What do you need to do to damage a DCX464?

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I have just had the strangest experience with my pair of DCX464. I bought them new, they sat in a box for around a year and then I took one out and did a few sweeps at living room SPL level to measure the FR and impedance with a horn. I put it back to the box and pulled it out another 9 months later. Again, I did a few sweeps with a horn and without it at comfortable levels - I would say 95 dB/m absolutely maximum, more probably less - since I did measure mostly a few cm from the mouth. I used the log chirp from Clio Pocket and a solid state amp I am normally using, which has no problems at all (except for a little noise and hum, which is another story). This time I took out both - measuring one on a horn and one without it. I got some strange results, so I did more measurements swapping the drivers, etc. The unused coil was always shorted or connected to the other channel of the test amplifier.

The drivers were sent back to BC for investigation since one unit had suspicious measurements on MF FR and Z, the other unit had suspicious measurements on the HF unit. They took the drivers apart and the membranes were smashed (they say, I have not seen any evidence of that). They suspect severe overload of the drivers (which has never happened!), but were kind enough to replace all the membranes free of charge.

I wonder what does one need to do to destroy the membranes of the drivers (all four!) - what voltage/frequency can damage these without a horn and on a 300 Hz horn? I did nothing but the FR sweeps. I have measured compression drivers like this for many years and none was destroyed by that (even the tiny 1" compression tweeters). I am puzzled, what could have happened? Either I have destroyed them with the (relatively silent) sweeps or they must have been damaged before I bought them new in a factory sealed box.
 
Wow pelanj, strange indeed.

I routinely sweep them at the level you described with no high pass filter.
And how on earth could all four sections be taken out ????

The amp your using....any chance it's putting out DC, or turn on / turn off transients?
What is the amp? Or at least, what's its voltage/power output?
I ask because,...... Is it a consumer amp?....I've had some whacky spikes come out of consumer amps
 
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The amp is the.amp S-150, there is no DC at the output (measured that), it has delayed turn on and speaker protection, but even then, I do not turn the amp on or off with a connecte driver.

Unfortunately, no measurements saved from the first session, I have only the ones before I filed the claim.

No HP on the sweep, I have not heard any bad stuff from the drivers - and I know how hammering of the membrane to the phase plug sounds from torturing cheap CDs.

Two of the membranes must have been damaged really badly, since the impedances were way off. The other two were just fine. I cannot really imagine how this could have happened - both were treated the same way and one lost the HF and the other one the MF...and there were not hundreds of sweeps, just 10-20 on each max.
 
which one were damaged - MF or HF? Strange, there were either already damaged when you picked them up or your amp has dc issues. I think it is hard to measure dc component when only present for some signal. To be safe then always use a large capacitor in series or use another amplifier.
 
They just stated all 4 were gone by overload and to be replaced. No more details.

The amp is very old and served well. It has DC and thermal protection on the output (managed to trip it once by overload when powering two PA boxes to the max).

Anyway, I have some large caps I wanted to use for protection while measuring the Grand Heils - so these will always be used from now on.
 
Well, now what would be the safe way to measure these then? I can e.g. check the voltage before connecting the driver, add a highpass (100 Hz?) either passive or in DSP, I can use a low wattage amplifier, battery powered (what voltage/power limit)? Can e.g. 1.5 V DC kill the driver?

I am sure that the DCX can take a lot of abuse on the road, it must be pretty hard to fry one, still cannot imagine what could have happened.

And my question is, could this sort of damage (allegedly from hammering the membranes against the phase plugs) result in shifting the low end cut off from 3k to 6 k for the HF part? And in decreasing the impedance peak to nonexistent for the HF and to much lower for the MF? But otherwise no big changes in FR when measured with Clio Pocket and low levels?

I will post the measurements when I find them.
 
I use this amp:

https://www.monacor.de/produkte/akb-60/?r=pdf

1703228536807.png

iirc the few shots we did together looked ok for one MF diaphragm. What I always take care about is the voltage set in CLIO Pocket. The device itself has a quite high internal impedance so for impedance shots I use at least 1V or more. But for a compression driver with an external amplifier I always start at 0.01V just to be safe und play a single 1k sinus tone to check the spl. Some amplifiers also have a high gain input setting for the driver stage. Too much level could cause clipping. Anyway this should not mean the other diaphragms were ok. All this is very strange and one may think about the robustness of these drivers when very short test impulses could damage them.
 
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Having a cap with a cutoff about an octave below expected use may be the only safe way to go.
You also have to consider what your amplifier or crossover will do in a brownout situation.

I have heard the sound of a larger midrange compression driver clapping against a stop when a brief power drop caused some LF offset. Venue sound engineer was not happy and agreed that the driver should have had some passive protection.
 
I have a 3W class D battery powered amp to try out. I will use a protection cap and also an L-Pad since that is what will be connected with them anyway.

Still, nothing catastrophic happened (no brownout, etc.) and many other drivers survived the same conditions many times...
 
Since you are in wtf is going on territory, I think everybody saying using a cap till it's figured out are being wise.
I've no clue what to use, as I've never bothered (due to no problems yet, knock on wood)

fwiw, i routinely make transfer functions and impulse without hpf using time averaged pink.
when i do sweeps for distortion (seldom) I do usually high pass, because often I'm testing that at high SPL, 120-130dB.

I typically use a channel of a 4-ch amp for each dcx section....a whopping 1150 watts per ch......without ever worrying about anything.
When I got a second dcx464 recently , I immediately compared it to the one I've used over several years of testing & listening with that amp setup.
Nearly identical, new vs old.
The driver really is robust I think....something is definitely amiss.

(the 4 ch amp is simply easier to run to speaker via NL4 speakon....and is the only reason for using big amp on CD)