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Old 4th Aug 2019, 8:04 pm   #21
LNBAudio
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Default Re: LNB Para-Lab 20’s

Really pleased that you're so happy with the 20s and I'll pass your thoughts on. Thanks
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Old 6th Aug 2019, 9:53 pm   #22
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Default Re: LNB Para-Lab 20’s

A kind chap on a Facebook group I’m in has sorted me out some stands nice and cheap.

A bit of height and stability has taken the slightly boominess out of the base and really opened up the soundstage. Sounding really very nice. Just bought a nice Sugden A21a off eBay -should really get them singing
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Old 1st Oct 2019, 9:11 pm   #23
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Default Re: LNB Para-Lab 20’s

Hi All

Just came across the forum when searching for a bit of information on my LNB speakers. I purchased these from new back around 1973/4, and they have moved around the country with me as I changed jobs. As a result the cabinets have suffered a bit over the years, and I've just starting to tidy them up a bit. Both B200s have been changed at different times as they started to suffer from what looked like de-lamination of the cone centers, but the T27 tweeter and super tweeter are original. I believe the super tweeter is the STC unit, but I'm not sure.

The crossovers were upgraded to a later version supplied by LNB, rather more years ago than I care to remember. But I know the speakers were originally built in 1973 as the date stamp is still on the speaker baffle.

I've always been please with the natural sound of these speakers, but have read elsewhere that the caps in vintage crossovers do deteriorate over time and can have a detrimental effect on the sound.

Is this a good idea, and if it is can anyone help me identify the caps I should fit. The one is clearly marked 0.1mf 10%, but can't see any voltage. The other just seems to have colour bands.

The speakers still sound pretty good, but a slow deterioration over time might not be that easy to notice.

Thanks
Paul
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Old 2nd Oct 2019, 10:52 am   #24
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Default Re: LNB Para-Lab 20’s

The one marked 0.15uF is a Mullard "mustard" an early plastic film capacitor now actually much sought after with an enviable reputation for reliability. The stripey one, not so sought after but nevertheless most likely to be just fine. It uses the same basic materials as the Mullard one at any rate.


Neither is a non polarised electrolytic type- they're the crossover caps with a reputation for failing that you've heard about.
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Old 2nd Oct 2019, 11:04 am   #25
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Default Re: LNB Para-Lab 20’s

I also have a pair - very nice speakers. Yes, super tweeters were STC (later branded Coles and still available I believe). Caps probably could benefit from a change, though having said that, I replaced the ones in one of my Radford T-lines (mid 60s) and couldn't hear the difference (might just be my great age!). The type you need are non-polarized electrolytics - 50/60V rating should be good enough. There are suppliers of 'specialist' crossover caps but I wouldn't want to comment on their superiority or otherwise.
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Old 2nd Oct 2019, 3:22 pm   #26
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Default Re: LNB Para-Lab 20’s

No no no.

The capacitors in the photo are NOT the usual non-polarised electrolytics often found in ordinary crossovers. They are much better types than that.

Non-polarised and also polarised electrolytic capacitors contain a fluid - the electrolyte. It is water based and over time water vapour escapes through the seals. They have a finite life expectancy as they slowly dry out.

The two capacitors in the photo are plastic (probably polyester) dielectric types. All solid. No liquids involved. Nothing to escape. They are also pretty much immune to moisture ingress that kills paper dielectric capacitors.

What you have is exactly the very best for the job. If you must replace them, use the same types. Don't fit electrolytics because they are much less reliable and they have more internal series resistance (which won't have been accommodated in the design of your crossover). It's like upgrading your jewellery from diamonds to glass.

In all likelihood, the capacitors you have should be fine, and will stay fine for a very long time.

David
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Old 2nd Oct 2019, 3:42 pm   #27
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Default Re: LNB Para-Lab 20’s

The crossover will be fairly high, ~ 3KHz. I guess this leads to smallish capasitor values which has allowed film capasitors to be used. Might be worth checking that they are the correct value in case someone has swapped them but, as pointed out already, these are types that were around when the speakers were originally build (sometimes know as 'mustard' and 'tropical fish').

dc
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Old 2nd Oct 2019, 4:00 pm   #28
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Default Re: LNB Para-Lab 20’s

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Originally Posted by Radio Wrangler View Post
No no no.

The capacitors in the photo are NOT the usual non-polarised electrolytics often found in ordinary crossovers. They are much better types than that.
That's worth knowing - I thought they had to be NP
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Old 2nd Oct 2019, 4:20 pm   #29
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Default Re: LNB Para-Lab 20’s

They do have to be non-polarised types in order to handle AC signals with no dc bias on them in the crossover

Non-electrolytic capacitors are inevitably non-polarised by their nature. and will handle AC without needing DC.

There are such things as non-polarised electrolytics, but they're a bit of a bodge-up containing two polarised electrolytics connected back-to-back in series. The idea is that self-rectification happens and that the two capacitors build up their own DC polarising charge.

Electrolytic capacitors need a polarising voltage to stop chemical reactions happening and destroying the aluminium oxide film (on one aluminium foil electrode) which is the dielectric of the capacitor. Electrolytic capcaitors are made with two aluminium foils and a permeable separator paper or film which is impregnated with conductive goo. As assembled, this isn't a capacitor. Next comes a forming operation - sticking voltage across it in the chosen direction and limiting the current. Two electro-chemical reactions happen at once. The natural oxide is stripped off of one electrode, and is built up on the other. The built up oxide is the dielectric of the finished capacitor. The aluminium it is built on is one capacitor plate. The conductive goo forms the other capacitor plate and by virtue of being liquid, does a good job of intimately coating all the craggy surface of the oxide. The naked aluminium plate is the connection to the goo.

Without good bias, the reactions which made the capacitor tend to run backwards and the capacitor is said to de-form or de-polarise.

If the aluminium foils are etched, they can have a far greater surface area at this microscopic scale than you'd expect, and the oxide is both thin and a good insulator, so you get a surprising amount of capacitance in a small package. The liquid electrode trick is needed to coat the 3-dimensional surface to make the most of all its area.

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Old 2nd Oct 2019, 5:32 pm   #30
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Default Re: LNB Para-Lab 20’s

Thank you David for taking the time to explain that so clearly. Capacitors have always been a bit of a mystery to me.
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Old 2nd Oct 2019, 5:40 pm   #31
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Default Re: LNB Para-Lab 20’s

Thanks for the info/advice guys. From what's been said seems little point in changing the caps if they are the best type already.

The only other changes I have made is to rewire with heavier cable, and fit heavy duty banana/screw terminal sockets on the rear. The originals always seemed a bit flimsy and small for modern loudspeaker cable. Next job is to make and fit new front panels as one of the originals (made from hardboard) is broken and the other looks well past it's prime.
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Old 2nd Oct 2019, 7:18 pm   #32
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Default Re: LNB Para-Lab 20’s

Glad that everyone is pleased with the 20s, thanks

Quote:
...de-lamination of the cone centers
You can buy the dome-shaped dust cover; careful use of a scalpel and some pva adhesive might sort that problem

Quote:
...originals (made from hardboard)
The originals would have been chipboard
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Old 2nd Oct 2019, 8:20 pm   #33
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Default Re: LNB Para-Lab 20’s

Yes I've seen cone repair kits for B200s advertised widely these days, but unfortunately this issue occurred many years ago when such things weren't widely available, so it was either source replacements or send them away for re-coning.

All I can say is that I purchased my LNBs new from, if memory serves, Wilmslow Audio back in the early 70s, and I don't remember ever replacing them until now. Not to mention that the cut outs are far too tidy for my limited carpentry skills.

Here's a photo of the back of the front panel which is covered in a black "plastic" speaker cloth.

I later purchased three pairs of LNBs for monitors in the theatre where I was working, purely because I was so pleased with my own pair. These were the later model with just the B200 and T27. from memory I think had nicer padded cloth front. A great design with an all important natural sound, which has stood the test of time.
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Old 2nd Oct 2019, 8:24 pm   #34
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Default Re: LNB Para-Lab 20’s

Well it would have helped if I'd actually posted the photo with my last post. Sorry old age is catching up with me fast
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Old 3rd Oct 2019, 7:37 pm   #35
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Default Re: LNB Para-Lab 20’s

Apologies
Checked with dad and you're correct; hardboard sI, chipboard sII
He doesn't remember supplying Wilmslow Audio (but he might have). He thinks that the nearest would have been suppliers in either Bolton or Manchester (Urmston?)
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Old 13th Oct 2019, 5:06 pm   #36
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Default Re: LNB Para-Lab 20’s

I could be wrong about Wilmslow Audio, it was a long time ago. But I'm pretty sure it was a dealer down South somewhere. Not that it really matters. My LNBs have had a long hard life, survived numerous moves and are still going strong. Hopefully with a couple of new cloth covered front panels shortly, if I can get a nice dry day to use the jig saw outdoors. Oh for a workshop.
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Old 13th Oct 2019, 11:15 pm   #37
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Default Re: LNB Para-Lab 20’s

I used self build Bailey transmission lines that I built myself (WW 1972) and used during the mid 70's to early 80's and enjoyed them greatly.

After a flirt with sealed box loudspeakers, which I never found satisfying, I went across to a series of dipoles (Quad 57's, Martin Logan, Magepan) and have ended up with Linkwitz LX521.

Like all of these designs, and including the LNB TL's, the perceived lack of bass is because the bass is not there in the first place in the recording. When true bass is indeed there, it comes out full bodied without overhang and boominess.

Craig
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Old 14th Oct 2019, 5:45 am   #38
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Default Re: LNB Para-Lab 20’s

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Like all of these designs, and including the LNB TL's, the perceived lack of bass is because the bass is not there in the first place in the recording. When true bass is indeed there, it comes out full bodied without overhang and boominess.
Craig
Indeed so.

The peak in the bass response of speakers employing that psychological trick, is made by a tuned resonance. The higher the peak, the higher the Q of the resonance must be, and so the narrower the bandwidth of the peak. The narrow bandwidth can be thought of as limiting the extent of the sidebands which describe the envelope shape rise and fall times. The peaked bass therefore takes extra time to arrive and takes extra time to decay. The other viewpoint is to look at the group delay associated with such a frequency-domain shape. Because of the low frequencies and narrow bandwidths, the times involved are significant.

Bumped bass responses are truly 'orrible. They make several things go wrong at once.

I never swallowed the 'The human ear isn't sensitive to phase' generalised simplification. We can hear differences between waveforms whose only differences are the phases of their harmonics... in some parts of the fundamental frequency range.

In the days when people thought that the usual B&K chart of frequency response was all there was to know about a speaker, some quite important factors were missed.

David
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