Omar 252 Fundamental Doc Mod Rebuild

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Simon Hickie
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Omar 252 Fundamental Doc Mod Rebuild

Unread post by Simon Hickie » 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.
“The reasonable man adapts himself to the world: the unreasonable one persists in trying to adapt the world to himself. Therefore all progress depends on the unreasonable man.” George Bernard Shaw.

Daniel Quinn
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Re: Omar 252 Fundamental Doc Mod Rebuild

Unread post by Daniel Quinn » Tue Mar 21, 2017 1:52 pm

Mass is not the only property of steel . It may be [ I do not know] analogous to a penis with more mass that is soft, not fit for purpose. Less mass and rigidity may be better. ;)
Even a stroke didn't result in me liking AOS

Simon Hickie
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Re: Omar 252 Fundamental Doc Mod Rebuild

Unread post by Simon Hickie » Tue Mar 21, 2017 2:02 pm

Nice analogy! Anyway, I'll try to post some snaps of the project as it develops. Sand/resin ought to be rigid if less massy.
“The reasonable man adapts himself to the world: the unreasonable one persists in trying to adapt the world to himself. Therefore all progress depends on the unreasonable man.” George Bernard Shaw.

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Dr Bunsen Honeydew
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Re: Omar 252 Fundamental Doc Mod Rebuild

Unread post by Dr Bunsen Honeydew » Tue Mar 21, 2017 3:29 pm

It doesn't do the same job, not suitable in Doc Mod projects. It will negate what the steel does.

Simon Hickie
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Re: Omar 252 Fundamental Doc Mod Rebuild

Unread post by Simon Hickie » Tue Mar 21, 2017 3:53 pm

OK Doc. I'll stick to the steel.
“The reasonable man adapts himself to the world: the unreasonable one persists in trying to adapt the world to himself. Therefore all progress depends on the unreasonable man.” George Bernard Shaw.

Simon Hickie
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Re: Omar 252 Fundamental Doc Mod Rebuild

Unread post by Simon Hickie » Wed Mar 22, 2017 5:23 pm

Steel quote received from a local supplier. If this project is a success, I plan to recycle the steel & drivers into a Cube 1 sized build (the steel is sized appropriately to accomplish this). When stuck down with Bison Kit, can the steel be removed easily for reuse (I've not used contact adhesive for ages)?
“The reasonable man adapts himself to the world: the unreasonable one persists in trying to adapt the world to himself. Therefore all progress depends on the unreasonable man.” George Bernard Shaw.

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Dr Bunsen Honeydew
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Re: Omar 252 Fundamental Doc Mod Rebuild

Unread post by Dr Bunsen Honeydew » Wed Mar 22, 2017 5:48 pm

Not easily but can be removed as long as you can lever under it. It get harder to remove the longer it is in place, but can always be got out.

Simon Hickie
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Re: Omar 252 Fundamental Doc Mod Rebuild

Unread post by Simon Hickie » Wed Mar 22, 2017 5:57 pm

Cheers Doc.
“The reasonable man adapts himself to the world: the unreasonable one persists in trying to adapt the world to himself. Therefore all progress depends on the unreasonable man.” George Bernard Shaw.

Simon Hickie
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Re: Omar 252 Fundamental Doc Mod Rebuild

Unread post by Simon Hickie » Thu Mar 23, 2017 12:57 pm

The doping material and 8 inch drivers have arrived and I have a good quotation for steel from a local supplier that works out cheaper than putting together bits of offcuts bought off fleapay. The cabs are stripped out including a bit of 38mm plastic piping used as the reflex tube. Each cabinet just had one large piece of fibreglass stuffed around the back of the bass driver.

I'm still mulling over the tweeter protection and padding and weighing up the merits of a single resistor versus conventional two resistor Lpad due to effect of the single resistor solution on driver impedance and, therefore, effective high pass frequency. I feel a trip to Maplins for a selection of resistors at 34p a pop coming on. I'll be using the SEAS 27TDFC tweeter with an efficiency of about 90db and 6 ohm nominal impedance - typically 5 ohm around the intended roll-off frequency. A 3.3uF cap will give a 6db slope at a roll-off frequency of around 5600hz which may or may not be too high, depending on how the 8 inch driver behaves after doping.

Let the mods begin!
“The reasonable man adapts himself to the world: the unreasonable one persists in trying to adapt the world to himself. Therefore all progress depends on the unreasonable man.” George Bernard Shaw.

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Dr Bunsen Honeydew
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Re: Omar 252 Fundamental Doc Mod Rebuild

Unread post by Dr Bunsen Honeydew » Thu Mar 23, 2017 1:05 pm

Doc Modding is a subjective empirical process. I have done that and a single resistor is better no matter what the *experts* and equations say. Now you can either just do it, or you can repeat the work I did. The point of Doc Mods is you don't have to.

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