Need help explaining an audio line-in circuit

[tomo]

Hi,

I am trying to add an additional audio input to my old Sony HCD-CPX1 stereo by re-purpose the tape deck. After looking at the service manual/schematics and the data sheet for the input selector chip I figured out that I would only have to copy the circuit of the already existing line-in input and wire it into the tape deck input.

But before I risk blowing up the audio selector chip I would really like to understand what the input circuit is actually doing (marked in red in the attachment). It would be great if somebody could point me in the right direction as I only have some basic electronics knowledge. I'm guessing it's input attenuation + some kind of filter. I am also wondering if I have to disconnect the tape deck audio lines before wiring in the new input.

Thanks

[Paul Price]

You would be better off using a CMOS analog switch to multiplex the existing input.
CD4066BE is a good choice.

Or use a dual op-amp to mix the two signals, if this is acceptable, assumes one input is inactive.

The circuit in red is simply a resistive divider, an input protection/attenuator with small caps to attenuate/filter out unwanted higher than audio frequencies and avoid the possibility of a high voltage transient applied to the inputs would damage the chip this circuit is feeding.

[Balaur]

Yes, it should be safe if you just duplicate the MD/video input circuitry and use that to drive the Tape TCLCH//TCRCH inputs on the selector chip.

However, after looking at the attached schematic and after browsing a bit through the service manual, I wonder if there is a mute control for the tape input.
I think that you should check this before doing any modifications. Try to inject a signal on the tape channels and see whether the signal is amplified when the tape isn't playing.