# Sub bottom or amp clip....Please Help!



## Vader (Jul 8, 2006)

OK,

I just started watching "The Matrix: Revolutions" on HD-DVD, using Dolby TrueHD via analog connections, and something kinda disturbing just happened (I shut down until I can determine what). During the explosion after the title screen, a loud "pop" eminated from one or both subs (I can't tell which). I have heard that a bottom is more of a loud metallic "clack", and this was not that. The first thing I thought of was that I clipped the BFD, so I turned the subs off, bypassed the BFD filters, and re-played the explosion. At the level I was listening at (-10dB on the MV), the level meter was clearly in the red. My question is, is my reasoning correct? Is there a safe way to determine which actually happened? Obviously, I don't want to repeat the bottom or clip and damage the sub(s). I considered turning the gain on the subs way down, and repeating the exposion, my reasoning being that if it were a bottom, it would likely not repeat, but if it were a clipped amp, it would still "pop" but at a much lower volume level. Thing is, a clipped signal can damage the driver, correct? So, I don't really want to repeat that, either....... 

The best case scenario at this point would be to re-calibrate the AVR trim to give some more headrrom before clipping, then compensting with the sub's gain. I am just hoping that I didn't bottom anything (I thought I had more headroom than I would ever use)...


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## Hakka (Sep 10, 2006)

I'm sort of in the same boat, I heard a pop/clack from the sub(s) when watching Lucky # Slevin, the clip lights on the amp were lit up, I ran through the scene again at 1db lower volume while looking at the drivers and they looked to be moving at least 35 - 40mm, which is over xmax. Lowering the volume by 1db stopped the amp from clipping.


Hakka.


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## drdoan (Aug 30, 2006)

If you can put your extended fingers around the dust cover of the woofer (try each one) and gently push the speaker in the direction of its travel, it should not feel restricted or stuck. If it doesn't move freely in and out, you might have warped the former, thus damaging the speaker. If it was a clip of the signal path, it might be just a matter of lowering inputs. Remember to start with the first controllable input to the signal path to lower the volume. If you just try to lower the signal toward the end of the signal path, you will still have the clipping, just at a lower lever. Keep us updated. Hope all is well with the system, Dennis


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## brucek (Apr 11, 2006)

> The first thing I thought of was that I clipped the BFD, so I turned the subs off, bypassed the BFD filters, and re-played the explosion. At the level I was listening at (-10dB on the MV), the level meter was clearly in the red.


Monitoring the input signal was a good idea to see if the BFD was overloading its ADC on the input signal. If it was, the noise was likely digital clipping. If you're familiar with the two's compliment math that a analog to digital converter uses, you'll know that a single bit over the maximum digital signal possible will force the output to a maximum negative signal. This transition of digital clipping creates extreme distortion in the signal. There is a bit of headroom left when the LED's turn red (for obvious reasons), but if you clip a digital device, it's an awful sound.......

brucek


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## Vader (Jul 8, 2006)

I am pretty sure it didn't warp the VC, as it still sounds fine (no distortion that would result from rubbing in the gap) if it did bottom, of which I am doubtful. The input level indicators were pretty solidly red when it happened, so I am hoping that I just clipped the amp (easy fix). The pop was more like a static discharge than a mechanical "clack". Tonight, I am going to bump the AVR trim down several dB (using the afore mentioned Matrix explosion as a guide) and allow for a couple of dB headroom before clipping, and then play the explosion back at the same levels (about -10dB from RL) but with the sub gain turned down - just to be safe. I have plenty of room to use the sub gain to compenstate - both are at about 11 o'clock. Thing is, for plain ol' Dolby or DTS, the trim is set for -2dB (coming in via optical) after calibration with DVE, allowing for the 10dB LFE discrepancy, while for Dolby TrueHD, it is set to +2.5dB (coming in via analog)... weird.


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## Vader (Jul 8, 2006)

Well,

I just finished re-calibrating the subs against the Matrix explosion from Revolutions. I was somwhat shocked that this required a reduction in AVR trim of -5.5dB..?!?. I adjusted the subs gain upwards to just past 12 o'clock on each. This is equivelent of 85dB using HD-DVE. And I tried the explosion sequence again at -10dB MV. And it (they) ckacked again. I am now quite sure that the sub(s) bottomed, and it was not the clip I had hoped. I don't understand why they are suddenly bottoming on material no louder than what I have done many times before. I thought I had a lot more headroom to play with, since I run duals in a mid-size room and do not listen anywhere close to reference. Am I correct in that HD-DVE is still 10dB hot on the LFE signal? If not, I am calibrating 10dB too hot, and that would explain a lot. Or could there be a problem with the drivers (the subs are still under warrenty)?


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## phaseshift (May 29, 2007)

> I am now quite sure that the sub(s) bottomed, and it was not the clip I had hoped. I don't understand why they are suddenly bottoming on material no louder than what I have done many times before.


 How long has the system been in place? Sounds like to me that the driver suspensions are becoming more compliant and you just needed to make some adjustments. Very unlikely a problem with the drivers. 

I do not mean to hijack this thread, but it is a good place to mention a few words about a surprisingly common driver failure mode- VC neck bond separation due to bottoming

A note on drivers "bottoming" - now this may sound perverted, but there are typically 2 conditions that you can experience- hard bottoming and soft bottoming. 

Hard bottoming is the VC former hitting the backplate and is typically a loud pop sound with an aluminum former (a lot of subs use aluminum formers) and typically a few times does not cause the thing to break. However, you must reset your levels to avoid hard bottoming as it does something you may not be aware of- weakens the adhesive bond between the VC and the damper. **EDIT Rev0606070108- Forgot to mention that the other former materials may not fare as well as aluminum depending on the condition. I am citing all examples here with the aluminum former in mind for simplicity sake, but for the record, have seen the same issues with fiberglass, Kapton and composite formers - no really significant differences except for the sound and the others typically failing earlier- often due to the lower end of the VC shorting out due to impact damage.

How does it do that? Well, the former hitting the backplate creates a significant amount of force and the resultant shockwave travels in the only direction it can- away from the lower end of the former and up through the coil eventually passing the adhesive bond area where the damper and cone neck joints are made. Typically, the joints are done with one of 3 adhesives- 2 part acrylic, 2 part expoxy and in some cases, cyanoacrylate. Of these, they are all typically somewhat brittle when cured; while that works well in their typical operation mode, it does not fare well when subjected to significant shock as in the case of a hard bottom condition. My experience is that the epoxies and rubber toughened CA's work best but the difference is certainly not significant enough to say that one or the other eliminates or really impacts the problem. 

Soft bottoming - where the damper or some other part of the suspension impacts the frame or even an intentionally placed object such as very tall screw head or a high density foam pad for instance. Typically, this is done with the damper since it will definitely give you a nice, loud pop when it hits something hard enough, but in general will not cause the damage as fast as the hard bottoming condition. 

Does the soft bottoming condition "save" the woofer? Not 100%, the conditions vary between driver designs and system parameters which impact actual excursion under use. I have found that a driver with a soft bottoming condition where we intentionally impact a soft object near the middle of the damper is probably the least invasive. Unfortunately, this is also the most difficult to predict and in most cases, probably will not work due to damper oscillation which can set up remarkably fast; particularly at lower frequencies. If the driver is designed to have a soft bottom condition near the VC - damper bond line, the results may not be much different than that of the hard bottom condition for obvious reasons- the bond area is hitting a hard surface with substantial force. 

When you are buying a system or raw driver, do a simple test. Carefully select a location on the diaphragm where you can push the suspension through it's inward range of motion. If you can feel the driver hit a hard stop before the surround runs out of travel (typically, you will see some buckling in the surround when it is near it's limit), you need to be aware that the driver may well bottom out under high power. You may want to consider this when selecting a driver for a DIY project or your system as it could be a factor in the overall lifespan of the system. If someone tells you that the driver will be damaged form this activity, invite them to observe what happens when the driver is operating under high power. Obviously, you have to use common sense and not punch in the DC or damage the driver, but in most cases, the woofer is going to take this, no problem.


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## Guest (Jul 25, 2007)

i would say you heard the sub reach P2P where the surround limits the travel of the cone...


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