# LM3886 Class-AB Power Amplifier

Non-inverting Overture™ audio power amp for the **midrange** channel of a 3-way active speaker. Closed-loop gain is **21 (26 dB)** into an **8 Ω** driver from **±25 V** rails.

## Circuit summary

| Block | Components | Why it exists |
|-------|------------|---------------|
| Input coupling | Cin = 1 µF film, Rin = 1 kΩ | Blocks source DC; sets input impedance with Rbias |
| Bias path | Rbias = 1 MΩ to GND | DC path for +IN bias current (keeps offset low) |
| Feedback | Rf = 20 kΩ, Ri = 1 kΩ | Gain = 1 + Rf/Ri = 21 |
| Feedback HF | Cf = 20 pF across Rf | Rolls off ultrasonic gain; aids stability |
| AC-coupled Ri | Ci = 250 µF to GND | DC gain = 1 → speaker DC offset ≈ Vos, not Vos×21 |
| Zobel | 10 Ω + 0.1 µF | Resistive HF load when speaker inductance rises |
| Output L | 0.7 µH \|\| 10 Ω | Isolates capacitive cables / EMC |
| Mute delay | 100 kΩ + 10 µF | ~1 s turn-on mute to avoid thump |
| Decoupling | 100 nF ceramic + 1000 µF electro per rail | HF bypass + bass current reservoir |

**VERIFY before PCB order:** pinout for your LM3886 package (T vs TF), mute pin polarity/threshold, SOA at ±25 V / 8 Ω, and heatsink θSA — see [TI LM3886 datasheet](https://www.ti.com/lit/ds/symlink/lm3886.pdf).

## Expected performance

| Parameter | Expected (typical) | Notes |
|-----------|-------------------|--------|
| Voltage gain | 26 dB (×21) | Flat midband |
| Bandwidth (−3 dB) | ~10 Hz to >50 kHz | LF set by Ci/Cin; HF by LM3886 GBW + Cf |
| THD+N (1 kHz, mid power) | <0.03% typical | Datasheet; behavioral SPICE will **not** match |
| Output power (8 Ω, ±25 V) | ~40–50 W continuous | Thermal/SOA limited — heatsink required |
| Mute delay | ~1 s | τ ≈ Rmute·Cmute |

## How to run the simulation

### ngspice (recommended for this repo)

```bash
cd lm3886-poweramp/spice
ngspice -b lm3886_poweramp.cir
```

The netlist runs:

1. **AC analysis** — 10 Hz–100 kHz, decade sweep → check Mag/Phase of `V(N_SPK)`
2. **Transient** — 1 kHz sine, 10 ms (mute pre-charged via `.ic`) → look for clipping
3. **`.four`** — Fourier / THD estimate on `V(N_SPK)`

### Official TI model (for accurate THD)

1. Download the LM3886 PSpice model from [ti.com/product/LM3886](https://www.ti.com/product/LM3886) → Design & development → Simulation models  
2. Place `LM3886.LIB` next to `lm3886_poweramp.cir`  
3. Uncomment `.include LM3886.LIB` and replace the behavioral VCVS block with the TI `.subckt` instance  
4. Re-run AC + TRAN

### LTspice

Open `lm3886_poweramp.asc`. Attach the TI symbol/model if available; otherwise use the netlist.

### What good results look like

- Midband gain ≈ **26 dB** (20·log10(|Vout/Vin|)) on AC plot  
- Phase smooth through audio band; no peaking near ultrasonic  
- Transient: clean 1 kHz sine at speaker node without flat-top clipping at 0.5 Vrms in  
- With TI model: THD well under 0.1% at moderate levels

## Files

| Path | Description |
|------|-------------|
| [`spice/lm3886_poweramp.cir`](spice/lm3886_poweramp.cir) | ngspice netlist (AC + TRAN + FOUR) |
| [`spice/lm3886_poweramp.asc`](spice/lm3886_poweramp.asc) | LTspice schematic |
| [`kicad/lm3886_poweramp.kicad_sch`](kicad/lm3886_poweramp.kicad_sch) | KiCad 8 schematic |
| [`docs/BOM.csv`](docs/BOM.csv) | Bill of materials |

## Heatsink note

At ±25 V into 8 Ω, plan for a heatsink with thermal resistance low enough that junction temp stays within datasheet limits under your duty cycle. Isolate the TO-220 tab (usually V−) from chassis with a proper insulator if needed — **VERIFY datasheet**.
