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Building a JBL 2420 External Wooden Horn for 12- and 15-Inch Full-Range Speakers
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posted by Vincent Zhang
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.
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.
- Dry-fit the driver, gasket and throat adaptor; check for a continuous bore with no exposed ledges.
- Support the driver from below, then tighten mounting bolts evenly. Do not use the bolts to pull misaligned parts together.
- Confirm that the assembly cannot tip forward when the cable is moved.
- Mark left and right modules and preserve consistent polarity.
- 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.
Find More
- IWISTAO Customized 1 Pair Hyperbolic Stable Diffusion Empty Wooden Horn 1 Inch Throat Birch Plywood Wide 444MM with Adapter
- IWSITAO Customized 1 Pair TAD TH4001 Hyperbolic Stable Diffusion Empty Wooden Horn 2 Inches Throat Birch Plywood Wide 612MM with Adapter
- IWSITAO 1 Pair Hyperbolic Stable Diffusion Empty Wooden Horn 1 Inch Throat Birch Plywood Wide 248MM with Adapter for TD2001 Driver
- IWISTAO 1 Piece Cast Aluminum Horn 2 Inch Throat Hole Suitable for JBL 2445/2440/375 TAD BC Wide 390
References
- 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
- JBL Professional, “2420 (Discontinued) — Product and Downloads.” https://jblpro.com/en-US/products/2420.html
- JBL Professional, “2340, 2345, 2350 and 2355 Radial Horns — Information.” https://jblpro.com/en-US/site_elements/2340-2345-2350-2355-information
- JBL Professional, “Horns and Horn Mounting Brackets.” https://jblpro.com/en-US/product_families/horns-and-horn-mounting-brackets
- JBL Customer Service Department, “Compression Driver Test Specifications,” Revision C, January 16, 1984 (archived document mirror). Archived JBL test specification PDF
- miniDSP, “Crossover — DSP Reference.” https://docs.minidsp.com/product-manuals/tide16/dsp-reference/crossover.html
- Room EQ Wizard, “All SPL Graph — Alignment Tool.” https://www.roomeqwizard.com/help/help_en-GB/html/graph_allspl.html
- Klippel GmbH, “Directional Radiation Characteristics.” https://www.klippel.de/know-how/measurements/sound-radiation-and-propagation/directional-radiation-characteristics.html
- Celestion, “Professional Loudspeakers Catalogue,” K12H-200TC extended-HF 12-inch driver. Celestion professional speaker catalogue PDF
- Fane International, “FC-152F01TC 15-inch Full Range Driver,” data sheet. Fane FC-152F01TC data sheet PDF
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