Small amp trashing big speakers

[glebert]

I have had a couple of speaker failures in a bass rig but I don't know if the problem I'm having is an amp problem or a speaker/enclosure problem. I have a regular jam with some friends and for a bass amp I use my Ibanez Promethean 300 watt 1x10 combo. The internal speaker doesn't give enough volume, so I use an external 1x15 speaker box (homemade) that belongs to one of the other guys. In this configuration I am running the external 8 ohm in parallel with the internal speaker (which is a dual coil speaker that will be 8 ohm when using an external speaker). I don't think the amp does 300W RMS, my guess is that is a peak or program number. I don't have the amp available to me right now and don't remember what kind of power amp it is using or voltage rails..

The problem is that I've trashed two 15" Peavey Black Widow speakers that should be able to handle 300W on their own, so assuming the Promethean is sending no more than 150 watts to the external speaker (and I suspect it's less) I don't know why the speakers are getting damaged. The last one it almost seemed like the speaker voice coil had travelled too far out and gotten dislocated. When I went to test the movement of the voice coil by pushing down it felt like it popped back into place, but it still giving distorted/ragged output.

After the first speaker died I tested the amp for DC on the output and it was fine. Neither of the BW speakers was new when I started using them, so there is a possibility they were both weak to start with. I'm wondering if anything with the construction of the speaker box could be causing/letting the speakers travel too far. The Promethean is voiced very low (probably to compensate for the small internal speaker enclosure). Would something like a too big port in the cabinet potentially cause damage? The port on the box to me looks too big (with essentially no length) but I'm not a speaker box guy. Or could it be that I am overdriving the amp and stressing the speakers that way? The bass doesn't sound distorted (when the speakers are working) but I do have the amp pretty much dimed to get enough volume out.

The setup is used for less than two hours once a week. The speakers have both worked well for a while (more than a year) before failing, so I can't even tell if there is a real problem or coincidence or something else.

Thoughts?

Greg

[g1]

Had you used those speakers at high power otherwise? I wonder if they may have been buckled near the surround which is sometimes hard to spot, but will allow over-excursion.
That they both may have come from the same unit, yet the internal speaker is not having issues seems a bit suspicious.

[The Dude]

What speaker cabinet is it? Can you link specs on it? If you take the basket off the BW speaker, what does the coil look like? Are there signs of high heat (darkened coil), or just deformation from over excursion? If it's an amp problem, I'd expect to see signs of the coil getting hot.

[glebert]

What speaker cabinet is it? Can you link specs on it? If you take the basket off the BW speaker, what does the coil look like? Are there signs of high heat (darkened coil), or just deformation from over excursion? If it's an amp problem, I'd expect to see signs of the coil getting hot.

It's a homemade plywood cabinet,basically a cube probably 30"x30"x30" with a 4"x6" rectangular port right above the speaker (all dimensions are wild guesses since I don't have it here in front of me). I haven't pulled the 2nd blown speaker from the cabinet yet, but will check the coil when I do. I didn't do an autopsy on the first one.

The two speakers that have gone bad came from completely different sources and both having unknown histories.

[The Dude]

A speaker autopsy can tell you lots.

[Chuck H]

The two speakers that have gone bad came from completely different sources and both having unknown histories.

But still both the same model of speaker and probable age? I have to wonder about the stability of any synthetics used in the surround material. This is a real problem for some speaker models. So there still may be a coincidence happening. It's hard to tell so far. The lack of errant DC registering at the output should rule out the amp. Maybe there's some errant anomaly that the tests aren't catching.

[loudthud]

There are ways to estimate the enclosure tuning. Basically you need to know the internal Volume of the box, and the dimensions of the port, width, height and depth even if it's only the thickness of the plywood. From this you can see how it would affect the speaker if you know it's resonant frequency, Qts and Vas. This takes some special software, should be able to find something online.

30x30x30 is a monster cabinet. Two to three times what is normally used for Bass Guitar cabinets.

[Mick Bailey]

I used WinISD when designing enclosures and it always worked out well - the results were close to the simulation so long as the speaker parameters were correctly chosen. It has models for many popular types and you can create a custom model by manually inputting the parameters.

[glebert]

I've used WinISD in the past, but I can't remember if it shows a transient response as well as a frequency response. If the problem is overexcursion to me that seems more of a transient problem. Also wondering if the box was designed to be a "bass bin" that assumed an external crossover that would limit the high frequency content going to the speaker. Seems like full range content (voltage step being worst case example) could cause more overexcursion.

I won't have access to the bin or the speaker for about a week so won't be able to answer some of these questions for a bit.

[Mick Bailey]

I has a problem with a bass amp destroying speakers at one time and I found a fairly simple circuit that limited transients by monitoring the output and interactively attenuating the input signal. It worked so well that you couldn't tell at all that the signal was being attenuated. Rather than turning the amp down to a safe level and everything being lower volume, it could be turned up without any risk to the speakers and the overall volume was much higher. I'll try to find the schematic.

[g1]

4"x6" rectangular port right above the speaker

Sounds like maybe was supposed to be for a horn rather than a port?

[glebert]

Sounds like maybe was supposed to be for a horn rather than a port?

It is entirely possible, and would not surprise me at all.

[J M Fahey]

Huge cabinet, very large port, Bass amp which strongly boosted Bass to compensate for its tiny speaker and enclosure, all add up to suspect overexcursion
Voice coil fully expelled from gap confirms it.

Which murders speakers way before they reach thermal limit.

And you had only 1 15" speaker at a time in that humongous enclosure?

*Start* by blocking the port with a piece of wood.

Then, if enclosure is not tolexed, grab some kind of saw and cut it in two, to 30" to 30" to 15" deep so you get two enclosures and mount speaker on one..

The day you get another 15" speaker you mount it in the other.

All you need is 2 "almost" 30" plywood pieces as a new back for one, a new speaker board for the other , and 8 1" by 1" by "almost" 30" cleats to be able to hold the new panels, burger money.

If you have no carpentry machines, a humble cheap jigsaw machine can do the whole job, it will only go slow.

In emergencies I have made usable cabinets using one of them.

There is not such a thing as "I have way too much equipment" although Wives often disagree

In any case, you have 2 BW speakers to repair .
. you have to mount them somewhere, don't you?

Bonus points: new enclosures now fit in a regular car: worst case in the back seat.

[glebert]

Interesting update on this problem. It seems like the issue may have had nothing to do with the amp or the speaker enclosure, or even the functional parts of the speaker. Since Hartley was nice enough to make Black Widows easy to take off the magnets I took off the three bolts, and when I looked down into the voice coil I saw this piece of foam sitting down in VC cap. Presumably this was a filter on the mesh vent on hole through the magnet.



When I looked at the voice coil itself I saw remnants of the foam had melted onto the kapton type insulator on the inside of the VC.



I vacuumed out the loose foam and used some naptha to remove it from the voice coil. I reassembled the speaker and it seems to be working well, I'm wondering if the earlier BW had the same problem, since they were very similar in age. I had never thought to look inside that vent to look for degrading foam.

[Jazz P Bass]

I have seen that many times.
A stupid piece of degraded foam bringing down the house.
Excellent idea using Naptha to clean out the gummy residue.