Omar 252 Fundamental Doc Mod Rebuild
Posted: Tue Mar 21, 2017 1:05 pm
I recently acquired a pair of Omar 252 monitors for very little money with a view to an experimental Doc Mod rebuild. They actually sounded pretty decent, but something was not right in the upper midrange / lower treble. Using a tone sweep it turned out to be some buzzing and distortion from one of the tweeters at around 2450hz. So yesterday evening I removed the tweeter and for good measure removed the woofer with a view to removing the fibreglass stuffing, whereupon the rubber surround decided to split and disintegrate.
As it happens I had a conversation with a chap at Scalford who had replaced the woofers in his Kefs with a pair of inexpensive 8 inch kevlar jobs. Overall, they sounded pretty good. I checked up on the drivers on my return and they look remarkably similar to the Doc's 8 inch drivers. So this week should see the arrival of 2 x 8 inch kevlar drivers, a goodly quantity of neoprene based contact adhesive, some plastidip, steel, fibreglass resin and sand. I'll either 'borrow' the tweeters from my JPW Mini Golds, or the SEAS from my modded ScanDynas. I have 3.3uF caps plus a range of resistors to try.
While the resin / sand mix is not on the list of mods, I thought that in the spirit of DIY and experimentation etc. it was worth trying a resin/sand mix to add still more rigidity to the enclosure plus a bit of mass along with the usual bits of steel. Needless to say it may go horribly wrong, but if not attempted then little would be learnt. Of course I'd get away with using less mass adding material if I used something denser (steel has a density of 7.82 compared with 1.0 for water and 0.6ish for wood) such as Platinum (21.5) or Osmium (22.5) but I suspect other materials offer better value.
Incidentally, I have considered lead flashing as an alternative to steel. Lead has a density about 40% higher than steel and is easily workable. Code 5 lead flashing is 2.24mm thick which in mass terms equates to a steel thickness of 3.25mm. 3 meters of 240mm Code 5 lead flashing would be around £57 plus delivery and would be sufficient for the interior of a pair of speakers around the size of the Cube 1s. Food for thought perhaps if I decide to attempt a pair of DIY cubes and in fact probably no more expensive than a steel/sand/resin mix and a lot more convenient.
As it happens I had a conversation with a chap at Scalford who had replaced the woofers in his Kefs with a pair of inexpensive 8 inch kevlar jobs. Overall, they sounded pretty good. I checked up on the drivers on my return and they look remarkably similar to the Doc's 8 inch drivers. So this week should see the arrival of 2 x 8 inch kevlar drivers, a goodly quantity of neoprene based contact adhesive, some plastidip, steel, fibreglass resin and sand. I'll either 'borrow' the tweeters from my JPW Mini Golds, or the SEAS from my modded ScanDynas. I have 3.3uF caps plus a range of resistors to try.
While the resin / sand mix is not on the list of mods, I thought that in the spirit of DIY and experimentation etc. it was worth trying a resin/sand mix to add still more rigidity to the enclosure plus a bit of mass along with the usual bits of steel. Needless to say it may go horribly wrong, but if not attempted then little would be learnt. Of course I'd get away with using less mass adding material if I used something denser (steel has a density of 7.82 compared with 1.0 for water and 0.6ish for wood) such as Platinum (21.5) or Osmium (22.5) but I suspect other materials offer better value.
Incidentally, I have considered lead flashing as an alternative to steel. Lead has a density about 40% higher than steel and is easily workable. Code 5 lead flashing is 2.24mm thick which in mass terms equates to a steel thickness of 3.25mm. 3 meters of 240mm Code 5 lead flashing would be around £57 plus delivery and would be sufficient for the interior of a pair of speakers around the size of the Cube 1s. Food for thought perhaps if I decide to attempt a pair of DIY cubes and in fact probably no more expensive than a steel/sand/resin mix and a lot more convenient.