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Adafruit MAX98357 I2S Class-D Mono Amp 5V on SD pin
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Adafruit MAX98357 I2S Class-D Mono Amp 5V on SD pin

by kingmob on Sat May 25, 2019 9:43 am

I had this neat little DAC/amp combo working fine but when I tried it out the other day I suddenly had no sound at all. I had been playing with adding a button to ground the SD pin for a quick disable, but other than that I have made no changes.

Some of the many tests I have done:

    If I measure the voltage between GND and SD now it is usually around 5V (VDD).
    Measuring the resistance between VDD and SD gives 1Mohm as expected.
    Measuring the resistance between GND and SD starts low but climbs to infinite, I assume because of capacitors? I had expected to measure the 100Kohm pulldown resistor here.
    While measuring ohms when the circuit is on (out of desperation) sound sometimes plays shortly. Resistance between VDD and SD than usually gives about 500 Ohm. This makes the SDD as the cause more suspect to me, although I would expect it to play one channel at 5V.

I made a simple circuit in Flastad to explain my understainding of the circuit here: Falstad link

I have not gone through the trouble of photographing the joints since it was working fine before. Measuring continuity is still as expected and in the short periods where the system works the sound is fine.

Does anyone know what the issue is and how I can circumvent it? I can't find the 100K pull-down resistor on the board, so I assume it is in the chip?

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Re: Adafruit MAX98357 I2S Class-D Mono Amp 5V on SD pin

by adafruit_support_mike on Sun May 26, 2019 2:05 am

Please do post photos of your hardware and connections. They may not tell you anything, but we've never seen them. 800x600 images usually work best.

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Re: Adafruit MAX98357 I2S Class-D Mono Amp 5V on SD pin

by kingmob on Sun May 26, 2019 6:49 am

Ok, you can find some pictures of the joints and the connections here: https://photos.app.goo.gl/X6iiFg1MJKuDBkMj7

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Re: Adafruit MAX98357 I2S Class-D Mono Amp 5V on SD pin

by adafruit_support_mike on Mon May 27, 2019 12:55 am

Thank you. Your soldering looks good and I don't see anything out of the ordinary in the connections.

From your measurements, it does sound like something has gone wrong with the internal pull-down on the SD pin. Try adding an external 100k resistor between SD and GND and see what voltage you get that way. Also check the I2S output and see if the MAX98357 produces output as you change the voltage on SD.

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Re: Adafruit MAX98357 I2S Class-D Mono Amp 5V on SD pin

by kingmob on Tue May 28, 2019 9:14 am

Thanks, i don't have a 100K resistor readily available, so that'll take a few days. My understanding is though that that won't have any effect if the pull-down resistor has shorted? In any case I'll try.

For the I2S I'm not sure how to check that? I have tried measuring voltage on the DIN and the LRC and BCLK pins. I measure a voltage there, but I'm not sure if that is an indication of activity. I don't have an oscilloscope, and I assume I'd need that to see any signal.
So I assume you mean to check if the pi is sending any data, but I don't know how to check for that? I've tried googling around, but can't seem to find anything.

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Re: Adafruit MAX98357 I2S Class-D Mono Amp 5V on SD pin

by adafruit_support_mike on Tue May 28, 2019 8:40 pm

kingmob wrote:Thanks, i don't have a 100K resistor readily available, so that'll take a few days.

You can also use a potentiometer if you have one. Any value between 1k and 100k should work for testing purposes. Connect the pot between 3.3V and GND, and connect the wiper to the SD pin.

kingmob wrote:My understanding is though that that won't have any effect if the pull-down resistor has shorted?

That's what we're going to find out.

kingmob wrote:For the I2S I'm not sure how to check that?

Just listen for output through the speaker.

Normally the MAX98357 switches its output from the left channel to the right channel, to both, to nothing as the voltage on SD changes. If that still works, then there's just a minor problem with the pull-up and pull-down resistors. If you don't get any output at all, there's a more serious problem.

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Re: Adafruit MAX98357 I2S Class-D Mono Amp 5V on SD pin

by kingmob on Fri May 31, 2019 9:07 am

Ok I tried putting 5 10K ohm resistors in series with a button in between the ground and the SD pin. With my multimeter this correctly shows 50Kohm. Measuring voltage between GND pin and SD pin has no measurable effect between pressing the button or not :(

Regarding the I2S pin, as said before, it was working before and intermittently while measuring the reistance while the circuit was running it would switch on. SO the problem doesn't appear to be there imo. (I haven't had that anymore though, I can't seem to replicate it reliably)

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Re: Adafruit MAX98357 I2S Class-D Mono Amp 5V on SD pin

by adafruit_support_mike on Sat Jun 01, 2019 3:21 am

Try adding another 50k as a pull-up resistor and measure the voltage at the SD pin. That will tell us whether there's a short to GND or the internal 1M pull-up has failed.

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Re: Adafruit MAX98357 I2S Class-D Mono Amp 5V on SD pin

by kingmob on Sun Jun 09, 2019 6:22 am

That doesn't seem to change any readings. I can measure 1Mohm between Vin and SD and this changes to 50Kohm when adding that rhat between those pins (that was what you meant right?).

Anyway, since I can measure the 1Mohm easily, I am confident that's working anyway. I'm afraid I'll just have to trash the amp, not sure what went wrong :(

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Re: Adafruit MAX98357 I2S Class-D Mono Amp 5V on SD pin

by adafruit_support_mike on Mon Jun 10, 2019 1:06 am

I was looking for the voltage on the SD pin with the 50k external pull-up resistor.

Sometimes you have to measure resistances indirectly, using a known resistance and a voltage to calculate the total effective resistance of what the resistor is connected to.

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Re: Adafruit MAX98357 I2S Class-D Mono Amp 5V on SD pin

by kingmob on Wed Jun 12, 2019 8:58 am

Ah I wasn't clear, the voltage between gnd eand the SD pin stays the same as before, around 4.9V

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Re: Adafruit MAX98357 I2S Class-D Mono Amp 5V on SD pin

by adafruit_support_mike on Sat Jun 15, 2019 12:33 am

Thank you. That makes it impossible for the SD pin to be shorted to GND.

Now let's flip it around: move the 50k resistor between SD and GND and measure the SD pin's voltage again. That will tell us whether SD is shorted to the 5V rail.

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