# Electronics & wiring — CAMS-1

## Block diagram

```
XLR/TRS combo ─► THAT1200/INA134 ─► Master pot ─► LR4 XO @ 2.5 kHz
                                                      │
                                         ┌────────────┴────────────┐
                                         ▼                         ▼
                                   LP→LP (mid)               HP→HP (HF)
                                         │                         │
                                         ▼                         ▼
                                   LM3886 mid                 HF pad → LM3886
                                         │                         │
                                         ▼                         ▼
                                      8″ mid                    1″ dome
```

Mains → IEC fuse → DPST → toroid 2×22 V → bridge → bulk caps → ±V to both LM3886  
Derived ±15 V (7815/7915) → op-amps.

## Reuse / derive

| Board | Base design |
|-------|-------------|
| Each power amp | [`../../lm3886-poweramp/`](../../lm3886-poweramp/) topology (gain 21, Zobel, Thiele, mute RC) |
| XO | [`../../active-crossover/`](../../active-crossover/) Sallen-Key LR4 — run calculator at **2500** Hz |

```bash
python3 ../../active-crossover/calculator/calculator.py 2500 --series E24
```

Use **low-pass** tables for mid path (2 stages) and **high-pass** tables for HF path (2 stages).

## Wiring rules (quality / safety)

1. Star earth at IEC inlet → chassis plate only once.  
2. Signal 0V and power 0V join at one star near bulk caps.  
3. Twisted pair mid and HF speaker leads; dress away from toroid.  
4. HF pad before HF LM3886 input (resistive) — set so on-axis FR is smooth in QC.  
5. Short mute RC on both LM3886 (~1 s) to avoid turn-on thump in sanctuary.  
6. Strain-relief all panel connectors.

## Torque / hardware

| Fastener | Torque (indicative) |
|----------|---------------------|
| LM3886 to heatsink M3 | 0.6–0.8 N·m |
| Heatsink to Al plate M4 | 1.5 N·m |
| XLR / IEC nuts | per Neutrik/Schaffner datasheet |
| Driver screws M5 | snug + 1/4 turn — do not crush gasket |

## Internal cable lengths (typical)

| From | To | Length |
|------|----|--------|
| Combo J1 | XO input | 120 mm shielded |
| XO mid out | Mid amp in | 80 mm |
| XO HF out | HF pad → HF amp | 100 mm |
| Mid amp out | Mid driver | 250 mm 1.5 mm² |
| HF amp out | Tweeter | 300 mm 1.0 mm² |

## VERIFY before power-up

- No shorts V+/V−/0V  
- Correct 7915 orientation  
- Transformer primary voltage matches site mains  
- Isolation: IEC earth ↔ signal floats only as designed (no accidental earth loops through mixer)