KEF 105 Series II (1980)
A three-way reference monitor that split itself in two — a 12-inch bass cabinet on the floor, a mid/tweeter head floating above it — and then defended itself with a self-powered protection circuit nobody else had.
Overview
The 105 Series II weighs 80 pounds and stands 38 inches tall, and it does not look like a domestic loudspeaker so much as a small studio installation that wandered into a living room. The architecture is the whole point: a 12-inch low-frequency driver lives in its own sealed enclosure — a 70-litre cabinet, per the technical notes — while the midrange and high-frequency units sit in a separate, vertically aligned head of roughly 8.5 litres perched above it. KEF's own literature explains the reasoning: the split was chosen to give a flat, even response both on and off axis while keeping inter-unit delay to a minimum. The crossover points land at 400Hz and 2.5kHz, so the 12-inch unit is doing only bass and lower-mid work, and the head is handling everything from the upper midrange up.
The numbers are the kind that make sense for a monitor first and a home speaker second. Impedance is 8 ohms. Sensitivity is quoted at 85dB with 1 watt at 3.3 feet — not a high-efficiency design, but the sort of figure that rewards a competent amplifier rather than a flea-watt tube amp. Frequency response runs 38Hz to 22kHz, which for a speaker of this vintage is a genuinely wide claim at both ends. Maximum power input is rated at 200W. According to KEF's own advertising, the system produces 107dB under typical listening conditions — a figure that, read alongside the 85dB sensitivity, tells you the 200W rating is real headroom and not marketing garnish.
What separates the Series II from the original 105 is the protection circuitry. KEF built a self-powered electronic guard called S-STOP — Steady State and Transient Overload Protection — and it watches three separate failure modes. In peak mode it trips whenever input voltages climb high enough to threaten the dividing network or to push program peaks into unacceptable distortion. In thermal mode it continuously monitors the input level to each drive-unit voice coil and cuts in when a safe operating temperature is exceeded. And in low-frequency excursion mode it watches the bass unit's travel, activating whenever program peaks would push the woofer past its maximum linear excursion. When limits are exceeded, the input signal to the drivers is reduced; normal operation restores itself automatically once the overload passes. That last detail matters more than it sounds — a protection circuit you never have to reset is one you'll actually leave in the chain.
KEF's manufacturing discipline is the other half of the Series II story, and it's where the company's computer-aided measurement work paid off in production rather than just in the lab. Every drive unit and filter network was measured individually, with performance data recorded digitally and used to group carefully matched component sets before assembly. Completed speakers were measured again. The claim KEF made in its 1980 advertising was striking: paired Model 105s differ in frequency response by less than 0.5dB, and vary from the original prototype by only 1dB across the entire range. Whether or not every pair left the factory at that tolerance, the intent is clear — this was a speaker sold on consistency, not on a golden sample.
Contemporary coverage treated the 105 as an established reference rather than a novelty. Audio (August 1980, p.13) noted that since the original Model 105's introduction in 1977 it had been adopted by audio testing laboratories and speaker manufacturers as a reference for evaluating other loudspeakers and audio products — a remarkable endorsement for a speaker with a domestic price tag. The same advertising text ran in High Fidelity (June 1980, p.65) and HiFI-Stereo (July 1980, p.31), which tells you how seriously KEF pushed the reference-monitor positioning. Studio-Sound (June 1980, p.121) carried the companion Model 101 in the same campaign and repeated the overload-protection pitch nearly word for word.
Practical ownership is where the 105 Series II earns its keep and also where it asks for patience. The two-box construction means setup is not a matter of sliding a speaker against a wall — the head has to be aligned over the bass cabinet, and the whole assembly is 16¼ inches wide by 38 inches tall by 18 inches deep, so it wants floor space and a listening position that isn't jammed into a corner. At 80 pounds it's a two-person lift. The S-STOP circuit is the thing collectors ask about first, because it's the Series II's defining feature and because a self-powered protection board is one more thing that can fail after four decades. KEF advertised the system as available with an optional full grille and in various wood finishes, and the period price was listed as "on application" rather than fixed — buyers were dealing with a dealer-quoted figure, not a catalog number.
Is it collectible? The 105 Series II sits in the sweet spot of KEF's reference line: earlier and rarer than the later Reference series, more ambitious than the compact two-ways, and documented well enough in period print that provenance is easy to establish. The 38Hz–22kHz window and the 200W rating make it a speaker that still works with modern amplification, and the vertically aligned head gives it an imaging character that's genuinely different from a conventional box. The caveats are the usual ones for a speaker of this age and complexity: the protection circuit, the crossover networks, and the drive units are all serviceable but not trivial, and the split-cabinet design means any restoration has to account for two enclosures rather than one. Buyers should expect to inspect the S-STOP board and the bass surround before committing, and should not assume the "on application" price of 1980 translates into a bargain today.
What the 105 Series II really represents is a moment when KEF stopped treating the home speaker as furniture and started treating it as an instrument that happened to live in a house. The separate bass enclosure, the aligned head, the computer-matched pairs, the self-powered overload protection — none of it is decorative. It's all in service of a flat response and a driver set that survives being driven hard. That's why the labs adopted the 105 as a reference, and it's why the Series II remains the version collectors seek out.
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