# PM_K-1 — Phase-1 Bench Prototype

Prove the **click → codec → transformer-isolated XLR** chain (and the all-important *no-buzz*
test) on bought boards **before** committing any custom PCB. Keeps the RP2350 firmware. This is
Phase 1 of the approved plan (`deep-honking-lemon`). Firmware lives in `pico-wm8960/code.py`.

## Why this shape (recap)
- The brain (MCU + codec + Bluetooth) is a commodity → buy it.
- The only genuinely-custom hardware is the **Pro Out** stage (line driver → output transformer
  → XLR) + the face → build only that, later.
- The past "buzz" was a **ground loop**; the cure is **galvanic isolation** (a transformer), not
  ±15 V. ±15 V was only studio headroom and is gone.

## Shopping list (~$40–80)
| Item | Why | ~$ |
|---|---|---|
| **Raspberry Pi Pico 2** (RP2350) | Keeps the existing RP2350 firmware/toolchain | 5 |
| **SparkFun WM8960 Audio Codec Breakout** | Stereo codec + HP amp + 1 W class-D speaker; **has an onboard 24 MHz oscillator → supplies the WM8960 MCLK** (resolves Risk R1), and the `adafruit_wm8960` driver is tested against exactly this board | 15 |
| **Output transformer** | The buzz-killer: galvanic isolation + balancing. Bench-start with a budget 1:1 600:600 Ω line transformer (Triad/Würth/Bourns); evaluate **Jensen JT-11** / Lundahl for the keeper. **Pick for full-level low-frequency handling** — a punchy click is transient-rich and small cores saturate/dull the thump | 10–50 |
| **Male XLR chassis connector** (Neutrik) + optional ¼" TRS | Stadium-standard, locking, unambiguous output | 5 |
| **Audio op-amp** (OPA1612 etc.) + breadboard/protoboard, jumpers | Low-rail line driver into the transformer primary | 5 |
| *(later)* **Microchip BM83 dev board** | Bluetooth leg (A2DP sink+source) for M5 | 30–40 |
| Measurement: USB audio interface + REW (free), or a phone analyzer app | Check output level + LF response (M3) | — |

## Wiring
**Pico 2 ↔ WM8960 breakout (digital):** match `pico-wm8960/code.py`.
| Pico 2 | WM8960 | Note |
|---|---|---|
| GP0 | BCLK | bit clock |
| GP1 | DACLRC (+ tie ADCLRC) | word/LR clock — must be GP(bit_clock)+1 |
| GP2 | DACDAT | Pico transmits the click |
| GP4 | SDA | Qwiic I²C |
| GP5 | SCL | Qwiic I²C |
| 3V3 / GND | 3V3 / GND | power the breakout |

**Analog out (Pro Out, breadboard):** WM8960 line/HP out → op-amp line driver (low rail) →
**transformer primary**; transformer **secondary** → XLR pin 2 (hot) / pin 3 (cold) / pin 1
(shield, tied to chassis). Optionally also a ¼" TRS off the same secondary (tip/ring/sleeve).
The transformer's secondary floats — that floating, isolated output is what cannot form a ground
loop.

## Firmware
1. Flash **CircuitPython** (9.x+) onto the Pico 2.
2. `circup install adafruit_wm8960`  (or copy `adafruit_wm8960/` into `CIRCUITPY/lib`).
3. Copy `pico-wm8960/code.py` to `CIRCUITPY/code.py`. It boots a steady 120 BPM click for M1.

## Bench validation milestones
- **M1 — click out:** the synthesized click plays cleanly via the WM8960 on headphones / speaker.
- **M2 — mix:** your guitar on the line input is heard *with* the click (enable the codec's
  analog input→output bypass for ~0 ms monitor latency — see the M2 hook in `code.py`).
- **M3 — pro output:** the transformer-isolated XLR drives a real console/interface at usable
  level, and **the click keeps its punch** — measure the low-frequency response through the
  transformer (this is where a too-small transformer shows up as a thin, gutless click).
- **M4 — the acid test (NO BUZZ):** power the rig from a **mains** USB charger **and** connect the
  XLR to a **mains-powered** console/interface. Confirm **no hum**. A/B against a non-isolated
  output (tap before the transformer) to prove the transformer is what kills the loop. **This is
  the acceptance gate for the whole approach.**
- **M5 (later):** add the BM83 — phone audio in (A2DP sink) mixes with the click; click out to a
  Bluetooth speaker (A2DP source).

## Then → Phase 2 (only if M1–M4 pass)
Design the **Pro Out module** PCB (op-amp → transformer → XLR) and optionally a custom RP2350 +
WM8960 brain, using the existing SKiDL designs as reference. Not before the bench proves it.

## Open risks to watch
- **Transformer LF/level** (M3) — the "rock the house" punch lives or dies here.
- **WM8960 MCLK** — relying on the SparkFun breakout's onboard 24 MHz osc; a bare WM8960 needs
  you to supply MCLK.
- **CircuitPython I²S/mixer throughput** on the Pico 2 — if it can't keep up at 44.1 k, fall back
  to the C/Rust path.