I2S Sound-Chip MAX98357A filtering away the 300kHz PWM from the BTL-output-signal

Other Hardware Audio
1

Hi Everybody,

The I2S-Sound-Chip has a bridge tied load (BTL) Class D amplifier
learn-adafruit explains
The audio input is I2S standard, you can use 3.3V or 5V logic data. The outputs are "Bridge Tied" - that means they connect directly to the outputs, no connection to ground.
The output is a ~300KHz square wave PWM that is then 'averaged out' by the speaker coil

  • the high frequencies are not heard. All the above means that you can't connect the output into another amplifier, it should drive the speakers directly.

Convenient to have a 3W-amplifier integrated if you want to use a small speaker.

The other I2S-decoder that would be suitable for line-in-input to amplifiers is discontinued

As the MAX98357 has this bridge-tied-load output I assume directly connecting this output to a line-in input of an amplifier does not work or maybe even damage the amplifier.

So I would like to try to filter away the 300 kHz PWM from the MAX98357-BTL-output to transform the signal into a 0,7V 1,0 V-line-in-signal.

Would a low-passfilter with an edge-frequency of 20 kHz be sufficient?

I apprecieate all kinds of hints and comments on this question.

best regards Stefan

2

Yes, a low pass filter (balanced) is required - you then have balanced signals and you'd ac-couple one of them to the line input. If your filter attenuates enough this will be safe for the line input, but adding 1k of series resistance is a simple way to protect from some over-voltage.

To drive a speaker an LC style filter is often used, but for picking off a line level signal RCRC filter might be more convenient, perhaps 1k, 4n7, 1k, 4n7?