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March 2013 Issue
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REFERENCE UPGRADE

Your magazine often features older equipment and also has some elements of DIY so I would like to ask a question and see if  I can get a little help. 

After a recent listening session organised by a friend I was able to hear a pair of KEF 103 Reference (original 1976/77 model). I really liked the sound of these and had not owned a proper sealed baffle speaker for a very long time. So like many here I did what we all do and scoured the second hand sales and finally eBay. By luck I found a pair and got them for the very good price (in my opinion) of £84. I have now collected them and put them in my front room for use with my TV and they sound as good there as I remembered they did originally. They also have the advantage of being able to work close to the wall making them more domestically acceptable .

However a little online research and some very welcome help from KEF has shown that the crossover is all electrolytic caps which after about 35 years it is very likely they will have dried out and possibly even changed there values. I am looking to replace with polypropylene film items and will also take the opportunity to replace the internal speaker wire with something a little better . 

 

KEF Reference 103 loudspeaker. "I really liked the sound of these"

says Andrew McBride.

 

 

Now it is almost certain that the new caps will be more electrically efficient than the originals and the replacements will then change the Q of the speaker. I am told it is possible to add a resistor in series to the HF unit which should adjust the Q back to were it was and about 150 R is the normal value to start with. As I do not have anyway of measuring is there any advice you could give on this ? I do not want to change the sound of these units too much as the original sound was what attracted me too them in the first place but if the changes can perhaps add a little transparency and bring them back closer to the original spec then it would be useful. 

When I removed the driver front mounting plate to view the Crossover it had been sealed with some form of self adhesive foam strip but this had long since dried out and perished can you advise on anywhere that could supply a replacement or alternative strip seal?

I have included a copy of the original KEF circuit diagram so that you have the details hope this will help .

Regards

Andrew McBride

 

 

 

KEF Reference 3 crossover circuit diagram, drawn 1975.

 

Restoring the KEFs raises some interesting issues. The drawing you sent, dated March 1975 and drawn by hand no less, shows large(ish) value capacitors that are physically quite small and these are likely Alcaps, bi-polar electrolytics recognisable by their black case with red end caps. They do not have a wet electrolyte, instead using a metallised polymer film, so should not dry out. However, other ageing processes may well occur and, looking at Alcap product info for ESR, or Effective Series Resistance, I see it carefully isn’t stated, an equation having DF as a variable whilst the DF equation has ESR as a variable, meaning you cannot solve either! This makes me suspect the capacitors may well have quite a high ESR even when new and I will presume 1 Ohm. Inserting new film capacitors will raise tweeter output by 3dB or so. But the original ESR is likely to have been compensated for in the design, so you will have to re-insert it by adding 2 Ohms in series with the T52 tweeter. This is a very approximate figure, because I have made some gross assumptions, but it gives some idea of what the issues are, and their magnitude. 

   The way I would tackle this is to get six 1 Ohm, carbon film resistors of 0.5 or 1 Watt, and connect them in series with the tweeter until I got the result I wanted. Start with one resistor of 1 Ohm. If it makes the sound too dull, then connect the other resistors progressively in parallel. If it is insufficient, then connect the other resistors progressively in series. 

You could also consider removing the crossover altogether, placing it outside the box. I would leave the original in place and re-build a new external crossover, with the inductors further apart (but still at right angles) and on separate bass and treble boards. The assembly can go into a solid plastic or aluminium case that sits on the floor. Do not solder leads onto the drive units as you may well unsolder connecting wires to the tags. I think you can lead cables out through the ports. 

   Leaving the old crossovers in place allows the speakers to be returned to original condition if desired, but if you think this unlikely you could use the original inductors. If you get new inductors make sure they have ferrite cores like the originals, so their d.c. resistance stays roughly the same. Do not use air cores, for this reason. 

   There is quite a lot of fun you can have here, experimenting with external crossovers that can be hacked about. New crossovers with better components will make a big improvement and you can tweak values until you get the sound balance you want. NK

 

ACTIVE CROSSOVERS

I very much enjoyed reading Peter Comeau’s musings a while back concerning various loudspeaker design concepts, and I thought I would share some of my experiences on the path I have chosen to follow – which has been fun and rewarding in equal measure. Having tried various types of loudspeakers over the years, from run-of-the-mill bass reflex designs, to full range horn loaded (e.g. Lowther Acousta), hybrid ribbon designs (Heil/AMT, Decca, Heybrook Sextets) and latterly Klipsch horns, I decided that the latter held the most potential in terms of bringing performers to life. Reading about bespoke horn projects from the likes of Definitive Audio also reinforced my feeling that horns must represent a sort of ‘ultimate’ reproducer. Given my small room constraints I quickly understood that the key to achieving decent results lay in getting the bass right, and in this respect the little Klipsch Heresy proved a most inspiring starting point. Its sealed enclosure produced tight bass and worked well against the rear wall, so from there it was a case of experimenting... 2 ways, 3 ways, different types of horns (constant directivity, Le Cleach’ ...) made out of solid wood, plaster, reconstituted stone... the options seem endless and trying the horns and driver combinations has been (and continues to be) a lot of fun. Best of all perfectly decent component parts can be had for reasonable money, especially in view of the scary prices which seem to have become the norm, and not just in ‘hi-end’ audio.

    Onto crossovers. Of course, lots of tools are now available online to help one work out the correct values, but if, like me, you are not particularly technically minded, the whole thing can be quite daunting. Nevertheless there is a solution, and with it another major breakthrough as far as I am concerned: go active! 

    Now, why more isn’t written about active crossovers in hi-fi magazines is a little perplexing. Your good selves once published a rave review of the BSS FDS-388, a landmark digital product in its day but not cheap and a little complex to the non-technically minded, but if I am not mistaken I believe that was the only time. There are, of course, myriad designs available from the golden age of analogue, products from the likes of Pioneer, Sony, Accuphase, Sansui, Audio Research, and even Kondo and the more exotic artisans from Japan. It seems to me that for the best part of 30 years, active crossovers were at the heart of state-of-the-art systems, so it seems a bit of a mystery as to why they seem to have disappeared off the map. Surely it can’t be just the cost of additional amps when some cable manufacturers have the cheek to ask five figure sums for single runs of their top loudspeaker cables? 

     From experience going active renders such tweaking less critical, and that route offers a number of other ways to adjust a system’s sound to one’s personal taste. And that’s without talking about the ‘life-size’ imaging, effortless dynamics, detail... I could go on. Of course there are some downsides. Aside from the aforementioned complexity (more amps required, cables, etc) – imaging is tricky to optimize, phasing issues might trigger bouts of paranoia, but at the end of the day it’s a case of trusting your ears and good old ‘trial and error’. A little bit of patience is all that’s needed really.  Anyway the whole ‘going active’ experience has really re-kindled my enthusiasm for audio, and I can’t recommend it too highly. I would be interested in hearing your thoughts on the subject, and perhaps suggest that it could form the subject of an interesting series of DIY articles?  

Keep up the good work,

Gabriel Topalian

 

 

A Rane active crossover: plenty of controls for altering crossover frequencies,

rate and gain. This is a great way to tailor your own loudspeaker says Gabriel Topalian. 

 

Hi Gabriel - and thanks for your views on the now-rare subject of active crossovers It’s great that you are experimenting and getting good results, with the sense of satisfaction that brings. An active crossover is for the experimenter though and they are, it seems, a dying breed. 

Also, the topology of an active loudspeaker fed by an active crossover, rather than a passive crossover, is a little daunting. And the bass unit needs to be tailored to the cabinet too, since active crossover cannot compensate for gross mismatches and the time domain problems mismatching may bring (overhang). So although active crossovers are nice to play with, they cannot cope with all issues. 

However, if you choose high quality drive units and follow their manufacturers recommendations about cabinet size etc then this is a great way to build a loudspeaker. Unfortunately, so few people nowadays would go down this path that the market for active crossovers will be miniscule. And this is why no one bothers to build or sell them any more I suspect. NK



 

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