In a passive crossover the capacitor sets the high-pass slope and takes the full signal voltage. Metallised polypropylene (MKP) is the standard: low losses, tolerance stable over time, sensible price.
Choose the voltage rating generously. A 400 V part on a tweeter branch is not extravagance: transients run far above the average voltage, and an under-rated capacitor drifts in value before it ever fails.
The premium ranges, Tri-Reference or Fine First, mainly earn their place as a small-value bypass across the main capacitor. Replacing a sound MKP with a part four times the price throughout the chain rarely produces an audible difference.
Bipolar electrolytics still appear in commercial speakers for reasons of cost and size. Their wide tolerance and ageing make them a poor choice for a careful build.
Variants
| References | Audyn Cap MKP-Q4 |
|---|---|
| MKP-QS 400V | |
| MKP-QS 630V | |
| KP-SN | |
| MKP PLUS | |
| Tri-Reference | |
| Fine First |
In practice
Capacitor value drift is the most common cause of an old speaker sounding wrong without anything visibly failing. Electrolytics in particular lose capacitance as they age, which raises the crossover point and thins the sound gradually enough that the owner adapts to it.
When rebuilding a crossover, measure the parts you remove before assuming the schematic is right. A twenty-year-old filter frequently measures several per cent away from its printed values, and that difference is part of how the speaker has been sounding.
Choosing a value and rating
| MKP | Metallised polypropylene | Standard choice | Low loss |
|---|---|---|---|
| Rating | 400 V on treble | Transients exceed average | Avoid drift |
| Bypass | Small value in parallel | Premium series | Where it earns it |
| Avoid | Bipolar electrolytic | Wide tolerance | Ages in service |