LS3/5A Crossover Network Analysis: The Evolution of 15Ω and 11Ω Circuit Design
Published by IWISTAO
Abstract
The BBC LS3/5A is not simply a compact loudspeaker, but a precision monitoring instrument. Its legendary status owes far more to its crossover network than to driver selection alone. This article presents a detailed engineering analysis of the LS3/5A crossover evolution, focusing on the 15Ω FL6/23 and 11Ω FL6/38 production designs.
1. Design Philosophy: Beyond a Conventional Crossover
Unlike typical two-way loudspeakers, the LS3/5A crossover is not a simple electrical filter. It functions as a combined acoustic equalisation and impedance management system, designed to correct the inherent response irregularities of the KEF B110 and T27 drivers.
- Driver response linearisation
- Midrange resonance suppression
- Phase and power response control
- Controlled impedance for broadcast amplifiers
2. The 15Ω Era — FL6/23 Production Network
2.1 Historical Context
The FL6/23 crossover became the standard production network during the 1970s and early 1980s, designed for the KEF B110 SP1003 and T27 SP1032. Due to significant unit-to-unit driver variation, BBC engineers accepted the necessity of individual crossover calibration during production.
2.2 FL6/23 Circuit Topology
The FL6/23 network is best understood as three interacting functional blocks:
Low-Frequency Equalisation (B110)
An inductor-resistor network (L1 + R1) compensates for the rising low-frequency response introduced by cabinet loading and driver characteristics.
Midrange Notch Compensation
A carefully tuned RLC notch network (C5, L2, R2) suppresses the characteristic 600–1000 Hz cone-related response hump of the B110.
High-Frequency Network with Tapped Inductor
The most distinctive feature is the tapped HF inductor (L3), acting as a passive auto-transformer, allowing precise tweeter level adjustment without dissipative resistive padding.
2.3 Typical FL6/23 Component Values (15Ω)
| Reference | Value | Function |
|---|---|---|
| L1 | 1.53 mH | LF equalisation |
| R1 | 82 Ω | LF attenuation |
| C5 | 6.2 → 10 µF | Midrange notch |
| L2 | 2.67 mH | Midrange notch |
| R2 | 33 → 22 Ω | Notch damping |
| L3 | 0.64 mH (tapped) | HF level control |
| C2 | Selected under test | HF crossover |
3. Transition to the 11Ω Version — FL6/38
3.1 Motivation for Redesign
In the mid-1980s, KEF introduced the B110 SP1228 with improved manufacturing consistency. This enabled a redesign of the crossover to reduce production variability and eliminate the need for individual HF trimming.
3.2 Key Architectural Changes
| Aspect | 15Ω (FL6/23) | 11Ω (FL6/38) |
|---|---|---|
| HF Level Control | Tapped inductor | Resistive ladder |
| Production Tuning | Individual | Fixed |
| Nominal Impedance | ~15 Ω | ~11 Ω |
3.3 11Ω Crossover Topology
The 11Ω crossover retains the conceptual LF and midrange correction networks but replaces the tapped HF inductor with a fixed resistive divider, resulting in improved consistency and smoother impedance behaviour.
4. Sonic and Engineering Implications
4.1 Measured Behaviour
- 15Ω: marginally smoother HF integration, more complex impedance
- 11Ω: improved consistency, simpler impedance curve
4.2 Listening Evaluation Notes
“The 15Ω system exhibits a slightly more organic HF integration, while the 11Ω version delivers improved focus and repeatability without compromising tonal neutrality.”
5. Conclusion
The evolution from FL6/23 to FL6/38 represents a shift from bespoke calibration to controlled manufacturing precision. Both designs remain authentic BBC solutions, demonstrating a level of passive crossover engineering rarely matched in modern loudspeakers.
