Sub-woofer fuse blown

Just been watching (again) The Dream is Alive on DVD.

Had the surround sound going - a must for this video

There are 2 or 3 shuttle launches with the deep & loud sounds from the rockets.

Towards the end, during one of the launches, there was a click, and the deep bass went. Clearly the sub-woofer was no longer doing its stuff. Checked and found the internal T 4.0A fuse had blown. The mains fuse is fine.

I haven't got a spare so will need to get one.

Question. Why might it have blown?

All the speakers are Bowers & Wilkins, the sub-woofer is a Bowers & Wilkins ASW608. The amplifier is a 14-year old faithful, it's specification shows that in surround mode, only channel driven, front power output is 65W+65W; centre power output is 65W; rear power output is 65W+65W. All at 8 ohms. There's no specific power output given for the sub-woofer - nor would I expect one as it's merely a signal that the sub-woofer takes and amplifies accordingly, though I can set a level on each channel, including the sub-woofer channel. As I don't have a centre speaker that output is disabled.

Could I have overloaded the sub-woofer?

Any thoughts or comments would be appreciated.

[Edit: The old, blown, fuse is ceramic; a Maplin's replacement is glass, is that significant?]

Edited by cheshire_man (Tue 31-Dec-13 23:30:02)

Replacing the fuse has not fixed it. Clearly there is a deeper internal problem.

I've written to the retailer detailing the problem and expressing my concern. The speaker cost £350 and is less than 3 years old, and probably not been used for more than c.50 hours.

We'll see what happens.

Glass or ceramic should make no difference.

By internal fuse do you mean the one on the back panel accessible externally or did you disassemble the subwoofer? There's another internal hidden fuse which, according to the destruction manual, should only be replaced by an authorized 'operative'

Accessing the internal fuse can be a bit tricky - you may need to cut a cable tie to release the wires sufficiently to withdraw the back panel far enough. Then you'll need to take the amplifier shielding cover off to get to the fuse. I know this from needing to pull a B&W ASW 610XP apart (not for a fuse problem but a snapped phono plug pin stuck in the connector).

Apart from that, good luck!

Edited by caffn8me (Wed 01-Jan-14 21:28:56)

Thanks for that Sarah, I'd missed that bit in the Owner's Manual. I'll see what the retailer says and I'll discuss things with him.

Any thoughts on why the fuse might have blown? I know the shuttle launch has very deep and powerful bass, but I thought that's exactly the sort of sound a sub-woofer is built to handle.

It's possible the recording was clipping at low frequencies - that never does amplifiers any good and could cause protection systems to activate.

I'm surprised that you managed to blow fuses too. The subwoofer has its own low pass filter but I wonder if the AV amplifier managed to generate a DC offset resulting in a high current draw.

I suspect that the internal fuse will be replaced, the subwoofer will work perfectly and nobody will be any the wiser!

Hopefully I'll get my own subwoofers up and running before too long. Also B&W but passive and I'll be running in stereo. The neighbours are going to love me

In reply to a post by caffn8me:

It's possible the recording was clipping at low frequencies - that never does amplifiers any good and could cause protection systems to activate.

...The subwoofer has its own low pass filter but I wonder if the AV amplifier managed to generate a DC offset resulting in a high current draw.

Forgive me, but could I have the bold bits in English - please

In reply to a post by caffn8me:

...The neighbours are going to love me

Clipping is where a signal reaches its maximum possible on high peaks and the top part of the original signal is 'clipped off'. This is explained by Wikipedia here. The result is a distortion of the original signal.

There is some debate about whether this can cause damage to audio components or not. It's certainly something that should be avoided in audio terms.

DC offset is where a direct current voltage, not part of the audio signal, is given out by an amplifier. The input of the subwoofer may filter for this. A lot of amplifiers do but some don't. If they don't the effect is to draw more power than necessary for the audio and either overload the amplifier or burn out the speakers.

Thinking about it, I'd be a bit surprised if either clipping or dc offset caused your problems.

Do you have a calibration facility to match the subwoofer output to that of the other speakers?

I'm not completely clued up on this stuff, but have done an audio engineering course some years ago. So I have a couple of questions.

The tech spec for the woofer says it is an active unit, meaning it has an in-built amplifier. This normally means it would take a line-level input. However, you mention that you are using your own AMP (14 years old) which is designed to power speakers which are passive. So, I am curious how you wired this woofer up to your existing setup. Can you elaborate on this?

I tend to agree with other posters here. One has to always be careful with woofers because, by definition, their job is to move substantial amounts of air to create deep base frequencies. If you have flat-lining happening (due to too much input coming into the woofer, and clipping of the signal), that's going to result in extended periods of time where the cone of the woofer is fully pushed out and in, and this can heat-up the coil inside the speaker mechanism to the point where it may burn out completely. If you have replaced the fuse, it may well be the cone coil that is burnt out. But even so, I would have thought units like these have built in safety circuitry to prevent events like this happening ... or maybe not (?)

Edited by mixt (Wed 01-Jan-14 23:32:27)

In reply to a post by cheshire_man:

Any thoughts on why the fuse might have blown? I know the shuttle launch has very deep and powerful bass, but I thought that's exactly the sort of sound a sub-woofer is built to handle.

The specification sheet quotes a power rating of 200 W but a power consumption of only 40 W so your sub-woofer is designed for intermittent bass signals, as you might expect in typical classical music. It might be that your shuttle DVD has more bass content than the speaker is designed to handle resulting in enough current being drawn by the unit to cause the fuses to blow. (If this is the case then getting the internal fuse replaced should restore correct operation. Just use less welly next time.)

In reply to a post by cheshire_man:

[Edit: The old, blown, fuse is ceramic; a Maplin's replacement is glass, is that significant?]

In the event of an overload a ceramic fuse can safely break a much higher current than a glass fuse. I don't know what specification of fuse you have but a Maplin 5 x 20 mm 4 A time delay glass fuse (order code N98JC) can break 40 A whilst a couple of otherwise similar ceramic fuses I found on the web can break 1,500 A (e.g. http://uk.farnell.com/schurter/0001-2510/fuse-antisu... ). As a UK mains socket can supply several 100 A (until a fuse/circuit breaker trips) it is safer to use ceramic fuses for mains supplies. (Glass fuses are better used on power supply outputs where the available current is lower.)

The B&W subwoofer can take either a line input (many AV amplifiers give a dedicated subwoofer line output) or speaker level inputs from each channel.

This is done by piggy-backing off the main speaker outputs but it isn't using the power of the AV amplifier to drive the subwoofer. The speaker inputs on the subwoofer are very high impedance and that signal is still fed into the subwoofer's own amplifier to drive the speaker.

In this case it seems that the AV amplifier has a dedicated subwoofer line-level (and mono) output. The OP mentions there is a separate level control for the subwoofer on the amplifier.

Edited by caffn8me (Thu 02-Jan-14 00:10:48)