Resurrected

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I have decided to republish my IB blog. A humour failure is no excuse not to give others a chance to achieve a serious upgrade in their sound quality. The rambling format is not conducive to finding a quick answer to everything. Or anything at all, for that matter. Perhaps this is the point? A hobby is supposed to kill time productively. Rather than the valuable time of others destructively. I put in the time so you can cut out the middle man. You can safely ignore everything you read here and go straight to the source: (Mentioned below)

I have spent far too much money over the years on Hi-fi. Endlessly searching for mere nuances. When if the truth were known most boxes of electronics sound exactly the same.

The IB subwoofer was easily the most serious and clearly audible upgrade in sound quality in nearly five decades of making an unnecessary mess at home.

Rule 1. Use all the same drivers in your IB and get them from a reliable source. Most of my original IB problems were due to being sold crap drivers and my total inexperience with the IB concept. There were no IB Cult FAQs back then.

My first four AE 15" drivers were so far off spec (and cosmetic acceptability) that I am still amazed that the manufacturer had the nerve to remain in business.

All four, straight taper, vinyl coned drivers actually have a measured Fs:32.5Hz instead of the claimed 16Hz. God only knows what the other specs look like in reality! The maker repeatedly denied there was anything wrong with these drivers on the AV forums.

Early model AEIB15 backplate. I was so ashamed of these that I have never published any pictures of them until now. I still have no idea why I have protected a company which would let these out of the factory door! If I'd made these myself I'd have thrown a tarpaulin over the scrap bin in case anybody ever saw them! 

His offer to replace them, free of charge, with the new model, was both very costly and frustrating. And, they still did not match the claimed specs! Though at least they erred on the right side of useful the second time around. Fs:13Hz instead of the still claimed 16Hz. The cosmetics were considerably improved too. It could hardly have been otherwise!

I probably learned far more from my struggle to tame the originals than I might have done had they actually been remotely to the published specs. That does not make me remotely grateful! Why should I be grateful for paying for quality and receiving crap quality drivers?

They (the older 32Hz plastic-coned drivers) only remain in use now because they complement the compromises of the newer drivers and add some free cone acreage. The newer AEIB15 drivers roll off very early and steeply above their peak output with a very narrow, useful power band. Which would  normally require a very low crossover point and huge main speakers. While the older drivers roll off very early in the deep bass unless massively boosted low down. I'm talking +16dB @20Hz, two octaves BW of boost from a Behringer DSP1124 Feedback Destroyer! Yet, together, somehow, these very different sets of drivers eventually learned to play together. It took me quite literally ages (years) to get the balance right.

I see the AEIB15 is now being listed again, at twice the original price, after a long hiatus. I wonder what the "real world" TS parameters are this time around? Given that you can buy an 18" from Fi for less money the new AE drivers are not quite the bargain they once were. One 18" Fi IB318 driver moves twice the air of the AEIB15. Making the Fi driver an even better bargain. You can get away with two Fi 18" instead of four AE 15" to make an IB. For the same money you can move huge volumes of air with 4 x IB318s.. You do the maths.

Rule 2: Never (ever) underestimate reaction forces from a row of large drivers in a flimsy structure! Ignore the knuckle-dragging ignoramuses who say a cone has no mass to speak of. They forget the huge mass of air involved and its resistance to violent acceleration and deceleration. Just build an opposed driver manifold first time round and do yourself an enormous personal favour.

Rule 2 (Get-out clause b) If you have a solid concrete or brick screen wall and you can cut holes in that for your drivers then you can safely ignore Rule 2 as well.

Rule 3,4,5-100. Don't think that you can get away without adding LFE for your film enjoyment. Adding a subwoofer to enjoy music is fine. When watching films it is utterly hopeless to just split the Main speaker channels with an active crossover to obtain a bass signal for your IB. I spent years wondering why my IB didn't thunder until I finally added LFE.

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If you feel you have any pressing questions on IB subwoofers then consult the Cult of the Infinitely Baffled forum. You could do worse than read the copious FAQs first. Or at least pretend you have before posting:

"[It is] better to be thought a fool than to speak out and remove all doubt."
(A corruption of some fictional, biblical nonsense, usually attributed to plagiarism by Lincoln or Twain)

I think you will find the contents of this blog are ample proof of the above sentiment.

I have my own saying:

"Better a failed clown than a successful bore."  

I seem to have spent a lifetime trying to perfect the latter. Imitate me at your own peril!

Keep smiling. Somebody might need one more than you can possibly imagine.

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Trial and user error!

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Two hours wasted on REW later and I was getting nowhere. I must have run 100 sweeps and still had no improvement. Thankfully, the limited number of sweeps before one has to delete earlier sweeps has gone. Thank you for that priceless improvement to your free software.

At one point I had a perfectly straight and level response from below 30 to over 110Hz. Then I made one small adjustment on the BFD and never got that flat curve back. So I ended the session without having achieved anything useful.

The following morning and I decided to reset the BFD to factory settings for the first time. I was getting odd effects which seemed intractable to any change. The reset produced a number of flashing red lights and a complete cancellation of all preset filters.

I soon found that I had been changing one IB set while imagining I was filtering the other. Quite inexplicable but true. I resolved to write down the channels affected by each box in the equipment chain. I had reversed the speaker cables from the EP2500 power amp since changing the position of the driver sets.

Behringer mixer, CX2310 and BFD (from top to bottom) 
It is usually too dark to capture the rack components without flash. 
So most of the usual lights are invisible in my pictures.

Behringer do not make it any easier by calling a channel one or right at random on their equipment. Of course the way the boxes are wired together will change which channel is right or one or two or left. It turned out the BFD was now reversed compared to all the others. One set of four 15" drivers is on each channel. The BFD and mixer affect only the bass channels. The speaker channels do not pass through either.

With two channels to play with REW is best run with stereo (or dual mono) connections. I use RCA/phono to stereo mini-jack, solid adaptors on the twin core, cable connections. This ensures that both sets of drivers can be tested together. Or one set alone, simply by muting one set, or not, on the CX2310 active crossover.

This is the result of another two hours of REW sweeping:

The bass is around 10dB hot but can be easily reduced on the active crossover gain controls to taste. Black line is the full combined IB with main speakers. Blue line is the four newer drivers. Violet the older four.

It looks quite a mess but will be further refined later. I now prefer to have a gentle hump between 30 and 60Hz to get some power and impact. My decades of chasing the infrasonics at the expense of higher bass frequencies are over. The IB has more than enough infrasonic power to wreck the house without worrying about silly house curves.

The peculiar, signature hump at 13Hz, with the associated higher frequency trough, is a known room effect. The older IB15 drivers are still getting their customary 20Hz, +16dB boost with 120 B/W. The newer drivers about 3dB boost at 20Hz. I also gave them a gentle lift in a daisy chain of gentle boost filters from 30-60Hz. Without which they follow a steep downward slope with rising frequency.

I tried every imaginable crossover point but nothing will affect the trough at 170Hz. This is obviously another fixed aspect of my system layout relative to the room. Previous testing shows it to be a cancellation effect between the IB in the left wall and the right, main speaker.

The sound quality is back to normal as I listen to my favourite organ CDs at fairly low levels. The clarity of the bass, as it ponders up and down in the acoustic basement, is exquisite. Even at the threshold of audibility the timbrel character of the great pipes is effortlessly portrayed.

Click on any image for an enlargement.

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An IB rebirth?

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After further REW testing and listening tests I decided the OB should be removed. It's humped response was too extreme to be useful in filling the 150Hz trough. The pair of older drivers, borrowed temporarily from the IB, were to be returned to the 6' tall, eight driver manifold.

While I was about it decided to move the four newer drivers down to the bottom four positions. The older drivers were moved up to the top four positions. I haven't tried this layout before. Logic suggests that the lower positions will enjoy more bass reinforcement. The upper drivers less so. Though this may be pure speculation given the manifold's position: Almost reaching the narrow ceiling. Just under the 45 degree overhang of the sloping attic walls.

The changeover of eight 15" drivers is not something to be undertaken lightly. Sixty four furniture screws and 64 T-nuts have to be removed and then refitted. One hundred and twenty eight loosening and fastening actions in all. Then there is the matter of the weather. If it is cold outside there is a hell of draught though each empty driver cut-out.

This time, I finally made a bit to drive the screws with a rechargeable drill. I simply cut off an Allen key close to the bend using a thin, diamond wheel in the angle grinder. Allen keys are too hard for hacksaws which just skate off.
 
I can't be both sides of the manifold at the same time. So I have developed a system. I leave a middle driver out on each side of the tall box. This allows me to reach through the empty cut-outs. So I can  fit the T-nuts to the inner ends of the screws inside the box.

Even this is not without difficulty. Because the screws are loose in their holes. So they want to push straight back out as I apply the T-nuts to the bare threads inside. I usually hold the screw thread with a finger nail while gently starting the nut. Once the threading is started the nut can be spun on until the locking spikes meet the inside of the plywood box.

The first screw to fit is always the bottom one on a driver fitted from the outside of the box. The heavy magnet inside the box dictates this order. The magnet wants to pull the back of the driver downwards. So the rim pushes out at the bottom. This is the complete opposite to my fuzzy logic. The first screw is pushed through the driver rim. That driver must now be held firmly against the baffle until the first T-nut is safely fitted.

This is where the missing drivers are helpful. All it takes is the bottom screw and T-nut to hold the driver perfectly safely. The basket sits in the cut-out so cannot move sideways. Now one can relax and work on all the other drivers. The final driver to be fitted is the real problem. One can't reach through an empty cut-out to fit the bottom T-nut.

My method is to push as many screws through the last driver as possible. To hold the driver in place by friction alone. I can then go around to the inside and fit as many T-nuts as possible. This ensures the driver won't suddenly slip. At least one screw will usually hold it safely in place. Once the first T-nut has a grip the battle is almost won.

Then I have only another 56 T-nuts left to fit. This usually leaves my finger tips rather sore. Even if I avoid the anchor spikes the stamped disks of the T-nuts are narrow and sharp. I never remember to have a tray to hold all the nuts and screws. Inevitably T- nuts drop as screws are removed or pushed through the holes. I always seem to end up with a nut or two missing.

Discipline demands that they must be found before they can wreck a driver! Usually they are hiding around the back of a driver magnet out of sight. The worst possible scenario is when a T-nut drops right inside the motor and coil area. There are small vent holes in the cast baskets which are just large enough for a T-nut to fall in and be held firmly by the magnetic field. Sometime a nut just sticks to the spider face by magnetism. These can be more easily rescued with extended fingers through the basket spokes. Those which make it inside the motor area need long nosed pliers, patience and some dexterity to get them out again.

One thing I noticed this time is that the older AEIB15 drivers are much heavier than the new ones. The older magnet back plates are much larger in diameter. As are the ferrite magnets. The baskets look almost identical.

The newer cones are flared, reinforced paper. The older ones are straight tapered and vinyl or polypropylene. Or whatever plastic was used in their manufacture. The older drivers have screw terminals. The newer ones are sprung. The sprung terminals will not take such large wire as the screw type. This isn't a problem with a single wire. Only when two wires must be twisted together in one terminal does it become a big problem. Unfortunately most terminals will have two wires inserted.

Now all I have to do is remember how to wire up the drivers again. The older drivers are 4 Ohms each. These will be wired in series-parallel for a total load of 4 Ohms on one channel of the EP2500 power amp. The newer drivers are 8 Ohms each. So will be wired all in parallel for a combined load of only 2 Ohms on the other channel. As usual, I will apply a 9 Volt battery to the amplifier end of the bared cables on each set. Just to ensure all eight cones jump outwards at the same time. I usually test the DC resistance as well with a DMM to ensure the combined loads are correct.

It will be interesting to see how moving the drivers around will affect the response and thus the BFD filters required. It could be a total disaster but was still worth trying. I hate "if-only I had tried that" situations. Usually the thought of removing eight heavy drivers and all those screws and nuts leads to sudden and permanent inertia! :-)

Click on any image for an enlargement.

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