Speaker Protection (24 V Relay)
Functions: (1) power-on delay to avoid thumps, (2) DC offset detection to protect speakers, (3) fast release on power-off.
Schematic
See speaker-protection.png. The relay coil is 24 V. Use a proper flyback diode (1N4148/1N4007) across the coil.
Parts list
| Item | Spec / Notes |
|---|---|
| Relay | 24 V DC coil, ≥8 A contacts (audio grade preferred) |
| Timing network | R-C for ~2–3 s delay (e.g., 100 k + 22 µF ≈ 2.2 s) |
| Comparator / transistor | LM358 / 2×NPN small-signal (for threshold + drive) |
| DC detect | ±0.7–1.0 V threshold with rectifier + RC average |
| Flyback diode | 1N4148 / 1N4007 across coil |
| Supply | 24 V DC (can be derived from amp’s aux rail + regulator) |
How it works
- Delay: RC charges after power-on; when threshold reached, transistor drives relay.
- DC detect: Senses average of speaker line. If DC magnitude > threshold, relay drops out.
- Power-off: Relay must release quickly—ensure coil supply collapses fast (series diode to prevent back-feed).
Build & wiring
- Route speaker lines through relay NO contacts; COM goes to speaker terminals.
- Keep detect wiring short; sense after power amp output Zobel/series resistor if present.
- Place flyback diode close to relay pins.
- Use star ground; keep power return away from small-signal ground.
Testing
- Confirm ~2–3 s delay on power-on before relay clicks.
- Inject slow ramp DC (bench supply via large resistor) to verify drop-out threshold ~0.7–1 V at output.
- Power-cycle quickly; relay should release immediately.