Technical Topics > The RF Workbench

Symptoms of an oscillating amplifier?

<< < (3/4) > >>

Antennae:
So it probably would have worked better!  Now, if I hadn't already pilfered parts from it...
 I'll amend the hfublog post when the blog comes back up.

ka1iic:
The audio problem is most likely from an impedance mis-match.

One must take into consideration that these chips are designed to operate with maximum power into a 4 to 8 ohm load and I must assume that the oscillator is showing a much higher impedance (27-30 ohms) than what the chip is designed for therefore a larger chip would be called for.

The higher impedance of the oscillator in reference to the audio chip, would cause the audio chip to produce much less current (watts)because of the load of the oscillator.

I just looked at the circuit again and... my gut tells me to place a very large value capacitor (250 uf or greater) in the line between the audio chip and the oscillator and put a resistor of say 16 ohms from the audio chips output to ground.  The Capacitor will pass the audio but will keep any DC current from entering the audio chip.  The chip might (I say might) not like having DC entering the output of the audio chip.

If I were to try this circuit I would use a 380 chip and not the 386 as shown.

Please take anything I say with a grain of salt... <heh> I just had my 3rd spinal operation and "they" have me all "doped up"... <heh>  ::)

p.s.  Syringomylia just plan sux... :'(

Antennae:
Sorry to hear about syringomylia, I just looked it up. A neighbor walks around with a walker because he had a tumor on his spinal cord and it slowly destroyed his leg control before they found it.

I have lots of questions:
How did you get the 27-30ohm range of the oscillator circuit? Is this about the same value for all circuits like this?

And how did you come up with the 16ohm resistor for the audio chip's output to ground?

So that its clear: Would I put the 16ohm resistor on the LM386 number 2 pin?

THANKS! The fires are kindled to make this thing work more better. 

ff:
All of what Vince said is true, Antennae.  If you are delving into this far enough to be concerned about impedance matching and fidelity, you should be forewarned that modulating an oscillator is very poor methodology and you probably will never be satisfied with the results.  MakeRF's circuit is more to demonstrate the principles involved, and to give the newbie a taste of generating a modulated signal.  If you are reaching for more than that, you should build a more ambitious design, like a Corsair:

 http://freeradiotx.blogspot.co.uk/2009/06/corsair-transmitter-by-dave-martin.html

Antennae:
Thank you ff, if I go for something more powerful I'll consider the Corsair. But I think it is now a personal vendetta against this wee little punk of a transmitter I have.   It took me a lot of time and effort to make this and it let me down (won't hardly transmit sound across the street).  I want to twist its nose to make it louder so to speak.

Navigation

[0] Message Index

[#] Next page

[*] Previous page