SpeakerTalk

[speakerguru]

The LS5/1AC was an active 2-way version of the BBC LS5/1A in a slightly larger cabinet. The BBC design used a low colouration Goodmans 15" woofer which featured a very soft pulp cone and soft pvc surround. The tweeters were Celestion HF1300s. Two were used to provide adequate power handling, with one being electrically rolled off at high frequencies to prevent HF lobing.

KEF LS5/1ACs were produced in the early to mid 70s for the then newly formed UK commercial radio stations; Capital Radio, Piccadilly Radio, etc.

The original KEF active design used HF1400s which were higher sensitivity than the 1300s. In the 2-way active design the tweeter outputs could therefore be turned down and the system would either play louder or run cooler. This design also featured a balanced transformer input stage and electronic speaker protection designed by the late great Peter Baxandall.

In the 80s, supplies of both Celestion tweeters ran out and as KEF were producing the KM1 at that time, the 5/1AC was redesigned to use a single KM1 tweeter.

Link to KEF LS5/1A paper

[speakerguru]

Henry,

I don't think there is anything you can do to improve these speakers. It's not that they are perfect, but they were among the best at that time for monitoring purposes.

Count yourself lucky if all the drive units are fully working and rattle and buzz free. Spares are unavailable. I've tried to get a Goodmans woofer reconed myself, without success. That soft pulp cone and pvc surround were very low colouration and resonance. All you can get these days are high res guitar cone assamblies.

Those early HH amplifiers were difficult to set up due to the fixed AOT resistor biasing and optimistic power transistor heatsinking. Later amps had a transistor Vbe multiplier circuit which was a bit more stable.

After all this time all of the electrolytic capacitors in the amplifer would probably need replacing if you want to approach the original performance and ensure reliability. Also get the amp PAT tested for safety's sake.

Happy listening.


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I bought these speakers out of reverential curiosity.I have owned and loved countless speakers over the years but
had never seen or heard these ones
I am a great admirer of early British speaker design and the opportunity came my way .

The pair i have are the modified type using the KM1 tweeter.
I bought them from a gentleman from Chester via EBAY , who said that he thought they had come out of an ITV studio in the Midlands.
I have enormous respect for KEF speakers and what was achieved by yourselves.
As usual I am now wondering whether I could do them further justice by updating them if at all possible? What do you think?
Perhaps the cables internally / Externally ?Replace the KM1's ?
I want to maximise their potential. Would you be able to suggest anything to improve them ? you would after all be the ideal person to ask.
I am currently running them from a Rotel pre amp so I can feed several inputs, control volume etc.
They are huge and My wife is not very happy although she is impressed with the sound
She is a keen opera singer and she feels that the midrange HF is confused / Coloured
Could we update this area?
Hope you can help
Kindest regards Henry

[speakerguru]

Henry,

Try to get hold of some of the acoustically transparent black foam and get rid of those ugly disco speaker grilles. The top one in particular must be producing horrendous diffraction of the tweeter output.

I know the foam is expensive and doesn't last but those big 5/1AC look really cool with smooth foam fronts.

SG