Cressy's Banned Omniset

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Dr Bunsen Honeydew
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Re: Doc Mod Drivers

Unread post by Dr Bunsen Honeydew » Sat Jul 22, 2017 5:15 pm

Are you using the 12 inch in the new ones or a 10 inch.

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SteveTheShadow
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Re: Doc Mod Drivers

Unread post by SteveTheShadow » Sat Jul 22, 2017 5:15 pm

Doc..can you move this post and the one before into the Cressy's banned Omniset thread
Gawd knows how I've managed to put them in the wrong thread. Ill reply to your post once this is sorted out. :oops:
DIY NVA amplification ( roughly equivalent to NVA A60) BTE Designs passive preamp, BTE Designs Lenco L75 turntable, Rega R200 arm, Goldring E3 cartridge, NVA Phono 1
Musical Fidelity M1 DAC, AppleTV4 with Apple Music subscription, ripped CDs streamed via WD MyCloud Home Duo NAS, with PLEX media server.
Speakers - Sealed cabs with Fane 12-250-TC 12" full range drivers.

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Dr Bunsen Honeydew
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Re: Doc Mod Drivers

Unread post by Dr Bunsen Honeydew » Sat Jul 22, 2017 5:26 pm

Can you PM Terry Booth he is tech admin for those things.

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SteveTheShadow
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Re: Cressy's Banned Omniset

Unread post by SteveTheShadow » Wed Jul 26, 2017 4:09 pm

More developments today with the flatties:
Image

Notice the slight forward tilt of the top surface, with the main driver.
DIY NVA amplification ( roughly equivalent to NVA A60) BTE Designs passive preamp, BTE Designs Lenco L75 turntable, Rega R200 arm, Goldring E3 cartridge, NVA Phono 1
Musical Fidelity M1 DAC, AppleTV4 with Apple Music subscription, ripped CDs streamed via WD MyCloud Home Duo NAS, with PLEX media server.
Speakers - Sealed cabs with Fane 12-250-TC 12" full range drivers.

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SteveTheShadow
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Re: Cressy's Banned Omniset

Unread post by SteveTheShadow » Wed Jul 26, 2017 4:09 pm

Here is a cross-section of the cabinet to scale, showing the tilt modification:
Image
Rear legs have simply been made longer than the front, to tilt the top about 5 degrees off the horizontal.
DIY NVA amplification ( roughly equivalent to NVA A60) BTE Designs passive preamp, BTE Designs Lenco L75 turntable, Rega R200 arm, Goldring E3 cartridge, NVA Phono 1
Musical Fidelity M1 DAC, AppleTV4 with Apple Music subscription, ripped CDs streamed via WD MyCloud Home Duo NAS, with PLEX media server.
Speakers - Sealed cabs with Fane 12-250-TC 12" full range drivers.

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SteveTheShadow
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Re: Cressy's Banned Omniset

Unread post by SteveTheShadow » Wed Jul 26, 2017 4:10 pm

The directivity has been slightly altered, bringing the direct to reflected sound ratio, slightly more in favour of the direct, and in addition, it makes better use of the ceiling reflection, lending a sense of air and height to the soundfield it generated within the room.
With the improvements made to the low bass, by tuning the port lower and the increased image focus, weight and solidity, brought about by the tilt mod,
I think we have a speaker that can easily hold its own in the company of any comparably sized transducer.

This design has now taken on board Quarknosis' comments from his week-long loan period and is a super little speaker because of it.
DIY NVA amplification ( roughly equivalent to NVA A60) BTE Designs passive preamp, BTE Designs Lenco L75 turntable, Rega R200 arm, Goldring E3 cartridge, NVA Phono 1
Musical Fidelity M1 DAC, AppleTV4 with Apple Music subscription, ripped CDs streamed via WD MyCloud Home Duo NAS, with PLEX media server.
Speakers - Sealed cabs with Fane 12-250-TC 12" full range drivers.

vrazjigwb
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Re: Cressy's Banned Omniset

Unread post by vrazjigwb » Thu Jul 27, 2017 9:43 am

karatestu wrote:
Sun Jul 09, 2017 8:06 am
I might give that white tack tweak a go on a pair of speakers i own. Kevlar 6 & 1/2 inch mid/bass. Do they do yellow tack ?

Stu
I'm guessing UHU PATAFIX is pretty similar to blue tack and you can get it in yellow.
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karatestu
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Re: Cressy's Banned Omniset

Unread post by karatestu » Sat Jul 29, 2017 8:06 pm

Ooooo, cheers. I will look into that.

Stu
Naim, Linn & Harbeth fanboy

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SteveTheShadow
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Re: Cressy's Banned Omniset

Unread post by SteveTheShadow » Thu Sep 14, 2017 9:14 pm

Anyway, I have been concerned about the looks of these things since day one to be quite honest.
So today I simply turned them sideways and mirrored them.

Image

The transformation in the looks is most pleasing. Now we're getting somewhere.
I simply fitted my other pair of SB ring tweeters and disconnected the existing.
It's not ideal as the speaker connections are now on the sides too, but the looks are worth the temporary loss of a tweeter until I figure out a decent looking way to blank off the hole.
The speaker terminals are another matter. The answer to that is a cabinet rebuild but that can wait a while.

The bass/mid unit is now as close to the wall as it can possibly get and despite it only being around 1.5" closer, the increased wall coupling is very evident in the resultant sound, which is now very solidly grounded. The lower bass has come up a tad due to the closer proximity of the line exit port to the back wall.
It is all tight, focussed and solid. Not a night and day difference, but I can't imagine going back to the other arrangement.
DIY NVA amplification ( roughly equivalent to NVA A60) BTE Designs passive preamp, BTE Designs Lenco L75 turntable, Rega R200 arm, Goldring E3 cartridge, NVA Phono 1
Musical Fidelity M1 DAC, AppleTV4 with Apple Music subscription, ripped CDs streamed via WD MyCloud Home Duo NAS, with PLEX media server.
Speakers - Sealed cabs with Fane 12-250-TC 12" full range drivers.

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SteveTheShadow
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Re: Cressy's Banned Omniset

Unread post by SteveTheShadow » Mon Sep 25, 2017 8:48 pm

Long post but may be useful to others wanting to design their own speakers.

OK,

So off I toddled to the MCRU speaker bakeoff with the semi-omnis and as soon as I saw the room, superb that it is, I thought I'd better get my coat. My farty little semi-omnis were dwarfed by everything else in the room. And I left them in the little vestibule, just outside the room so I could sneak them back into the car before anyone noticed.
However after the Doc and Savvy had demmed a pair of cubes I felt a bit better.

The OmniMets performed better than I expected, given the size of the room and produced a sound devoid of boom and with plenty of rhythm, tunes and drive. The bass end worked very well and the near wall placement incorporated into the design of the things was spot on. Those were the positives.

HOWEVER all was far from well at the upper mid and lower treble boundary. Out of the bass and lower mids area they sounded as flat as a fart, swathes of detail and harmonics missing in action, boring as hell and at least 70% of the people in the room just ignored them and started having conversations.

That absolutely had not happened with any of the other speakers in the room and frankly I was fucking pissed off, big style, quite angry in fact and it was written all over my face how I was feeling. Poker faced I am not unfortunately, which is not good in these sorts of "public" circumstances because it inevitably comes over as "sore loser" syndrome.

After leaving the room, taking a walk to Tescos and calming down I pondered what had happened and put on my objective head. Some of the audience had lost interest for a good reason, they didn't do it on purpose just to piss me me off, and a couple of the guys there struck up a conversation about harmonic details of guitar strings, cymbals being shut off because there was a hole in the treble response, air and space just not happening, dry as dust.

PRaT was excellent, bass even and punchy, not a lot else but they played alright. Jesus! I began to realise that I had actually gone and designed a bloody Flat Earth speaker and after all I had said about never going there again, there I was once more, and to make matters worse, I had done it without any help from Linn/Naim :angry-screaming:

This is not something I had noticed at home and neither had anyone else who had heard them in my own room, in fact they had been used in several rooms and the problem had never reared its head, but it bloody well had now! Indeed they had measured relatively flat through the third octave spectrum analyser, the measurements appearing to confirm their goodness.

BUT this had by now happened twice; once at Owston and now in MCRU's dem room, so something was going pear shaped when they were taken out of their domestic context, which I'm afraid is just not good enough in my book. Just goes to show that measurements need to be treated with a healthy degree of skepticism and caution.

OK so between us, myself Colin and Chris, after a long conversation out of the dem room had agreed that it had to be the crossover to the tweeter that was causing the problem with the missing bits of the music and I resolved to do something about it as soon as I got home.

Looking at my calculations for the crossover capacitor value, it all looked right and double checking it all, it was still correct, so no clue as to what was going on. I examined the 45 degree, off-axis response curve of the bass mid driver, with its white tack induced roll off and the curve that the first-order tweeter crossover capacitor gave with my calculation, and in theory, the curves married up to give a flat response. In other words, on paper, everything lined up perfectly. There was much head scratching and beard ruffling (well there would have been, if I'd had a beard) I mean, what the hell was going on?

This was getting a bit silly now. I've been building speakers for nigh on forty years. OK, yes, granted; this is a semi-omni, but come on, I do know what I'm doing. I'm missing something obvious, but I'm damned if I can work out what it is, these things should work and they do.....yet they don't if you get my drift.

As a last resort I went right back to the SB Acoustics catalogue and got out the spec for the tweeter:
SB.Acoustics SB29RDNC-C000-4 Tweeter

Efficiency, 94dB
Impedance - 8 ohms?.....err...no...4 Ohms.....OH FOR FUCK'S SAKE :doh:

The carefully chosen crossover cap was half the value it should have been, meaning the crossover frequency was twice as high as it should have been, hence the hole in the treble. Oh poo!

So the cap value was doubled with another one the same value in parallel with it and it all fell into place soundwise, musicwise and everything elsewise. The other three omnis I have built all had the same error in the crossover, due to me somehow misreading the HF driver data.

Here endeth the first lesson. Check, check and check again. Make sure you have the bloody right data before you design your crossover!
DIY NVA amplification ( roughly equivalent to NVA A60) BTE Designs passive preamp, BTE Designs Lenco L75 turntable, Rega R200 arm, Goldring E3 cartridge, NVA Phono 1
Musical Fidelity M1 DAC, AppleTV4 with Apple Music subscription, ripped CDs streamed via WD MyCloud Home Duo NAS, with PLEX media server.
Speakers - Sealed cabs with Fane 12-250-TC 12" full range drivers.

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