# PCM1808+PCM5102 I2S HAT — Bill of Materials + Wiring Diagram

> **Project:** Pi Multi-FX Pedal  
> **HAT:** Custom PCM1808 ADC + PCM5102 DAC I2S audio HAT  
> **Board:** 65×56mm 2-layer PCB (1590B compatible)  
> **Revision:** 1.0 — 2026-06-07

---

## Bill of Materials

### Audio ICs

| Ref | Part | Description | Package | Qty | Est. Cost | Source |
|-----|------|-------------|---------|:---:|:---------:|--------|
| U3  | **PCM1808** | 24-bit stereo ADC, I2S output | TSSOP-14 | 1 | $3.50 | Mouser/Digikey |
| U4  | **PCM5102A** | 32-bit stereo DAC, I2S input | TSSOP-20 | 1 | $3.00 | Mouser/Digikey |
| U2  | **TL072CN** | Dual JFET op-amp (preamp + buffer) | DIP-8 | 1 | $1.50 | Mouser/Digikey |

### Power Regulation

| Ref | Part | Description | Package | Qty | Est. Cost | Source |
|-----|------|-------------|---------|:---:|:---------:|--------|
| U1  | **AMS1117-3.3** | 3.3V LDO regulator, 1A | SOT-223 | 1 | $0.50 | Mouser/Digikey |
| C1  | 10µF electrolytic | Input decoupling (5V rail) | 5mm radial | 1 | $0.15 | — |
| C2  | 100nF ceramic | Input decoupling (5V rail) | 0805 | 1 | $0.05 | — |
| C3  | 10µF electrolytic | Output decoupling (3.3V rail) | 5mm radial | 1 | $0.15 | — |
| C4  | 100nF ceramic | Output decoupling (3.3V rail) | 0805 | 1 | $0.05 | — |

### Preamp Stage (Guitar Input → PCM1808)

| Ref | Part | Description | Package | Qty | Est. Cost | Source |
|-----|------|-------------|---------|:---:|:---------:|--------|
| J2  | 6.35mm TS jack | Guitar input jack, switched (NC contact for bypass) | Panel-mount | 1 | $2.00 | Switchcraft/Neutrik |
| R1  | **1MΩ** | Input pulldown (prevents floating input noise) | 1/4W axial | 1 | $0.05 | — |
| R2  | **10kΩ** | Series resistor to op-amp non-inverting input | 1/4W axial | 1 | $0.05 | — |
| R3  | **100kΩ** | Feedback resistor (gain = 1 + R3/R4) | 1/4W axial | 1 | $0.05 | — |
| R4  | **10kΩ** | Ground reference for feedback divider | 1/4W axial | 1 | $0.05 | — |
| C5  | 10µF electrolytic | DC-blocking cap after op-amp output | 5mm radial | 1 | $0.15 | — |

### ADC Decoupling + Configuration

| Ref | Part | Description | Package | Qty | Est. Cost | Source |
|-----|------|-------------|---------|:---:|:---------:|--------|
| C6  | 100nF ceramic | PCM1808 VCC decoupling | 0805 | 1 | $0.05 | — |
| C7  | 10µF electrolytic | PCM1808 VCC bulk decoupling | 5mm radial | 1 | $0.15 | — |

### DAC + Output Stage (+ config resistor)

| Ref | Part | Description | Package | Qty | Est. Cost | Source |
|-----|------|-------------|---------|:---:|:---------:|--------|
| J3  | 6.35mm TS jack | Audio output jack | Panel-mount | 1 | $2.00 | Switchcraft/Neutrik |
| C8  | 100nF ceramic | PCM5102 VCC decoupling | 0805 | 1 | $0.05 | — |
| C9  | 10µF electrolytic | PCM5102 VCC bulk decoupling | 5mm radial | 1 | $0.15 | — |
| C10 | 10µF electrolytic | DC-blocking cap on DAC output (before output jack) | 5mm radial | 1 | $0.15 | — |
| R5  | **330Ω** | Output series resistor (PCM5102 hiss reduction filter) | 1/4W axial | 1 | $0.05 | — |
| R6  | **10kΩ** | Output stage resistor (output buffer feedback) | 1/4W axial | 1 | $0.05 | — |

### Connectors

| Ref | Part | Description | Qty | Est. Cost | Source |
|-----|------|-------------|:---:|:---------:|--------|
| J1  | 2×20 female header | RPi 40-pin GPIO stacking header, 8.5mm height | 1 | $2.50 | Adafruit/Amazon |
| —   | 2×20 male pin strip | Breakaway header for HAT-to-PCB mount | 1 | $1.00 | — |

### Hardware

| Part | Description | Qty | Est. Cost | Source |
|------|-------------|:---:|:---------:|--------|
| M2.5 standoffs | Brass, 12mm female-female, for RPi mounting | 4 | $2.00 | Amazon |
| M2.5 screws | Pan head, 6mm, for RPi + HAT sandwich | 8 | $1.50 | — |
| Pedal enclosure | 1590B diecast aluminum (112×60×31mm) | 1 | $8.00 | Hammond/Amazon |
| Rubber feet | adhesive, 10mm diameter | 4 | $1.00 | — |

---

## Cost Summary

| Section | Est. Cost |
|---------|:---------:|
| Audio ICs (PCM1808 + PCM5102 + TL072) | $8.00 |
| Power regulation | $0.90 |
| Preamp passives | $2.40 |
| ADC/DAC decoupling + output | $2.45 |
| Connectors + stacking header | $3.50 |
| Hardware (standoffs, screws, enclosure) | $12.50 |
| **Total HAT (board + component)** | **~$29.25** |
| + Enclosure + hardware | **~$41.75** |

Compare to AudioInjector Stereo HAT at $35-40 (no preamp, no enclosure) — the custom build costs about the same but is purpose-designed for a stompbox.

---

## Wiring Diagram

```
RPi 4B 40-pin Header (J1)
┌─────────────────────────────────────────┐
│  ●  ●  ●  ●  ●  ●  ●  ●  ●  ●  ●  ● ● │  Pins 1-20
│  ●  ●  ●  ●  ●  ●  ●  ●  ●  ●  ●  ● ● │  Pins 21-40
└─────────────────────────────────────────┘
    │  │  │  │  │  │  │  │  │  │
    │  │  │  │  │  │  │  │  │  │
    ▼  ▼  ▼  ▼  ▼  ▼  ▼  ▼  ▼  ▼
┌─────────────────────────────────────────┐
│  P1 P9 P17 P25 P33  |  P2 P10 ...      │  HAT PCB (top view)
│  GND 3V3 5V  BCLK LRCLK                │
│                                         │
│  ┌─────┐   ┌─────┐   ┌──────────────┐  │
│  │ U3  │   │ U4  │   │    U2        │  │
│  │PCM1808│ │PCM5102│ │    TL072     │  │
│  │ ADC  │  │ DAC  │  │ dual op-amp │  │
│  └──┬──┘   └──┬──┘   └──────┬───────┘  │
│     │         │              │          │
│     └────┬────┘              │          │
│          │ (I2S bus)        │          │
│          │                   ├── J2     │
│     ┌────┴────┐              │ Guitar In│
│     │   U1    │              │          │
│     │AMS1117  │              └──────────│
│     │  3.3V   │                         │
│     └────────┘                          │
│    5V → 3.3V                            │
│                                         │
│                              ┌───────┐  │
│                         J3 ──│ C10   │  │
│                Output ───────│ 10µF  │  │
│                              └───────┘  │
└─────────────────────────────────────────┘
```

### Detailed I2S Connections

```
GPIO18 (BCLK)  ───── PCM1808 pin 8 (BCK)  ───── PCM5102 pin 14 (BCK)
GPIO19 (LRCLK) ───── PCM1808 pin 7 (LRCK) ───── PCM5102 pin 13 (LRCK)
GPIO20 (DIN)   ────────────────────────────────→ PCM5102 pin 12 (DIN)
GPIO21 (DOUT)  ←──── PCM1808 pin 9 (DOUT)
```

### PCM1808 Configuration Pins

| Pin | Name | Connection | Function |
|-----|------|-----------|----------|
| 12  | FMT  | GND       | I2S format (24-bit left-justified) |
| 13  | MD1  | 3.3V      | 48kHz sampling rate |
| 14  | MD0  | GND       | Standby disabled (always active) |

### PCM5102 Configuration Pins

| Pin | Name | Connection | Function |
|-----|------|-----------|----------|
| 15  | FLT  | GND       | Normal operation, low-latency filter |
| 16  | DEMP | GND       | No de-emphasis |
| 17  | XSMT | 3.3V      | Unmuted (always active) |

### Preamp Stage Circuit

```
Guitar → J2 (TS jack)
  │
  ├── R1 (1MΩ) → GND       (input pulldown)
  │
  └── R2 (10kΩ) ──┬── TL072 (non-inverting input, pin 3)
                  │
                  │     TL072 (pin 6, output)
                  │       │
                  └── R3 (100kΩ) ──┐
                                   │
                                  TL072 (inverting input, pin 2)
                                   │
                                  R4 (10kΩ) → GND
```

Non-inverting gain = 1 + R3/R4 = 1 + 100k/10k = **11 (~20.8dB)**

For high-output humbuckers, this is sufficient to bring ~200mV pickup output to ~2.2V (near line level). Single-coils may need a second gain stage; omit R4 and R3 becomes a 470kΩ feedback resistor for gain of 2 (~6dB).

### Power Distribution

```
RPi 5V (pin 2/4)
  │
  ├── U1 (AMS1117-3.3) → 3.3V
  │   ├── C1 (10µF) + C2 (100nF)  ← input decoupling
  │   ├── C3 (10µF) + C4 (100nF)  ← output decoupling
  │   │
  │   ├── U3 (PCM1808) — VCC pin 11
  │   │   └── C6 (100nF) + C7 (10µF)  ← local decoupling
  │   │
  │   ├── U4 (PCM5102) — Vin pin 1
  │   │   └── C8 (100nF) + C9 (10µF)  ← local decoupling
  │   │
  │   ├── U2 (TL072) — VCC+ pin 8
  │   └── PCM1808 MD1 (pin 13) — pull to 3.3V for 48kHz
  │
  └── PCM5102 XSMT (pin 17) — pull to 3.3V for unmuted
```

### Ground Strategy

- **Star ground** at the RPi GND pin: all audio circuit grounds return to a single point
- Separate analog and digital ground traces on PCB
- Tie analog/digital ground planes together at the star point only
- Keep I2S digital lines separate from analog audio traces
- Input jack ground: connect to star via 10Ω + 100nF for ground loop isolation

---

## PCB Design Notes

- **2-layer board**, 65×56mm, 1.6mm FR4, HASL finish
- **Trace width:** 0.254mm signal, 0.5mm power
- **Clearance:** 0.2mm min, 0.3mm for audio traces
- **Via:** 0.6mm/0.3mm (outer/inner diameter)
- **Copper pour:** GND flood fill on both layers
- Keep I2S traces (BCLK, LRCLK, DIN, DOUT) equal length ±2mm
- Place decoupling caps within 5mm of IC power pins
- Separate analog audio section (left side of board) from digital I2S (right side)