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Saturday, September 26, 2026

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.

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 fs. 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]

A — Impedance-compensation trapAmplifier (≈ constant-voltage source)DriverCLRCan flatten a matched impedance peak; little or no SPL change under a low-Z amplifier.B — Acoustic-pressure notchAmplifierRsDriverCLRSeries Rs forms a voltage divider with the low-impedance trap, attenuating SPL at f₀.

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:

Ztrap = Rtotal + j(2πfL − 1/(2πfC))
Zp = Zd || Ztrap
Vd / Vin = Zp / (Zseries + Zp)

If Zseries approaches zero, Vd approaches Vin 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.

SPL (relative dB)Frequency (illustrative)2001k4k10k20kBreakup peak ~4 kHzBefore notchAfter notch

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) / Rtotal
BW = Rtotal / (2πL)   (so BW = f₀ / Q)

where Rtotal 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 Zseries, 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 Rtotal is 4.0 Ω, Qbranch is approximately 3.14. These values describe only the LCR branch; they do not predict acoustic attenuation until Zseries 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 / (Re · Qes · fs)
L = 0.1592 · Qes · Re / fs
R = Re + (Qes · Re / Qms)

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 (fs = 436 Hz, Qes = 0.54, Qms = 2.5, Re = 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 Rtotal, 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 Rtotal when selecting the external resistor.
  7. Verify by measurement and listening. If the correction is too deep, raise Rtotal or reduce Zseries, 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 Rtotal 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.
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Wednesday, September 9, 2026

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

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.

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
© 2026 IWISTAO. All rights reserved. JBL is a trademark of its respective owner; this independent DIY article is not affiliated with or endorsed by JBL Professional.
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Friday, September 4, 2026

CSR (Qualcomm) Bluetooth Chips and Their Bluetooth Versions: A Reference

 PUBLISHED BY IWISTAO · TECHNOLOGY / WIRELESS AUDIO

From the BlueCore era to the Qualcomm QCC and Snapdragon Sound platforms — which Bluetooth specification each CSR / Qualcomm chip implements, and what a version number does and does not guarantee.

Cambridge Silicon Radio (CSR) was, for over a decade, one of the leading suppliers of Bluetooth silicon. Qualcomm announced its acquisition of CSR in October 2014 and completed it on August 13, 2015, with the business renamed Qualcomm Technologies International, Ltd. (QTIL).[4] The product families CSR built — BlueCore, CSR8xx, CSR86xx — and the QCC series and Snapdragon Sound / S1 platforms that followed under Qualcomm together make up a large share of the Bluetooth audio and connectivity silicon on the market.

This guide maps each major CSR / Qualcomm chip family to the Bluetooth specification version it implements. It is a reference, not an exhaustive datasheet: Qualcomm's audio portfolio spans dozens of part numbers, and new platforms (e.g., the S-series and S1-series Sound Platforms) continue to ship. Where a claim rests on a distributor or third-party page rather than a Qualcomm primary document, it is flagged as such.

Bluetooth Versions at a Glance

Before mapping chips to versions, recall what each Bluetooth Core specification step delivered. Note that several headline user features — most importantly LE Audio — are separate specifications built on top of a Core version, not part of the Core release itself (see the feature caveat).

SpecificationYearHeadline capability at the Core level
Bluetooth 1.1 / 1.22001 / 2003First interoperable baseline; 1.2 adds adaptive frequency hopping
Bluetooth 2.0 + EDR2004Enhanced Data Rate (~3 Mbps)
Bluetooth 2.1 + EDR2007Secure Simple Pairing, extended inquiry response
Bluetooth 3.0 + HS2009High-speed via 802.11 AMP
Bluetooth 4.02010Introduces Bluetooth Low Energy (LE)
Bluetooth 4.1 / 4.22013 / 2014Coexistence; LE data length extension, privacy
Bluetooth 5.020164× range, 2× speed, 8× advertising capacity
Bluetooth 5.12019Direction finding (AoA / AoD)
Bluetooth 5.22020LE Isochronous Channels (the foundation that enables LE Audio), EATT, LE Power Control
Bluetooth 5.32021Periodic Advertising enhancements (ADI), Connection Subrating, Encryption Key Size change
Bluetooth 5.42023PAwR (Periodic Advertising with Responses), Encrypted Advertising Data
Bluetooth 6.02024Channel Sounding (secure two-way ranging)[6]
Bluetooth 6.12025Randomized RPA (Resolvable Private Address) Updates[19]
Bluetooth 6.22025Shorter Connection Intervals; Channel Sounding amplitude-based attack resilience; HCI USB LE Isochronous Support; LE Test Mode enhancements[20]
Bluetooth 6.32026Ranging precision (Channel Sounding inline PCT transfer, PHY-specific RTT accuracy); HCI capacity expansion (Running Out of Bits); RF requirement alignment (ACP / C/I limit relaxation)[21]

The exact adoption dates and feature lists are maintained by the Bluetooth SIG.[6]

The BlueCore Era: CSR's Foundation (BC1–BC6)

CSR's original BlueCore family carried Bluetooth from its earliest days through the 2.1 + EDR generation. BlueCore1 (BlueCore01) launched around 2000 and is widely credited as the first true single-chip Bluetooth device; it operated in the Bluetooth 1.0B era — the BlueCore01 data sheet predates the adoption of Bluetooth Core 1.1 in February 2001.[5b][6] An EE Times teardown documents the version compliance of the later numbered generations:

  • BlueCore 2 (BC2) and BlueCore 3 (BC3) — compliant with Bluetooth v1.1 and v1.2, built on a 0.18-µm RF CMOS process.[5]
  • BlueCore 4 (BC4) — compliant with Bluetooth v2.0 + EDR.[5]
  • BlueCore 5 (BC5 / BlueCore 5 Media) — compatible with Bluetooth v2.1 + EDR and backward-compatible with v2.0 + EDR; added an integrated 64-MIPS Kalimba DSP and stereo codec.[5]
  • BlueCore 6 (BC6) — the CSR 61xx series — this generation is marketed as the CSR 61xx family of single-chip mono-headset ROM devices (e.g., BC6110, BC6130, BC6140, BC6145, BC6150). All are qualified to Bluetooth v2.1 + EDR with extended SCO (eSCO); CSR's AuriStream (ADPCM) voice codec debuted here, cutting power versus legacy CVSD.[5][22] A third-party selection guide notes the BC6145 variant can be configured up to Bluetooth 3.0 and adds A2DP — treat that as a variant-specific claim, not a blanket BC6 figure.[2]
  • BlueCore 7 (BC7) — the CSR 65xx series — launched in 2008 as CSR's seventh-generation BlueCore and its most highly integrated part. The 65xx family (e.g., BC6540 / BC6570 / BC6590 class devices) is the first single chip to combine Bluetooth v2.1 + EDR with Bluetooth low energy (then branded "ULP Bluetooth" / Wibree), plus eGPS and FM transmit/receive. It kept the AuriStream voice codec and added in-built speaker drivers; note that the "Bluetooth low energy" here predates the formal Bluetooth 4.0 LE branding, so it is best described as an early/legacy LE implementation rather than a 4.0-qualified dual-mode stack.[24][25]

The 61xx / BC6 family was the workhorse of early mono Bluetooth headsets and hands-free car kits, where its low cost and long battery life mattered most. (Stereo speakers and USB dongles came later, with the CSR86xx and CSR8510 generations.)

The CSR8xx and CSR10xx Families

After the numbered BlueCore generations, CSR rebranded its silicon under the "CSR8xx" banner, and introduced a dedicated Low Energy line:

  • CSR8510 — a dual-mode controller widely used in USB Bluetooth 4.0 dongles. Third-party documentation describes it as compliant with the Bluetooth 4.0 specification (dual-mode: Classic + LE) and backward-compatible with 2.0, 2.1, and 3.0; this is consistent with CSR's positioning of the CSR8xx generation as its first 4.0 dual-mode line.[7]
  • CSR101x family (CSR1010 / CSR1011 / CSR1012 / CSR1013) — Qualcomm's own product page qualifies the CSR1010 to Bluetooth 4.1 (single-mode LE).[15]
  • CSR102x family (CSR1020 and others) — these parts are documented by Qualcomm as configurable with a Bluetooth stack up to 4.2 in some implementations, so the CSR10xx generation should not be collapsed into a single "4.0/4.1" label. The earlier CSR101x is 4.1; the CSR102x can reach 4.2 depending on the qualified stack.

The CSR86xx Audio SoCs (CSR8615, 8635, 8645, 8670, 8675)

The CSR86xx portfolio is where CSR's audio reputation was cemented. Bluetooth versions across this family vary by member:

  • CSR8615 — Qualcomm lists this ROM audio device at Bluetooth 4.1 (targeted at 1-mic mono speakers / headsets / car kits).[13]
  • CSR8635 — Qualcomm's current product page lists this part at Bluetooth 4.1 (dual-mode audio). Some older third-party / distributor listings show 4.0; where they differ, the Qualcomm page is the authoritative source.[18]
  • CSR8645 — the older Qualcomm product brief states "Bluetooth 4.0 specification compliant" / "Bluetooth v4.0 firmware" (dual-mode ROM audio platform with aptX and cVc).[12] Qualcomm's current product page, however, lists the specification field as Bluetooth 4.1. As with CSR8675, treat the qualified version per the specific brief / firmware you are using.[18b]
  • CSR8670 — dual-mode Bluetooth v4.2 firmware; 80 MHz Kalimba DSP; aptX, aptX Low Latency, AAC, SBC, MP3; 16 Mb integrated flash. Qualcomm's product brief explicitly lists "Bluetooth v4.2 firmware."[1]
  • CSR8675 — Qualcomm's CSR8675 product brief lists Bluetooth version 4.2 (120 MHz Kalimba DSP; aptX HD; dedicated hardware ANC).[3] A caveat: some Qualcomm web pages present inconsistent specification fields for this part (e.g., a "5.0" value in one field versus "4.2" in the brief's Features section). Because of that inconsistency, treat "CSR8675 = Bluetooth 4.2" as the figure from the official product brief and firmware version, and verify the exact qualified version against the specific brief and firmware you are using rather than assuming 5.x support.

Note — the CSRA64xx ROM audio line (e.g., CSR64215). Parallel to the programmable CSR86xx parts, CSR also shipped a family of ROM-only stereo audio SoCs branded CSRA64xx (e.g., CSR64215 / CSRA64215, plus CSRA64110, CSRA64210). These are Bluetooth v4.2 single-chip ROM solutions with an 80 MHz RISC MCU and an 80 MIPS Kalimba DSP, built for TrueWireless Stereo (TWS), stereo headsets/speakers, and car audio. They support aptX, aptX Low Latency, AAC, SBC and the sixth-generation cVc voice enhancement. Because they are ROM-masked (not flash-programmable like the CSR8670/8675), they are not field-upgradable to a new Core version — but that is a hardware limitation of the ROM part, consistent with the field-upgrade note earlier in this article.[26]

The Qualcomm QCC Series (QCC300x, QCC30xx, QCC51xx)

Under Qualcomm, the audio roadmap moved to the QCC brand. Version splits within a family matter, so they are stated explicitly:

  • QCC300x (QCC3001–QCC3008, entry-level flash audio SoCs) — Qualcomm's product brief qualifies the family to Bluetooth 5.0.[14]
  • QCC30xx — per Qualcomm's QCC30xx brief: QCC302x / QCC303x → 5.1; QCC304x → 5.2; QCC305x / QCC307x → 5.3; QCC308x / QCC309x → 5.4. The 307x/308x/309x parts are designed for LE Audio use cases.[8]
  • QCC51xx — per Qualcomm's QCC5100 series brief, the split is: QCC512x → 5.1; QCC514x → 5.2; QCC515x and QCC517x → 5.3; QCC5181 → 5.4. The QCC517x / QCC518x parts are designed to support the LE Audio standard.[11]

Newer Platforms: Snapdragon Sound / S1 (Bluetooth 6.0+)

The "QCC" naming is no longer the top of Qualcomm's audio stack. The Snapdragon Sound and S-series / S1-series Sound Platforms extend well beyond Bluetooth 5.4:

  • Qualcomm S1 Gen 1 Sound Platform (QCC1228) — Qualcomm's product page lists Bluetooth 6.0, targeting value-tier true-wireless earbuds with aptX and Adaptive ANC.[10]
  • Snapdragon S5 / S7 series — newer S7 Sound Platforms are documented with a Bluetooth 6.2 radio and LE Audio / Auracast experiences.[17]

So as of 2026, the newest Qualcomm audio silicon is listed with a Bluetooth 6.x specification version (e.g., a Bluetooth 6.2 radio on the S7 platforms, Bluetooth 6.0 on the S1 Gen 1) rather than topping out at 5.4.

BlueCoreBT 1.0B–2.1BC1–BC7CSR8xx / 10xxBT 4.0–4.28510 / 101x / 102xCSR86xxBT 4.0–4.28615–8675QCC30xx / 51xxBT 5.0–5.4300x–518xS1 / S7BT 6.0–6.2QCC1228, S7CSR founded 1998 · Acquired by Qualcomm Aug 2015

Figure 1: The CSR → Qualcomm Bluetooth silicon lineage and the specification range each generation spans.

Quick Reference: Chip-to-Bluetooth-Version Matrix

Family / PartBluetooth versionTypical use
BlueCore 1 (BC1 / BlueCore01)1.0B (2000)First single-chip BT modules
BlueCore 2 / 3 (BC2/BC3)1.1 / 1.2Early headsets, data modules
BlueCore 4 (BC4)2.0 + EDRUSB dongles, data links
BlueCore 5 (BC5)2.1 + EDRStereo headsets, speakers
BlueCore 6 / CSR 61xx (BC6110 / 6130 / 6140 / 6145 / 6150)2.1 + EDR (eSCO); BC6145 to 3.0Mono headsets, hands-free kits
BlueCore 7 / CSR 65xx (BC6540 / 6570 / 6590 class)2.1 + EDR + early Bluetooth low energy; + eGPS + FMMobile phone connectivity hub
CSR85104.0 (dual-mode)USB Bluetooth adapters
CSR101x4.1 (LE)Sensors, beacons, peripherals
CSR102xup to 4.2 (LE)LE peripherals (config-dependent)
CSR86154.1Mono headsets, car kits, speakers
CSR86354.1 (per Qualcomm page; some listings 4.0)Stereo headsets, speakers
CSR86454.1 (current page; old brief / firmware 4.0)Mid-tier wireless audio
CSR86704.2Premium wireless audio
CSR86754.2 (per brief; see field caveat)ANC premium headphones
CSR64215 (CSRA64xx ROM audio)4.2TWS / stereo headsets / speakers / car audio
QCC300x (3001–3008)5.0Entry-level headsets / speakers
QCC302x / 303x5.1Entry true-wireless earbuds
QCC304x5.2Mid true-wireless, ANC
QCC305x5.3Snapdragon Sound (mid true-wireless)
QCC307x5.3Snapdragon Sound, LE Audio
QCC308x / 309x5.4LE Audio / Auracast
QCC512x5.1Premium earbuds / headsets
QCC514x5.2Premium, Snapdragon Sound
QCC515x / 517x5.3LE Audio-ready premium
QCC51815.4LE Audio-ready premium
S1 Gen 1 (QCC1228)6.0Value-tier true-wireless
Snapdragon S7 series6.2Flagship, LE Audio / Auracast
Read the matrix with care: a "Bluetooth version" column shows the Core specification a part is qualified against. It does not by itself prove support for any specific higher-layer feature. LE Audio, the LC3 codec, Isochronous Channels, and Auracast are qualified separately (e.g., under BAP, PBP, and related specifications). See the next section.

What a "Bluetooth Version" Does and Doesn't Tell You

It is tempting to read a version number as a checklist of features. The Bluetooth SIG is explicit that most capabilities are optional,[16b] and a Core version alone does not prove a product supports a given function. Two common confusions:

  • LE Audio is not "Bluetooth 5.3" or "5.4." LE Audio is built on the LE Isochronous Channels introduced in Bluetooth Core 5.2. A chip qualified to 5.2 (or 5.3/5.4) may implement LE Audio, but only if it has also passed the relevant separate qualifications (BAP, PBP, LC3, etc.). A "5.2-qualified" radio is therefore not automatically LE-Audio-capable.[16a]
  • Auracast is part of the LE Audio specification set, not a Bluetooth 5.4 feature. Auracast broadcast audio is enabled by LE Audio qualifications (notably PBP); it is not a Core 5.4 addition. The 5.4 Core release's own headline additions are PAwR and Encrypted Advertising Data.[16c]

For selection, verify the specific feature qualifications (BAP, PBP, LC3, and so on) for the exact part and firmware, rather than inferring them from the Core version.

How to Choose the Right Chip

  • For new true-wireless designs wanting LE Audio / Auracast, start at QCC307x / 308x / 309x (5.3 / 5.4) or the S1 / S7 platforms (6.0 / 6.2) — and confirm the specific LE Audio / Auracast qualifications, not just the Core version.
  • For cost-sensitive earbuds with solid aptX Adaptive, QCC304x (5.2) remains a strong, widely available choice.
  • For legacy product maintenance or aptX HD audio, CSR8670 / 8675 (4.2) are still supported through the ADK toolchain.
  • For USB host-side adapters, CSR8510 (4.0) is the classic, mature option.

A chip's Bluetooth specification version is tied to its radio hardware and original qualification. Firmware cannot add PHY or controller capabilities the silicon does not physically support. However, where the hardware already includes the necessary radio/controller features, a vendor can enable additional functionality — for example, some Auracast-related capabilities — through firmware, stack updates, and re-qualification; the Bluetooth SIG notes that the specification allows certain existing products to be upgraded in the field. Treat "version locked" as "locked by hardware," not as an absolute software prohibition.

Frequently Asked Questions

Is CSR still a separate company?

No. Qualcomm announced the acquisition in October 2014 and completed it on August 13, 2015; the business now operates as Qualcomm Technologies International, Ltd. (QTIL). "CSR" survives mainly as a product-name legacy on older parts.[4]

Does the CSR8670 support Bluetooth 5?

No. The CSR8670 is qualified to Bluetooth 4.2 per its product brief.[1] For Bluetooth 5 features you need a Qualcomm QCC-series part (or newer). Note the CSR8675 carries the same 4.2 brief figure, with some Qualcomm pages showing inconsistent fields — verify the exact qualified version against the brief and firmware you use.

What is the newest Bluetooth version in the CSR / Qualcomm audio lineup?

As of 2026 it is not 5.4. Qualcomm's S1 Gen 1 Sound Platform (QCC1228) is qualified to Bluetooth 6.0, and the newer Snapdragon S7 series documents a Bluetooth 6.2 radio.[10][17] The QCC308x / 309x (5.4) are simply the top of the older QCC30xx family.

If a chip is "qualified to Bluetooth 5.2," does it support LE Audio?

Not necessarily. Bluetooth 5.2 introduced the LE Isochronous Channels that enable LE Audio, but LE Audio itself (BAP/PBP/LC3 and related qualifications) is optional.[16a][16b] A 5.2-qualified part may or may not implement LE Audio — check the specific feature qualifications for that exact part and firmware.

Can a chip's Bluetooth version be upgraded through a firmware update?

Generally not at the Core level if the required radio/controller hardware is absent — firmware cannot invent PHY or controller capabilities the silicon lacks. But where the hardware already supports the necessary features, a vendor can add functionality (for instance, some Auracast capabilities) via firmware, stack updates, and re-qualification; the Bluetooth SIG states the specification permits some existing products to be field-upgraded.[16b][16c] So "version locked" means "locked by hardware," not an absolute software rule.

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References

  1. Qualcomm, "CSR8670 Bluetooth Audio Platform — Product Brief" (PDF). Lists Bluetooth v4.2 firmware.
  2. CSR Bluetooth Chip Selection Guide (third-party) — BlueCore BC03–BC06 generation mapping; notes BC6145 up to Bluetooth 3.0 and CSR8635 at 4.0. Distributor/third-party source; verify against Qualcomm briefs.
  3. Qualcomm, "CSR8675 Bluetooth Audio Platform — Product Brief" (PDF). Features section lists "Bluetooth version 4.2 compliant." Note: some Qualcomm web pages show inconsistent specification fields for this part.
  4. Qualcomm, "Qualcomm Completes $2.4 Billion Acquisition of CSR" — official press release (distributed via PR Newswire), Aug. 13, 2015. Acquisition completed; CSR renamed Qualcomm Technologies International, Ltd. (Announced Oct. 2014.)
  5. EE Times, "Under the Hood: CSR evolves along with Bluetooth" — BlueCore 2–6 Bluetooth version compliance (BC2/3: 1.1/1.2; BC4: 2.0+EDR; BC5: 2.1+EDR; BC6: 2.1+EDR eSCO).
  6. Business Weekly (Cambridge), founder account — BlueCore1 described as the world's first single-chip Bluetooth device, launched around 2000 (predating Bluetooth Core 1.1, adopted Feb. 2001).
  7. Bluetooth SIG, Specification Adopted Versions (official release dates and feature scope).
  8. DFRobot, Bluetooth CSR 4.0 Dongle (CSR8510) — Bluetooth 4.0 dual-mode specification (third-party; consistent with CSR8xx 4.0 dual-mode positioning).
  9. Qualcomm, "QCC30xx Series Bluetooth Audio SoCs — Product Brief" (PDF). QCC302x/303x=5.1, QCC304x=5.2, QCC305x/307x=5.3, QCC308x/309x=5.4; LE Audio on 307x/308x/309x.
  10. Qualcomm, "S1 Gen 1 Sound Platform (QCC1228)" product page — Bluetooth Specification Version: Bluetooth 6.0.
  11. Qualcomm, "QCC5100 Series Bluetooth Audio SoCs — Product Brief" (PDF). QCC512x qualified to 5.1; QCC514x to 5.2; QCC515x and QCC517x to 5.3; QCC5181 to 5.4; QCC517x / QCC518x designed to support LE Audio.
  12. Qualcomm, "CSR8645 — Bluetooth Audio Platform" Product Brief (PDF). "Bluetooth 4.0 specification compliant" / "Bluetooth v4.0 firmware."
  13. Qualcomm, CSR8615 product page — Bluetooth Specification Version: Bluetooth 4.1.
  14. Qualcomm, "QCC300x Family — Product Brief" (PDF). "Bluetooth 5.0 qualified" (QCC3001–QCC3008).
  15. Qualcomm, CSR1010 product page — Bluetooth Specification Version: Bluetooth 4.1 (CSR101x family, single-mode LE).
  16. Bluetooth SIG — LE Audio FAQs: LE Audio is built on the LE Isochronous Channels introduced in Bluetooth Core 5.2; it is a separate specification set (BAP / PBP / LC3), not part of any single Core release.
  17. Bluetooth SIG — "Communicating supported Bluetooth functionality": most Bluetooth capabilities are optional; the SIG advises members not to use the Core Specification version as the sole indicator of supported functionality, and to state specific qualified features instead.
  18. Bluetooth SIG — Auracast FAQ: Auracast broadcast audio is defined by the Public Broadcast Profile (PBP) within the LE Audio set; the specification allows some existing products to be upgraded in the field (subject to underlying hardware and supplier strategy).
  19. Qualcomm, "Snapdragon S7 and S7 Gen 1 Sound Platforms — Product Brief" (PDF). Bluetooth 6.2 radio; LE Audio / Auracast experiences.
  20. Qualcomm, CSR8635 product page — Bluetooth Specification Version: Bluetooth 4.1 (dual-mode audio ROM platform).
  21. Qualcomm, CSR8645 product page — current Specification field: Bluetooth 4.1 (older product brief lists "Bluetooth v4.0 firmware"; see ref12).
  22. Bluetooth SIG, "Bluetooth Core 6.1 is here" (published 6 May 2025) — introduces Randomized RPA (Resolvable Private Address) Updates.
  23. Bluetooth SIG, "Bluetooth Core 6.2 feature overview" — Shorter Connection Intervals, Channel Sounding amplitude-based attack resilience, HCI USB LE Isochronous Support, LE Test Mode enhancements.
  24. Bluetooth SIG, "Bluetooth Core 6.3 technical overview" (published 5 May 2026) — ranging precision (Channel Sounding inline PCT transfer, PHY-specific RTT accuracy), HCI capacity expansion (Running Out of Bits), RF requirement alignment (ACP / C/I limit relaxation).
  25. EDN, "CSR launches Bluetooth ROM device line" — BlueCore6 mono ROM headset family BC6130 / BC6140 / BC6150, fully compatible with Bluetooth v2.1 + EDR; introduces Proximity Pairing and CVC 5.0.
  26. EE Times, "Sixth generation Bluetooth brings fixed-line-quality voice" — BlueCore6 silicon supports v2.1 + EDR; debuts CSR AuriStream (ADPCM) voice codec for fixed-line-quality calls and lower power vs. CVSD.
  27. EDN / EE Times, "CSR trumpets tightest integrated wireless chip" — BlueCore7 (BC7), launched 2008, first single chip to combine Bluetooth v2.1 + EDR, Bluetooth low energy (ULP / Wibree, pre-4.0 branding), eGPS and FM transmit/receive; includes AuriStream voice codec.
  28. ESM China, "CSR launches 7th-gen BlueCore (BC7)" — details BlueCore7's v2.1 + EDR radio (+10 dBm Tx / –91 dBm Rx), AuriStream (–30% power), eGPS and FM; samples in 2008, volume in Q4.
  29. CSRA64215 teardown / module notes (Qualcomm CSRA64215) — single-chip ROM audio solution; compliant with Bluetooth v4.2; 80 MHz RISC MCU + 80 MIPS Kalimba DSP; TrueWireless Stereo (TWS), aptX / aptX-LL / AAC / SBC, 6th-gen cVc. Third-party module/manufacturer source; treat ROM-mask and exact qualified version per the specific ordering code.
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