Technical help

[peter h]

Anyone any ideas on this problem of blown woofers?
This is the third time I have a problem with woofers blowing on left hand channel- this time all 3 woofers have blown on left hand channel – luckily I think the separate subwoofer has survived.
Was listening quietly to a cd, then experienced horrible loud humming then speaker blown - smell of cone burning – I have a video- had phone close to hand(cant download it here) – hope this gives you an idea of problem. This is now third time this has happened

Can anyone please advise with any technical expertise out there whether there is any possibility this could be a fault with the subwoofer, amp or speakers – if not, then any advice on what could be the cause(eg. Faulty amp or faulty speakers)would be much appreciated – I think one of the subwoofer engineers mentioned last time that it sounded like amp was going into DC mode??

The dealer has, I believe approached both manufacturers of amp(musical fidelity m6i) and speakers(Focal Chorus 836vw) and am awaiting their feedback The subwoofer is a BKxxls400

Thanks if anyone can help

 

[davedotco]

It really does seem like you are getting DC on you amplifier on that channel.

What is really odd is that the amp seems to recover and then behave normally, until the next time.

Transient DC offset is one thing, but for your woofers to be blown that comprehensively suggests something else. Sounds like a power supply rail going down, but I have never seen that sort of failure in an intermittant way, normally once it goes it blows everything up.

 

[Covenanter]

Assuming everything is wired up correctly then it has to be something happening in the amp but like Dave I'm at a loss to know precisely what. As Dave says, normally anything that would blow speakers up would kill the amp as well.

I think it has to go in for investigation.

Chris

 

[peter h]

What happened can be viewed at https://youtu.be/677hThcnRNI

 

[Thompsonuxb]

What happened can be viewed at https://youtu.be/677hThcnRNI

For the link.

 

[peter h]

The wiring can be viwed at https://youtu.be/nYH8BzZy8vs

Cheers Peter

 

[Thompsonuxb]

Were they bought new?

If not get the internal wiring in them checked and the cross over circuit if the mid cone did not blow too.

If it's just one speaker blown and your amp did not blow its fuse get the speakers checked by proper techs.

I don't mean to laugh, but that video made me jump.... *mosking*

 

[peter h]

The speakers are new - however I put on another pair last time it happened and it did same to them also

Thanks

peter

 

[peter h]

I didnt know whether to laugh or cry - sure as hell made me jump when it happened, hence the rapid movement away at end of video*yahoo*

 

[Covenanter]

That very much sounds like mains hum to me! If so you have some dangerous wiring issue. Take care!

Chris

 

[Covenanter]

I've just looked at the wiring clip. What are the red, white and black wires? That looks like a mains cable to me!!!!!! If it is, disconnect it straight away (having switched everything off everywhere) and get someone who knows what they are doing to wire it up for you.

Chris

 

[matt49]

I'd have thought those wires connect the sub, no?

 

[peter h]

Yes indeed - you are right - they are the cable linking the sub at high level via a neutrik connector.

 

[peter h]

Hello Chris - thanks for the concern but it is not mains wiring. It is the cable connecting the amp to the subwoofer.

 

[jonathanRD]

Yes indeed - you are right - they are the cable linking the sub at high level via a neutrik connector.

+ 1 - they are exactly the same high level wires I have too with my BK XXLS400.

 

[Covenanter]

Just 3 wires? 2 to one set of speaker terminals and 1 to one of the others?

Weird! I stand to be corrected but that simply can't be right.

Chris

 

[Vladimir]

You need to check for DC voltage.

Grab a multimeter, set it to volts AC and connect the probes to the left channel (+ to +, - to -).

Have the speakers and sub disconnected, no music playing.

Turn the amp on and see how much voltage you're getting. If nothing is showing, switch to milivolts range on the multimeter.

Repeat for the right channel as well.

Smell the amp if it has some burnt electronics.

 

[Covenanter]

Ok, I'm pretty certain it's your sub wiring that's the problem. I've looked up the M6i manual and it has two separate mono amps with separate power supplies. A 3 wire connection to a sub would work if it was a vanilla amp with one power supply with a "common rail" configuration, which most do (I would think as it is a simple way of doing things).

I would disconnect the sub and try again. I'd be willing to bet it will be fine.

Chris

 

[Vladimir]

That could be it Chris. However, best not to risk another pair of speakers and use a multimeter to rule out a faulty amp.

 

[davedotco]

Just 3 wires? 2 to one set of speaker terminals and 1 to one of the others?

Weird! I stand to be corrected but that simply can't be right.

Chris

It is.

BK subs all use a common earth, hence a nice bit of heavy duty mains cable is fine.

But then with dual mono amps, who knows, there should not be a problem but this is Musical Fidelity we are talking about here, their designs are sometimes 'different'.

 

[Covenanter]

Just 3 wires? 2 to one set of speaker terminals and 1 to one of the others?

Weird! I stand to be corrected but that simply can't be right.

Chris

It is.

BK subs all use a common earth, hence a nice bit of heavy duty mains cable is fine.

But then with dual mono amps, who knows, there should not be a problem but this is Musical Fidelity we are talking about here, their designs are sometimes 'different'.

I guessed that was how the sub was wired BUT if the two amps don't have a common earth then you have created one and you would be routing mains from one power supply through the circuit which would explain the hum (and if that's not mains hum I'm a Dutchman) and the exploding speakers.

The simple solution as to ask MF how to connect it up.

Chris

 

[davedotco]

Just 3 wires? 2 to one set of speaker terminals and 1 to one of the others?

Weird! I stand to be corrected but that simply can't be right.

Chris

It is.

BK subs all use a common earth, hence a nice bit of heavy duty mains cable is fine.

But then with dual mono amps, who knows, there should not be a problem but this is Musical Fidelity we are talking about here, their designs are sometimes 'different'.

I guessed that was how the sub was wired BUT if the two amps don't have a common earth then you have created one and you would be routing mains from one power supply through the circuit which would explain the hum (and if that's not mains hum I'm a Dutchman) and the exploding speakers.

The simple solution as to ask MF how to connect it up.

Chris

I would like more details, but it strikes me as a crappy way to build an amplifier.

I have used the '3 wire' subwoofer setup on mono blocks in the past without trouble, what is it about the way MF design and build their amplifiers?

 

[Covenanter]

Well I can't be certain I'm right but the evidence points that way. What I can see on the web suggests connecting an active sub to the pre-outs.

Chris

 

[Vladimir]

On the back of the unit there is a sticker alarming that the amp must be earthed, which means it has a common ground and not a floating ground. That Dutchman's hum at the begining was probably ground loop. Perhaps with the subwoofer connected the amp went into oscilation and threw a bunch of DC into the speakers. Who knows what safety compromises MF made to add more 'purity' to the signal. I'm not seeing any relays. Possibly one on the left channel but why would there be just for one channel?

 

[Covenanter]

On the back of the unit there is a sticker alarming that the amp must be earthed, which means it has a common ground and not a floating ground. That Dutchman's hum at the begining was probably ground loop. Perhaps with the subwoofer connected the amp went into oscilation and threw a bunch of DC into the speakers. Who knows what safety compromises MF made to add more 'purity' to the signal. I'm not seeing any relays. Possibly one on the left channel but why would there be just for one channel?

Is that the right image? It only has one power supply whereas the manual says two.

Chris