Who call tell me the theory of this two circuits?

[Junker_eu]

Hi everybody,

Who can tell me the theory of the attached circuit schematic? Thanks!

 

[duffy]

First one's an integrator, works like a low-pass filter, but one of the inputs is connected to Vcc, which is a mistake. The second one makes even less sense, that Audio_in signal should be killed off at the +2V junction and never make it to the transistor.

 

[Junker_eu]

Attached is the whole circuit schematic.Please comment what's the using of LM324 and the transistor BC856.

 

[dougy83]

The first circuit is a low pass filter, -3dB point @ 24kHz. The second looks like an unmute type circuit - apply a low voltage to the base of the transistor and the current passed increases, thus increasing the volume. The full schematic you posted was pretty much unreadable. It looks like it just buffers the left & right channels, with weak mute function and a tone injection into both ears (e.g. for beep indications)

 

[audioguru]

The schematic is unreadable because it was saved as a fuzzy JPG file type instead of as a very clear GIF or PNG file type.

 

[Junker_eu]

This time should be readabled.Please refer to attached picture.

What did the "-3dB point @ 24kHz" mean?And how to calculate it?

 

[dougy83]

The output from the buffer/gain-control section feeds into a headphone amp (LM4881). I might guess the circuit's allows the audio level to be reduced while it plays tones - e.g. to notify you that the phone's ringing while you're listening to music.

The -3dB point refers to where a filter's cut off frequency. It's calculated for that little snippet of circuit from the value of the resistor and capacitor in the negative feedback path. the 24kHz is calcuated using Fc = 1/(2*pi*R*C). google filters for more info

 

[audioguru]

I see a lousy old LM324 quad opamp. It has 3% of crossover distortion and poor high frequency response.