Helps: xover and drivers damaged? (very long)

[kalbi_hana]

I am into this hifi hobby for more than 20 years and I believe that I have established a base systematic way to troubleshoot hifi related problems.

Now, I have experienced problem(s) which I do not know what is happening. I hope that forumers are able to unlock the mystery.

Here how thing goes:

Last Saturday, I played some female vocal jazz at repeat mode, set the volume control at 8 o’clock, and leave home for leisure.

Normally, the loudest I can go is between 10:00 o’clock -10.30 o’clock.

Few hours later, my family member called and told me that there was a very loud “pop” from the speakers, followed by decrementing of sound or noise, and went silent.

My family member switched off the power amp immediately.


My system consists of:
1. Pre-amp
2. Power-amp: after power has switched on, the relay activates few seconds later; relay activate immediately after power has switched off.
3. CD player
4. 1 pair of speakers – a 2-way acoustic 4th order Linkwitz-Riley crossover at 1.75 KHz
5. 1 pair of active sub-woofer
-each sub-woofer comes with to drivers
-Signal input has signal-sensing automatic on/off circuit
-The sub-amp has 3 operation modes: Off , On an Standby. The Off setting disconnects it from the power outlet completely, the On setting keeps it constantly operational, and the Standby setting turns it on when it receives a music signal through its input.
-I always set the sub-amp on Standby setting
- Come with protection against short-circuit, DC, overheating and clipping ensures no damage to the transducers – even at extreme listing levels


I conducted an investigation immediately after I reached home, the below are my findings:

General:

1. The weather on this day is good, no lighting strike happened on that day
2. The circuit breaker of the domestic does not trip

Power status of the equipment:

1. All the fuses of the equipment and sub-amp are intact. No blown fuse detected
2. CD Player is still On
3. Pre-amp is still On
4. Power-amp was switched off by my family member
5. The left sub-amp is still On
6. The right sub-amp was at Off setting. I suspect the sub-amp switch from Standby setting to Off setting automatically after the big ‘pop’

Health status of the equipment, speakers and sub-woofers:
1. CD player is fully functioning
2. The pre and power-amp is fully functioning. The relay of the power-amp is working fine
3. The left speaker, including the left sub-woofer amp is perfectly alright
4. The problematic sub-woofer (right) amp is able to turn on
5. One of the drivers of the problematic sub-woofer (right) is able to move in and out normally when music is played but with distortion. The distortion sound like internal wire is loosed somewhere.
6. The other sub-woofer driver is totally jammed.
7. There is no sound from the mid-woofers of speaker (right) when music played. The resistance of this mid-woofer is ~6.3ohm, same reading as the one on the working speaker (left).
8. I swapped the mid-woofer between left and right speakers. The result is reserve therefore I suspect the crossover of the right speaker has problem.
9. There are sounds from the tweeter of the right speaker. Compare it to the left speaker, it sounds softer, noisy, lost of harmonics and dry.

I suspect that either the right channel of the pre-amp module or the cd output giving problem, or DC leaking, or something else. But there are protection circuits in the amp and sub-woofer amps, and I played the music at low level, therefore I am puzzling.

I would like to seek everyone advice to enlighten me what are the possible causes, so that I can prevent them to be happened again.

Sorry for the inconvenience caused!

[JLC7]

What do you mean by one of the drivers in the right subwoofer being totally jammed? Is it stuck in place like you can't move it even by pushing it?

How do you know your pre/power amplifiers are fine? Does the problem follow the speakers or does it stay with the amplifier channels?

[kalbi_hana]

Your are right, the driver is stuck in place, and need to use a little bit more force to move it compare to a good one.

To be precise, my sub-woofer comes with two boxes, each box has it own driver.

The amp is housed in the lower box, there are speaker cable link the two boxes together.

The driver in the lower box has mechanical sound when the music signal is feed.

The driver in the upper box is stuck.

The left speaker, including the left sub-woofer and sub-amp is perfectly alright.

I swapped the outputs of the power-amp, from left-to-right and right-to-left, and the left speaker is alright.

I swapped the outputs of the pre-amp, from left-to-right and right-to-left, and both left speaker and sub-woofer are alright.

I swapped the outputs of the CD player, from left-to-right and right-to-left, and both left speaker and sub-woofer are alright.

For all tests, the problems of the right speaker remain, i.e.

1. There is no sound from the mid-woofers of speaker (right) when music played.

2. There are sounds from the tweeter of the right speaker. Compare it to the tweeter of the left speaker, it sounds softer, noisy, lost of harmonics and dry.

it seems like the outputs of the CD player, pre and power-amp are working fine.

your inputs is appreciated.

[kalbi_hana]

Dear JCL7,

Under what circumtances the crossover will blow but not the driver?

Thanks!

[JLC7]

If one of the subwoofer drivers is physically stuck, it's probably because the voice coil has overheated, expanded, and is now dragging against the sidewall. Either that happened, or something physically fell into the driver somewhere and is blocking its movement which I doubt happened. You say the other subwoofer driver can still move but it sounds like a loose wire which I think confirms that it's a voice coil problem in both.

I think the most common failure in crossovers is due to capacitors. Your more likely to get a bad batch of capacitors than inductors. Inductors are for the most part just wound up wire. It's hard to mess that up in manufacturing.

Can you open the speakers and subwoofer to look inside? Maybe take some pictures of the crossover components?

[kalbi_hana]

Thanks JLC7!

More info to add:
- The speakers are bi-wired, so the crossovers of the high and mid/bass are separated.
- The pre has two outputs: each feed to the power and sub-woofer amp directly; there is not link between the speakers and sub-woofer amp.

If the capacitors in the crossover failed, will it able to cause such a big damage to the sub-woofer amp and tweeter? Or the capacitors are damaged by the DC leakage of the pre-amp?

Am I right to say that when there is a dc leakage from the amp, usually the mid/bass driver will be the first one to blow but not the tweeter, this is because there is resister to protect the tweeter?

It is not easy to open the enclosed of the speakers, and I’ll try to.

Sorry, could not take pic because I don’t own a digital camera, still old fashion.

I hope that from this incident, everyone is able to learn something new.

[JLC7]

Have you been able to get inside to see the crossover of the speaker?

Have you tried switching the tweeter between speakers to see if it's also the crossover causing the problem and not the tweeter?

[kalbi_hana]

Dear JLC7,

It is not so easy to dismantle the cabinet, and need to unscrew the tweeter from inside of the cabinet.

My speaker is bi-wired design and I used two pairs of speaker cables.

As there is sound from the right tweeter therefore its hard to tell the crossover is completely damaged unless I swapped the tweeter.

Therefore I took the easier way out, the measure of the DCs.

1.1.dc pre-out to power amp (left channel) is vary between 9mv to 12mv.

1.2.dc pre-out to power amp (right channel) is vary between 9mv to 13.9mv.

2.1 dc pre-out to sub-amp (left channel) is vary between 8mv to 21mv.

2.1 dc pre-out to sub-amp (right channel) is vary between 8mv to 17mv.

3.1 dc speaker-out to rigt speaker is stable at 17mv

3.2 dc speaker-out to left speaker is stable at 20mv

4. the dc output of the sub amps to the two drivers is stable at 2mv

All the readings look fine, puzzling...