Capacitor Signal Path Issues

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Net 71

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Looking for "typical application circuit input coupling capacitors INP INN recommendation" in 2209161630_Shenzhen-Nsiway-Tech-NS4150B_C189961.pdf

Looking for "input pins INP INN input impedance and bias requirements" in 2209161630_Shenzhen-Nsiway-Tech-NS4150B_C189961.pdf

Looking for "recommended input network components Rin Cin differential input" in 2209161630_Shenzhen-Nsiway-Tech-NS4150B_C189961.pdf

Looking for "electrical characteristics input impedance input common-mode" in 2209161630_Shenzhen-Nsiway-Tech-NS4150B_C189961.pdf

Looking for "pin description INP INN" in 2209161630_Shenzhen-Nsiway-Tech-NS4150B_C189961.pdf

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Thanks, that helps. In your schematic, C36 and C37 are the AC‑coupling caps from the codec into the amp; they should pass audio from Net 67/66 to Net 70/71.

What the NS4150B requires (datasheet):

“音频信号通过隔直电容和输入电阻输入到 NS4150B 的 INP 与 INN。输入电容 Cin 与输入电阻 Rin 构成一个高通滤波器。截止频率为 fc = 1/(2π · Rin · Cin).”
“Bypass: 内部共模电压旁路电容脚,接 1uF 电容至 GND.”

Your current network and likely causes:

Nets: codec on [Net 67] → [C36] → [Net 70] → [R26] → [U2:INP]; codec on [Net 66] → [C37] → [Net 71] → [R25] → [U2:INN]. This is the intended series coupling path.
Parts: [C36]/[C37]/[C38]/[C39]/[C31] are 0.1 µF (CC0603KRX7R9BB104). [R25]/[R26] are 150 kΩ (RC0603FR-07150KL). The high‑pass cutoff with Cin = 0.1 µF and Rin = 150 kΩ is fc ≈ 10.61032953945969 Hz, so bandwidth is fine.
You have 0.1 µF 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 common‑mode via BYPASS, but your C31 is 0.1 µF; the datasheet explicitly calls for 1 µF. Too small a BYPASS capacitor can upset the internal bias and effectively kill the post‑cap signal.
Also note R24 ties U2 low via [Net 22]/[Net 19], which puts the amp in shutdown. That won’t stop signal across [C36]/[C37], but it will keep U2 inputs at unusual bias.

What to do next

Replace C31 with 1 µF to GND as required by the datasheet, then retest.
For debug, add a temporary 100 kΩ 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 bring‑up.
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 “H/Open = normal, L = Shutdown”.
Measure with a scope at [C36:P1] (Net 70), [R26:P2] (Net 68), and [U2:INP]/U2. A DMM often won’t show small AC on a high‑impedance, AC‑coupled node.

If you want, I can wire [OUTP]/[OUTN] to the correct nets and update [C31] to 1 µF in the design.

Replace bypass capacitor

Add input bias resistors

Lift C38/C39

Trace net connectivity

142 days

Ready.

R24

Resistance

10k Ω

FB2

OUTP

C36

Capacitance

0.1u F

FB1

AU_GND

C37

Capacitance

0.1u F

PA_OUTL-

R25

Resistance

150k Ω

Manufacturer Part Number

NS4150B

AU_GND

GPIO21

VCC

GPIO21

C39

Capacitance

22p F

PA_OUTL+

GND

OUTN

C31

Capacitance

1u F

R26

Resistance

150k Ω

AU_GND

C38

Capacitance

22p F

AU_GND

PA_OUTL-

PA_OUTL+

C30

Capacitance

1u F

C29

Capacitance

0.1u F

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