Resound — Small Build (the buildable v1 target)

The focused, buildable unit. Beast (6/12 broadcasters) is deferred — this drops all the hard scaling parts. See [Hardware Design v0.1] for the full picture and the detailed [Power and Battery] / [Carrier PCB and BOM] / [Stack Connector and Addressing] / [LED and Music Sync] pages.

Config

5 boards: 1 HUD + 1 SINK + 3 BROADCASTERS (channels: Headset, Speaker 1, Guest).

SINK: iPhone A2DP in -> I2S master.
3 BROADCASTERS: I2S -> A2DP to each speaker (I2C 0x11 / 0x10 / 0x12).
HUD: ESP32-S3 round LCD, control over I2C. All audio boards = ESP32-WROOM-32E (one carrier PCB, SINK vs BROADCASTER assembly).

Runtime target: ~8 h (start)

Load ~2.9 W @ 5 V (5 boards). At ~61% usable derate, 8 h needs ~38 Wh nameplate.

Battery: 2S2P 18650 (4 cells, ~50.4 Wh) -> ~10 h with margin (comfortably clears 8 h).

~225 g of cells; compact. (Upgrade path to 2S3P/6 cells later for >12 h with no design change.)
Charger: TI BQ25887 (2S, I2C, balancing, ~2 A) over USB-C 5 V — fine at this pack size (no USB-PD needed for the small build).
Reg: single TPS563201 (3 A) buck to the 5 V bus. NTC on pack -> charger (JEITA). Reverse-polarity P-FET + TVS + fuse; soft-latch power switch.

What the small build SIMPLIFIES (vs beast)

No backplane, no I2C mux — a single vertical 5-board stack on the 26-pin 2.54 mm pass-through connector.
I2S: one buffer at the SINK (e.g. 74LVC125) + 22-33 R series term, fanning to just 3 broadcaster inputs — short, easy.
I2C: 100 kHz, a single 2.2-4.7 kOhm pull-up pair on the SINK. ~5 nodes -> well under the cap budget.
Addressing: 4-bit solder straps (GPIO13/14/27/26) — only 3 broadcasters, so no auto-enumeration needed. Headset=0x11, Speaker1=0x10, Guest=0x12.
Power: single 2S buck + a basic 2 A USB-C charger; no PD, no high-current multi-phase.

Small-build BOM (rough)

Block	Parts	~$
4x carrier PCB (1 sink + 3 bcast)	WROOM-32E + connectors + passives, ~$7 ea	~28
HUD	ESP32-S3-Touch-LCD-1.28	~15
Battery	4x INR18650-35E + 2S2P holder/weld	~35
Power electronics	BQ25887 + TPS563201 + USB-C + BMS + fuse/TVS/switch	~20
Misc	I2S buffer (74LVC125), stacking connectors, standoffs, enclosure	~15
Total (excl. speakers)		~$113

Wiring (single stack)

I2S: SINK GPIO5/25/23 (BCK/WS/DATA) -> buffer -> bus -> each broadcaster GPIO19/18/22. Common GND.
I2C: HUD GPIO15/16 + each board GPIO32/33 on the shared bus; one pull-up pair at the SINK.
Power: 2S pack -> buck -> 5 V bus up the stack; each board's onboard LDO makes 3V3.
Antenna: still mind WROOM antenna keep-out / orientation even at 4 boards (overhang + rotate). Lower risk than beast but real.

Next engineering step

Hand this + the detailed pages to a KiCad layout of the single carrier PCB (45x45 mm, 4-layer, antenna keep-out), then fab 4 + assemble. LEDs (APA102) are an optional later add per [LED and Music Sync].