Omar 252 Fundamental Doc Mod Rebuild

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Simon Hickie
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Re: Omar 252 Fundamental Doc Mod Rebuild

Unread post by Simon Hickie » Thu Mar 23, 2017 6:13 pm

I certainly favour simplicity and will see how it sounds with the single resistor solution first.
“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 » Tue Mar 28, 2017 10:07 am

Picked up the steel yesterday and spent a slightly frustrating evening gluing it into cabinet one. Using a glue-gun had mixed results: the glue bonded well to the cabinet but was patchy on the steel despite degreasing it first. Some steel stuck fine, but other bits just fell off. Therefore, it's back to the good old contact adhesive which has worked well.

Each cabinet will have 7.7kg of steel added, which together with the rest of the cab and drivers will mean each speaker will weigh in at around 15kg.

The drivers are doped too & just waiting for the Plastidip. I also now have a variety of resistors to try with the 3,3uF capacitor and will attempt to tune by ear. However, I might also try a quick measure of the response of the bass driver alone using a mic and REW to get a ball-park figure of where the driver rolls off.
“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 » Tue Mar 28, 2017 11:27 pm

One speaker now completed. I initially tried a 3.3uF cap and 8.2 ohm resistor, but the REW response showed a scooped midrange and lower treble which I put down either to the tweeter roll-off being too high and/or suppressed response from the doped driver in the 3-6k region. Either way, I dropped the roll-off to around 2.2khz (quite feasible with the tweeter I'm using as it has an Fs of 550hz) by using a single 8.2uF cap and dumped the resistor. Apart from some lumps and bumps which may well be down to room interactions, it's broadly flat from 40hz to when the mic seems to run out of sensitivity at about 13khz (the tweeter is theoretically good for approaching 30khz).

The second speaker should be ready by Friday, although a house viewing on Saturday means I might have less time than I'd like to finish the job.
“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 28, 2017 11:39 pm

Feck what the test equipment says, if you do it like that it is NOT Doc Mods, use your bloody ears. Why do you think the test gear will tell you more than your ears, do you want to listen to music with them or test tones.
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Simon Hickie
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Re: Omar 252 Fundamental Doc Mod Rebuild

Unread post by Simon Hickie » Wed Mar 29, 2017 12:08 am

Early days Doc, early days! They are a bit hard to evaluate with just one speaker done, hence straying to the dark side for a short while (something I do only rarely as I do indeed trust my ears). However, what I heard broadly echoes what I saw. I'll be delighted if when the second one is completed that it sounds good to my ears with just the cap on the tweeter.
“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 29, 2017 1:20 am

Well don't bother to tell us, you are insulting the process and you will just make me react. I (we) couldn't give a stuff what your test gear says as it just tell lies like it does to everyone else.
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Simon Hickie
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Re: Omar 252 Fundamental Doc Mod Rebuild

Unread post by Simon Hickie » Thu Mar 30, 2017 11:02 pm

OK Doc, you were right. What I "measured" turned out to be utter :Bllocks: 3.3uF caps in place as recommended. 8.2 ohm tweeter padding too much on my SEAS TFFC, 2.2 ohm possibly a touch hot, 3.9 ohm pretty close, but 3.3 ohm may just hit the spot. The bottom line is that there's no substitute for using ones ears - as you've been at pains to point out.
“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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karatestu
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Re: Omar 252 Fundamental Doc Mod Rebuild

Unread post by karatestu » Wed Sep 13, 2017 8:38 pm

How are you getting on with these speakers Simon ?

There is a shortage of fellow doc modders around at the moment to talk to regarding all things doc mod :guiness;

Stu
Naim, Linn & Harbeth fanboy

Simon Hickie
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Re: Omar 252 Fundamental Doc Mod Rebuild

Unread post by Simon Hickie » Wed Sep 13, 2017 11:36 pm

Hi. The Omars went pretty well, although I'm not convinced my tweeter mated well with the doc modded bass driver (very similar to the one used in the Cube 1s). The bottom end was pretty tight though and lived up to my expectations. Therefore, I turned my attention to my old Scandynas. These have a 10 inch bass driver that ran full range in the original speakers. I built some larger cabs and modelled them on the larger Dynaco A35 with an aperiodic vent between two chambers.

The problem with these is that as the bass driver doesn't need doping thanks to a very smooth natural roll-off, the backwave then seems to penetrate the speaker cone more readily than with a doped driver. The 3mm steel (in both chambers) may also be insufficient. So I've had to resort to putting some fibreglass back in.

So the conclusion I've drawn is that with a Doc Mod project you need to consider doing the full range of mods including driver doping, but in the Doc's recommended order.

My next project after the house move which looks on for late next month is to build a pair of semi-omnis using the 8 inch doc modded drivers and 5mm steel lining. My drivers sim at 27 litres for a Q of 0.71. Dropping the volume to 17 litres raises the Q to about 0.8 which still quite acceptable for sealed acoustic suspension job. I've a pair of filled Atacama SE20 stands doing nothing which should be about the right height for the design - tweeter at about chest height.

One more thing to try for me is some more speaker cables. My DIY solid core jobs are using cores that are too thick (3 x 1.5mm2 per connector). I went for low inductance at the expense of higher capacitance too (OK with my amp). So I'm going to try 8 x 0.20mm2 runs per connector in a low capacitance arrangement. Off topic I know, but my digging around on speaker cable suggests that an individual wire with a greater CSA of 0.5mm2 is going to give some undesirable effects. That's one of the reason the Doc's speaker cables work so well: there's a complete avoidance of skin effect distortion due to having an appropriate current density throughout each strand.
“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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