OK,
AI content follows -- it asserts that the issue is the relatively high ESR and ESL of electrolytics and that the practice "is a very common practice in cost-effective yet high-quality speaker designs.":
_____
The reason for wiring a large electrolytic capacitor in parallel with a smaller film capacitor in a loudspeaker crossover is to
optimize performance across the entire audio frequency spectrum by combining the strengths of both capacitor types.
Here is the breakdown of why this specific pairing is used:
1. The Electrolytic Capacitor (The "Bulk" Storage)
- Role: Provides the high capacitance needed for low-frequency filtering.
- Why: To create a crossover point at low frequencies (e.g., for a woofer), you need a large capacitance value (often hundreds of microfarads). Electrolytic capacitors are the only practical choice here because they offer high capacitance in a small, affordable package.
- Weakness: Electrolytics have high Equivalent Series Inductance (ESL) and higher internal resistance (ESR). At high frequencies, their internal inductance causes them to lose effectiveness or even resonate, behaving like an inductor rather than a capacitor.
2. The Film Capacitor (The "High-Frequency" Cleaner)
- Role: Handles the high frequencies that the electrolytic struggles with.
- Why: Film capacitors (polypropylene, polyester, etc.) have extremely low ESL and low ESR. They remain stable and accurate at high frequencies where electrolytics fail.
- The Problem: They are physically large and prohibitively expensive to manufacture in the high capacitance values required for low-frequency crossovers.
3. The Combined Effect (Parallel Operation) When placed in parallel:
- Low Frequencies: The large electrolytic does the heavy lifting, providing the necessary reactance to block low frequencies from tweeters or pass them to woofers.
- High Frequencies: As frequency rises, the impedance of the electrolytic's internal inductance increases, making it ineffective. The smaller film capacitor, with its low inductance, takes over, ensuring the crossover circuit maintains its intended frequency response and phase characteristics in the high-frequency range.
Summary of Benefits:
- Cost Efficiency: You avoid buying a massive, expensive film capacitor by using a cheap electrolytic for the bulk of the value.
- Performance: You avoid the poor high-frequency response and distortion that would occur if you used only an electrolytic.
- Stability: It prevents the "self-resonance" of the electrolytic from causing peaks or dips in the speaker's frequency response.
In short, the electrolytic handles the
power and low-end, while the film capacitor ensures
clean, accurate high-end performance. This is a very common practice in cost-effective yet high-quality speaker designs.
A silly AI answer based on the wrong information.
This is the main problem for AI as it tends to generate answers the one asking wants to hear.
And lots of nonsense combined with some actual science is combined to create a 'very nice' looking answer people want to hear based on wrong info on many websites.
The paralleling of caps with very different values only works for decoupling capacitors at RF frequencies.
Because it works there some folks also used it for coupling caps where it also works but only at RF frequencies but has no influence at audible frequencies.
Of course that came with the usual 'audiophile' findings and then people started doing it for those reasons.
The irony is even that for RF this only really does something in electronics when it is done closely to the power supply pins of the (very fast) circuit itself.
For power supply caps in SMPS it can reduce HF currents in reservoir caps when they are on their way out making them 'live' a little longer.
It does not do anything for coupling caps and in case of crossover parts it could be used as a way to fine tune the filter frequency because caps are not available in just any value but only in standard values and have tolerances as well.
In this case when one requires/wants to can parallel a smaller value to a larger one but this won't be the same value and it won't be a 10nF across every cap.
Using a 10nF in parallel to a larger value (some even use 100nF) as a
local decoupling cap is common practice in circuits switching at RF frequencies or 'analog' circuits that might oscillate at such frequencies.
That's the only case where this works.... and, perhaps, in some feedback loops in very fast amplifier circuits.
So yes, the practice is common and based on actual science and does work well .... but NOT for coupling caps in the audio range NOR in passive XO's for speakers.
The fact that some manufacturers do this is no evidence the 'trick' works here too. In fact science says it cannot even work in this scenario (see post #21).
