blew up my driver...time for something more

[mpiotro]

I posted probably 7 months ago in BFD forum about a GR Research PR sub I built.



The BFD is awesome but I used lots of cuts.... And the gain knob just kept getting cranked up. I bottomed the driver a couple times messing with Flight of the Phoenix when the plane rolls with the subsonic stuff- I think that is the killer test of all I have tried. Until I was watching V for Vendetta last weekend and it clanged away before I could get to the mute button.

Very musical, but lacking for HT IMO.

So I prefer minimal additional outlay in $$$... So I am stuck with a 2.4cf box a pretty box, but kind of small ultimately (69L net) and I am stuck with a plate amp that pushes 280@8 ohms or 350@4ohms (Apex Sr- same mfr as Rthymik). I spent so much labor with the evil Walnut Burl veneer that I feel obligated to utilize this box, no matter how much of a compromise it is. WAF is good though- even she was impressed with how it looks in the living room.

So now I am playing with Unibox and I now know I have difficulty being responsible with the gain knob.

Woofer is a Treo Engineering SSI 12" 27mm Xmax 700WRMS unit. PR is the new 18" CSS (overkill but inexpensive and lots of Vasp!). This "mobile audio" driver modeled better for me than any other TC Sounds/SS/ED/FiCar model...

So what am I missing???? this shows 100db@15Hz... 104dB@20Hz.




And no way for me to push cone to overexcursion with current amp (275W is max)




Please advise if this all looks reasonable... It will naturally be BFD'ed with about a 6-8db drop to 80Hz from the 104dB peak at 20Hz. I would consider other options also... Seems like QSC or Behringers can be picked up used for less than $300. Any other driver/PRs I should be considering with my tiny lil' 69L box???? Any magic tricks??? SDX15 with Two CSS PRs? If somebody can show me a magic bullet in this box I may hit the credit card...

[brent_s]

The first problem I see is the 12.44 you're looking at is a dual 4 ohm coil design. How do you plan to wire it? Your current plate amp won't like a 2 ohm load which leaves you driving an 8 ohm load with 280 watts.

Secondly, what improvement are you expecting compared to the GR-Research SW12a. I modeled the 12.44 vs. a SW12a in 2.4 tuned to 19hz...I'm too lazy to run as PRs, especially since GR doesn't publish the necessary specs to complete the WinISD model. Should be valid for the purposes of this comparison. The SW12A actually has a slight (2db or less) output advantage above 22hz. The SW12 doesn't hit Xmax until about 16hz, assuming no rumble filter, which I think the Apex Sr. should have. So, it's only below 16hz that the 12.44 would protect you from Gain Knob Abuse, again, assuming no rumble filter.

Given the models, are you sure you bottomed the driver? PR's have limits too and should make some sort of noise when they get there. In my experience, a clipping amp can produce a sound that's resembles a bottoming driver. Unless you've modified the Senior, it comes with 6db of boost at 30hz, which probably indicates an f3 of 20hz or higher for the rumble filter...in other words, you shouldn't be able to bottom the SW12A with 350 watts, but you do have enough excursion to run the amp into clipping. Above 20hz, your excursion limited power handling is good for 500 watts except around 31hz, where it dips to 440ish.

Looking even deeper, I forgot,the SW12 is an 8ohm nominal driver. The simulated impedance curve only dips to a low of 7.8 ohms so you're getting 280 watts or less sustained out of the Senior...making it highly unlikely you've bottomed the driver with that amp.

Standard questions. How did you calibrate your subs to your mains? What size room? What's the distance to your seat from the sub? How loud do you listen in terms of SPL or relative to a 75db/85db calibrated reference level?

In the end, output is about air displacement...a function of Xmax and Power. The SSi has more potential output and a decent looking response curve in that box, but you'll need more power than the Senior can provide. With a suitable rumble filter, that SSi in 2.4/19 will handle 1500 watts nicely for >111db from 20hz up at 1m ground plane. No idea if the CSS PR can handle that kind of excursion. A suitable port would be too large to fit inside the box, not that it actually has to for proper function...SAF is generally kind of low with an outie port, though.

In short, a driver change alone is not going to move the needle in a meaningful way, IMO.

-Brent

[mpiotro]

Thanks for reading and the detailed follow-up questions... I'll answer as best I know how.

How do you plan to wire it? 8 ohm

What improvement are you expecting? more output below 20Hz, not damaging another driver

Rumble filter info:Amp is modded with corner freq of 18Hz with a 2dB boost at 21-22Hz, so rumble filter starts at 18Hz right?

Given the models, are you sure you bottomed the driver? I heard multiple,repeated mechanical/metal clanging noises watching first exposion scene in "V for Vendetta". The dust cap was clearly damaged and the voice coil is rubbing badly. I cut off the damaged dust cover, attempted to center the assy and it still rubs enough I doubt it could be fixed

How did you calibrate your subs to your mains? Std BFD setup using REW; used "Flight of the Phoenix" crash scene as loud as I would ever ever listen to it to get the BFD to be just shy of clipping on the input signal by adjusting sub calibration level. BFD filter used significant cuts to get a flat to 17Hz in room response and then a -6db house curve after that. Then used built in test tones on my Onko 602 to level match. Level matching of sub channel was accomplished with gain knob on Apex Sr amp. I then double checked with DVE disk for DVD playback- no change needed as DVE test tones were within 1-2 db on all channels from Onkyo test tones. I feel confident that level setting was done properly. Gain knob was probably a bit more than 3/4 of 100%.

What size room? 2200cf with 3 open doors to entire house ( entire home is only 1400sf or 11200cf)

What's the distance to your seat from the sub? 6 feet

How loud do you listen in terms of SPL or relative to a 75db/85db calibrated reference level? Pretty loud for action movies. Normal dialog in the 80dB SPL range. Maybe 105db in louder dynamic areas of movie

Is it possible I was just hearing clipping??? but how would that have caused the damage I describe?

Is to be assumed a proper rumble filter will cut the FR enough to avoid the over excursion below 20Hz? Maybe I tried to EQ the sub too much to extend more than was intelligent used all my headroom and clipped bad enough to somehow damage the driver... I think I understand the rumble filter effectively eliminating the overexcursion issue. So I modeled the Treo @8ohm and 800W ( I could do this with a QSC RMX1450 for $200 off eBay) and a single CSS PR at 2100 grams just stays under it's excursion limits. 800W gives me 109dB from 20Hz and up.

One more dumb question... how many dB is a noticable improvement that a non-audiophile person would notice 3,4,5 dB?

[brent_s]

That does sound like excursion damage. I was expecting you to say the driver was cosmetically fine, but the voice coil no longer passed continuity...meaning you ran the amp at a high enough sustained level to melt the coil, an unusual failure in a sub, but possible.

I can only think of a few reasons that you were able to excursion damage the driver in the described alignment.

A) An out of spec driver. Really no way to tell now.

B) Somehow your cabinet "appears" larger than modeled and the driver is unloading higher up than the ported model suggests. A small air leak could do this. I'm not sure if exceeding the excursion limits of the PR could cause a similar effect. I doubt a single GR 12" PR is capable of 2x the displacement of the SW12A, which is the recommended ratio. That's why most PR systems use the next standard frame size larger PR with a driver such as 12"/15" to get the required displacement via surface area. Otherwise, a same sized PR would need double the suspension travel. Without the necessary parameters for the PR, we can't model how much power is needed to over drive it. It's possible the 200 watts power handling that Danny specs for the SW12A is actually that number, but that's pure speculation on my part.

C) Somehow your BFD EQ is nullifying the rumble filter roll off, allowing sub 16hz material to bottom the driver.

My best guess is mostly "C" with maybe a bit of "B" from PR limits. The corner frequency of your rumble filter should be the -3db point when a 0db signal passes through it...the rumble filter is really just a highpass filter. The Senior should have a roughly 12db/octave roll off starting somewhere just above your corner frequency. Depending on how much BFD boost you've got dialed in around 20hz, your corner frequency could actually be shifted much lower than 18hz.

Actually that's a rather astute question for determining an upgrade path. The research I've read says the smallest SPL change the average human can reliably detect is a 1db difference between pure tones in a clinical environment. In the real world of complex signals like music/ht and typical environmental noise, the threshold is more like 3db. We perceive a 10db increase as twice as loud.

-Brent

[brent_s]

BTW, your calibration and listening levels/distance sound reasonable. Maybe a bit hotter than my tastes, but not the excessive 115+db I see some folks looking for. At 6' to the sub, assuming at least one wall is reasonably close, you should be able to add at least another 3db to the ground plane models unless you're sitting in a deep null.