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I'm trying to extract audio signal from my car stereo and use it as an input to my external amp. The place I want to get it from is marked in red in this picture: Level diagram I used the IC31-34 pin12 to get the signal, but when I connected it to my external amp line in inputs it went to protective mode. I measured the voltage and it was 4V dc. Will it work if I put some resistors to lower the voltage?

My external amp has these pre-in parameters:

Pre-In sensitivity: 0.3 ÷ 5 V RMS
Pre-In impedance: 15 kΩ

Here is the full service manual of the stereo.

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    \$\begingroup\$ Welcome to the site. Why did you pick pin 12? I don't think your amp wants DC. \$\endgroup\$ – mike65535 Oct 10 '18 at 17:04
  • \$\begingroup\$ I think this EQ-FIX is modifying the signal so it sounds good on the factory low quality speakers. I followed the 4 channels outputs from IC22 controller and they all go to pin 12 of IC31-34. \$\endgroup\$ – myexec Oct 10 '18 at 17:21
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    \$\begingroup\$ What is protective mode and which device went into that mode. Your question is unclear. \$\endgroup\$ – Andy aka Oct 10 '18 at 18:25
  • \$\begingroup\$ You probably want at least a series capacitor for coupling the AC audio, and not the DC bias. However that may not be sufficient to do this safely. \$\endgroup\$ – Chris Stratton Oct 10 '18 at 18:40
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You have a 0-14V supply available in automotive systems, there's no negative supply available unless you generate one (some power amps do that with a switching supply), so to deal with bipolar signals the easy thing to do is to have a reference at some mid voltage so that the amps can swing either side of that reference. In this case, IC34 generates a 4V reference that is used by ICs 31-34, so the signal you need is at pin 12, but is relative to this 4V reference. If you follow the lines to and from these amps, they are all decoupled with capacitors, to prevent the DC offset being amplified by following stages, and that's what you also need to add in - plus a resistor at the far side to bias the output around zero volts. Your external amp probably also has some decoupling to allow its input stage to operate offset, but may be sensing that DC voltage causing the shutdown. The IC 31-34 amps do have some RC filtering in their feedback circuits, so the pin 12 signals do represent the least mucked about with signals you are looking for. The datasheet for these amps is at https://www.njr.com/semicon/PDF/NJM2060_E.pdf if you want to work out the schematic.

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  • \$\begingroup\$ So if I understand you right this answer is the solution: electronics.stackexchange.com/a/178938/200858. But I see there is already a capacitor right after the outputs from the microcontroller and still there is 4v DC after that. \$\endgroup\$ – myexec Oct 10 '18 at 19:01
  • \$\begingroup\$ Exactly that. C223 and C224 decouple the outputs from IC23 going into IC31 and IC32 for the front signals, but all of these are using the same 4V reference (though buffered through the other opamp in IC35), so there likely wouldn't be much difference across those two caps, just the amp offset errors. The inputs to IC31-34 are biased separately via R301, R302 so that the caps will assume an appropriate DC differential. All these caps having to settle is likely to cause a good thump on power-up, hopefully your external amp has an anti-thump circuit that blanks the input at power-up. \$\endgroup\$ – Phil G Oct 10 '18 at 19:25
  • \$\begingroup\$ Would a 4.7uF capacitor with 10Kohm resistor do the job? \$\endgroup\$ – myexec Oct 10 '18 at 19:37
  • \$\begingroup\$ That'd work. Depending on the input impedance of the amp you might get away with a smaller cap to reduce the settling time. \$\endgroup\$ – Phil G Oct 10 '18 at 20:04
  • \$\begingroup\$ Well from the amp documentation the pre-in impedance is 15 kΩ. \$\endgroup\$ – myexec Oct 10 '18 at 20:10

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