KEF B139 SP1044

Name: KEF B139 SP1044
SKU: 47110
Price: 100.00 GBP
Availability: InStock

£100.00

1 in stock

8481

KEF B139 SP1044 Bass Drive Units — Matched Pair | 8Ω | Iconic EPS Piston | Made in England | Checked & Tested by Andrew

The KEF B139 SP1044 is one of the most historically significant and widely discussed bass drive units in British loudspeaker history. An oval, flat-piston design manufactured by KEF Electronics in Maidstone, Kent, it uses a diaphragm constructed from expanded polystyrene foam covered with a thin black anodised aluminium foil skin — a highly unconventional approach at the time, chosen for its combination of extremely low mass and high stiffness. The B139 was used in a remarkable range of distinguished speaker designs, among them the Linn Isobarik, the IMF TLS80 and Reference Monitor, Bailey’s original transmission line and his later triangulated line, the Atkinson State-of-the-Art TL, and numerous other classic DIY and commercial transmission-line and isobaric designs of the 1970s and 1980s. Both units have been tested and confirmed working by Andrew in our workshop.

Condition — Both units are in used condition commensurate with their age. The rubber surrounds are intact on both drivers. The EPS foam pistons show age-related surface marking: the left-hand driver has noticeable mottling, spotting, and some deterioration to the aluminium foil skin surface, while the right-hand driver is in comparatively better order. Both units test electrically sound and function correctly. Buyers purchasing for use should be aware of the cosmetic state of the pistons; buyers considering these as a restoration starting point should note that Falcon Acoustics, working with original B139 designer Malcolm Jones, produce a genuine replacement piston assembly. Please refer to our photographs for a full and honest assessment of condition.

1 in stock

Categories: All Used Equipment, Bass units, Classic & Vintage, speakerchassis

Description

Description

Key Features

The B139 SP1044 is an oval drive unit, its distinctive shape dictated by the need to maximise diaphragm area within a cabinet baffle while maintaining a low profile depth — a critical requirement for the transmission-line and isobaric cabinet designs for which it became the preferred driver. The flat EPS piston moves as a rigid body rather than flexing like a conventional cone, which suppresses break-up modes and gives the driver an unusually extended and clean frequency response for a bass unit of its size. Combined with its neoprene rubber roll surround and a relatively high compliance for its period, the B139 has a characteristically low resonant frequency making it well suited to sealed, reflex, and transmission-line loading alike.

The genuine replacement for the SP1044 is the Falcon Acoustics B139, designed by Malcolm Jones — the same engineer who created the original — and built in Great Britain using tooling matched to the original KEF dimensions and specification. This is noted for the benefit of buyers who may wish to restore a system to original performance using these units as a reference.

Technical Specifications

(Sources: t-linespeakers.org, citing KEF original specification data; audio-database.com; speakerboxlite.com driver database; Falcon Acoustics B139 datasheet, citing KEF SP1044 reference specifications)

Type: Oval flat-piston bass driver
Diaphragm: Expanded polystyrene foam, aluminium foil skin
Surround: Neoprene rubber roll
Nominal Impedance: 8Ω
Resonant Frequency (Fs): approximately 25–43Hz (varies by production revision; see notes)
Vas: approximately 200 litres
Qts: approximately 0.4
Qms: approximately 2.7
Qes: approximately 0.48
BL: approximately 6.0 Tm
Power Handling: 50W (nominal)
Dimensions: Oval chassis, approximately 330 × 240mm overall
Country of Origin: England (Tovil, Maidstone, Kent)

A note on resonant frequency: Published Fs figures for the SP1044 vary between approximately 25Hz and 43Hz across different sources and production dates. The nominal 25Hz figure was acknowledged internally at KEF as aspirational rather than typical; measured values in the field are commonly higher. This does not materially affect the driver’s suitability for the applications it was designed for, where cabinet alignment is the primary determinant of bass extension.

Why Buy From Us

With over 40 years of experience in the Hi-Fi industry, we are well placed to assess and test equipment of this type. Every item we list is fully inspected and tested before sale — this pair was checked by Andrew in our workshop.

When you buy from us you can expect:

Full inspection and testing prior to listing — not simply a visual check
Transparent descriptions with no omissions
Professional packaging to ensure safe delivery
After-sales support available by phone or email should you have any questions

Summary

This is a matched pair listed as a single lot. The KEF B139 SP1044 is one of the most sought-after vintage bass drive units for Linn Isobarik, IMF, and transmission-line speaker restoration and DIY projects. Both units are confirmed working by Andrew. Condition is commensurate with age — please refer to our photographs carefully, particularly regarding the foil skin surface on the left-hand driver. Please feel free to get in touch with any questions prior to purchase.