Main studio monitors these days are packed full of technology — but that wasn’t always the case, and it took an innovative British hi‑fi brand to drag mains out of the dark ages.
If you visit almost any high‑end recording studio, particularly in the UK and Europe, there’s a pretty good chance you’ll see a pair of either freestanding or soffit‑mounted main monitors comprising multiple bass drivers arranged around a direct‑radiating (as opposed to horn‑loaded) midrange and tweeter module — quite often with multiple midrange drivers. I’m thinking of monitors like the Genelec 1235A, Dynaudio M3VE, ADAM S6X or PMC QB1A, for example. The role of all of these monitors, and others like them, is to generate very high sound pressure levels over an extremely wide bandwidth, without significant distortion, compression or coloration, and to do so reliably for long periods without rest. They need to reach PA volume levels yet combine that with the ultimate in electro‑acoustic precision. It’s a genuinely tough job, and it demands as much electro‑acoustic know‑how as it does manufacturing budget. So you’ll probably imagine that one of the aforementioned monitor manufacturers would lay claim to have developed the first such studio monitor, but you’d be wrong, because that honour belongs to British hi‑fi speaker company KEF with their KM1 of 1982.
KEF, starting in the 1960s, had a close relationship with the R&D department at the BBC, where all the in‑house monitor designs were created. A number of BBC designs used KEF drivers, the best known being the B110 bass/midrange driver and T27 dome tweeter employed in the compact LS3/5A monitor, but KEF also made complete systems aimed at studio applications. The 5/1AC (which, ironically, didn’t use KEF drivers) was used widely in the UK’s commercial radio sector, as was the later P60, a largely forgotten but technically interesting semi‑active nearfield monitor.
In the late 1970s and early 1980s, the BBC found itself with a monitoring problem in the studios used to record live sessions for its rock and pop stations, Radio 1 and Radio 2 respectively. The main monitors in those studios, primarily Maida Vale and Golders Green in London, were BBC LS5/8s. And although the LS5/8 was designed to satisfy “monitoring requirements from pop to serious music” (as described in a BBC R&D technical report available at www.bbc.co.uk/rd/publications/rdreport_1979_22), it wasn’t in practice able to manage the volume levels required and it kept failing on the job. The option to equip the studios with Tannoy‑ or JBL‑style monitors, the latter comprising large‑diameter, lightweight‑cone bass/midrange drivers and horn‑loaded compression tweeters, was of course available. But those styles of monitor, effectively repurposed PA speakers, simply wouldn’t be able to meet the BBC’s needs for tonal accuracy, minimal coloration and low distortion. So something new was required and KEF were asked to make a proposal. The KM1 was what they came up with, and it was like nothing that had gone before.
The KEF KM1 monitoring setup at the BBC’s Hippodrome Studio, 1987.Photo: Nick Jennings
Active Learning
At a time when studio mains almost exclusively used horn‑loaded tweeters, KEF’s KM1 employed a heavily modified T52 direct‑radiating dome.Photo: Pete ThomasKEF had at the time a success on their hands in the shape of the top‑of‑the‑range hi‑fi speaker, the Reference Series Model 105. The 105 was a three‑way passive system incorporating a B300, 300mm‑diameter bass driver; a B110, 110mm‑diameter midrange driver; and a T52, 38mm dome tweeter.
KEF’s electro‑acoustic calculations suggested that a system comprising four of the B300 bass drivers, two modified B110 midrange drivers and one very heavily modified T52 tweeter ought to be able to reach the required BBC spec of at least 120dB (60Hz to 20kHz) at 2m from a pair of monitors. But KEF also realised that any practical speaker designed along those lines and able to reach the required volume levels would need to be active, so they set about working with another iconic UK audio manufacturer, Quad, to help develop the necessary amplifier and electronic crossover modules.
The KM1’s amplification, developed in collaboration with Quad, was built into a sub‑assembly that slid out of the bottom of the speaker like a drawer. The central part of the cabinet functioned like a chimney, drawing heat away from the amp and speaker heatsinks.Photo: Pete ThomasThe amplification built into each KM1, in a slide‑out, drawer‑style sub‑assembly, comprised eight single‑channel modules derived from the Quad 405 power amplifier. The amp channels were distributed across the drivers such that a bridged pair of channels was dedicated to each pair of bass drivers, another bridged pair of channels was dedicated to the midrange drivers, and a final bridged pair of channels was dedicated to the tweeter.
The bridged amp channels were rated at 400 Watts into 8Ω, so the total power available was, in theory, 1.6kW per KM1. It’s said that both the amplifier power supplies weren’t quite up to the job of delivering 1.6kW, and that the system’s current limiting protection would often cry “enough!” a little early, but even so, the KM1 seemed well furnished with power. And it needed to be. Its drivers were derived from hi‑fi units, fundamentally designed for flat frequency response and low distortion rather than electro‑acoustic efficiency, so even in the loudspeaker realm where anything over five percent efficiency is going some, their efficiency was poor. The upshot was that over 95 percent of each KM1’s 1.6kW rated power could potentially be dissipated as heat. And then if you factor in that the Class‑A/B power amplifiers themselves would be only around 50 percent efficient, the KM1 begins to look like a room heater first and a high‑performance monitor second. A KM1 working hard could quite feasibly be dissipating something like 2kW.
So a major challenge with the KM1 was to manage heat such that it was drawn away from the driver voice coils and didn’t result in thermal compression or driver failure. To that end, both the midrange drivers and tweeter incorporated Ferrofluid‑filled voice‑coil gaps, both firsts for KEF, and the KM1 cabinet architecture was designed to create a central chimney, drawing air from underneath and routing it over 64mm‑diameter cylindrical aluminium heatsinks attached directly to the back of the midrange driver magnets and then to a rear heatsink panel. The warmed air would vent through the central top panel of the cabinet. The tweeter magnet system also was able to sink significant heat energy — it was twice the size of that used on the 300mm bass drivers.
Each KM1 amp module housed eight single‑channel Quad 405 amplifier boards.Photo: Pete Thomas
Photo: Pete ThomasBut it wasn’t only thermal issues that KEF faced in the context of hi‑fi drivers working at sustained high levels. For example, the standard Bextrene thermoplastic diaphragm of the B110 midrange driver was found to suffer from fatigue splits at the neck when subject to the kind of sustained accelerative shocks that 120dB imply. The solution was to swap to a polypropylene diaphragm (also lighter, so it improved efficiency), however this meant not only a near complete redesign of the driver, but also the development of new adhesive techniques to ensure that the diaphragm would remain attached to its surround and voice‑coil. The low‑frequency power‑handling performance also came under the spotlight because, although the KM1 had originally been specified by BBC Radio as a reflex‑loaded design, the first monitors delivered were of closed‑box format (KEF technical director Laurie Fincham was well known to be a reflex‑loading sceptic). Unfortunately, the closed‑box KM1s were found wanting by the BBC studio managers in terms of low‑frequency power handling and had to be converted to reflex‑loaded format. It’s interesting that even though all KM1s delivered to the BBC were reflex-loaded, the KEF sales brochure of the time illustrates and describes a closed‑box system. It’s likely that customers other than the BBC would have received KM1s in closed‑box format.
Electronics
Along with integrated power amplifiers, the KM1 also necessarily incorporated active filtering. The filters were designed in‑house at KEF, however the renowned analogue electronics specialist Peter Baxandall (original inventor of the style of active EQ circuit used widely throughout audio electronics) was employed as a consultant to help refine the circuits and to develop a new input stage for the power amplifiers.
In addition to its active filter circuits, the KM1’s electronics also incorporated a novel method of protecting the drivers from thermal overload. The system initially relied on a continuous low‑voltage DC signal routed through the driver voice‑coils. Measurement of the resulting DC current could then be used to calculate the real‑time voice‑coil resistance and hence the voice‑coil temperature (temperature and resistance are linked via the voice‑coil material’s temperature coefficient of resistance: 0.39 percent per degree C for copper). When the temperature approached preset limits, the KM1 overload indicator would illuminate followed by automatic driver attenuation if necessary. The driver protection system worked, however it had the significant problem that if any driver was replaced, the system would require recalibration to account for the production‑line variations in voice‑coil resistance. Later KM1s relied on a protection system based on thermal modelling of the drivers that enabled accurate voice‑coil temperature predictions to be made based on input signal history.
With its ambition, technology, engineering and sheer bulk, the KEF KM1 effectively redefined very high‑performance studio monitoring.
The KM1 Legacy
With its ambition, technology, engineering and sheer bulk (a single monitor weighed 140kg), the KEF KM1 effectively redefined very high‑performance studio monitoring. But was it a success? The answer to that isn’t simple. The KM1 achieved its basic performance aims and served in the Maida Vale studios for around six years before being superseded by the first PMC monitor, the BB5 (PMC were co‑founded by Pete Thomas, previously a BBC studio manager with extensive experience of the KM1). However, the KM1 also had a reputation for unreliability, and for that reason was reportedly not well liked by BBC studio managers. A common subjective reaction to the KM1 was said to have been that, despite what the studio SPL meters were saying, it didn’t really go loud enough, and that perhaps led to unreliability. Even at 120dB, the KM1 was said to sound uncompressed and undistorted. It seems it was loud, but to some ears it didn’t sound it.
Perhaps the fundamental shortcoming was that the use of inefficient hi‑fi drivers to reach very high sound pressure levels over a very wide bandwidth required heaps of amplifier power — most of which would wind up as heat. Viewed from our perspective 40 years later, the KM1 has a whiff of white elephant about it. It was perhaps something of a technical cul de sac at KEF in that its development didn’t appear to contribute much towards the core business of somewhat more affordable hi‑fi speakers.
Ultimately, 72 pairs of KM1s were made, and sold for a reported £20,000 each (around £75,000 nowadays), so in commercial terms it represented a significant slice of turnover — and the KM1 undoubtedly prepared the ground for a whole new style of monitor, ground that subsequently grew businesses for the likes of PMC, Genelec and Dynaudio. Sadly, it appears that the whereabouts or operational state is known for very few of the 72 pairs of KM1s, which is a shame, particularly because I never had the opportunity to experience the KM1 back in its heyday, and I would really quite like to see and hear a pair.
Many thanks to Pete Thomas of PMC for his help with this feature.
KEF At 60
Along with the KM1’s ground‑breaking technical achievement, one of the motivations for celebrating it in the ‘Golden Gear’ series is to mark KEF’s 60th anniversary. Compared to KEF in the early 1980s when the KM1 was launched, the company are a very different organisation today. Despite their HQ still being at the same Maidstone premises in the South East of the UK, KEF are no longer a privately owned UK company and, along with Maidstone, their products are manufactured at multiple sites across the world.
In an important respect, however, nothing has changed: KEF are still one of the most audacious, innovative and electro‑acoustic engineering‑led of all speaker companies. A recent example of innovation is the development, by Vice President of Technology Jack Oclee‑Brown and his team, of a ‘metamaterial’ wide‑band acoustic absorber. The metamaterial absorber is fitted behind, and almost entirely absorbs the rear radiation of, the tweeter on the recently launched KEF LS50 Meta.