Hi i built a very simple audio amp-circuit for my little speaker.

i built it from another schematic i found.

but there is a problem of the sound quality of it, actually if i connect it to my smartphone and listen there is noise/distortion in the background of the music.
and if i connect it to my PC the sound quality is very good which i am satisfied with. i've recorded samples of it if you want to listen. https://drive.google.com/file/d/1E6YHDswBZBKyTp1XUo60Dz8H64Rfo84r/view
https://drive.google.com/file/d/1ZOXj2FkDy244TNFrhLZe43cbzIwtKrac/view

my questions are:
how could i reduce the noise if i plug in the smartphone ? some low-pass filter ?

in the idea-schematic why is there a PNP-BJT ? this circuit works with and without it.

is R1 really necessary ?

 

Did you try turning down the volume on the iPhone? It sounds like the level is just too high.

Bob

 

The circuit has several issues. Unlike a true speaker amp like the LM386, the opamp is not designed for a single supply *and* a ground-referenced input.

There is no coupling capacitor on the input, so whatever residual DC is on the signal source is amplified by the opamp.

When running on 5 V, the highest output voltage is approx. 3.8 V, minus the Vbe drop of the transistor, so around 3.2 V.

There is no coupling capacitor to the speaker, so there would be 1.6 Vdc across the speaker and 200 mAdc current through the speaker at all times if the output were properly centered, which it is not. That is 0.32 W, enough to severely limit the cone's travel in one direction, leading to an overdriven, clipped sound.

Unless you really want to improve this circuit, I suggest starting over with an LM386 circuit. It handles all of the standard issues with IC's and speakers and is a good learning platform.

ak

 

Hi i built a very simple audio amp-circuit for my little speaker.
in the idea-schematic why is there a PNP-BJT ? this circuit works with and without it. Is R1 really necessary ?

The transistors are in the circuit to drive the speaker. I breadboarded the circuit using the LM358 and without a negative supply on pin4 you are only amplifying the positive side of the waveform. R1 in this case is not needed. Go with the LM386 as AK suggested.

EDIT: The PNP transistor is used to drive the speaker with the negative half of the signal, but since you don't have a negative supply it's not used in this case. BTW the second circuit shown is no better and the pinout is incorrect. The output of the TLC272 should be pin1 not pin6 and + supply is pin8 not pin7.
SG

 

Actually, both circuits can be salvaged by referring the inpits to something other than ground. This is done by creating a "virtual ground" that has a low impedance and a non-zero DC value. The DC value should equal the midpoint of the output voltage swing range (not just Vcc/2). The audio quality still will be poor, but better.

ak