Cloning BBC LS 5/8 monitors -- worthwhile or even possible? - diyAudio
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Old 28th November 2013, 09:28 AM   #1
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Default Cloning BBC LS 5/8 monitors -- worthwhile or even possible?

All,

Having heard a pair of these big BBC LS 5/8 monitors about a year ago, I was quite impressed. Now I'm wondering whether it would be worthwhile to attempt making a clone.

That is, I have several Quad 405/2 amps and a friend has spare Audax TWO34XO tweeters, so I "only" would need to build the electronic crossover, the cabinets (12mm birch plywood dampened with bitumen) and decent stands. At least for electronics and woodworking, I think I can deliver a decent job. That doesn't guarantee good sonic results, of course.

Now I don't have the polypropylene woofers, somebody of my acquaintances suggested the Monacor SPH-300KE (kevlar cone). But the LS 5/8 has vents (if the design info I have is correct) -- not sure if these Monacor woofers would be the prime choice.

In the worst case, I end up with active speakers consisting of a Monacor woofer/midrange and an Audax tweeter and I will have learnt from the experience.

Some info sources, like the Harbeth users forum or Mark Hennessy's pages don't attempt to suggest replacement woofers, or at least not that I've found.

I also read about peaks in the frequency spectrum and adjustments in the electronic crossover to flatten these out. With different drivers, all this has to be re-considered.

I can, of course, go for a pair of those big monitors like Harbeth M40s which claim to be the descendants of the big BBC boxes (and they are much better according to the leaflets!), but the price tag is probably at least 20 times as high and they are less fun to get.

Or should I consider other designs? My preferred music is all sorts of "classical" music (from the 16th century to the 20th) and many jazz recordings (often from the 30s - 40s), to give an idea of what I expect to drive through those speakers.

Thank you for any thought and/or advice!

Jacques
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Old 28th November 2013, 09:54 AM   #2
tvrgeek is offline tvrgeek  United States
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Driver technology, understanding of issues like center distances and diffraction, and materials have come a long way in 35 years. Good for us, but it does leave "vintage" system like that in the dust. Please go listen to some more contemporary well executed speakers. Then, with your competency in construction, build an established documented design or kit.

Speaker design is not slapping some drivers into a box and hooking up an active crossover.
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Old 28th November 2013, 10:44 AM   #3
sreten is offline sreten  United Kingdom
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Hi.

There is not much chance of sourcing a suitable similar 12" driver nowadays.

rgds, sreten.
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Old 28th November 2013, 11:48 AM   #4
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Quote:
Originally Posted by tvrgeek View Post
Driver technology, understanding of issues like center distances and diffraction, and materials have come a long way in 35 years. Good for us, but it does leave "vintage" system like that in the dust. Please go listen to some more contemporary well executed speakers.
Technology may have advanced a lot (it certainly has, generally speaking), but many listening tests seem to indicate that you need to go to rather expensive areas of speaker offerings in order to get something similar in terms of convincing midrange rendition like the ugly, hyper-directional, position-critical Quad ESL 57 or even that other pièce de résistance, the AR3a. And even other boxes from an era long gone by.
When I listen to a recording of Andreas Staier playing a Conrad Graf pianoforte from 1824, I want it to sound similar to his live concert on the same instrument which I was happy to attend.
Many modern speakers are harsh, don't reproduce well the subtle harmonics from such instruments. Recordings of a Streicher pianoforte from around 1820 sound on most modern speakers like some mean out-of-tune honky-tonky piano.

OK, I must be getting old (I actually am) and from a certain age, strong high-frequencies start to hurt the ear, it seems. I'll have to live with that.


Quote:
Originally Posted by tvrgeek View Post
Speaker design is not slapping some drivers into a box...
That was never my intention, having access to test facilities and the possibility to rely on the competence of several others with strong technical background. I just want to experiment and if the result is worthless, so be it. In the end, not much more than some panels of plywood will be spoiled, I suppose. There are worse activities to keep a retired person off the street :-)
I have built several kits, the first one was a Wharfedale Unit 5, drivers from the Dovedale III, more than 40 years ago. Results usually were good enough (compared to equivalent ready-made speakers), all that may have given me too much confidence.

Thanks for the advice!

Cheers,
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Old 28th November 2013, 12:13 PM   #5
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There is more to loudspeaker design the deeper you dig. I don't think you will be able to build a true competing speaker without years of research and experience.

As a project and a learning exercise I think you will be able to build something reasonable as you have access to measuring equipment. But as sreten has said, finding a suitable 12 inch drive unit could be hard these days and you might have to pay a lot and buy something from a professional range. A quick look at the Monacor SPH-300KE and it doesn't look that bad though...

Have fun! I'm sure you'll enjoy it.
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Last edited by richie00boy; 28th November 2013 at 12:16 PM.
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Old 28th November 2013, 12:15 PM   #6
tvrgeek is offline tvrgeek  United States
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Well Jacq, I think you have not heard the right speakers. Some out there are really terrible. Might go find something like Vanderstein 2ce's. Not at all harsh. My last Seas based set are not at all harsh.

If you listen to a lot of vintage recordings, you may consider doing some filtering as what you are hearing now may be a result of the truth of the recording, where the vintage speakers did not have the bandwidth to reproduce the upper harshness. Listen on some good cans to get a baseline. Even the baby Grado's are far more accurate than the best speakers.

My wife has hyper critical sensitivity to harshness, i.e. tweeter distortion. Horns & strings would cause actual pain, but when we heard it live, no problem. After 30 years of building speakers, I discovered it is all about the tweeter quality. Once I moved up to the Seas 27tbfc/g level, the problem was solved. Suddenly we could use my Parasound amps rather than the Rotels and got an entire level of detail added with the same balance and smoothness. The problem was nothing but harmonic distortion caused by breakup IM producing sub-harmonics that were objectionable.

You have an advantage with competence in construction, now find an established modern design to put the effort into. I would love to do the Santori build being worked up over in the ventor's forum.
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Old 28th November 2013, 04:28 PM   #7
system7 is offline system7  United Kingdom
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I often look at a design and ask myself what is GOOD about it.

Rogers Loudspeakers › LS5/8

In this case it is the big Audax TW034 tweeter crossed low around 1-2kHz with the big woofer. This is quite a common idea if you look at the Devore Orangutan or SEAS A26 designs. It crosses over below the bass cone breakup.

They sure don't make polycones with PVC surrounds anymore. In fact PVC was a bit of a disaster in the long run, tending to deform.

You mention Spendor and Harbeth. They do keep the tradition alive in passive speakers.

Click the image to open in full size.

Click the image to open in full size.

You might consider this H1659-08 U22REX/P-SL woofer with a E0055-06 T35C002 that will cross low. Quite a design job, but it's fairly familiar territory.
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Old 29th November 2013, 10:03 AM   #8
tvrgeek is offline tvrgeek  United States
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I look at these boxes and see sharp front edges, crossovers too low for the tweeter, and drivers that have been surpassed by the cheapest no-names on the market with cabinets made from PB that vibrated all over the place. They were the best we could buy back then. They are not worthy of a big-box store now.
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Old 29th November 2013, 11:34 AM   #9
sreten is offline sreten  United Kingdom
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Hi,

FWIW IMO building a classic BBC style 2 cuft speaker would
worthwhile and that Seas 8" mentioned by S7 would be on
the top of my list for such an endeavour.

The 35mm tweeter would not be, nor would the 34mm Audax.

Tweeters have come on leaps and bounds since the days
http://downloads.bbc.co.uk/rd/pubs/reports/1979-22.pdf
the above report was written, also see : TW034

Power handling, distortion and SPL of modern drivers
goes way beyond the drivers of 35 years ago and IMO
a 2cuft BBC style speaker would be very worthwhile.

Seas make some entirely suitable high quality modestly
priced 25mm dome tweeters that will do the job well :
http://www.seas.no/index.php?option=...=81&Itemid=105
http://www.seas.no/index.php?option=...=80&Itemid=104
The latter more suitable for the active route than the former.

Active arrangements like the LS 5/8 are very possible,
and a 405 for bassmid with a JLH10 for treble would
be a very nice contemporary amplifier combination.
(Or a Death of Zen for the treble, similar to JLH.)

Active x/o should be based on contemporary 5532
op-amps, which surprisingly are still very difficult
to better with any real significance nowadays.

Quick modelling indicates the Seas bass driver
its well suited to a vented box of 42L tuned
to 35Hz (my sort of room friendly alignment).

Actively correcting the bass-mid to nominally the equivalent of 87dB
will give a lot of clipping headroom in the midrange, and the tweeter
run off 10-20w class A unattenuated at 91-92dB/W will also be fine.

For the cabinets read up on the BC1, and its clones, typically
front baffle 3/4" flooring garade chipboard, all sides 3/8" ply,
rear 1/2" ply. Cabinets edge battened and screw on front
and back. All sides and back bitumen damped. I'd also
do the front. Thickness the same on all I reckon.

Add a cross member to the baffle between the drivers.
Properly off set the tweeters in mirror image pairs.
(That isn't typical BBC style though ....)

rgds, sreten.
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Last edited by sreten; 29th November 2013 at 11:56 AM.
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Old 29th November 2013, 12:06 PM   #10
DrBoar is offline DrBoar  Sweden
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Those speakers has a baffle step that is at least one octave below all those deep narrowfaced moderns speakers with woofers consisting of two or three 5" or 6.5" woofers. This baffle step is also in the range were the most energy content is. For cabinet look on my "Dorspen" that is a Spendor inspirated design. Troed Gravesen DIY site also describe the BC 1 with its 12mm plywood cabinet no braces but lightweight bitumenloaded fibre board lamination
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