I’ve been enjoying my KEF Reference Series 107 loudspeakers since I bought them new in 1986, originally they lived in Hammersmith but are now in Keswick. All three drive units have excellent clarity and lack of distortions or “sonic footprint” due to a highly-innovative cabinet design using units which were already esteemed in conventional enclosures. The brochure claimed a frequency response of 20 Hz - 20 kHz to the tight tolerance of ±2 dB. KEF intended the 107 speakers to be used with an electronic equaliser (“Kube”) but it has been problematic to include this in a modern system or replace it with a digital equivalent. The choice has been to put up with the limitations of the Eighties equaliser or to have cleaner, clearer sound but with inaccurate tonal balance, limited by conventional tone controls.
I tried first a Sony AV amplifier with multiple HDMI connections and very modern A to D converters. It also includes a room equalisation routine which I hoped would help. Unfortunately it would neither equalise for my KEF 107 speakers on its own nor does it have a tape monitor loop, either digital or analogue, that I would use either for KEF’s original Kube equaliser or for a modern equivalent.
I tried instead a much simpler Marantz amplifier, much the same as the Denon I have been using since I arrived here. Neither have tape monitor loops but it’s possible to use the Record output of the Denon through an equaliser to an Aux input on the Marantz. The Marantz has the greater power output so it’s that which drives the speakers. One more remote control in the basket...
I have a small wideband microphone, amplifier and meter to measure audio volume roughly and display it in decibels on my Android phone. The Kube is fine and gives the reference frequency response (and tonal balance) as intended by KEF but it’s bad for analogue noise by current standards. So a bit of measurement with that and a test CD from the Hi-Fi News magazine of years ago.
Repatching to a modern analogue graphic equaliser cleans up the sound, both analogue noise and harmonic/intermodulation distortions that I could hear from the period integrated circuits and components of the Kube.
I used my test results to set the equaliser to much the same response as the Kube had given, turned it round so that the many LEDs do not distract, and am now enjoying the KEF 107s like never before: neutral, good definition with bass lines coming out where I had forgotten they existed, etc.
This is much the same approach used by the room set-up routines in home cinema systems. An installation for a professional sound control room might also include phase control but that’s less relevant in a domestic setting without specific acoustic treatment. This is not how the traditional hi-fi world operates but I’m happy, it has been achievable with readily-available equipment and now lets me get on with enjoying the music.
The sound source can be up to 48/96 and is decoded to analogue in the Denon, which sounds fine to me and gets good reviews. The rest of the signal chain is analogue. I normally use the Source Direct feature on the Marantz, bypassing its tone controls. There’s a phono input on the Denon for when/if I bring my turntable and LPs here, also an analogue input for my Revox A77.
I refined my basic set-up to take account of the highest obvious room resonance, between ceiling and floor. That works out mathematically at about 75 Hz; putting a small notch in the equalisation at that frequency has made a further worthwhile improvement, for example making clear the tuning of timpani.
The main downside is the extra remote control and needing to switch everything on and off individually rather than the automation associated with HDMI connections.
The low ambient noise floor here allows the inner detail of the recorded sound to be heard; being able to play loud without neighbour worries (me disturbing them or them disturbing me) is a pleasure and a privilege. Then there’s the fantastic view of the Lake District beyond the screen and the speakers.
I think KEF never published the in/out frequency characteristic of the Kube (or the circuit diagram) but the equalisation I have ended up with is a boost from 200 Hz and below, shelving at about +18 dB below 100 Hz except for a notch around 75 Hz. This is specific to my room and won’t be right anywhere else.
Kit list
KEF Reference Series 107 loudspeakers
Denon PMA-600NE Integrated Amplifier
Behringer FBQ3102HD Ultragraph Pro 31-Band Stereo Graphic Equaliser
Marantz PM6007 Integrated Amplifier