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Lucas_G 9th November 2006 10:40 AM

Smoking Chipamp
 
Hi,

I had just finished my LM3886 chipamp using the pcbs of chipamp.com

I thought this was an easy job compared to other projects that I finished succesfully.

However, when I switsched on power there was a huge flash at the LM3886 chip, followed by some hefty smoke.

I checked the LM3886 chip and pin 4 an 5 (looking at the front from left to right) seem to have had the shortage.

I doublechecked the connections, and also the resistorvalues, but I still have no clue. I hope someone can help me in tracing the problem.

I enclose some pictures of the boards.

There are some things I thought of:

1. I did not attach the CHG from the ampboard. It should eventually be connected to the chassis, but for now I had everything running on a piece of wood.

2. I used Welwyn resistors at the amp board. They are a bit longer so I had to mount them a bit upward. I checked whether they made a shortage but this is not the case. However they present a longer path. Could it be oscillation? Could oscillation bring the hole thing to shortage in such a short moment?

3. Could it be that the LM3886 is faulty? Has anyone encountered such failures?

I am still reluctant to check whether the second mono amp is alright. I would like to pin the problem down in order to avoid the chance of frying my second LM3886...


Any suggestions are highly appreciated.

Lucas

Lucas_G 9th November 2006 10:41 AM

Picture 1
 
2 Attachment(s)
Picture 1

Lucas_G 9th November 2006 10:42 AM

2 Attachment(s)
Picture 2

Lucas_G 9th November 2006 10:43 AM

2 Attachment(s)
Picture 3

Nordic 9th November 2006 11:03 AM

So you connected the + and - rails but not the ground rail to the PSU?
If so, thats your problem right there.... connect the PSU ground the the amp board's ground. Then use an earth decoupling network between the PSU ground and the case.wich in turns goes to mains earth if you have 3 prong plugs.

Lucas_G 9th November 2006 11:28 AM

Quote:

Originally posted by Nordic
So you connected the + and - rails but not the ground rail to the PSU?
If so, thats your problem right there.... connect the PSU ground the the amp board's ground. Then use an earth decoupling network between the PSU ground and the case.wich in turns goes to mains earth if you have 3 prong plugs.

Hi Nordic,

Thanks for looking.

I checked this too.

Indeed I did not connect the CHG to chassis (since I still don't have a chassis)

However, from the PSU board there are two PG (power ground) connections that meet at the ampboard. So there is a positve voltage, a common ground and a negative voltage supplied to the amp. This is alright, isn't it?

I am still wondering whether oscillation/instabillity might cause this.

I opted for the standard configuration with just R3 (680 Ohm) and no Ci (47 uF) feedback capacitor as described on page 8 of this pdf: http://www.chipamp.com/docs/lm3886-manual.pdf

Regards,

Lucas

sayang001 9th November 2006 11:54 AM

Quote:

However, from the PSU board there are two PG (power ground) connections that meet at the ampboard. So there is a positve voltage, a common ground and a negative voltage supplied to the amp. This is alright, isn't it?
Yes it is alright. What are your voltages ?

Nordic 9th November 2006 03:35 PM

I don't THINK oscillation would kill off straight on the flip of a switch like that....
If the power rails shorted with either the input or outpu pins it would kill it like that right away, also if you connected power the wrong way around (I have certainly done that... nowadays I keep an LED in series with a 20K resistor between the positive and negative rail to show properly working PSU.

Lucas_G 9th November 2006 08:28 PM

Quote:

Originally posted by sayang001


Yes it is alright. What are your voltages ?


Hi Sayang,

I will check them tomorrow.
As you can see on the photos, I was so sure that nothing could go wrong with this "simple" propject, that I connected the the PSU and Ampboard with soldered wires, and no connectors in between. So I will have to deconnect them tomorrow.

I did check the other (100% identical) PSU board. I measured 25 Volt AC and 34 Volt DC without any load. I guess that is alright?

Regards,

Lucas

Lucas_G 9th November 2006 08:31 PM

Quote:

Originally posted by Nordic
I don't THINK oscillation would kill off straight on the flip of a switch like that....
If the power rails shorted with either the input or outpu pins it would kill it like that right away, also if you connected power the wrong way around (I have certainly done that... nowadays I keep an LED in series with a 20K resistor between the positive and negative rail to show properly working PSU.


Hi Nordic,

Yes I know. We all make such ridiculous mistakes from time to time. With this board design, things are very straightforward. All the connections on both boards are facing each other...

Still no clue, but as I said, I will check voltages of THIS Psu tomorrow...

Regards,

Lucas


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