Hi,
Last day I gave a life to a retired cassette player power amplifier. Removed the cassette player mechanism and added 3.5mm input to the audio power amplifier. Now it's compatible to modern players like cellphone.

Without attenuator, amplifier was overloaded because of high audio output of cellphone. With an attenuator resistor (15k and 3.3k) and a 10nF high frequency booster, audio output was awesome from the very old power amplifier.

During experiment I got two condition, see the attached diagram. First did not work (or did I make any mistake?) but 2nd did work. Is there any theory behind it? (The power audio amplifier has a general purposes transistor circuit as preamp at input.)

 

Hi,
Last day I gave a life to a retired cassette player power amplifier. Removed the cassette player mechanism and added 3.5mm input to the audio power amplifier. Now it's compatible to modern players like cellphone.

Without attenuator, amplifier was overloaded because of high audio output of cellphone. With an attenuator resistor (15k and 3.3k) and a 10nF high frequency booster, audio output was awesome from the very old power amplifier.

During experiment I got two condition, see the attached diagram. First did not work (or did I make any mistake?) but 2nd did work. Is there any theory behind it? (The power audio amplifier has a general purposes transistor circuit as preamp at input.)

Because it really depends what is to the right of the image.

I assume the ground of your input is a battery (-). Then imagine if the amplifiers input is biased to a virtual ground at Vcc/2.

On the top option, you are pulling that virtual ground to (-) with a 3300 ohm resistor. In the bottom, the capacitor floats the connection to the virtual ground and everyone is happy.

EDIT, it may not be a. Ritual ground but your 3300 ohm resistor on the bottom circuit is screwing up the bias of the transistor input stage.

 

I did not understand the answer completely. Can I say in this simple way too- The first not working circuit, directly connected to Base of BC547 of audio power amp has a 3.3k resistor directly added to the base to ground which is pulling down the volume. I just saw this.

 

I did not understand the answer completely. Can I say in this simple way too- The first not working circuit, directly connected to Base of BC547 of audio power amp has a 3.3k resistor directly added to the base to ground which is pulling down the volume. I just saw this.

Look below, the transistor bias is screwed up when the resistor is to the right of the input capacitor.

 

Not that since the amplifier is from a tape player, its frequency response is not flat. Not even close to flat. Look into the NAB tape recorder curve to see why the amplifier probably sounds mushy when fed with normal audio instead of the output from a tape head.

ak