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I have connected a MAX98357A board to my ESP32 and am playing some audio files through it.

I've seen that it accepts 2.5-5.5 V as input and I'm wondering if it has any impact on the output. Do I get louder sound if I put 5 V in? Do I get noisier sound?

I've also seen this in the technical documentation:

Power-Supply Input
VDD, which ranges from 2.5V to 5.5V, powers the IC, including the speaker amplifier. Bypass VDD with a 0.1uF and 10uF capacitor to GND. Some applications might require only the 10uF bypass capacitor, making it possible to operate with a single external component. Apply additional bulk capacitance at the ICs if long input traces between VDD and the power source are used.

I'm a bit confused about this bypassing (I'm a total beginner, so it might be silly):

  1. From what I understood, by "bypassing" they mean to connect a capacitor in parallel to the input source, right? When they require both 0.1 μF and 10 μF caps, I guess they are both in parallel, like in this schematic, right?

schematic

simulate this circuit – Schematic created using CircuitLab

  1. If bypassing is required, why isn't it a part of the MAX98257A board itself? Should I adjust the values to my specific use-case? If so - how?

  2. What exactly do I protect here? Noise coming from the power supply or noise that the amplifier generates (on the speaker cables)? Should I also put capacitors on the speaker cables to get rid of "ticks and some static noises"?

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  • \$\begingroup\$ The Adafruit board has the capacitors fitted. The datasheet does give some fluffy criteria as to whether the caps are required. If in doubt, add the capacitors as there’s no negative effect. Balance that with not adding the capacitors and finding that you do need them. The bypass caps are to provide a low impedance voltage source for the chip. Wiring has inductance, so the caps counter this. As for caps on the speaker wiring - does the datasheet suggest anything here? As for input voltage, more volts equals more output. \$\endgroup\$
    – Kartman
    Commented Sep 20, 2022 at 21:54
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    \$\begingroup\$ The datasheet you linked is for the IC only. If you have the IC mounted on a board, you would have to find a schematic for the board to see if it includes the bypass capacitors and any other recommended components. \$\endgroup\$ Commented Sep 20, 2022 at 21:58
  • \$\begingroup\$ In general , if two parallel capacitors are recommended the lower value should go as close as possible to the IC and the higher value can be place somewhat further away. \$\endgroup\$ Commented Sep 20, 2022 at 22:46
  • \$\begingroup\$ Sorry for the IC/board confusion. This is the board I'm using: aliexpress.us/item/3256803032478005.html. I guess it has the default caps in place? \$\endgroup\$
    – Zach Moshe
    Commented Sep 24, 2022 at 18:06

2 Answers 2

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I've seen that [MAX98357A] accepts 2.5-5.5 V as input and I'm wondering if it has any impact on the output. Do I get louder sound if I put 5 V in? Do I get noisier sound?

The datasheet claims "3.2W Output Power into 4Ω at 5V." If the voltage is reduced, the power it can deliver will be reduced as well, but probably more than expected. Claims 0.4W at 2.5V. There's a chart of this on page 11. So that's the main reason for using 5V if possible.

The charts on page 9 show generally, that noise is very low (-70dB or so) for all but the highest power outputs. So, it should sound fine at any low volume, but crank it to max, and it'll start sounding bad.

By "bypassing" they mean to connect a capacitor in parallel to the input source, right? When they require both 0.1uF and 10uF caps, I guess they are both in parallel, like in this schematic?

Correct.

If bypassing is required, why isn't it a part of the MAX98257A board itself? Should I adjust the values to my specific use-case? If so - how?

It isn't part of the board itself because they assume you're not going to power it from a long (and resistive) cable. It's also cheaper to make the board with less parts on it (offloading design caveats to you.)

If you are going to power the board from a long cable, then add the 0.1µF at the board. And if powering it from a mile of cable, even more capacitance will be needed.

Generally this is analyzed using an oscilloscope. The oscilloscope is setup to measure the voltage changes at the board, and capacitance is added to reduce these voltage changes. The amount of capacitance depends on how good (low-impedance) the power supply is, and how lossy the wiring is. Math could be used, but a lot of variables are involved (calculating resistance from wire diameter/length, etc.) It's easier to just measure it. The datasheet says that a 0.1µF may be needed and that should cover most use-cases.

What exactly do I protect here? Noise coming from the power supply or noise that the amplifier generates (on the speaker cables)?

Think about what would happen if instead of being directly powered from the power supply, it was instead powered through two 10 Ohm resistors. When the audio volume would peak, the voltage would sag badly... likely enough to cause undesired operation.

Well very fast changes in this audio ("transients") can put a tremendous strain on any power supply, such that even 0.1 Ohm of resistance (i.e. a long wire) will cause the voltage to briefly dip, leading to undesired operation. The 0.1µF cap next to the board is to briefly supply power during these events, so that the volts doesn't decrease too much. You should see a noticeable reduction in variation when the cap is added.

Should I also put capacitors on the speaker cables to get rid of "ticks and some static noises"?

In general, it is possible to put a capacitor (such as 10µF) in series with a speaker. This is because capacitors couple AC and block DC, so have the effect of eliminating any DC offset present in the output. But a capacitor should not be placed across a speaker, as this will be shorting the AC signal out.

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  1. You have this 100% correct. Typically you would place the smaller cap closest to the pins.
  2. What board? The MAX98257A is an IC chip. If you mean that you bought a demo board of some kind based on this chip, then yes, I would expect it to already have bypass caps installed.
  3. You are trying to reduce noise from entering the chip via its power supply lines. This noise can be from the power source itself, or noise picked up by the lines leading into the chip. As far as putting caps on the speaker cables, that should be its own question but the answer is most likely a no. If you want to filter your output, that is a different story.
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  • \$\begingroup\$ Thanks! I bought a board similar to these (not 100% sure this was the same link, but probably the same board): aliexpress.us/item/3256803032478005.html So I guess caps are already installed? If I still have noises in the output, does it make sense to add additional caps? \$\endgroup\$
    – Zach Moshe
    Commented Sep 24, 2022 at 18:05
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    \$\begingroup\$ Yes, the bypass caps are already on there. Adding more will not help with your noise. \$\endgroup\$ Commented Sep 25, 2022 at 14:30

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