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May someone explain the operation of this car amplifier input stage...

Hi everyone, May someone explain for me how the grounding system is working in this car power amplifier. It's a Sony XM-222. The schematic can be found at:

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The components in the red circle is of interest. In car audio systems grounding noise can be a problem, and it seems like manufacturers tend to isolate the input jacks as much as they can with help of differential front ends, or other ways to "not connecting the ground directly". First, explain to me what components C858 / R861 is doing, then look at R860 / C853 (everything in the red circle) and explain why they're there.

The main thick groundline that can be seen in the schematic is the secondary ground in the DC-DC and NOT the battery ground. By the way, there are a similair RC network between battery ground and secondary ground. I guess this also helps minimizing noise.

I appreciate your time,

Regards Stefan

Reply to
Steve
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Stefan, As a rookie EE that went Power my Electronics background is not as strong as some would like. However, my first thought would be a filter (RC filter is Low pass). Meaning that Freq. lower than 153.92 kHz would be allowed to pass though (on C858/R861, assuming that the

220 on the Cap is in Pico's, If Nano, 153.92 Hz). This leads to the problem that the R860/C853 "filter" would pass Freq. of 723.43 Ghz. Which I would feel is highly unlikely to be present in any stage of an amp, let alone the input. This leads me to beleive that this second set's function is in fact not for filtering. Without doing more digging and remembering horrific memories from my classes I cannot help any further. Someone tell me where I am wrong!

KT

Reply to
Kit Talich

"Steve"

** The 220uF cap provides a low AC impedance from the Left input ground to audio ground, the 4.7 k keeps the cap discharged. The polarity of the 220uF cap suggests that it is possible for the audio ground to become 12 volt positive if the battery negative is not connected.
** Everything depends on the electrical characteristics of what you * imagine * is plugged into the L and R jacks and even a few scenarios involving wiring errors and missing grounds on items.

The scheme looks to be basically an anti ground loop induced noise measure PLUS a measure against damage ( ie blown tracks) if battery voltages appear on the jack grounds.

....... Phil

Reply to
Phil Allison

Ah yes, reminds me of my Electronics II class. Take my best guess and it is wrong! haha. Thank goodness for High Power!

KT

Reply to
Kit Talich

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