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.

Shop Speaker Components →

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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/

© 2026 IWISTAO. All rights reserved.

Building a JBL 2420 External Wooden Horn for 12- and 15-Inch Full-Range Speakers

PUBLISHED BY IWISTAO · DIY Audio

A measurement-led approach to using a classic 1-inch compression driver and handcrafted wooden horn as an adjustable high-frequency compensation module for large full-range loudspeakers.

Table of Contents

1. What the JBL 2420 Actually Is
2. Inspect the Vintage Driver First
3. Choose and Build the Wooden Horn
4. Assemble the External Module
5. Protect, Cross Over and Level-Match It
6. Add High-Frequency Compensation to a 12- or 15-Inch Full-Range Speaker
7. Place and Time-Align the Horn
8. Measure Before Final Listening

Figure 1: a freestanding wooden high-frequency horn module. (IWISTAO original image)

An external horn can restore presence, air and listening-area coverage above a large full-range cone’s practical high-frequency region without rebuilding the main loudspeaker cabinet. This is especially useful with 12- and 15-inch single-cone, whizzer-cone or triple-cone systems. The concept looks simple—attach a driver to a horn and place it on top of the speaker—but a successful result depends on acoustic loading, crossover protection, output matching and physical alignment.

1. What the JBL 2420 Actually Is

The JBL 2420 is a discontinued professional compression driver with a 25 mm (1-inch) throat, an Alnico V magnetic assembly and a nominal 16-ohm impedance. JBL’s March 1970 data sheet specifies a 1.75-inch edge-wound aluminium-ribbon voice coil, a 0.002-inch duraluminum-alloy diaphragm and 30 watts of continuous programme capacity.^[1] These are original production specifications, not a promise that every surviving example still performs as it did when new.

Inside the driver, the phase plug uses machined concentric exponential passages, while a pure-silver impedance-control ring on the pole piece was intended to counter the voice coil’s inductive component at high frequencies. The sheet also specifies a 19,000-gauss flux density.^[1]

Throat	25 mm / 1 inch
Nominal impedance	16 ohms
Programme capacity	30 W continuous programme
Sensitivity	118 dB, measured with 1 mW on a 1-inch terminated tube
Frequency range	500 Hz–20 kHz, as listed in the specification table
Detailed response statement	Usable 500 Hz–15 kHz on a terminated tube; 500 Hz–17 kHz on a JBL 2350 horn (±3 dB)
Recommended crossover	500 Hz or higher, per the original JBL sheet
Diaphragm / voice coil	0.002-inch duraluminum alloy; 1.75-inch edge-wound aluminium ribbon
Flux density	19,000 gauss
Dimensions / weight	5.75-inch diameter × 3.875-inch depth; 11 lb (approximately 5 kg)

The data sheet uses three different descriptions that should not be collapsed into one claim: a 500 Hz–20 kHz specification-table range, a 500 Hz–15 kHz usable terminated-tube response, and a 500 Hz–17 kHz response on the JBL 2350 horn within ±3 dB.^[1] This is why “external high-frequency horn” is more accurate than “super tweeter.” JBL’s quoted 118 dB sensitivity was measured with 1 mW on a terminated tube, so it should not be compared directly with a conventional loudspeaker’s 1 W/1 m rating.

The horn, crossover and physical condition of the driver determine the usable result. The model number alone does not.

2. Inspect the Vintage Driver First

Before cutting wood, inspect both drivers as a matched pair. Look for impact damage, loose rear covers, oxidised terminals, damaged mounting threads and evidence of non-original diaphragms. JBL’s historical service test document lists 4.8–5.8 ohms as the DC-resistance range for the 16-ohm 2420.^[5] A normal resistance reading does not prove that the diaphragm is centred or undamaged, but an open circuit or a major left/right difference is a clear reason to stop.

• Measure DC resistance with the driver disconnected from all crossovers.
• Use a very low-level swept tone only after installing a protective high-pass filter.
• Listen for rubbing, buzzes and sudden response discontinuities.
• If a diaphragm must be replaced, service both channels consistently and remeasure them.

3. Choose and Build the Wooden Horn

The throat must join the 1-inch driver exit through a smooth, airtight transition. Do not assume that every generic “1-inch horn” has the correct bolt pattern or throat depth; measure the actual driver and adaptor before drilling. A thin closed-cell gasket prevents air leakage, while a rigid mounting plate carries the driver’s weight without loading the horn throat.

The horn’s flare and mouth dimensions influence low-frequency loading and radiation pattern. The supplied 2420 sheet documents ±3 dB response from 500 Hz to 17 kHz specifically on a JBL 2350 horn.^[1] JBL’s horn catalogue further notes that the compact 1-inch 2345 was intended for crossover at 800 Hz or higher, while the larger 2350 could be used from 500 Hz with the appropriate throat adaptor.^[3] A custom wooden horn has no validated minimum frequency until it is measured, so do not transfer the 2350 result directly to a smaller homemade flare.

For a home system, a practical wooden construction uses laminated hardwood, high-quality plywood or a combination of a machined throat insert and laminated sidewalls. Keep the internal flare symmetrical, sand away steps at layer joints and round the mouth edges consistently. Dense, well-braced walls reduce panel vibration, but decorative timber does not correct a poor flare profile.

Design boundary: the illustration in this article communicates assembly and signal flow. Generate the horn contour with an established acoustic model, then verify the prototype acoustically before treating it as a finished product.

4. Assemble the External Module

Build the horn as a stable freestanding object rather than allowing the driver to hang from a thin wooden throat. A rear cradle or metal bracket should support the compression driver. Add soft feet under the base, terminal posts or a locking connector at the rear, and strain relief so the cable cannot pull on the driver terminals.

1. Dry-fit the driver, gasket and throat adaptor; check for a continuous bore with no exposed ledges.
2. Support the driver from below, then tighten mounting bolts evenly. Do not use the bolts to pull misaligned parts together.
3. Confirm that the assembly cannot tip forward when the cable is moved.
4. Mark left and right modules and preserve consistent polarity.
5. Add a removable rear guard if the unit will be used around children or pets.

Figure 2: Correct active-DSP topology—the source reaches the DSP first, then separate power-amplifier channels drive the full-range speaker and protected 2420 horn. (IWISTAO original diagram)

5. Protect, Cross Over and Level-Match It

Never connect the 2420 to an unfiltered full-range signal. A compression driver needs a high-pass filter to remove low-frequency energy. Choose one of the following topologies and keep its signal path consistent.

Option A: active DSP with a separate HF amplifier

Route the source or preamplifier into the DSP first. Send one DSP output to the main-speaker power amplifier and a second, high-passed output to a separate HF power amplifier for the 2420. Gain, EQ, delay and polarity adjustment all occur before power amplification. DSP crossovers provide independent output filters and flexible slopes for matching the measured acoustic behaviour of the drivers.^[6]

Option B: passive high-pass on a shared amplifier

Keep the full-range speaker on the main amplifier output and feed the 2420 through a correctly designed passive high-pass network followed by a power-rated L-pad. This arrangement cannot provide independent delay or DSP EQ. The complete network must be calculated from the measured driver impedance and required attenuation, and the combined load presented to the amplifier must remain safe. A single series resistor alone changes both level and filter behaviour.

The original JBL sheet recommends 500 Hz or higher, but that figure applies to the driver’s published application context and does not validate an arbitrary wooden horn.^[1] A measured large-format horn might initially be evaluated around 1.5–2 kHz, while additive high-frequency compensation is often first explored higher. These are author-derived commissioning examples—not JBL crossover specifications—and the final point and slope must be determined from horn loading, distortion, impedance and acoustic summation.

6. Add High-Frequency Compensation to a 12- or 15-Inch Full-Range Speaker

A 12- or 15-inch full-range driver may publish impressive on-axis extension, yet its high-frequency output and radiation pattern can change substantially away from the centre line. Manufacturer data illustrates the range of possibilities: Celestion’s 12-inch K12H-200TC uses a secondary cone to extend output to 10 kHz, while Fane’s 15-inch triple-cone FC-152F01TC is specified to 15 kHz and publishes a separate 45-degree response trace.^[9][10] The useful handover point must therefore come from the actual speaker’s on- and off-axis measurements, not diameter alone. Klippel’s directivity guidance likewise shows that cone geometry and material influence both sound-power response and directional behaviour.^[8]

High-pass-only compensation

For an add-on arrangement, leave the full-range driver unfiltered and high-pass only the 2420 branch. An author-derived first experiment can place the horn around 3–5 kHz with a 12–24 dB/octave slope and its level well below the main speaker, then raise it gradually while measuring. This is not a JBL specification or a universal recommendation: a different full-range driver, horn flare or listening geometry can require a different setting. Overlap can create comb filtering, so this method is compensation rather than an acoustically complete crossover.

Controlled two-way handover

If measurements show a broad overlap peak or deep cancellation that placement cannot solve, add a matching low-pass filter to the 12- or 15-inch driver and operate the system as a two-way loudspeaker. This changes the original full-range concept, but gives more control over summation, level and directivity through the crossover region.

Adjustment target: do not tune for the brightest on-axis sound. Aim for a smooth transition at the main seat and stable tonal balance at several positions 15–45 degrees off axis.

7. Place and Time-Align the Horn

Start with the horn mouth near ear height and close to the vertical plane of the main speaker’s acoustic output. Toe-in controls how much on-axis energy reaches the listening seat. Small position changes can alter the crossover region because the main driver and external horn are separated in space.

Measure each source separately from the same microphone position, then examine their summed response. Room EQ Wizard’s alignment tools use impulse-response data to compare timing and can indicate when a polarity inversion gives a better crossover sum.^[7] Use that result as evidence, not as a reason to reverse polarity automatically. Confirm the final setting at several nearby seats because a narrow correction at one point may not generalise across the listening area.

8. Measure Before Final Listening

Commission the system at low level. Measure left and right horn responses, check for symmetry, then add the main loudspeaker and inspect the crossover sum. Look for a broad peak caused by excessive horn level, a notch caused by timing or polarity, and narrow irregularities that may indicate throat discontinuities or diaphragm trouble.

• Frequency response: smooth the broad balance, but do not boost deep narrow cancellations.
• Impulse response: use it to assess delay and polarity around the crossover.
• Off-axis checks: measure a few horizontal angles to understand the horn’s real coverage.
• Thermal check: after moderate listening, confirm that the driver, resistors and connectors are not overheating.
• Listening check: use familiar voices, cymbals and strings; the horn should integrate rather than call attention to itself.

A well-integrated JBL 2420 wooden-horn module is less about “adding more treble” than restoring high-frequency balance and coverage where a large full-range cone needs support. Build the mechanics carefully, protect the vintage diaphragm, and let measurements determine the final crossover, attenuation and placement.

Frequently Asked Questions

Is the JBL 2420 a true super tweeter?

No. JBL’s sheet lists a broad 500 Hz–20 kHz frequency range, but the detailed text specifies usable response to 15 kHz on a terminated tube and ±3 dB response to 17 kHz on a 2350 horn. It is a wide-range high-frequency compression driver, not an ultrasonic super tweeter.

Can I connect it in parallel with my main speaker?

Not directly. It needs a high-pass filter and level control, and the resulting amplifier load must be checked. A DSP crossover with a separate amplifier channel is the safest and most adjustable development method.

What crossover frequency should I use?

There is no universal value. JBL’s 500 Hz-or-higher figure is a historical driver specification, not a prescription for a custom horn. The article’s 1.5–2 kHz and 3–5 kHz bands are author-derived commissioning examples only; the final frequency, slope and level must come from measurements of the actual horn and main speaker.

Does the horn have to be solid wood?

No. Laminated hardwood, quality plywood and composite constructions can all work. The priorities are a correct smooth flare, a sealed throat, rigid walls and proper support for the driver.

Will it work with every 12- or 15-inch full-range speaker?

No universal network fits every model. Cone response, sensitivity, impedance and off-axis behaviour vary, so the horn level and high-pass point must be adjusted for the specific loudspeaker. Measure first and avoid using published upper-frequency limits as crossover instructions.

Shop External Horn for JBL 2420 Driver →

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References

1. James B. Lansing Sound, Inc., “Professional Series Model 2420 Compression Driver,” PPB 2420 3/70, two-page original data sheet supplied for this article. Open the supplied JBL 2420 PDF
2. JBL Professional, “2420 (Discontinued) — Product and Downloads.” https://jblpro.com/en-US/products/2420.html
3. JBL Professional, “2340, 2345, 2350 and 2355 Radial Horns — Information.” https://jblpro.com/en-US/site_elements/2340-2345-2350-2355-information
4. JBL Professional, “Horns and Horn Mounting Brackets.” https://jblpro.com/en-US/product_families/horns-and-horn-mounting-brackets
5. JBL Customer Service Department, “Compression Driver Test Specifications,” Revision C, January 16, 1984 (archived document mirror). Archived JBL test specification PDF
6. miniDSP, “Crossover — DSP Reference.” https://docs.minidsp.com/product-manuals/tide16/dsp-reference/crossover.html
7. Room EQ Wizard, “All SPL Graph — Alignment Tool.” https://www.roomeqwizard.com/help/help_en-GB/html/graph_allspl.html
8. Klippel GmbH, “Directional Radiation Characteristics.” https://www.klippel.de/know-how/measurements/sound-radiation-and-propagation/directional-radiation-characteristics.html
9. Celestion, “Professional Loudspeakers Catalogue,” K12H-200TC extended-HF 12-inch driver. Celestion professional speaker catalogue PDF
10. Fane International, “FC-152F01TC 15-inch Full Range Driver,” data sheet. Fane FC-152F01TC data sheet PDF

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