The series capacitor sealed box

Open the crossover schematic for the SB Acoustics Kinnara and one component stops you dead: a 1000µF bipolar electrolytic, in series with the woofer. It is not a filter and it is not a mistake. It is the bass alignment — and it is the reason the woofer that sits behind it had to be designed from scratch.

A sealed loudspeaker rolls off below its resonance at second order — 12 dB per octave. Put a large capacitor in series with the woofer and you add one more reactive element to that system. The result is a third-order high-pass: the resonance hump flattens, the useful response reaches a little lower than the box alone would manage, and everything below the corner is progressively starved of voltage.

The short version: the capacitor is not filtering the woofer. It is part of the enclosure alignment. Remove it and you do not get more bass — you get a peak, a higher corner frequency, and a woofer flapping uselessly at 20 Hz.

That last point is the one most often missed. Below the corner the capacitor's impedance climbs steeply, so the driver simply stops being driven. You get a subsonic filter for free, which on a driver rated 40 W with 11 mm of linear travel is not a small gift.

The Q hump flattens — the electrical rolloff pulls down exactly where the box resonance pushes up.
The corner moves lower than an unassisted sealed box of the same volume.
Sub-30 Hz excursion disappears, so power handling and clean output both improve.
The enclosure can be smaller than a conventional sealed alignment would allow.

This is not a new idea

The technique has a proper paper trail. The canonical reference is US Patent 4,383,134, “Loudspeaker systems”, granted in 1983 to Daniel R. von Recklinghausen and assigned to Electro Audio Dynamics. Von Recklinghausen was president of the Audio Engineering Society in 1967 and edited its journal from 1991 to 2004, so this is not fringe material.

The patent gives the design rule for the third-order case: the series capacitor should be roughly three times the driver's electrical equivalent capacitance, C_mes, where C_mes = M_ms / Bl². It also describes a fourth-order version, which needs a shunt inductor as well as the series capacitor — and claims a maximally flat fourth-order design needs only 23% of the cabinet volume of the equivalent second-order box.

Worth being precise about: a series capacitor on its own gives third order, not fourth. Fourth order requires the shunt inductor too. The Kinnara has only the capacitor, so the Kinnara is third order.

Run the Kinnara's numbers. The SB20PFCS30-7 has M_ms of 19.6 g and Bl of 5.6 Tm, so C_mes = 0.0196 / 5.6² = 625 µF. The textbook value would therefore be around 1875 µF. SB Acoustics chose 1000 µF — about 1.6 × C_mes, a deliberately more conservative alignment than maximally flat. Less extension, in exchange for better damping and group delay.

There is also a practical ceiling in play. 1000 µF is about as large as a capacitor can sensibly get inside a passive crossover before size and cost become absurd — which quietly constrains what any designer can do here.

The driver has to be built for it

This is the part that makes the Kinnara interesting rather than merely clever. A series-capacitor alignment needs a driver with a high Q_ts — an under-damped woofer that would misbehave in an ordinary sealed box. Most 8-inch woofers are the opposite.

So SB Acoustics built one. Put the SB20PFCS30-7 next to the 4 Ω and 8 Ω versions of its stablemate, the SB20PFCR30, and the intent is unmistakable.

Specification	SB20PFCR30-4	SB20PFCR30-8	SB20PFCS30-7
Magnet weight	0.54 kg	0.54 kg	0.36 kg
Gap flux density	1.2 T	1.2 T	0.91 T
Force factor, Bl	6.6 Tm	7.7 Tm	5.6 Tm
Total Q-factor, Q_ts	0.33	0.37	0.66
Free air resonance, F_s	34 Hz	34 Hz	39 Hz
Sensitivity (2.83 V / 1 m)	92.5 dB	90.5 dB	88 dB
DC resistance, R_e	3.4 Ω	5.6 Ω	5.0 Ω
Surround	Rubber	Rubber	Foam

Read that magnet row again. The -7 is not a rewind of the 4 Ω and 8 Ω models. It is the same 8-inch platform with a third of the magnet removed — flux down from 1.2 T to 0.91 T, Bl down from 7.7 to 5.6, and 2.5 dB of sensitivity deliberately thrown away — purely to push Q_ts from 0.37 to 0.66.

That is a driver designed backwards, from the alignment inwards. Every specification that looks like a weakness on the datasheet is a requirement of the topology it was built for.

As for why SB call it seven ohms rather than eight, when they already describe a 5.6 Ω driver as 8 Ω elsewhere in the range — we do not know, and we are not going to invent a reason. If SB publish one we will update this page.

SB Acoustics Kinnara floorstander seen from the front, showing the 8-inch paper woofer below a 26 mm aluminium dome tweeter on a tall narrow baffle. — The Kinnara. The 1000 µF capacitor behind that woofer is doing more for the bass than the cabinet is.

Reading the rest of the Kinnara crossover

The Kinnara is published as an open-source kit, so the full schematic is free to download and every claim below can be checked against it. Nothing in the circuit is decorative.

C3 — 1000 µF bipolar electrolytic, in series with the woofer. The alignment, as above.
L2 1.2 mH → C4 3.3 µF shunt → L3 1.5 mH, with R2 3.3 Ω across L3 — a third-order low-pass with the final inductor damped to control the knee.
L4 0.18 mH + R3 5.6 Ω + C5 6.8 µF to ground — a damped series-RLC trap. It resonates at roughly 4.5 kHz, well above the 2300 Hz crossover point, so it is suppressing paper cone breakup rather than shaping the crossover.
Tweeter: C1 6.8 µF → L1 0.22 mH shunt → C2 15 µF → R1 1.2 Ω — a third-order electrical high-pass on the 4 Ω SB26ADC-C000-4.

Total series inductance in the woofer path is 2.7 mH, which is a great deal. That is heavy built-in baffle-step compensation: the woofer is an 88 dB driver and the finished system is rated 84 dB, so roughly 4 dB has been spent buying flat in-room response instead of sensitivity. It is also why SB recommend 100–200 W.

One consequence worth stating plainly, because it will annoy some people: C3 cannot be upgraded. There is no 1000 µF film capacitor anyone is realistically putting in a loudspeaker. The single most consequential component in this crossover is a bipolar electrolytic, and it is staying one.

Should you try this yourself?

You can, but the driver decides whether it will work. The technique wants a woofer with a fairly high Q_ts — broadly above 0.4, and comfortably higher if you want the full benefit — in a box tuned to a total system Q around 1.0. Feed a modern low-Q_ts woofer through a big capacitor and you will simply lose bass.

Two practical cautions. First, the capacitor sits in the direct signal path carrying the full woofer current, so it needs an appropriate voltage rating and a bipolar type — SB specify 1000 µF / 100 V. Second, this alignment trades sensitivity for extension, every time. If you need loud from few watts, look elsewhere.

A genuinely useful side effect: because the capacitor rather than the cabinet is setting the low-frequency behaviour, the design is unusually forgiving of enclosure volume errors. We built a pair in a box roughly a third smaller than the reference and lost only a few hertz at the bottom — see the build notes linked below.

Our Kinnara build at Noising 2026

About the Kinnara kit

Sources

Daniel R. von Recklinghausen, US Patent 4,383,134 — Loudspeaker systems (Electro Audio Dynamics, 1983)
SB Acoustics — Kinnara kit page, crossover schematic and cabinet drawing
SB Acoustics — SB20PFCS30-7 specifications; SB20PFCR30-4 and SB20PFCR30-8 datasheets
diyAudio — About a series capacitor with closed-box loudspeaker
diyAudio — Disadvantages of a capacitor in series with the main driver
diyAudio — Third order sealed sub