kHz which is good for his stereo encoder.

 

[Willen]

ok if i add 100nf coupling capacitor and opamp additive mixer on Harry's encoder according to your, suggestion then can i connect it to general fm transmitter like mod 4?

 

[audioguru]

ok if i add 100nf coupling capacitor and opamp additive mixer on Harry's encoder according to your, suggestion then can i connect it to general fm transmitter like mod 4?

Yes, but then the MOD4 transmitter doers not need its preamp transistor that has too much gain and it has unwanted pre-emphasis (the stereo encoder should have two input channels with pre-emphasis).

 

[Willen]

Unfortunately, I have only one 330nf capacitor. Then which is the best use of it in your pre-emphasis:- 330nf as an input coupling? Or 330nf as an output coupling? I am using another coupling capacitor is 1uf or 4.7uf. (I think- using 330nf as an 'Input Coupling' is correct because you said- It has 21 Hz cut-off in Input.) Anything wrong?