Aaargh - amp disaster

Posted: Wed Mar 08, 2006 4:41 am

Hi,

I averted it, but I am hoping somebody can help me identify this amp disaster and how to prevent it/what caused it.

I upgraded recently to a Carver PM900 amp and Dynaudio BM15 speakers.

I plugged it all up, testwd it, and everything seemed fine.

I then went in to track some acoustic 12-string on Saturday, got all set up and suddenly said to my 2nd engineer 'can you smell burning?'.

I am 99 sure it was coming from the speakers. I have quite a sensitive nose, and by the time I had powered down and dropped in a backup Samson Servo, only I could smell it.

I am sure it was there and am having the Carver sent for testing.

What I am hoping for help with is:

- can an amp make speaker drivers burn?

- what is the cause - sending current to them? clipped files with a voltage element? I don't get it - they were fine for extended testing on the same amp, what has changed?

- how can i prevent this in the future

I just feel if I had a bit of knowledge, I'd be able to operate my power stage more safely.

Thanks in advance,

Jeem

Posted: Wed Mar 08, 2006 4:45 am

Oh and also.....am I likely to have done any long-term damage to the speakers? I think it was the woofer, but can't be sure; the second I smelt it I powered the room down and then tried to track it by smell. I worked on for a good 607 hours on the other amp with no funny issues so I think I am ok....do you think there could be anything internal messed up in the speakers? Wiring or other?

Posted: Wed Mar 08, 2006 5:29 am

What type of wire are you using for the conductors. This might be your problem. It is very important that you use the right type.
Also you need to narrow down where the smell is coming from. Even if that means taking the whole thing apart, piece by piece until you find the one piece (wire, speaker, whatever) that smells like it is burnt.
You might also want to keep a bucket of water near by, incase things get out of hand in the future Embarassed

Posted: Wed Mar 08, 2006 5:41 am

Its pretty standard multistrand copper wire. Quote thick, maybe 3mm core, maybe 24 strand or more?

What would the right type be? I have read most of the forum and have seen nothing but preference vis multistrand vs single.

If it is the cable, would the burning (which I am certain is coming from the driver section of the woofer, call it a hunch) be something I could risk experiencing again without checking the amp?

And how come it works fine on the Samson? Does the Carver put out so much more power it needs different loading from the cable or something?

Sorry if I sound challenging, just trying to ascertain best method to proceed.....thanks for your help.....

Posted: Wed Mar 08, 2006 3:02 pm

Jeemy wrote:

- can an amp make speaker drivers burn?

- what is the cause - sending current to them? clipped files with a voltage element? I don't get it - they were fine for extended testing on the same amp, what has changed?

- how can i prevent this in the future

yes

DC Direct current on the output
or
Oscillation at high frequency

many amps now have DC protection but the oscillation can be tricky

people may suggest fuses and poly switches and racked speaker protection devices
but they do sit between amp and speaker so there must be some involvement

there are some things that just can't be protected for

when the amp is in good condition and your method of use is good
the amp/speaker combination can be near bullet proof and give years of excellent service

Posted: Wed Mar 08, 2006 6:19 pm

As Kev wrote, DC at the output.

I have a Carver TFM22 which did exactly what you have described, except I was not so lucky to grab it before it killed my woofers. (they were only old Technics speakers fortunately though)

What occurred in mine is the amp overheated and hurt the first 2 transistors in the chain. These 2 transistors still actually functioned, but began leaking about half a volt of DC.

After several more gain stages, that DC had been cranked up to about 30 odd volts at the speaker terminals, which my speakers didn't like.

2 $3 transistors later and the amp was fine.

Most consumer amps protect for this sort of thing, with fuses or Hipass filters etc. But pro amps in general have less "junk" in the way of the signal chain.

You can test for it with a basic volt meter across the speaker terminals, the less you read, the better. but more than 0.25% of the amps rated power means there is probably a problem. (eg: 400w amp = max 1volt DC)

sorry to also say, if you smelt burning, your speakers have probably suffered some damage, they may still work but will have changed the freq response and powerhandling I Imagine.

HOpe this all helps you a bit

Al Wink

Posted: Thu Mar 09, 2006 12:09 am

All of the above posters are correct. Much of our electronics these days can pass DC which is fatal to speakers. Another problem used to be with analog tape recorders and the bias oscillator used for recording could leak into the output (if the biased traps were not properly adjusted). If your signal chain can pass 100kHz, your tweeters could be fried quickly.

Turn it down
Ms. Remy Ann David

Posted: Thu Mar 09, 2006 1:23 am

It all helps. I still need to know what to do about cabling - what should I change to?

I guess I am going to have the amp thoroughly tested first. When I reinstall it I am going to put it in its own rack where there is less chance of overheating than in its previous position under a 2u joemeek vc1qs in a 4u rack.

I will have my amp technician test for any possibility of DC voltage at the terminals. If he is unsure about it I will sell it (letting the buyer know the reason) and go to something else - even if its psychological at this point.

And finally I guess I will have the speakers returned to Dynaudio for a service and checkup - if they even do this - but I have no B speakers for a while, so they will have to perform as they are. They seemed fine to me on the Samson but I only tracked 12-string, I haven't gone back to my reference CDs yet.

Hopefully the damage was light and may have affected a small amount of frequency response which hopefully won't affect what I hear too much, and as for powerhandling, they handle more than my room would so if I lose something off the top end there, I don't think its a drama. I remember reading in a previous post that Dynaudio woofers are about $140 each or something, so hopefully they can be swopped in. I imagine this type of damage is actually quite hard to test for.

So if somebody could give me a verdict on the speaker cable, I would be grateful. Thanks to all. I guess forewarned is forearmed but how can you legislate for this?

Posted: Thu Mar 09, 2006 1:31 am

your cable sounds appropriate - 2.5 - 3mm multistrand copper, should be more than adequate.

This topic sponsored by: