Rogers AB1 Bass Extender: The Engineering Behind the Legendary LS3/5A Bass Stand

Published by IWISTAO | Vintage Hi-Fi | LS3/5A Engineering

Few loudspeakers have achieved the legendary status of the BBC LS3/5A. Its extraordinarily natural midrange, precise stereo imaging and ability to reproduce voices have made it one of the most admired compact monitors in audio history.

But the LS3/5A also has an unavoidable limitation: enclosure volume. A cabinet this small cannot reproduce truly deep bass at meaningful sound pressure levels.

Rogers approached this problem in a particularly elegant way. Instead of simply placing a conventional subwoofer beside the LS3/5A, the company developed the AB1 Auxiliary Bass System — a tall, narrow bass enclosure designed to sit directly underneath the LS3/5A and simultaneously function as its speaker stand.

The important point: The Rogers AB1 should be understood primarily as a bass extender, not as a modern 20–30 Hz home-theater subwoofer. Its purpose is to extend and strengthen the lower octave of the LS3/5A without destroying the tonal balance that made the monitor famous.

Table of Contents

1. What Is the Rogers AB1?
2. Why the LS3/5A Needed a Bass Extender
3. Rogers AB1 Specifications
4. The KEF B110 SP1228 Driver
5. The Fourth-Order Band-Pass Enclosure
6. The AB1 Passive Crossover
7. Crossover Component Values
8. How the Crossover Actually Works
9. Why the AB1 B110 Appears Reverse-Connected
10. Correct System Connection
11. Cabinet Construction and Port
12. 55–120 Hz: Why AB1 Is Not a Conventional Subwoofer
13. The Jo Ki Method
14. What We Can Learn From the AB1 Today
15. A Modern AB1-Inspired Design
16. Frequently Asked Questions
17. Conclusion

1. What Is the Rogers AB1?

The Rogers AB1 — generally understood as Auxiliary Bass One — was introduced in 1995 as a dedicated low-frequency extension system for the LS3/5A.

The design is associated with Rogers engineer Andy Whittle, who later provided technical information about the original crossover to LS3/5A enthusiasts and DIY constructors.

Visually, the AB1 looks more like an unusually substantial LS3/5A stand than a conventional subwoofer. That appearance is intentional.

Rogers LS3/5A with AB1 bass extenders in the original stand-style configuration. Image source: StereoNET.

Instead of wasting the space occupied by a conventional loudspeaker stand, Rogers turned that volume into an acoustic system capable of extending the LS3/5A's bass response.

2. Why the LS3/5A Needed a Bass Extender

The LS3/5A was originally conceived as a compact monitor for environments where physical space was extremely limited. Its design priorities were midrange accuracy, consistency and near-field monitoring — not deep bass.

Its small cabinet and approximately 110 mm B110 bass/midrange driver impose fundamental acoustic limitations.

The LS3/5A therefore cannot move the volume of air expected from a large floor-standing loudspeaker.

Simply increasing bass electrically is not an ideal solution. Below the natural operating range of the cabinet, additional amplifier power primarily produces greater cone excursion rather than proportional acoustic output.

• increased B110 excursion;
• higher distortion;
• lower maximum usable SPL;
• greater amplifier demand;
• possible masking of the LS3/5A's famous midrange clarity.

The AB1 solves the problem acoustically and electrically by transferring part of the lowest-frequency workload to an additional B110 driver.

3. Rogers AB1 Specifications

Specification	Rogers AB1
System type	Passive bass extender / auxiliary bass system
Enclosure	Fourth-order band-pass
Bass loading	Symmetrically loaded band-pass, approximately 24 dB/octave acoustic slope
Driver	KEF B110 family, normally SP1228
Driver diameter	Approximately 125 mm
Frequency response	Approximately 55–120 Hz (-3 dB)
Crossover frequency	Approximately 120 Hz
Nominal impedance	Approximately 8 Ω
Power handling (combined LS3/5A + AB1 system)	40 W unclipped programme*
Recommended amplifier	Approximately 25–100 W
Cabinet height	Approximately 570 mm
Width	Approximately 190 mm
Depth	Approximately 160–162 mm
Original plinth height	Approximately 24–25 mm
Total approximate height	595 mm

Power-rating note: Rogers published the combined LS3/5A + AB1 system at 40 W unclipped programme. Some secondary databases list the AB1 at approximately 50 W, so surviving literature may show both figures.

4. The KEF B110 SP1228 Driver

One of the most interesting aspects of the AB1 is Rogers' choice of driver. Rather than using a large conventional woofer, Rogers employed a member of the KEF B110 family.

The later 11-ohm LS3/5A used the KEF B110 SP1228, and this version was also normally used in the AB1.

• similar diaphragm material and mechanical character;
• similar transient behaviour;
• good tonal compatibility with the LS3/5A;
• controlled rather than exaggerated bass output;
• relatively easy integration around the 100–120 Hz region.

5. The Fourth-Order Band-Pass Enclosure

The B110 in the AB1 is not mounted conventionally on the front panel. Instead, it operates inside a fourth-order band-pass enclosure.

The enclosure consists conceptually of a sealed chamber and a vented chamber. The B110 is mounted on an internal partition separating these chambers, so the driver itself does not radiate directly into the listening room.

6. The AB1 Passive Crossover

The electrical crossover is arguably the most elegant part of the AB1. It requires only one large inductor and two capacitors, yet simultaneously creates a complementary relationship between the AB1 bass driver and the LS3/5A.

Rogers AB1 Bass Extender for LS3/5A crossover schematic. The network uses L1 = 16 mH (DCR ≈ 0.8 Ω), C1 = 4.7 µF polypropylene, and C2 = 220 µF non-polar/bipolar electrolytic. The AB1 B110 polarity and the “To LS3/5A” output are shown exactly as in the historical circuit drawing.

Physical AB1-Style Crossover Hardware

 

A modern pair of AB1-style passive crossover boards sold for LS3/5A / KEF B110 applications. The large inductors are marked 15 mH. 

Close-up of one reproduction board, showing the large 15 mH inductor and connection markings. 



An original crossover, just for you information.

Important identification note: These photographs show a later reproduction / replacement crossover, not a confirmed original Rogers factory AB1 crossover board. The historical Rogers/Andy Whittle schematic specifies L1 = 16 mH with approximately 224.7 µF total capacitance (220 µF + 4.7 µF). The photographed reproduction uses a 15 mH inductor, so it should not be labelled “Original Rogers AB1 Crossover.”

Shop Rogers AB1-Style Passive Crossover for LS3/5A Bass Extender System and Loudspeakers →

7. Original Crossover Component Values

Component	Value	Original Specification / Function
L1	16 mH	Main crossover inductor
L1 DCR	Approx. 0.8 Ω	Part of the intended electrical behaviour
Inductor wire	Approx. 1 mm	Original winding information
C1	4.7 µF	Polypropylene, 63 V, 5%
C2	220 µF	Original drawing: electrolytic, 63 V, 5%. For modern replacement, use a non-polarised / bipolar crossover capacitor.

Ctotal = C1 + C2 = 4.7 µF + 220 µF = 224.7 µF

An AB1-inspired crossover PCB implementation. This reconstruction uses a 15 mH ferrite-core inductor with adjusted capacitance rather than the exact original 16 mH value. Source: LS3/5A.org.

8. How the Crossover Actually Works

For the LS3/5A, the capacitor network is effectively in the signal path, while the 16 mH inductor provides complementary low-frequency shunt behaviour. The result is a second-order high-pass characteristic for the LS3/5A.

• B110 cone excursion inside the LS3/5A is reduced;
• intermodulation distortion can be reduced;
• the monitor can operate more comfortably at higher levels;
• the AB1 assumes responsibility for the lowest musical frequencies.

The AB1 driver receives the complementary low-frequency energy, and its electrical response combines with the natural acoustic behaviour of the band-pass enclosure.

9. Why Does the AB1 B110 Appear Reverse-Connected?

The original AB1 crossover drawing contains an unusual detail: the amplifier positive terminal connects to the terminal marked negative on the AB1 B110, while the B110 positive terminal connects toward the crossover output node.

See the Rogers AB1 crossover schematic above ↑

DIY note: When reproducing a historical crossover, do not change driver polarity simply because it appears unconventional. The acoustic phase relationship, not the visual polarity marking alone, determines correct summation.

10. Correct System Connection

Rogers AB1 cabinet and rear binding posts. The AB1 sits electrically between the amplifier and the LS3/5A in the original arrangement. Image source: Rogers HiFi Germany.

This arrangement differs fundamentally from simply connecting a passive woofer in parallel with the LS3/5A. The LS3/5A receives the high-pass-filtered signal, which is a major part of the engineering concept behind the AB1.

11. Cabinet Construction and Port

Cabinet Feature	Approximate Value
Main cabinet height	570 mm
Width	190 mm
Depth	160–162 mm
Original plinth height	Approximately 24–25 mm
Original Rogers cabinet specification	15 mm Medite, critically damped
Reconstruction / measured cabinet walls	Approximately 18 mm MDF reported in a well-known AB1 reconstruction
Internal B110 partition (reconstruction)	Approximately 12 mm MDF
Original port diameter	Approximately 42 mm
Original port length	Approximately 85 mm

Cabinet-material note: Published Rogers specifications describe the AB1 cabinet as 15 mm Medite, critically damped. A widely referenced reconstruction based on measurements of original cabinets reports approximately 18 mm MDF cabinet walls and a 12 mm internal B110 baffle. These should be treated as two different source descriptions rather than as identical factory specifications.

Foam lining and installed B110 drivers in an AB1-inspired reconstruction. Source: LS3/5A.org.

Airtight Construction Is Critical

• airtight cabinet joints;
• sealed driver gasket;
• sealed internal partition;
• well-sealed removable rear panel;
• controlled damping material;
• correct port dimensions.

Why Port Diameter and Length Matter

The vented chamber behaves as a Helmholtz resonator. Changing port diameter without changing length will alter the tuning frequency.

For example, documentation from an AB1 reconstruction project reports that when a larger approximately 46 mm internal-diameter port was used, the port length was increased to roughly 105 mm to maintain a similar acoustic tuning.

12. 55–120 Hz: Why the AB1 Is Not a Conventional Subwoofer

55 Hz – 120 Hz (-3 dB)

The AB1 fills the region immediately below the LS3/5A rather than attempting to reproduce the lowest effects channel of a modern cinema soundtrack.

System	Primary Objective
LS3/5A alone	Compact monitor with exceptional midrange and imaging
LS3/5A + AB1	Extend useful musical bass while preserving LS3/5A character
Modern large subwoofer	Very deep low-frequency extension and high acoustic output

13. The Jo Ki Method

Over the years, LS3/5A enthusiasts have experimented with alternative ways of integrating the AB1. One well-known approach became associated with audiophile Jo Ki.

Instead of necessarily placing the LS3/5A directly on the AB1 and feeding the monitor through the original crossover, this method can use the LS3/5A on a separate stand and connect the AB1 in parallel. This changes the electrical and acoustic behaviour and should therefore not be confused with the original Rogers configuration.

 

Jo Ki's alternative placement method: the LS3/5A remains on a dedicated stand while the AB1 is positioned separately behind it. Source: LS3/5A.org.

Original Rogers principle:
Amplifier → AB1 crossover → LS3/5A

Alternative enthusiast approach:
AB1 and LS3/5A operated more independently or in parallel.

14. What Can Modern DIY Designers Learn From the AB1?

14.1 Do Not Automatically Use a Large Woofer

For bass extension around 50–120 Hz, integration can matter more than maximum cone area.

14.2 Relieve the Main Monitor of Deep Bass

A properly designed high-pass filter can offer benefits beyond frequency response extension. Reducing low-frequency excursion in the main speaker can improve usable headroom and reduce distortion.

14.3 Acoustic and Electrical Filters Work Together

The AB1 illustrates that crossover design cannot be separated from enclosure design. The electrical crossover, sealed chamber, vented chamber, port tuning, driver response and LS3/5A acoustic roll-off all interact.

14.4 Bass Extension Does Not Have to Mean More Bass

Good low-frequency extension should make the system sound larger and more complete, not necessarily more obviously bass-heavy.

15. A Modern AB1-Inspired Design

• one bass extender per channel;
• 5-inch or 6.5-inch low-mass woofer;
• approximately 50–55 Hz lower system target;
• approximately 90–120 Hz crossover region;
• passive or active high-pass filtering for the main monitor;
• cabinet dimensions compatible with LS3/5A stand height;
• stereo bass rather than a single centrally positioned subwoofer;
• adjustable bypass option for comparison.

Engineering warning: The 16 mH / 224.7 µF network is part of a complete system. Substituting another woofer while retaining the original crossover values does not automatically reproduce the Rogers AB1 response. Driver impedance, Fs, Qts, Vas, enclosure volume and acoustic response must all be considered.

16. Frequently Asked Questions

Is the Rogers AB1 a subwoofer?

Technically it performs a subwoofer-like function, but bass extender is a more accurate description. Its primary operating region is approximately 55–120 Hz rather than the 20–80 Hz range commonly associated with modern deep-bass subwoofers.

Was the AB1 designed specifically for the LS3/5A?

Yes. Its cabinet proportions, crossover arrangement and use of a KEF B110 driver were intended to complement the LS3/5A.

Which driver did the Rogers AB1 use?

The AB1 normally used the KEF B110 SP1228, the later B110 version associated with the 11-ohm LS3/5A.

What is the AB1 crossover frequency?

The commonly published crossover frequency is approximately 120 Hz.

What are the original crossover values?

The original crossover information supplied by AB1 designer Andy Whittle specifies a 16 mH inductor, a 4.7 µF polypropylene capacitor and a 220 µF capacitor.

Should the 220 µF capacitor be polarised?

The historical drawing specifies a 220 µF electrolytic capacitor, 63 V, 5%. For a modern replacement in this loudspeaker crossover, use a non-polarised / bipolar electrolytic crossover capacitor.

Can a 15 mH inductor replace the original 16 mH part?

It is possible to redesign the network around another inductor value, but the capacitance should be recalculated rather than simply substituting the component.

Can I use a different woofer?

Yes, but the crossover and enclosure must be redesigned around the new driver's Thiele/Small parameters and impedance.

Should the LS3/5A always sit directly on top of the AB1?

That was the original physical concept, but some enthusiasts prefer using the LS3/5A on separate stands and positioning the AB1 independently.

Does AB1 work with both 15-ohm and 11-ohm LS3/5A?

Yes. Rogers intended the AB1 for use with both earlier 15-ohm LS3/5A systems and later lower-impedance versions, including standard and bi-wire models. The AB1 itself normally used the KEF B110 SP1228. Because different LS3/5A generations have different impedance characteristics, the exact electrical behaviour is not identical, so precision restoration or measurement work should still be based on the actual loudspeaker being used.

17. Conclusion

The Rogers AB1 is a fascinating example of loudspeaker engineering because its apparent simplicity hides a carefully integrated system.

A KEF B110 driver, two acoustic chambers, a tuned port and an unusually simple passive crossover transform what appears to be an LS3/5A stand into a genuine low-frequency extension system.

Its approximately 55 Hz lower limit may seem modest compared with modern high-powered subwoofers, but that comparison misses the point.

The objective of the AB1 was never maximum bass. The objective was to preserve what the LS3/5A already did exceptionally well while extending its useful low-frequency range and reducing stress on its own B110 driver.

Rogers AB1 design philosophy in one sentence:

Extend the bass without changing the character of the LS3/5A.

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Technical References

Technical information in this article has been cross-checked against published Rogers AB1 specifications, LS3/5A archival resources, KEF B110 documentation and crossover information supplied to enthusiasts by original AB1 designer Andy Whittle. Where published Rogers specifications and later reconstruction measurements differ, both are identified separately rather than treated as the same source.

As with any vintage loudspeaker, production variations and differences between surviving examples may exist. Measurements of an actual unit are recommended before attempting restoration or exact reproduction.

© IWISTAO HIFI MINIMART. This article is intended for technical, historical and educational discussion of vintage audio engineering. Rogers, KEF and LS3/5A-related trademarks belong to their respective owners.

How to Choose Audio Power Amplifier Chassis

Published by IWISTAO · Speaker Design

From the Sonab OA-5 to the Carlsson OA-52, Stig Carlsson designed loudspeakers around the reflections, boundaries, and furniture of an ordinary living room.

Most loudspeakers are explained as if the cabinet ends at the grille. Carlsson loudspeakers make more sense when the explanation begins outside the box: with the wall behind it, the floor below it, the first reflected sound, and the listener seated across the room.

Swedish engineer Stig Carlsson developed this room-aware approach over several decades. His designs changed substantially—from the multi-tweeter Sonab models of the 1960s to the absorber-equipped two-way systems of the 1980s—but the recurring goal was to make a loudspeaker behave predictably in a normally furnished domestic room. The term associated with that work is Ortho-Acoustic (often written “ortho-acoustical”).^[1]

Table of Contents

1. The loudspeaker-room system
2. Three design generations
3. Key models at a glance
4. Why the OA-52 matters
5. Placement and restoration
6. Lessons for modern DIY

1. The Loudspeaker and Room Are One Acoustic System

In a domestic room, the listener receives direct sound first and then a succession of reflections from the floor, nearby walls, ceiling, and furniture. A conventional free-standing speaker is often placed away from boundaries to reduce their influence. Carlsson’s later designs took a different route: they specified the nearby wall as part of the operating condition, used broad radiation where it was useful, and placed absorbers where selected early reflections would otherwise interfere.

The defining idea is not “make every frequency omnidirectional.” It is “control direct and reflected sound together, in the room where the speaker will actually be used.”

Carlsson documentation describes the OA-52 as a floor-standing model intended for placement against a wall, with a damped floor and a reflecting ceiling. Its built-in absorber panels address selected early reflections near the loudspeaker.^[2] This is why the driver angle, cabinet corner, wall, rug, and visible absorber cannot be treated as unrelated styling details.

Figure 1: Conceptual room interaction—not a dimensional drawing. Carlsson designs use boundaries deliberately and control selected early reflections.

2. Three Generations of Carlsson Thinking

1960s: OA-5 and the multi-tweeter approach

Sonab introduced the OA-5 in 1966. Its model number literally referred to its four tweeters plus one mid-woofer. Rather than mounting every driver on a vertical front baffle, the OA-5 used an upward-facing arrangement intended to distribute energy into the room. The raised-baffle OA-5 Type II later addressed a high-frequency problem associated with the recessed baffle of the original version.^[1]

1970s: more direct sound for stereo

The OA-12, OA-14, OA-116, and OA-2212 increased the proportion of sound directed toward the listening area. Their moulded baffles angled the mid/bass or midrange drivers toward the listener, reflecting Carlsson’s adaptation from earlier mono-oriented, broadly radiating systems to stereo reproduction.^[1]

The compact OD-11, launched in 1974, reduced the formula to one mid-woofer and one tweeter in a recessed, angled baffle. Its “11” designation referred to that one-plus-one driver layout. Teenage Engineering later developed a licensed modern OD-11 with the Stig Carlsson Foundation, demonstrating the continuing appeal of the original cube.^[3]

1980s: fewer drivers, deliberate absorption

The OA-51 was publicly presented in 1982, followed by the floor-standing OA-50 and OA-52 in 1984. These were two-way systems with driver baffles placed near a room boundary and absorbent material close to the drivers. The OA-51 was designed for elevated wall placement; the OA-50 and OA-52 were designed to work on the floor against a wall.^[4][2]

The model-number logic also changed. Earlier names such as OA-5 and OD-11 described driver counts, but OA-50, OA-51, and OA-52 are all two-way systems; the “50-series” labels should therefore not be read as a simple count of installed drivers.^[1][2]

3. Key Models at a Glance

Model	Introduced	Basic architecture	Design significance
OA-5	1966	1 mid-woofer + 4 tweeters	Early, broadly radiating Sonab success with an upward-facing driver platform.
OA-12 / OA-14	1973	1 woofer + 2 or 4 tweeters	More direct sound and mirrored stereo pairs.
OD-11	1974	1 mid-woofer + 1 tweeter	Compact “Carlsson Cube” with recessed, angled drivers.
OA-51	1982	Wall-mounted two-way	Scan-Speak 7-inch mid-woofer, Peerless tweeter, and a removable local absorber.
OA-50	1984	Floor-standing two-way	Peerless SC165 mid/bass; lower-cost entry in the OA-50 series.
OA-52	1984	Floor-standing two-way bass reflex	Flagship form with Scan-Speak 7-inch mid/bass and visible absorber system.

4. Why the OA-52 Deserves Special Attention

The OA-52 is a useful case study because its architecture makes the Ortho-Acoustic method visible. The original version used a Scan-Speak 18W/8542 paper-cone mid/bass driver and a Peerless 115DT26 25 mm textile-dome tweeter in an approximately 20-litre enclosure. CarlssonPlanet lists 30 Hz bass-reflex tuning and a nominal 2.6 kHz crossover. The cabinet was built as a mirrored pair, intended to stand on the floor against the wall.^[2]

Those specifications are only part of the design. The drivers sit on a raised corner baffle. Absorbent material lies close to the radiating area and extends toward the rear wall. This geometry helps manage the earliest local reflections while allowing later room energy to contribute to the listening impression. The enclosure, absorber, wall, and floor treatment therefore form a single installation.

Figure 2: A conceptual OA-52-inspired installation. It illustrates relationships, not original cabinet dimensions or driver angles.

Engineering caution: The published OA-52 data describe a complete system, not a universal recipe. Substituting a modern seven-inch driver changes the impedance, acoustic output, bass alignment, and crossover target. Copying the original filter into a visually similar cabinet is unlikely to reproduce the original result.

5. Placement, Buying, and Restoration

A vintage Carlsson should first be evaluated as the correct model and revision. OA-50, OA-51, and OA-52 upgrade generations can use different woofers, tweeters, and crossover networks. Confirm the label, driver part numbers, crossover version, and whether both speakers remain a matched mirrored pair before ordering parts.

• Follow the intended boundary condition. Do not audition an OA-50 or OA-52 far into the room and assume the result represents its design target.
• Inspect the absorbers. Missing, compressed, replaced, or incorrectly positioned material can change the early-reflection behaviour.
• Check ageing parts carefully. The original OA-52's Scan-Speak 18W/8542 foam surround is a known age-related service item and should be inspected before the speaker is played. Tweeter output and pair matching, capacitors, wiring, and enclosure seals should also be checked; replacement decisions should follow inspection and measurement rather than age alone.^[5]
• Restore symmetrically. Component tolerances and left/right geometry matter in a stereo pair.
• Measure in the intended room position. Near-field driver tests, gated measurements, impedance sweeps, and listening-position data answer different questions; no single trace describes the whole system.

6. What Modern DIY Designers Can Learn

A useful Carlsson-inspired project does not need to copy an antique driver list. It should preserve the method: define the wall and floor position first, choose the desired directivity, identify harmful early reflection paths, and then design the baffle, absorber, enclosure, and crossover as a coordinated system.

1. Write the placement specification before drawing the cabinet. Wall distance and listening height are design variables.
2. Measure drivers on the real angled baffle. Manufacturer curves on an IEC baffle cannot predict the finished off-axis field.
3. Model boundary gain and bass alignment together. A response that looks lean in free space may be intentional near a wall.
4. Prototype the absorber. Its size, density, thickness, covering, and position affect a frequency-dependent reflection path.
5. Use crossover or DSP only after acoustic measurements. Electrical textbook filters do not guarantee an acoustic sum on an angled, boundary-coupled baffle.
6. Validate more than the sweet spot. Compare listening-window, early-reflection, and in-room measurements to see whether the design behaves consistently across seats.

The lasting contribution of Stig Carlsson is therefore larger than any one cabinet shape. He treated the domestic room as an unavoidable—and potentially useful—part of loudspeaker engineering. That principle remains relevant whenever a designer asks not only “How does this speaker measure?” but also “Where will it stand, what will reflect its sound, and what will the listener actually hear?”

Frequently Asked Questions

Are all Carlsson speakers omnidirectional?

No. Early models used very broad, multi-directional radiation, but later stereo models increased direct sound. The 1980s OA-50 series combined wide radiation with specific driver aiming and local absorption. “Room-aware” is more accurate than treating every Carlsson as equally omnidirectional.

Should an OA-50 or OA-52 be pulled away from the wall?

Normally no. These models were designed for floor placement against a wall, with the room boundary included in their acoustic balance. Small adjustments may help in a particular room, but conventional free-standing placement changes the intended condition.

Why does the OA-52 have visible absorbent material?

It helps attenuate selected early reflections near the driver and rear wall. It is a functional acoustic element, not simply cabinet decoration or internal stuffing.

Can an original Carlsson crossover be used with modern drivers?

Not reliably. Even similarly sized drivers differ in sensitivity, impedance, resonance, directivity, and acoustic centre. A modern replacement requires new measurements and usually a redesigned crossover or DSP setup.

Which Carlsson model is the best starting point for a modern DIY study?

The OA-52 is especially instructive because its wall placement, angled two-way baffle, bass-reflex cabinet, and absorber system are easy to identify as interacting design elements. The OA-51 is equally valuable when wall mounting and compact form are priorities.

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References

1. CarlssonPlanet, “History of the Carlsson Loudspeakers.” https://carlssonplanet.com/en/speakers/history-of-the-carlsson-loudpeakers/
2. CarlssonPlanet, “Carlsson OA-52,” original-model overview, placement guidance, and technical data. https://carlssonplanet.com/en/speakers/produced/carlsson-oa-52/
3. Teenage Engineering, “Carlsson Story: OD-11.” https://teenage.engineering/products/od-11/carlssonstory
4. CarlssonPlanet, “Carlsson OA-51,” documenting its first presentation in May 1982, wall placement, and original drivers (Swedish). https://carlssonplanet.com/hogtalare/producerade/carlsson-oa-51/
5. CarlssonPlanet, “Renovation & Restoration,” identifying the foam-surround Scan-Speak 18W/8542 used in the OA-51 and OA-52. https://carlssonplanet.com/en/renovation-restoration/

© 2026 IWISTAO. All rights reserved.

Full-Range Speaker Notch Filters: Correct Topologies, Measurement, and Safe Design

 PUBLISHED BY IWISTAO · AUDIO ENGINEERING

An LCR network can correct a narrow response peak, but only when the complete source, filter, and driver circuit is designed together.

A full-range driver may develop an upper-midrange or treble peak when its cone, whizzer, or dust cap no longer moves as a single rigid piston. A notch filter built around an LCR (inductor–capacitor–resistor) network can help, but exactly what it changes depends on how it is wired. A network intended to flatten electrical impedance is not automatically a filter for an acoustic response peak.

Table of Contents

1. What Is a Full-Range Speaker Notch Filter?
2. Why Full-Range Drivers Develop Harsh Peaks
3. Two Different Jobs: Impedance Compensation vs. Acoustic Notch
4. Why a Series Impedance Is Required
5. Designing an Acoustic Notch: Q, Bandwidth, Depth
6. The Impedance-Compensation Variant (Thiele/Small Formulas)
7. Step-by-Step Implementation
8. Amplifier and Component Safety Checks
9. Common Mistakes to Avoid
10. When You May Not Need One
11. Frequently Asked Questions

1. What Is a Full-Range Speaker Notch Filter?

A notch filter (also called a band-stop or band-reject filter) attenuates a limited frequency band. One useful loudspeaker topology is a series LCR shunt: an inductor (L), capacitor (C), and resistor (R) wired in series, with that branch connected in parallel with the driver. This branch creates an acoustic notch only when the surrounding circuit supplies the required series impedance.^[1][2]

The same LCR building block is used for two different jobs, and conflating them is the most common mistake in this corner of DIY audio. One job is to flatten the driver’s electrical impedance curve; the other is to cut a peak in its acoustic output. Which one you actually achieve depends entirely on what impedance sits in series with the trap — covered in sections 3–6.

2. Why Full-Range Drivers Develop Harsh Peaks

Depending on its construction, a driver may stop behaving as a rigid piston and develop one or more breakup modes. The resulting response may contain a narrow peak, a broad rise, several smaller ripples, or no serious on-axis problem at all. Cone material, diameter, whizzer geometry, baffle, enclosure, and listening axis all matter.

Harshness at higher listening levels is not automatically breakup. Driver nonlinearity, compression, amplifier clipping, or room reflections can produce similar symptoms. Measure the driver in its intended baffle with a calibrated microphone, repeat the sweep, and compare several listening angles before adding parts.^[4]

3. Two Different Jobs: Impedance Compensation vs. Acoustic Notch

The same LCR trap is used for two distinct tasks, and treating them as interchangeable is the central error this article corrects:

• Impedance compensation. A complementary series-LCR branch can flatten an electrical impedance peak — commonly the peak at the driver’s resonance f_s. This makes a passive crossover see a more predictable load. It does not, by itself, guarantee an acoustic response correction.^[2][3]
• Acoustic notch. Cutting a peak in the driver’s SPL requires the network to reduce voltage or current delivered to the driver around the target frequency. In the shunt series-LCR topology, that requires a meaningful upstream series impedance.^[1][3]

4. Why a Series Impedance Is Required

At resonance the LCR trap’s reactances cancel, so the branch impedance collapses to roughly R (plus the coil’s DC resistance). What that low impedance does depends on what is in series with it:

• Shunted straight across the driver, driven by a low-output-impedance amplifier (an approximation of a constant-voltage source), the trap mainly draws extra current and lowers the total load impedance. The amplifier holds the driver’s terminal voltage nearly constant, so the driver’s acoustic output changes very little.
• With a defined series impedance in the signal path — a series resistor, or the series elements of a passive crossover — the trap at resonance forms a voltage divider. The series element then drops a larger share of the source voltage, so the driver receives less and its output dips at f₀.^[1][3]

In short, the trap alone does not “steal energy” from the driver under a stiff voltage source. The series impedance is what converts the trap into an attenuation. Standard shunt-resonator examples therefore include a series element and warn that the resonant branch can present a heavy load to the source.^[1]

Figure 1: Two passive notch configurations. A shunts an LCR trap directly across the driver and can flatten a matched electrical-impedance peak. B inserts a series impedance Rs upstream so the trap actually attenuates the driver’s acoustic output at f₀.

The complete divider, not the resonant-frequency formula alone, determines the driver voltage:

Z_trap = R_total + j(2πfL − 1/(2πfC))
Z_p = Z_d || Z_trap
V_d / V_in = Z_p / (Z_series + Z_p)

If Z_series approaches zero, V_d approaches V_in even at resonance. The trap may still draw substantial current, so a circuit that produces little acoustic change can create significant electrical stress.

Figure 2 is an illustrative response shape, not a measurement. It shows a sharp breakup peak near 4 kHz before correction and the smoother result after a notch is applied. For publication, replace it with your own REW or CLIO sweep, including axes, measurement distance, and baffle conditions.

Figure 2: Illustrative on-axis response shape of a full-range driver (schematic — not measured, no calibrated axes or measurement conditions). The notch smooths the localized breakup peak; actual depth and width depend on the series impedance and driver impedance (section 5).

5. Designing an Acoustic Notch: Q, Bandwidth, Depth

For an acoustic notch, start from a measurement of the driver’s on-axis SPL and impedance in its final baffle. Locate the peak frequency f₀ and its bandwidth, then size the trap:

f₀ = 1 / (2π√(L·C))
Q = √(L / C) / R_total
BW = R_total / (2πL)   (so BW = f₀ / Q)

where R_total is the total series loss inside the trap: the external resistor, the inductor’s DC resistance, capacitor ESR, and any material wiring or contact resistance. These equations describe the isolated series-LCR branch current and its half-power bandwidth (high Q = narrow). They do not directly give the acoustic SPL notch’s −3 dB bandwidth. Both the acoustic depth and width depend on Z_series, the driver’s complex impedance and phase, and the rest of the passive network. A complete design therefore needs measured impedance and the complete circuit, not just L, C, and R.^[1][3][8]

Choose the starting Q from the measured peak bandwidth. If the response peak implies a Q near 3–5, that can be a useful starting range, but it is not a universal rule. Begin with a conservative correction and deepen it only after re-measuring the complete system.

Illustrative 4 kHz branch

With L = 0.50 mH, the resonance equation gives C ≈ 3.17 µF at 4 kHz. If R_total is 4.0 Ω, Q_branch is approximately 3.14. These values describe only the LCR branch; they do not predict acoustic attenuation until Z_series and the measured driver impedance are included in the divider above.

6. The Impedance-Compensation Variant (Thiele/Small Formulas)

If the goal is to flatten a single electrical impedance peak associated with the driver’s fundamental resonance — rather than a 3–8 kHz acoustic breakup peak — starting LCR values can be estimated from Thiele/Small parameters^[2]:

C = 0.1592 / (R_e · Q_es · f_s)
L = 0.1592 · Q_es · R_e / f_s
R = R_e + (Q_es · R_e / Q_ms)

These formulas apply most directly when the driver is characterized under the same condition in which the compensation network will be used, such as a free-air driver or a closed-back tweeter with one dominant resonance. A cabinet changes the system impedance: a sealed enclosure shifts the resonance, while a vented enclosure normally produces two low-frequency impedance peaks. For an installed full-range driver, design from the measured in-box impedance curve; one network calculated only from datasheet free-air parameters may not be sufficient.^[7]

Worked example using a Vifa XT25TG30-04 tweeter (f_s = 436 Hz, Q_es = 0.54, Q_ms = 2.5, R_e = 3 Ω): C ≈ 225 µF, L ≈ 0.59 mH, R ≈ 3.65 Ω.^[6] This example flattens impedance; it is not a model for an acoustic breakup notch. The required R is total branch resistance, so the inductor’s measured DCR must be subtracted when selecting the external resistor.^[2]

7. Step-by-Step Implementation

1. Measure. Capture the driver’s SPL and impedance in the final baffle. Check on-axis and several off-axis responses, locate the peak frequency and bandwidth, and repeat the sweep to confirm it.^[4]
2. Decide the goal. Impedance compensation, or a true acoustic notch? They use the same LCR parts but different wiring.
3. Choose the topology for the complete circuit. A shunt series-LCR branch needs upstream series impedance. A series-inserted resonant topology or DSP may be more appropriate for a directly driven full-range unit.
4. Simulate before building. Import measured frequency and impedance data into a loudspeaker simulator such as VituixCAD, model the entire network, and inspect on-axis, off-axis, phase, and impedance results.^[5]
5. Set f₀ and Q. Choose L and C for f₀; set the electrical branch Q through R_total, then simulate and adjust the complete divider for the intended acoustic depth and width.
6. Account for series losses. Subtract the coil’s measured DCR and any other material series losses from the target R_total when selecting the external resistor.
7. Verify by measurement and listening. If the correction is too deep, raise R_total or reduce Z_series, then re-check the response and minimum impedance.

8. Amplifier and Component Safety Checks

A shunt trap lowers the parallel load impedance near f₀. Reducing R_total makes that local load more demanding; increasing the upstream series impedance can deepen acoustic attenuation but also changes passband level and amplifier loading. Before finalizing:

• Minimum system impedance. The driver and trap are in parallel, so near resonance the combined load drops. If the driver is approximately resistive and measures 8 Ω at f₀, placing it in parallel with a ~3.65 Ω trap branch gives about 2.5 Ω; a real driver may have a different magnitude and phase there. Confirm the amplifier is stable and within its rated load, and include any series resistor in the complete load calculation^[1]. (Under a near-constant-voltage source the driver voltage stays roughly constant — the trap mostly draws current — which is exactly why this wiring alone does not cut SPL.)
• Impedance phase. A passive trap adds reactive phase; check the combined impedance phase near f₀.
• Coil saturation. Resonant current through the inductor can be high; prefer air-core or adequately rated parts.
• Resistor power. Calculate continuous and peak dissipation at the intended drive voltage, then provide suitable wattage and ventilation.
• Capacitor type. Use a suitably voltage-rated non-polar component. Film capacitors are common at modest values; bipolar electrolytics may be practical when the required capacitance is large.

9. Common Mistakes to Avoid

• Assuming a shunt trap alone cuts SPL. Without a series impedance, a correctly tuned and damped branch can flatten a matched electrical-impedance peak, but it does not by itself correct the acoustic peak under a stiff voltage source^[3].
• Equating impedance flattening with an acoustic notch. They are different circuit functions and different formulas apply^[2].
• Over-notching. Too wide or too deep makes the speaker dull, hollow, or lifeless even when the graph looks tidy^[1].
• Ignoring the inductor DCR and the series element. Both set the real damping and depth.
• Trusting the nominal impedance. Use the measured driver impedance at the peak, not the “8 Ω” label — its value there is not predictable from the nameplate.

10. When You May Not Need One

A notch is not universal. If the breakup peak is already well outside the intended passband, a sufficiently steep crossover may suppress it. In an active system the mechanical breakup does not disappear, but a parametric EQ cut before the power amplifier can reduce drive at the measured peak without creating a low passive load. Passive impedance compensation is generally unnecessary when no passive crossover depends on a flattened driver impedance.^[3] Some well-behaved full-range drivers need no narrow correction at all.

Frequently Asked Questions

Will a notch filter affect the rest of the frequency range?

Yes, to a limited extent. Every finite-Q notch has skirts, loss, tolerance, and phase effects. A well-designed high-Q correction can keep the affected band narrow, but it never changes only one mathematical point.^[1]

Can I use just a resistor instead of an LCR trap?

A shunt resistor can reduce the height of an impedance peak, but it lowers impedance over a broad range. It is not a frequency-selective substitute for a correctly designed LCR network.^[2]

How do I find the right frequency?

Measure the installed driver in REW with a calibrated measurement microphone and the appropriate calibration file. Confirm the peak on repeated sweeps and inspect more than one listening angle before treating the on-axis maximum as f₀.^[4]

Is Q the same as depth?

No. Branch Q describes the isolated LCR branch’s electrical bandwidth. The final acoustic notch depth and width depend on the complete divider, including series impedance and the driver’s complex impedance at f₀.^[1][8]

Do active or DSP systems remove breakup?

No. DSP can reduce drive at the measured peak, but the mechanical breakup mechanism remains. The correction still has to be measured and verified.^[4]

Shop Full-Range Speaker Notch Filter →

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References

1. All About Circuits. “Resonant Filters.” Series- and parallel-resonant band-stop circuits, source resistance, and loading. https://www.allaboutcircuits.com/textbook/alternating-current/chpt-8/resonant-filters/
2. Elliott Sound Products (Rod Elliott). “Impedance Compensation for Passive Crossovers.” https://sound-au.com/articles/z-compensation.htm
3. Elliott Sound Products (Rod Elliott). “Passive Crossover Network Design.” https://sound-au.com/lr-passive.htm
4. Room EQ Wizard. “Making Measurements.” https://www.roomeqwizard.com/help/help_en-GB/html/makingmeasurements.html
5. Kimmo Saunisto. “VituixCAD Features.” https://kimmosaunisto.net/Software/Software.html
6. Tymphany HK Ltd. “XT25TG30-04 Transducer Specification Sheet,” Rev. 1.0, September 10, 2009. https://audioalchemy.ro/difuzoare/vifa/xt25tg30-04e.pdf
7. Dayton Audio. “DATS LA Product Manual,” section “Using DATS LA to Evaluate a Vented Box Loudspeaker.” https://www.daytonaudio.com/images/resources/390-805--dayton-audio-dats-la-manual.pdf
8. All About Circuits. “Q Factor and Bandwidth of a Resonant Circuit.” https://www.allaboutcircuits.com/textbook/alternating-current/chpt-6/q-and-bandwidth-resonant-circuit/

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