MAX98357A I2S amplifier

The MAX98357A breakout: samples in over I2S and up to 3 W out to a speaker, with no DAC in between. Aliases: I2S amp, MAX98357, class D amplifier.

The board is the I2S master. It drives BCLK and LRC and sends the samples on DIN; the chip plays them into a speaker on its terminal. It takes 16, 24 or 32 bits a sample (32, 48 or 64 clocks a frame) at 8 to 96 kHz, and needs no MCLK.

Pins

Pin	What it does
LRC	Word select, the frame clock, from the board. Low left, high right.
BCLK	The bit clock, from the board.
DIN	The samples, from the board.
GAIN	Picks the gain by what it is tied to (below). Open is 9 dB.
SD	Shutdown and channel select, by its voltage (below).
GND	Ground.
VIN	Supply, 2.5 V to 5.5 V.
SPK+, SPK-	The speaker, 4 ohms or more. Neither side is ground.

SD and GAIN

SD under 0.16 V shuts the amp down; 0.16 to 0.77 V plays (left + right) / 2; 0.77 to 1.4 V the right channel; over 1.4 V the left. The breakout has a 1 M pull-up to VIN against the chip's 100 k pull-down, so left open it sits at 0.45 V on 5 V and plays the mix. Wired to a pin, HIGH plays the left channel and LOW shuts it down.

GAIN: 15 dB through 100 k to GND, 12 dB to GND, 9 dB open, 6 dB to VIN, 3 dB through 100 k to VIN. The chip reads it as it powers up, so a change takes effect after the supply goes off and on, as on the real board.

While it runs

The board prints the gain it read and the channel SD picks, and a bar shows how loud it is playing. A speaker wired to SPK+ and SPK- shows the note it hears.

What the model gets right

The level. The gain is referred to 2.1 dBV: a full-scale sample is sqrt(2) x 10^((2.1 + gain) / 20) volts peak across the speaker, 5.1 V at 9 dB, clipped at the supply.

The frame. Samples are read most significant bit first, one clock after LRC changes, on the rising edge of BCLK. A frame rate outside 8 to 96 kHz, or a frame that is not 32, 48 or 64 clocks, plays nothing.

The bridge. Both speaker pins sit at half of VIN and move apart by the sample's voltage. With no frames for 2 ms they return to the middle; shut down, they let go.

What it does not model

The real outputs are a 300 kHz PWM that the speaker coil averages; Mokxi drives that average, updated once a frame. The chip's filters, click and pop suppression, TDM and left-justified modes, and its thermal limits are not modeled. No sound comes out of the page: the speaker shows the note.

Libraries

On the ESP32 boards, the ESP-IDF driver (driver/i2s.h: i2s_write), Arduino-ESP32 3.x's ESP_I2S.h (I2SClass) and 2.x's I2S.h. Mokxi bit-bangs the I2S bus on the pins you set, at the sample rate you ask for, while the sketch is writing.

Common mistakes

Wiring one speaker pin to ground. It is a bridge output: both pins move.

Changing GAIN while it runs and expecting a change. Power it off and on.

Stopping writes. The amp goes quiet 2 ms after the last frame; keep i2s_write fed.

See it in action

ESP32 I2S sine player plays a 440 Hz sine into a speaker. Open it at /templates.