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.