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filters

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mstechca

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Filters seem to screw audio up when used improperly.

AFAIK, in a standard NPN common emitter amplifier, a filter can be obtained by connecting a capacitor between collector and ground (correct me if I am wrong).

So in my radio, I have three of them, one which is in my push-pull amplifier. When I change one value by a small amount, the whole radio goes wacko. I change it to another, it goes special.

The IF frequency for most commercially made FM radios is 10.7Khz isnt it?

if it is, I think I turned my d**n receiver into a jackpot!

LMAO :lol:
 
Mstechca,
The IF frequency for FM radios is 10.7MHz, not kHz. It is far above the audio band so that the IF inductors and tuning capacitors can be small and have a high Q (low resistance).

I suspect that changing the value of the cap in the audio lowpass filter in your audio amp is affecting the radio's quench oscillator, possibly due to feedback through the supply.
 
Does Q = 1 / XL (impedance of an inductor)?

Correct me if I am wrong, but I think what you are trying to say is that I have some frequency going on in the superregen oscillator and the filter frequency is too low for it.
 
Mstechca,
No, the Q of an inductor is XL/R. The Q of an unloaded LC tuned circuit is its peak frequency divided by its bandwidth.

The time-constant of the audio lowpass filter in an FM broadcast band radio is supposed to match the pre-emphasis (treble boost) transmitted, for the proper balance of bass and treble sounds.

Since your super-regen tuner is probably powered by a battery that has a high internal impedance, I suspect that its audio amplifier is modulating the supply voltage, causing weird feedback effects to the quench oscillation. Your tuner has only 0.1uF as an RF supply bypass, but it needs an additional 100uF to 1000uF as an audio frequency supply bypass to stop the supply from being modulated.
 
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