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I'm trying to feed an audio signal which has around 5v DC bias to an amplifier and then an ADC. the final signal should have 2.5v DC bias. i've mixed things i've learned off forums but i'm still not getting the desired result. i don't have an oscilloscope to check but when the samples are played back in android, the volume is too low so i'm guessing the DC bias is not set up correctly.

any input is appreciated Schematic

the output from stage 1 is biased at exactly 2.5v while the final output is biased at 3.68Volt!

EDITS

  • C8 is 1uF in the actual soldered circuit
  • the output from stage 1 is biased at exactly 2.5v while the final output is biased at 3.68Volt!
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    \$\begingroup\$ Your bias setup looks OK and you have lots of gain. Are your OpAmps capable of rail-to-rail output? \$\endgroup\$
    – user1582568
    Jan 27, 2016 at 19:26
  • \$\begingroup\$ What is your Vcc? If you want to check the DC bias on the input to the ADC, make sure no signal is going into your circuit and put a volt meter set to VDC across c6 \$\endgroup\$
    – Voltage Spike ♦
    Jan 27, 2016 at 19:26
  • \$\begingroup\$ You don't need to have an oscilloscope to confirm output bias - just measure output voltage using any multi-meter or voltmeter without any input signal or input shorted to ground - output should be exactly 2,5V. \$\endgroup\$
    – Jakub Rakus
    Jan 27, 2016 at 20:53
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    \$\begingroup\$ Maybe C2 is damaged or leaky. If you disconnect "stage 2 input" does the output change to 2.5V? \$\endgroup\$
    – Spehro Pefhany
    Jan 28, 2016 at 18:08
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    \$\begingroup\$ You've seen it before when dealing digital - decouple, when dealing analog - decouple, when dealing hybrid - double decouple. \$\endgroup\$
    – ChrisR
    Jan 28, 2016 at 19:56

1 Answer 1

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As the output voltage is not at the correct DC level, but the DC bias is correct, change the amplifier. There is lots of other good advide here, particularly @ChrisR decoupling comments. Another point to note is the the virtual earth connections (ive Opamp inputs) are very sensitive to capacative pickup, so ensure that they are routed close to the amp and do not do near potentially noisy signals.

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