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Old 2nd April 2003, 09:47 AM   #1
SHiFTY is offline SHiFTY  New Zealand
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Talking Interesting amp building experience... BANG!

Well, I have been building a 320W @ 4ohm subwoofer amp kit. It is from Dick Smith Electronics (cat K-5618) and features 3 pairs of 2SK1058 & 2SJ162 MOSFET outputs. It is a fairly powerful beast!

Well at least it would be, if it didnt have some serious issues...

When I first hooked it up, I put a cheap 10cm speaker across the outputs, and then turned the amp on. There was a loud bang, and the voicecoil, glowing red hot, ripped out of the centre of the speaker! There was a fair bit of smoke too!

Lucky, I was just about to connect it to my nice subwoofer!!

The clip LED on the PCB seems to go on more or less immediately when the amp is turned on, so I suspect oscillation. This would mean that poor 10cm speaker had 320W through it!

Gah, anyway no real point to all this, just thought I would tell my story. Anyone else had some "explosive" projects?

/me goes off to search for random oscillations...
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Old 3rd April 2003, 02:53 AM   #2
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I have built this amplifier myself and it is an awsome window shaking amp. I had a similar problem when I first built mine. Check for shorts between the PCB tracks where the MOSFETS are mounted and the heatsink (this is what caused my problem but I didn't have a speaker hooked up at the time).
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Old 3rd April 2003, 05:52 AM   #3
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I suggest nasty DC offset. Oscillation generally occurs at such a high frequency that the speaker, especially a woofer, will appear to be a pretty high impedence.

Didja look at offset?
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Old 3rd April 2003, 06:02 AM   #4
SHiFTY is offline SHiFTY  New Zealand
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What would cause DC offset in this type of amp? And why would the clip indicator be activated? Thx!
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Old 3rd April 2003, 06:10 AM   #5
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Has the output of your amp any turn-on delay? Otherwise the turn-on thump might have killed your speaker if it isn't because of DC offset as someone already suggested.

It is always wise to use a cheap speaker for the first test-run of an amp - as you did.

But it is even wiser to turn it on for the first time without any speaker and measure the DC offset at the output.

Regards

Charles
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Old 3rd April 2003, 11:39 AM   #6
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Quote:
When I first hooked it up, I put a cheap 10cm speaker across the outputs, and then turned the amp on. There was a loud bang, and the voicecoil, glowing red hot, ripped out of the centre of the speaker! There was a fair bit of smoke too!
No way a 'turn-on thump' did this. The only way to do that kind of damage is with a (un)healthy dose of DC at the output.

As far as what would cause the offset, damn near any dead transistor in the output stage or solder screw-up or resistor screw-up, or..............

Man, ya gotta get in there and troubleshoot!!!

Do not give up so easy. I have had amps that it took me an embarrasssing amount of time to troubleshoot, but in the end I kicked its butt. I suggest you take your time and figure it out. Your self-esteem and self-confidence will thank you in the end, regardless of how long it takes.
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Old 3rd April 2003, 12:56 PM   #7
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Quote:
No way a 'turn-on thump' did this. The only way to do that kind of damage is with a (un)healthy dose of DC at the output.
This depends on how long it takes for an amp to settle. If it takes several seconds then you could easily fry a small driver with such a large amp.

Because of the statement about the clipping LED I agree with you though that it must be something else.
I don't know the circuit of this amp so I don't know how they control the clipping LED. I guess full DC output or oscillation.

DC offset can be measured quite easily.

Regards

Charles
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Old 3rd April 2003, 01:39 PM   #8
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Hey shifty. One of my friends recently finished building one of there sub amp modules, like the one you have, except his did nothing, no blowing speaker no nothing, he did some basic measurments, getting voltage rails up properly etc, nothing on the output. He is going to bring it round in a week or so, and I'll take to is see if i can find out whats going wrong with it. but it seems that DC has a reputation of selling kitsets with dodgey componetry, so it may just be something some where is dead. like i said ill have a look, and keep you posted....
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Old 3rd April 2003, 09:47 PM   #9
SHiFTY is offline SHiFTY  New Zealand
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Quote:
I have built this amplifier myself and it is an awsome window shaking amp. I had a similar problem when I first built mine. Check for shorts between the PCB tracks where the MOSFETS are mounted and the heatsink (this is what caused my problem but I didn't have a speaker hooked up at the time).
Yeah, unfortunately its not that, I haven't connected the heatsinks while I was testing it... Did you have any other problems? Glad to hear that someone has got it working BTW!!!

Quote:
it seems that DC has a reputation of selling kitsets with dodgey componetry, so it may just be something some where is dead. like i said ill have a look, and keep you posted....
Yeah one of the diagrams for the transistor wiring is around the wrong way, for the BC640. Watch for that! I have already replaced all the small transistors in the thing, might try replacing the signal diodes & zeners next.

Cheers!
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