Capacitor Signal Path Issues
Thought for 2 mins 24 sec
Thanks, that helps. In your schematic, C36 and C37 are the ACโcoupling caps from the codec into the amp; they should pass audio from Net 67/66 to Net 70/71.
What the NS4150B requires (datasheet):
- โ้ณ้ขไฟกๅท้่ฟ้็ด็ตๅฎนๅ่พๅ ฅ็ต้ป่พๅ ฅๅฐ NS4150B ็ INP ไธ INNใ่พๅ ฅ็ตๅฎน Cin ไธ่พๅ ฅ็ต้ป Rin ๆๆไธไธช้ซ้ๆปคๆณขๅจใๆชๆญข้ข็ไธบ fc = 1/(2ฯ ยท Rin ยท Cin).โ
- โBypass: ๅ ้จๅ ฑๆจก็ตๅๆ่ทฏ็ตๅฎน่,ๆฅ 1uF ็ตๅฎน่ณ GND.โ
Your current network and likely causes:
- Nets: codec on [Net 67] โ [C36] โ [Net 70] โ [R26] โ [U2:INP]; codec on [Net 66] โ [C37] โ [Net 71] โ [R25] โ [U2:INN]. This is the intended series coupling path.
- Parts: [C36]/[C37]/[C38]/[C39]/[C31] are 0.1 ยตF (CC0603KRX7R9BB104). [R25]/[R26] are 150 kฮฉ (RC0603FR-07150KL). The highโpass cutoff with Cin = 0.1 ยตF and Rin = 150 kฮฉ is fc โ 10.61032953945969 Hz, so bandwidth is fine.
- You have 0.1 ยตF shunt caps on the codec side: C39 hangs from Net 67 to AU_GND, and C38 from Net 66 to GND. These load the codec at higher audio frequencies and, together with [C36]/[C37], make a capacitive divider. They should not zero the signal, but they can attenuate it.
- Bias/return on the amp side: The amp provides its own commonโmode via BYPASS, but your C31 is 0.1 ยตF; the datasheet explicitly calls for 1 ยตF. Too small a BYPASS capacitor can upset the internal bias and effectively kill the postโcap signal.
- Also note R24 ties U2 low via [Net 22]/[Net 19], which puts the amp in shutdown. That wonโt stop signal across [C36]/[C37], but it will keep U2 inputs at unusual bias.
What to do next
- Replace C31 with 1 ยตF to GND as required by the datasheet, then retest.
- For debug, add a temporary 100 kฮฉ from [Net 70] to AU_GND and another from [Net 71] to GND. If signal appears after [C36]/[C37], the issue was lack of a solid bias/return on the U2 side.
- Try lifting C39/C38 to remove the capacitive divider and codec loading during bringโup.
- Verify actual connectivity: in this file, OUTP/OUTN portals are on different nets than 67/66. Ensure your codec outputs are truly tied to [C36:P2]/[C37:P2] (Net 67/66).
- Take U2 out of shutdown: float or pull up U2 (remove or rewire [R24]) per datasheet โH/Open = normal, L = Shutdownโ.
- Measure with a scope at [C36:P1] (Net 70), [R26:P2] (Net 68), and [U2:INP]/U2. A DMM often wonโt show small AC on a highโimpedance, ACโcoupled node.
If you want, I can wire [OUTP]/[OUTN] to the correct nets and update [C31] to 1 ยตF in the design.
Replace bypass capacitor
Add input bias resistors
Lift C38/C39
Trace net connectivity
Welcome ๐
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