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I'm using a 3.3V DC-DC boost converter (https://www.torexsemi.com/file/xcl101/XCL101.pdf) as the power source for my audio circuit. There is a popping sound (headphone output) that happens when the circuit is turned on. There is no audible noise after power up. I viewed the boost converter's output on my scope at power up, and there's an AC-coupled voltage spike of about 2.5V that quickly decays back to zero:

enter image description here

I'm using 10 uF capacitors at the input and output of the boost converter, as shown in the datasheet:

enter image description here

How can I eliminate this AC-coupled voltage transient that's creating the popping sound?

Edit: I don't have any popping noise issue when using a bench power supply. It's only a problem using the boost converter. This is my audio circuit:

enter image description here

I put the 1k resistor at the output in an attempt to eliminate the popping sound, as a discharge resistor. There is a large decoupling capacitance at the output because I needed a low cut off frequency. The op amps are power op amps that have the required current capability.

I tried using a different 3.3V booster circuit, and did not hear the pop sound. The booster IC is different, and the circuit includes a diode and 68 uF output cap: enter image description here

Datasheet of 3.3V booster: https://cdn.sparkfun.com/datasheets/BreakoutBoards/NCP1402.pdf

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  • 1
    \$\begingroup\$ "I viewed the boost converter's output on my scope at power up, and there's an AC-coupled voltage spike of about 2.5V that quickly decays back to zero": well yes, the voltage was 0 before turning on and then became 3.3V after turning on, that's not DC. \$\endgroup\$ – τεκ Feb 10 '18 at 5:16
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The voltage spike you're seeing is because you're switching the power on. When you go from 0 volts to 3.3v in a couple of ms, that will appear as AC when your scope is set to AC. The issue is not your power supply, the issue is your audio driver that doesn't compensate for startup pops.

Edit:

One way might be to follow this suggestion like so

schematic

simulate this circuit – Schematic created using CircuitLab

R2 charges C2 up slowly, gradually turning on the mosfet over a couple of milliseconds, reducing the volume of the pop. You'll need to select an op amp that can handle the amount of power that you're putting into your speaker.

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  • \$\begingroup\$ When I use my bench power supply, there is no popping sound on power up. It's only an issue using the boost converter. How do you compensate for the start up pop? \$\endgroup\$ – donut Feb 10 '18 at 2:00
  • \$\begingroup\$ In order to answer that, it'd help if you posted your audio circuit \$\endgroup\$ – C_Elegans Feb 10 '18 at 2:09
  • \$\begingroup\$ I just edited in the audio circuit. \$\endgroup\$ – donut Feb 10 '18 at 3:01
  • \$\begingroup\$ In that case, a small mosfet such as a BS170 would work \$\endgroup\$ – C_Elegans Feb 10 '18 at 3:04
  • \$\begingroup\$ Is there a way to eliminate the pop rather than reduce it? I actually tried using a digital delay circuit that delayed the connection of the headphones to audio, so the circuit powers up, and then half a second later the headphones are connected to audio. There was still a small popping sound on power up, and a slightly louder popping sound at half a second. I'm not sure why. ...Would you expect this mosfet/RC configuration to have a different effect than this type of delay circuit, because it seems like you're doing the same thing? \$\endgroup\$ – donut Feb 10 '18 at 3:14
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Looking at your power supply, Cl is only 10 uF, which allows a fast rise time of the output voltage.

To dampen that 'pulse' you can add a series resistor of 5 to 10 ohms at 3 to 5 watts, and another capacitor of 470 uF to ground so you have a heavy duty power filter.

It may not be wise to replace Cl with a high value capacitor, as it may become unstable, which is why I isolated them with a resistor.

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  • \$\begingroup\$ I just tried a different 3.3V booster (breakout board from Sparkfun) and did not hear a popping sound. This is the schematic: cdn.sparkfun.com/datasheets/BreakoutBoards/… Could it be the diode from LX to VOUT that prevents the popping? \$\endgroup\$ – donut Feb 10 '18 at 4:05
  • \$\begingroup\$ No. It is the value of the Vcc capacitor that dampens the Vcc rise time, and helps with bass response \$\endgroup\$ – Sparky256 Feb 10 '18 at 4:11
  • \$\begingroup\$ What's the purpose of the diode? Could it be that the different booster chip on the Sparkfun breakout board is better? \$\endgroup\$ – donut Feb 10 '18 at 4:20
  • \$\begingroup\$ I can't use the link as I am on my cell phone. \$\endgroup\$ – Sparky256 Feb 10 '18 at 4:22
  • \$\begingroup\$ I just added the Sparkfun breakout schematic as an edit to my question. \$\endgroup\$ – donut Feb 10 '18 at 4:38
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In this case you will not get away with just more capacitance here and there. You need to specifically enable th7ngs after voltage is stable. Most straightforward option is to disconnect the speaker until everything stabilize and then to connect it. You can do it with mosfet or ssr. Maybe using amplifier with "enable" input could also help. I recall that audio amplifiers usually suppress those pops by themselves.

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  • \$\begingroup\$ The op amps have pop suppression listed as a feature, and they can't be causing the problem because there is no popping sound when I use my bench power supply instead of the 3.3V booster. I actually just tried using a different 3.3V booster (Sparkfun breakout board: cdn.sparkfun.com/datasheets/BreakoutBoards/…) and there's no popping sound. Now I'm wondering whether the issue is with the different booster chip, or is it because the Sparkfun circuit includes a diode from LX to VOUT? The only other difference is Sparkfun's 68 uF on the output instead of 10 uF. \$\endgroup\$ – donut Feb 10 '18 at 4:17
  • \$\begingroup\$ I would guess that the other boosters have a lower slew rate. It kind of makes sense because much lower frequency and bigger inductor. \$\endgroup\$ – Gregory Kornblum Feb 10 '18 at 5:57
  • \$\begingroup\$ The Sparkfun 3.3V booster does have a bigger inductor- it's a separate part. The inductor is actually built in to the small Torex 3.3V booster. I thought it was an advantage to not need a separate inductor component! Maybe not. I wonder if using a 68 uF capacitor on the Torex booster output would make a difference. \$\endgroup\$ – donut Feb 10 '18 at 6:47
  • \$\begingroup\$ I know, i am just saying that torex inductor is small \$\endgroup\$ – Gregory Kornblum Feb 10 '18 at 6:48
  • \$\begingroup\$ So the question is whether the issue will be solved by using a larger 68 uF capacitor at the booster's output, or if the inductor built into the booster is too small for that capacitance to fix the problem, and I'll need to use a booster with a larger inductor, correct? \$\endgroup\$ – donut Feb 10 '18 at 7:02
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@C_Elegans answer is correct, this turn-on pop is caused by the coupling capacitors at the output charging up.

Here is another way to eliminate turn-on pop.

schematic

simulate this circuit – Schematic created using CircuitLab

When the power is turned on, the voltage across C1 is 0 (it is discharged by R3 when the power is off), so current is flowing through D1 and the opamp will output 0. As C1 charges through R5 and R6, the opamp will assume it's steady-state output of +V/2. Choose C1R5 to prevent a 'pop' from being heard (i.e. much more than 8ms).

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