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Derek Hughes external cross-overs for the Rogers LS 8/5

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Derek Hughes external cross-overs for the Rogers LS 8/5

Postby CeruttiA » Wed Nov 28, 2018 12:27 am

Hello !

I have just bought a pair of external cross overs, developed by Derek Hughes and marketed by Graham audio especially for the Rogers 5/8 to enable the user to use any power amp and not be limited by the Quad 405 and its rather badly designed cross-over.

Has anyone had some positive experiences with this ?

Can it really improve the quality of the good old Rogers ?

Thanks

Alexander
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Re: Derek Hughes external cross-overs for the Rogers LS 8/5

Postby Phil Ward » Wed Nov 28, 2018 1:33 pm

Well if Derek says they're good I wouldn't argue.

Derek's dad, Spencer, was a major part of the team responsible for all the BBC (and BBC derived) monitors of the period and Derek carries the flag with great commitment and enthusiasm. He's also got a great pair of ears and definitely knows his stuff technically. Spencer left the BBC to found Spendor (with his wife Dorothy - see what he did there?) and when he retired Derek took over the reins until the company was sold (to Philip Swift of Audiolab fame I believe).

As to whether external crossovers are fundamentally a good thing, I'd say generally, yes. Partly, the inside of a speaker is not a great place for potentially microphonic components to be sited, and secondly, the opportunity to use alternative or (depending on how the crossover is configured) multiple power amps might well be of benefit (especially if the Quad 405 is one of the early ones with its allegedly over cautious protect systems). Having said that, any benefit of the new crossovers will probably be more likely found in more modern components - the capacitors especially.

Hope that helps
Phil
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Re: Derek Hughes external cross-overs for the Rogers LS 8/5

Postby Hugh Robjohns » Wed Nov 28, 2018 1:54 pm

Phil Ward wrote:As to whether external crossovers are fundamentally a good thing, I'd say generally, yes. Partly, the inside of a speaker is not a great place for potentially microphonic components to be sited, and secondly, the opportunity to use alternative or (depending on how the crossover is configured) multiple power amps might well be of benefit (especially if the Quad 405 is one of the early ones with its allegedly over cautious protect systems). Having said that, any benefit of the new crossovers will probably be more likely found in more modern components - the capacitors especially.

The LS5/8 was designed as a two-way active speaker with a line-level crossover originally built into the chassis of a Quad 405 power amp, such that one channel powered the tweeter and the other the woofer. It was a very elegant, reliable, and effective design.

I don't know why you think the original active crossover was 'badly designed'. It seems a perfectly good design to me -- albeit a 'Neve-esque' discrete transistor circuit rather than op-amp based -- and it was used without alteration across the entire production life of the LS5/8, including in the last models which employed more powerful customised Chord amplifiers. The only significant change was the use of a different unbalancing transformer on the amplifier input which cured the tendency of the original to roll off the extreme HF a little prematurely.

There have been lots of attempts to 'improve' the LS5/8 over the years, some of them being very good... but fundamentally this is a very old speaker design now and the world has moved on considerably in driver, cabinet and crossover technology.

I'm sure Derek's new passive crossover does a great job. Whether it's benefits outweigh those of a true active design I couldn't say not having heard it -- but the Beeb could have gone down the passive x-over route if they'd wanted when they designed the thing and decided that their active design was significantly better... even with the Quad power amps!

Personally, while the LS5/8 was undoubtedly outstanding in its time, and I spent many many years using them every day, I wouldn't give it house room today as there are so many better monitor speakers around now. Even the Beeb has moved on... But the LS5/8 has gained a cult status and following in the hi-fi world I suppose.

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Re: Derek Hughes external cross-overs for the Rogers LS 8/5

Postby CeruttiA » Wed Nov 28, 2018 3:05 pm

Thank you Phil and Hugh, most helpful and very informative. I paid 750 UKP for 2 of them, will update yiu how they sound. The advantage to use any amplufier is a considerable one, i am not convinced that the Quads keep up with the power requirements of the rogers 8/5, so i am intending to get a good old Audio Research with 125 or 150w.

Thanks again for your great advice
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Re: Derek Hughes external cross-overs for the Rogers LS 8/5

Postby Hugh Robjohns » Wed Nov 28, 2018 3:21 pm

Most passive speakers benefit from having plenty of power on hand in my experience, and there were certainly a few occasions where the LS5/8 +Quad combo didn't go loud enough at the Beeb, but for 99% of the time it did what it was designed to do very well -- which is basically to play speech at a slightly higher than normal level.

It's also worth bearing in mind that one reason the Quad 405 was used was because of its built-in voltage limiter which was designed to protect Quad's electrostatic speakers, but was used in the LS5/8 to protect the tweeter... That essential protection would be absent if a standard but more powerful substitute amp is used (unless the revised passive crossover incorporates some protection circuitry)

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Voltage limiter fir the Rogers 5/8

Postby CeruttiA » Thu Nov 29, 2018 12:23 am

Does the absence of a voltage limiter render the passive crossover ( by Derek Hughes) useless ? I mean , how can i protect the tweeter and base unit from damage. I do not listen at high volume but thought of getting a more powerful amplifier - perhaps not a good idea after all !
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Re: Derek Hughes external cross-overs for the Rogers LS 8/5

Postby Hugh Robjohns » Thu Nov 29, 2018 12:30 am

I know the original tweeters (back in the 70s) were prone to blowing, hence using the Quad limiters. I'm afraid I don't know if that remained the case -- it may be that later tweeters were more robust, and I don't think the last-model Chord amps had limiters.

I'm sure Derek's design works perfectly well, but perhaps he should be your advisor here as he knows what his intentions were as far as amp power and driver protection etc is concerned.

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Re: Derek Hughes external cross-overs for the Rogers LS 8/5

Postby CeruttiA » Thu Nov 29, 2018 12:38 am

Thank you, Hugh !
I have rarely come across a more professional forum and such knowledgeable members as you and Phil !
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Re: Derek Hughes external cross-overs for the Rogers LS 8/5

Postby James Perrett » Thu Nov 29, 2018 9:49 pm

The OP asked the same question on another forum inhabited by some ex-BBC people with some interesting answers.

https://www.vintage-radio.net/forum/sho ... p?t=151823
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Re: Derek Hughes external cross-overs for the Rogers LS 8/5

Postby Hugh Robjohns » Thu Nov 29, 2018 10:48 pm

:lol: :beamup:
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