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KEF LS5/1AC Reference series studio monitors
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ColinR
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PostPosted: Mon May 08, 2006 10:15 pm    Post subject: Reply with quote

Thanks for the mug shots.

As the HH logo is visible on the circuit board in the third image of the amplifier's innards, plus the power switch, transistors on the case and red international voltage regulator are similar to a HH built AM8/12 amplifier I've seen, I'll say job done Very Happy .
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proffski
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PostPosted: Mon May 08, 2006 11:13 pm    Post subject: Reply with quote

I'll second trhat Terry. A breath of fresh air and so informative as well as entertaining!
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henryc
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PostPosted: Tue May 09, 2006 7:18 am    Post subject: Reply with quote

Guys this is sterling detective work thanks a mil..
The plot thickens.. with your HH reference I came across this web link:
BBC Radiophonic Workshop "An Engineering Perspective"
1986-1988 which refers to the BBC AM8/12 amplifiers, could these be the same? interesting stuff and worth a read of the whole article

http://myweb.tiscali.co.uk/whitefiles/b2_h/4_radiophonic_workshop/rws/pgs/r07.htm

EXTRACT: The Workshop also standardised its loudspeakers. The old BBC LS5/5 units in the Piano Room and the small LS3/5A speakers (with BBC AM8/12 amplifiers) in the temporary Studio F, were replaced by Roger’s LS5/8 units. Each loudspeaker had a single stereo Quad amplifier that the BBC had fitted with an active ‘crossover’ circuit. The amplifier then gave two outputs, one for the ‘woofer’ and one for the ‘tweeter’. There was also a need for alternative loudspeakers that would simulate a domestic hi-fi system. The Fostex model 6301B was used, a small unit with inbuilt amplifier and volume control. Some composers used these in stereo pairs, others singly to check mono compatibility.
By the end of 1987, the department experimented with removing the input transformers on the LS5/8 loudspeaker amplifiers. This gave a 3 dB improvement in performance at 15 kHz, which was quite audible. Also, in Studios A, B, C and D, the loudspeaker amplifiers were plugged directly into the consoles, rather than via the original 4-pole F&E (EP) pattress sockets. In Studio E, however, the EP connectors were replaced by XLRs. All the amplifiers were then fitted with short ‘gender bender’ cables that converted the audio inputs to the commercial ‘pins point in the direction of signal’ standard.
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ColinR
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PostPosted: Tue May 09, 2006 7:59 am    Post subject: Reply with quote

Two more tails for you to chase Very Happy .

As long established large pro & domestic amplification shops with staff who remember these things are few and far between Sad . Have a word with SMC in Shrewsbury about HH practice amplifiers if you want some background information in that area.

For general information about the speakers, why not talk to a "top flight" organisation - Granada Television in Manchester Cool .
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henryc
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PostPosted: Tue May 09, 2006 8:30 am    Post subject: Reply with quote

Couple of web links
LS5's in BBC Studios/ workshops
1. http://homepage.ntlworld.com/paul.whatton/JABBC5.JPG.html
2. http://homepage.ntlworld.com/paul.whatton/JABBC7.JPG.html

HH amps views from music heads etc
http://www.vintageamps.com/plexiboard/viewtopic.php?t=35994&highlight=&sid=ee6319ab0589604f9241d518fb1898a7
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terry
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PostPosted: Tue May 09, 2006 10:15 am    Post subject: Reply with quote

Please see Henry's photos here. They are now published on the site with links from Personal Treasures and from the main KEF speaker system index page.
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henryc
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PostPosted: Tue May 09, 2006 10:42 am    Post subject: Reply with quote

Thanks for that link Terry..Great Job
The speaker and Amp history hunt continues hopefully we can find out more about them and where all the other examples are out there probably gathering dust at the back of a studio somewhere in Broadcast land.
I'd love to hook up with someone in Gloucestershire who may be able to check the amp components see if all's ok and if possible update them in anyway. any help on this front would very welcome
Much as I have tried I can't find anyone with sufficient experience and knowledge, the local Hifi outlets don't know anyone who can repair anything
other than the original manufacturers.
Surely there must be a trusted engineer out there who can advise.
Look forward to the contact
Thanks in advance
henry
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proffski
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PostPosted: Tue May 09, 2006 11:20 am    Post subject: Reply with quote

Do not hold your breath, but...

I know an excellent chap in Cheltenham now working for the NHS who has left the TV repair business as well as AV because work simply did not come in enough to make the business worth running.

I'll contact you if I can find him and if he says yes.
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henryc
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PostPosted: Sat May 13, 2006 9:50 am    Post subject: Reply with quote

Speaker Guru' letter extracts which add more history info:
Thanks javascript:emoticon('Very Happy')

Hi Henry,
Thanks for the pictures.
They bring back lots of great memories.
I can still remember the first time I heard a pair playing the
Hawaii-5-0 theme in the KEF listening room. They were very impressive. I still have a pair of these speakers in my garage but sadly missing a
woofer.

The amp.: It is branded KEF but the power amp boards, power supply and the case are definitely H-H. I don’t know who assembled them. I can try to find out but it may take some time.

That was all just a bit before my time at KEF. I joined in1975 – just in
time to see the last of the 5/1ACs being set up before shipping to Japan.
The active crossovers were KEF designs and the transformer input stage and protection circuitry were designed by Peter Baxandall for KEF. The
tweeters were Celestion HF1300 and later on HF1400.

Later on, in my KM1 Professional Series era, there were still a few 5/1ACs coming in for repair and tweeter supplies had dried up. We decided to offer a revamp using the KM1 tweeter. The active crossover and eq was redone. The protection circuits were redesigned around the Reference Series modules. I doubt that we modified more than a dozen in total, so you have a very rare pair indeed.

Happy Listening and Best wishes,

SG
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Parts
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PostPosted: Sun May 14, 2006 12:01 pm    Post subject: Reply with quote

Earlier on someone said the bass drivers were unidentfied.

The are made by goodmans, (originaly a guitar speaker).
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proffski
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PostPosted: Sun May 14, 2006 4:11 pm    Post subject: Reply with quote

Would you know if it was a variant on the Goodmans Audiom 81 or Audiom 15P series? The Audiom 81 had an Fs of 30Hz. No info on the 15P.
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henryc
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Location: Gloucestershire

PostPosted: Sun May 14, 2006 8:43 pm    Post subject: Reply with quote

Thanks parts & Proff
Hi guys some more questions this time its about the speaker front baffle ..

Can someone explain to me why the LF 15 inch unit bafle opening is oblong on the LS5/1ac?
Why was this done?
Was it to funnel or direct the sound into a tighter beam ?
Would it sound better if the opening was made round to reflect the true diameter of the speaker?
How would the sound change if the opening shape was changed ?
I ask because I think that it would look better if the opening were round
in shape?
I'm also asking because the wife(and many others have mentioned it as well ) hates the square shaped hole and I would do anything to keep her happy !
As she said" it would make them more liveable with ..."
Any learned views verry welcome please
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Last edited by henryc on Mon May 15, 2006 12:16 pm; edited 5 times in total
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proffski
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PostPosted: Sun May 14, 2006 10:51 pm    Post subject: Reply with quote

Indeed it is there to modify the dispersion characteristics of the transducer.

I just would like to measure the extra resonances and artefacts that in effect another resonant cavity provides...
At some time both versions must have been teried I suppose, colouration Vs directivity up to a point?
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speakerguru
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PostPosted: Mon May 15, 2006 11:49 am    Post subject: Reply with quote

The slot is to increase the dispersion of the 15" unit, which would otherwise be very narrow, in the horizontal plane.

From antenna theory the beam width is the Fourier Transform of the antenna aperture. The narrower the aperture the wider the beam.

I remember reading a BBC Research Dept. paper on it where they also discuss the trade off of cavity colouration vs beamwidth for good stereo sound stage. Anyone got a copy?
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henryc
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PostPosted: Mon May 15, 2006 12:18 pm    Post subject: Reply with quote

Hello SG
Thanks again for that info which is very ineteresting
I have no intention in changing the aperture size or shape even more determined than ever now to beat off views to the contrary
you will be pleased to hear.
Hope we find the paper you refer to
Kindest regards
Henry
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