Technical help

[peter haynes]

Anyone any ideas on this problem?
This is the third 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 for anyone who can help with this :scratch:http://theartofsound.net/images/smilies/17428.gif

[Firebottle]

Sounds like you possibly have some dc component going into the amp, can't tell from the user instructions to whether the amp is direct connected or not.

Get hold of a multimeter of find a friend who has one, then measure to see if there is any dc present on the input leads/connectors.

:)

[peter haynes]

Thanks for reply Alan
I have attached photos of connections of speakers and Subwoofer- don't know wether this makes it any clearer
Peter

[Firebottle]

I don't think it has anything to do with your connections, which look fine.

On re-reading it sounds like the amp is probably at fault, do not switch it back on with speakers connected until you have it checked out, it could blow more speakers.

:(

[Reffc]

DC on the output, sounds like your amp has developed a fault and fried one of its output transistors, op amps or an output smoothing cap may have developed a short. Best not connect any more speakers until the amp is repaired. Your tweeters will have been protected by the crossover capacitors. This can be a common problem for anyone buying used old amps which haven't ever been serviced or checked or where the amp has been run into an over-demanding load which has caused the output stage to fail. I've lost count of the number of times I've seen this.

To check the amp; use a multimeter set to DC, then switch to the voltage on the lower range (0-10V something), place an 8 Ohm resistor across the output terminals of each speaker output on the amp, rated for 20W, have the volume fairly low, say 1/4 volume, and put the multimeter across the terminals in parallel with the load resistor. If you're getting anything much over 0.1V on each channel, you've a fried output stage component or components so you'll know its the amp needing a service.

[Arkless Electronics]

95% likelihood of blown output transistors. The mains hum as it blew is a dead give away as well. I'd loose the sub if I was you.... or at least power it from a line level output.
You can indeed check for DC with a meter. You don't need any load resistors or anything. Don't connect any speakers to it as you'll almost certainly blow them!
You can't check for likelihood of output transistors failing or for signs of abuse in the past I'm afraid. Nor will servicing mitigate against it. Either they blow or they don't and that's that.

[anthonyTD]

Really sorry to hear this,:(
Yep,
Agree with the advice given, That is unfortunetly the main reason i bang on about Transistor amps having DC detection circuits on their outputs!
Like Arkless has stated, you never know when an output Transistor is going to fail. :(
FET output amps are safer to a degree, as the device just stops conducting, where as with Bipolar devices, they generally go dead short, and seen as most conventionally designed bipolar output stages have an NPN and a PNP device [or banks of them] when one or the other goes, you get the full power supply voltage, and current capability of either the positive or negative rail on your speakers.:doh:
A...

[Marco]

Yes, and another good tip is not to have inflammable curtains... ;)

Marco.

[StanleyB]

This used to be quite a regular problem that I used to come across when I was in the audio repair business. Almost all of them were due to soldering dry joints, especially around transistors and voltage regulators in the power supply and the output stage. As the amp heats up during play, the solder at a dry joint location starts to separate. That's when you start hearing the hum, which normally is soon followed by the protection relay cutting in. But if your speakers are blowing before the relay trips, I would head straight for the output stage and resolder any non-shiny soldering spot.

[anthonyTD]

Yep,
I remember my Dad repairing a lot of solid state stuff in the 70's 80's, and as you say, some of it was indeed down to bad joints etc, mostly in high powered Amps used for Disco...:D
This used to be quite a regular problem that I used to come across when I was in the audio repair business. Almost all of them were due to soldering dry joints, especially around transistors and voltage regulators in the power supply and the output stage. As the amp heats up during play, the solder at a dry joint location starts to separate. That's when you start hearing the hum, which normally is soon followed by the protection relay cutting in. But if your speakers are blowing before the relay trips, I would head straight for the output stage and resolder any non-shiny soldering spot.

[struth]

The sub may be causing an abnormal load on the outputs which could be causing the problems. but as said above there are plenty of options. Are the 2 negatives on the speakers terminals connected to each other?

[Marco]

Yes, and most importantly remember that if the amp catches fire, whilst still connected to your speakers, that the voice-coils are up to the job of acting as a grill!

Marco.

[Arkless Electronics]

This used to be quite a regular problem that I used to come across when I was in the audio repair business. Almost all of them were due to soldering dry joints, especially around transistors and voltage regulators in the power supply and the output stage. As the amp heats up during play, the solder at a dry joint location starts to separate. That's when you start hearing the hum, which normally is soon followed by the protection relay cutting in. But if your speakers are blowing before the relay trips, I would head straight for the output stage and resolder any non-shiny soldering spot.

Whilst what you describe can certainly happen, in the main the hum comes from the power supply trying to dump it's full output into the speaker coil after an output device has blown. Such is the current draw that the smoothing cap has little effect and hence all the hum. It's usually shortly later (a few seconds) followed by silence as the driver blows or the amps mains fuse blows.

The single biggest cause of such amp blow ups is user abuse in my experience. Things like trying to change speaker cables with the amp still on, failing to spot that single strand of cable that's shorting between the amp output or speaker input binding posts. Another one is connecting many sets of speakers to an amp at the same time.

[Arkless Electronics]

Yes, and most importantly remember that if the amp catches fire, whilst still connected to your speakers, that the voice-coils are up to the job of acting as a grill!

Marco.

I've seen speakers catch fire due to an amp going DC!

[Marco]

Yes, it can certainly happen!

In those circumstances, would you blame the amp for the fire, or the speakers for not being 'tough enough' to stand it before combusting?

Marco.

[Arkless Electronics]

Yes, it can certainly happen!

In those circumstances, would you blame the amp for the fire, or the speakers for not being 'tough enough' to stand it before combusting?

Marco.

Just in case you're serious..... THE AMP!! No speaker could take it without blowing (fire is a secondary issue).

[Marco]

I am serious, Jez... So, in your professional opinion, in that scenario, there would be no acceptable reason to blame the speakers for catching fire?

Marco.

[struth]

Amp needs a dc fault detection circuit added. Trouble is what monitors the monitor?;)

[Arkless Electronics]

I am serious, Jez... So, in your professional opinion, in that scenario, there would be no acceptable reason to blame the speakers for catching fire?

Marco.

There is no acceptable reason for blaming the speakers for blowing. Whether or not they catch fire is another issue... It is only the cone around the voice coil that can catch fire and they are well buggered by that time anyway!

[Marco]

Thanks, Jez. And in your opinion, would a DC fault protection circuit prevent that from happening, which I also presume is a good idea to be fitted on a commercial amplifier?

One other thing, if designed and implemented properly, would such a protection circuit affect sound quality?

Marco.

[walpurgis]

I've seen speakers catch fire due to an amp going DC!

I've seen speakers catch fire 'cos some prat connected them to the mains (yes! seriously!). :rolleyes:


(nice new pair of Harbeths it was. not mine fortunately)

[Arkless Electronics]

Amp needs a dc fault detection circuit added. Trouble is what monitors the monitor?;)

It's all more money so some amps don't have protection. At the sharp end of the market the protection circuitry and probable relay could cost nearly as much as one channel of the power amp section... and all for something that only does anything in the rare event of a fault. Caveat emptor. Some expensive amps eschew protection as well! A relay can of course suffer from dirty contacts which can cause intermittent output or a reduction in sound quality. I've had loads of repairs in where the protection circuitry or its relay has been the cause of the fault. The other main protection method is a "crowbar" protector. This avoids the relay but it works by sacrificing the amplifier to protect the speaker, not so bad if an output device has blown but if the DC is caused by input DC offset or dry joints or whatever then it will (usually) destroy the output devices anyway rather than just cause a relay to pull in...

[Arkless Electronics]

For completeness... Fuses are also sometimes used but are wholly unsatisfactory IMO. If they are small enough to offer a good chance of protecting the woofer then they will generate distortion. If meaty enough for distortion to be negligible then they probably wont protect the speaker! Negative feedback can be taken after the fuse or relay contacts to help ameliorate any distortion from them. In the few amps with regulated output rails the protection can be arranged to shut down the regulators in the event of DC on the output (a good technique). Switching off the power at the mains in the event of DC by using a relay or triac can also be done.

[peter haynes]

On each occasion I was just sat listening to music - the amp has two speakers + sub connected to it per the photos I have downloaded - these connections were made when amp was switched off - it has played fine for about 5 weeks. It has not been abused and there is def no single strand of cable thats shorting. The amp is 5 months old and was bought new - it has been back to musical fidelity last time it happened and they say no fault found. I do hope the manufactueres manage to sort it

[peter haynes]

Hello Grant
The sub manufacturers have got back to me today and are satisfied with my connections etc. Only 1 negative is connected - it is on left hand channel though

[Arkless Electronics]

On each occasion I was just sat listening to music - the amp has two speakers + sub connected to it per the photos I have downloaded - these connections were made when amp was switched off - it has played fine for about 5 weeks. It has not been abused and there is def no single strand of cable thats shorting. The amp is 5 months old and was bought new - it has been back to musical fidelity last time it happened and they say no fault found. I do hope the manufactueres manage to sort it

MF didn't have to repair it then? It's very unusual for an amp to go DC, blowing the speaker, and not be destroyed itself. In fact it's usually the case that the amp is already destroyed and this is why it has took out the speaker!

[peter haynes]

I love the sound including the sub(when it is all working though :lol: ) I really don't want to lose the sub. This connection seems to be pretty common place - if its ok for all others I don't understand why the sub would be causing me problems with the musical fidelity amp
The fault occurs after a period of weeks - I bet I could connect it up again and all would be well for a few weeks, then blow again..
Thanks all for your help
peter

[Reffc]

This may seem a daft question but is the sub passive or active? I've seen passive subs connected this way, and that's almost always bad news for the driving amp as load impedance is usually halved at the most demanding load point.

[peter haynes]

MF didn't have to repair it then? It's very unusual for an amp to go DC, blowing the speaker, and not be destroyed itself. In fact it's usually the case that the amp is already destroyed and this is why it has took out the speaker!
Hi Jez
They claim amp was on 'soak test' and nothing wrong found. It took me a while to get a system I was happy with - I got it and now it just keeps blowing up :doh:
I am sure the amp still works
Peter

[peter haynes]

This may seem a daft question but is the sub passive or active? I've seen passive subs connected this way, and that's almost always bad news for the driving amp as load impedance is usually halved at the most demanding load point.

Hi
The sub is active
Cheers
Peter

[Arkless Electronics]

Hi Jez
They claim amp was on 'soak test' and nothing wrong found. It took me a while to get a system I was happy with - I got it and now it just keeps blowing up :doh:
I am sure the amp still works
Peter

It has a very meaty output stage then!!!
Sounds like an intermittent fault in the amp. If it's still under guarantee I'd try and get them to replace it.

[struth]

You could try running it for several weeks without the sub and see if it is causing the problem or try another amp with the sub connected and ditti.

[anthonyTD]

It could also be something plugged into it, ie; a CD player or some other source equipment, that has an intermitent fault on its output, putting DC into the front of the amp on one channel, if nothing has been found with the amp, i guess you have to suspect anything that is part of the chain now.
A...

[peter haynes]

You could try running it for several weeks without the sub and see if it is causing the problem or try another amp with the sub connected and ditti.

Hi Grant
The last time the amp was being tested by MF, I borrowed my sons Sony amp and all seemed fine - however it was only for 3 weeks or so. This seems to happen after about 5 or 6 weeks - the whole thing seems very strange
Cheers
Peter

[struth]

think i would be tempted to leave sub off once speaker is fixed.

[peter haynes]

It could also be something plugged into it, ie; a CD player or some other source equipment, that has an intermitent fault on its output, putting DC into the front of the amp on one channel, if nothing has been found with the amp, i guess you have to suspect anything that is part of the chain now.
A...

I have managed to get what happened up on you tube - maybe this makes it a bit clearer? The amp is going back to manufacturer together with blown woofers

Maybe the video will shed some light on it

Thanks to all for your help and input

Peter

677hThcnRNI


nYH8BzZy8vs

[Marco]

I've embedded the videos for you now, Peter :)

Marco.

[peter haynes]

I've embedded the videos for you now, Peter :)

Marco.

Thank you

[anthonyTD]

Definitely DC there Peter, if you look, you will see how the cone is being pulled inwards just before it lets go!
Unfortunately, This still doesent prove whether its the amp at fault though, or a piece of equipment connected to it.
I have managed to get what happened up on you tube - maybe this makes it a bit clearer? The amp is going back to manufacturer together with blown woofers

Maybe the video will shed some light on it

Thanks to all for your help and input

Peter

677hThcnRNI


nYH8BzZy8vs

[Arkless Electronics]

Not many amps are DC coupled input to output so I suspect the amp rather than anything connected to it.

[anthonyTD]

I agree Jez,
It just seems strange that its been back to the manufacturers, and nothing was found, hopefully, this time they will do a more thorough investigation!
Not many amps are DC coupled input to output so I suspect the amp rather than anything connected to it.

[peter haynes]

:):) Thanks to all for your input - I will let you know the outcome of the tests by Musical Fidelity:scratch:

Peter

[anthonyTD]

It will certainly be interesting Peter!
:):) Thanks to all for your input - I will let you know the outcome of the tests by Musical Fidelity:scratch:

Peter

[dowser]

If your sub is active, why have you got it connected to speaker outputs of amp? Tape out line level would be better I think?

[anthonyTD]

Yes, that is a good point!
If your sub is active, why have you got it connected to speaker outputs of amp? Tape out line level would be better I think?

[CageyH]

Surely, it depends on what connections have been made on the sub?
If it is a high level connection, it should be ok, but I thought it was better taking the negative to a common ground, such as the case earth?

[CageyH]

It seems a bit of a coincidence that the sub is fully connected on the left channel of a dual mono amp, and the left hand speakers are blowing the woofers.
It would appear to me that the sub connection is the problem.

[anthonyTD]

You could well be right, as have already been sugested by others, earlier on in the thread, guess we'll just have to wait for the verdict!
It seems a bit of a coincidence that the sub is fully connected on the left channel of a dual mono amp, and the left hand speakers are blowing the woofers.
It would appear to me that the sub connection is the problem.

[dowser]

Depends on connections at sub end - I assume the amp is full dual mono - so the 2 earth connections on left and right speaker probably do not have continuity. How this translates to DC on one channel no idea...but I almost bet it doesn't do it with the sub removed :)

[peter haynes]

If your sub is active, why have you got it connected to speaker outputs of amp? Tape out line level would be better I think?

The recommended connections by the sub manufacturer are how I have connected it:mental:

Peter

[dowser]

Fair enough - I'm not familiar with connecting subs up, and I guess having at speaker output fixes any level matching issues. It is still a potential cause though - are you sending the sub off with the amp when they check it out?

Richard

[peter haynes]

Fair enough - I'm not familiar with connecting subs up, and I guess having at speaker output fixes any level matching issues. It is still a potential cause though - are you sending the sub off with the amp when they check it out?

Richard

Hi Richard

I am not sending the sub - I used it with my sons sony with no problems - I have now been offered an Arcam A39 as a replacement for the Musical Fidelity and going to accept I think:)

[awkwardbydesign]

Does this mean we will never find out what the cause is? :(

[peter haynes]

Does this mean we will never find out what the cause is? :(

Hello Richard

I hope I will get feedback from the dealer- however, I get the impression that Musical Fidelity are not exactly forthcoming with info - I may be wrong and if so apologise to Musical Fidelity.
Bk Electrics (the sub manufacturers) have responded very quickly to each query I have raised about this problem

Its a shame as I finished up with just the sound I was looking for - its just that I don't get to hear it very often as the amp and speaker drivers are either in transit or non functional:lol: By the way, this is the second Musical Fidelity amp(the first one was faulty )- and this was before I bought the sub:(

If I get anymore info on the problem I will post it on here

Cheers

Peter

[Arkless Electronics]

Faults like this are an engineers nightmare! Thankfully pretty rare.... When an amp only malfunctions once a month (or whatever) it's almost impossible to effect a repair, after all it has to be faulty in order to fault find and if it functions perfectly for all but a few minutes a month then there's not much to go on.
Like I say faults as described above are very rare but, taking into account labour charges, it can be a case of replacing all the components other than ones it couldn't possibly be can be the best solution.... if the amp is valuable enough to warrant what would be a large bill of course!
BTW I used to be the Musical Fidelity repair department but it was many years before the OP's model came out.... I ain't even seen one of these in the flesh.

[peter haynes]

Faults like this are an engineers nightmare! Thankfully pretty rare.... When an amp only malfunctions once a month (or whatever) it's almost impossible to effect a repair, after all it has to be faulty in order to fault find and if it functions perfectly for all but a few minutes a month then there's not much to go on.
Like I say faults as described above are very rare but, taking into account labour charges, it can be a case of replacing all the components other than ones it couldn't possibly be can be the best solution.... if the amp is valuable enough to warrant what would be a large bill of course!
BTW I used to be the Musical Fidelity repair department but it was many years before the OP's model came out.... I ain't even seen one of these in the flesh.

Agreed - its been a nightmare for everyone. However,I just cant believe all the problems I have had with a bit of kit costing £2500(this is the second M6i I have had - the first one went faulty also) Hope the Arcam proves to be more reliable. It would be good if the fault was identified as there has been a lot of input on this forum for which I am grateful
Cheers
peter

[oscarsaudio]

I notice you had bare wires at the amp end for the sub , what connections did you use for the sub end ?

[peter haynes]

I notice you had bare wires at the amp end for the sub , what connections did you use for the sub end ?

Hello

The connections at sub end are via a Neutrik connection

http://www.bkelec.com/HiFi/Sub_Woofers/hll.htm

peter

[peter haynes]

Does this mean we will never find out what the cause is? :(
Final outcome is do not really know what fault was with amp - one part was replaced as a precaution but do not have the details-sorry. I now have a brand new Arcam A39 thanks to Nintronics- good customer service. Good bass and warm sound but not as open and detailed as the Musical Fidelity - think I may have preferred the MF:doh: Will keep for a while but may change back to MF.
Arcam did tell me though not to connect the sub unless it is via Pre-outs as it may damage equipment. I believe that the fault was therefore probably with the connection of the subwoofer all along!! :scratch:
The good news is I don't need the sub with the Arcam
Thanks again to all for your input

[peter haynes]

The saga is now over - I have bought a new Musical Fidelity - the M6si after a thorough demo with some other amps. Got it home - great match for my speakers (Focal Chorus 836vw) and now really happy with sound - no subwoofer needed!!
All i have to do now is sell the Arcam and subwoofer - and then look to replace the Marantz cd player:lol
Thanks again to all for your input
:)