JLM 60Watt Power AMP Kit

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frazzman
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Re: JLM 60Watt Power AMP Kit

Postby frazzman » Tue Feb 21, 2017 8:12 pm

Hi guys,

I had a nicely working bridged version of this power amp running well, albeit without a proper enclosure. The enclosure had since arrived and everything has been rehoused (without any change to the wiring configuration) - quite literally a transplant.

Anyhow, upon first power up the LM3886 on one of the channels fried up instantly, smoke and sparks from the chip...

The pcbs are mounted on metal stand offs, and the lm3886s are directly attached to the side of the case with a thin layer of paste. The case sides are entire heatsinks themselves.

I am running a 160vA toroid with 18v secondaries. After disconnecting both channels, I measured my DC voltage from the regulator. I am getting an expected -19v and +19v.

My initial thoughts were that the chips were shorting against the heatsinks, but that does not appear to be the case.

I tried to reconnect just a single channel for further testing by just simply connecting the - and + supply from the regulator (and no othe connections) but the board smoked immediately. This may be "normal" given that the chips are probably internally shorted.

I have attached some photos for reference. I appreciate any insight. Thanks in advance.
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Re: JLM 60Watt Power AMP Kit

Postby Joe Malone » Tue Feb 21, 2017 8:46 pm

frazzman wrote:Hi guys,

I had a nicely working bridged version of this power amp running well, albeit without a proper enclosure. The enclosure had since arrived and everything has been rehoused (without any change to the wiring configuration) - quite literally a transplant.

Anyhow, upon first power up the LM3886 on one of the channels fried up instantly, smoke and sparks from the chip...

The pcbs are mounted on metal stand offs, and the lm3886s are directly attached to the side of the case with a thin layer of paste. The case sides are entire heatsinks themselves.

I am running a 160vA toroid with 18v secondaries. After disconnecting both channels, I measured my DC voltage from the regulator. I am getting an expected -19v and +19v.

The expected +/-voltages from 2 x 18vac are +/-25vdc (not 19vdc)? By Regulator do you mean diode bridge?

My initial thoughts were that the chips were shorting against the heatsinks, but that does not appear to be the case.

The photo shows a couple of bright copper marks on one of the LM3886 legs which is usually the centre of any spark as it is the only place no black carbon forms. So my thoughts would be a tiny bit of metal metal swarf or component leg etc has fallen in there. Also the cut bare part on the 1000uF cap plastic cover should not be allowed to touch the heatsink or case.

I tried to reconnect just a single channel for further testing by just simply connecting the - and + supply from the regulator (and no other connections) but the board smoked immediately. This may be "normal" given that the chips are probably internally shorted.

If 0v was not connected as well it would be unpredictable as to what the lm3886 would do. Also I would check the diode bridge doesn't have a shorted diode as this would also blow the lm3886
Joe :-)
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frazzman
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Re: JLM 60Watt Power AMP Kit

Postby frazzman » Tue Feb 21, 2017 9:21 pm

Joe Malone wrote:
frazzman wrote:Hi guys,

I had a nicely working bridged version of this power amp running well, albeit without a proper enclosure. The enclosure had since arrived and everything has been rehoused (without any change to the wiring configuration) - quite literally a transplant.

Anyhow, upon first power up the LM3886 on one of the channels fried up instantly, smoke and sparks from the chip...

The pcbs are mounted on metal stand offs, and the lm3886s are directly attached to the side of the case with a thin layer of paste. The case sides are entire heatsinks themselves.

I am running a 160vA toroid with 18v secondaries. After disconnecting both channels, I measured my DC voltage from the regulator. I am getting an expected -19v and +19v.

The expected +/-voltages from 2 x 18vac are +/-25vdc (not 19vdc)? By Regulator do you mean diode bridge?

My initial thoughts were that the chips were shorting against the heatsinks, but that does not appear to be the case.

The photo shows a couple of bright copper marks on one of the LM3886 legs which is usually the centre of any spark as it is the only place no black carbon forms. So my thoughts would be a tiny bit of metal metal swarf or component leg etc has fallen in there. Also the cut bare part on the 1000uF cap plastic cover should not be allowed to touch the heatsink or case.

I tried to reconnect just a single channel for further testing by just simply connecting the - and + supply from the regulator (and no other connections) but the board smoked immediately. This may be "normal" given that the chips are probably internally shorted.

If 0v was not connected as well it would be unpredictable as to what the lm3886 would do. Also I would check the diode bridge doesn't have a shorted diode as this would also blow the lm3886


Hi Joe,

Thanks for the reply.

Sorry, I typed regulator but meant to say bridge rectifier - I used your stocked 25A bridge rectifier for the job.

That exposed part of the cap has been covered.

As I mentioned, the LM3886 on other channel has also fried, so I am inclined to think there is some issue that is common to both, as opposed to a tiny bit of metal causing a short.

With regards to the voltage readings, I do measure 19V without load, but I do recall when the unit was up and running that I was seeing the 25V under load.

I can't can't test either channel under load now as they both smoke.

I'll desolder and replace the chips tomorrow and reassess.

I forgot to mention I did have 0v connected in the isolated test as well.

Is something going awry with the bridge rectifier not out of the realms of possibility ?

Cheers

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Re: JLM 60Watt Power AMP Kit

Postby Joe Malone » Tue Feb 21, 2017 9:41 pm

As I mentioned, the LM3886 on other channel has also fried, so I am inclined to think there is some issue that is common to both, as opposed to a tiny bit of metal causing a short.

A short between one power rail and output or 0v would pull over 50v to the other rail for a split second which would kill both lm3886 ic's on the same power rails.

With regards to the voltage readings, I do measure 19V without load, but I do recall when the unit was up and running that I was seeing the 25V under load.

Ok this is a fake reading from your meter because you have no capacitance to store the dc after the bridge.


I forgot to mention I did have 0v connected in the isolated test as well.

Is something going awry with the bridge rectifier not out of the realms of possibility ?

Cheers

The bridge would only be damaged after a high current surge like the short that seems to have happen to those ic legs.
What value mains fuse did you have in line with the power transformer and did it blow?
Joe :-)

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frazzman
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Re: JLM 60Watt Power AMP Kit

Postby frazzman » Tue Feb 21, 2017 10:05 pm

Joe Malone wrote:
As I mentioned, the LM3886 on other channel has also fried, so I am inclined to think there is some issue that is common to both, as opposed to a tiny bit of metal causing a short.

A short between one power rail and output or 0v would pull over 50v to the other rail for a split second which would kill both lm3886 ic's on the same power rails.

With regards to the voltage readings, I do measure 19V without load, but I do recall when the unit was up and running that I was seeing the 25V under load.

Ok this is a fake reading from your meter because you have no capacitance to store the dc after the bridge.


I forgot to mention I did have 0v connected in the isolated test as well.

Is something going awry with the bridge rectifier not out of the realms of possibility ?

Cheers

The bridge would only be damaged after a high current surge like the short that seems to have happen to those ic legs.
What value mains fuse did you have in line with the power transformer and did it blow?


Of course... It's obvious now that those DMM readings were invalid with out any capacitance.

I had a 2A slow blow fuse in there, it did not blow.

I'll desolder and replace the chips, with the high current surge I cant help but think it may be prudent to replace that bridge rectifier as well.

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Re: JLM 60Watt Power AMP Kit

Postby Joe Malone » Wed Feb 22, 2017 9:41 am

I had a 2A slow blow fuse in there, it did not blow.

160VA should only need 1A slow blow

Might be worth adding + and - rail fuses for extra protection. You can fit low 1A or 2A fast ones to test and then so to 5A for normal running etc.

I'll desolder and replace the chips, with the high current surge I cant help but think it may be prudent to replace that bridge rectifier as well.


You can test the diode bridge diodes with a meter easily and since they are 25A continuous and about 100A surge it is unlikely but worth testing.
Joe :-)

JLM Audio

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frazzman
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Re: JLM 60Watt Power AMP Kit

Postby frazzman » Wed Feb 22, 2017 3:52 pm

I'll test the bridge rectifier tonight.

In the interim, here's the post-Mortem.. Not pretty.

Obviously a lot of the top pads are ruined but the bottom pads have now cleaned up relatively well.

Given the state of the PCB - Is there a schematic or PCB overlay that I can use so I can measure continuity between each of the IC pads and whatever components they connect to ?

Thanks again
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Re: JLM 60Watt Power AMP Kit

Postby frazzman » Wed Feb 22, 2017 3:58 pm

Bottom view
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Re: JLM 60Watt Power AMP Kit

Postby Joe Malone » Thu Feb 23, 2017 7:32 am

frazzman wrote:I'll test the bridge rectifier tonight.

In the interim, here's the post-Mortem.. Not pretty.

Obviously a lot of the top pads are ruined but the bottom pads have now cleaned up relatively well.

Given the state of the PCB - Is there a schematic or PCB overlay that I can use so I can measure continuity between each of the IC pads and whatever components they connect to ?

Thanks again


Drop me a email so I can send you PCB layers and schematic

Best way to remove these is cut all legs off and set your soldering iron to just above solder melting point and softly pull on each leg with pointy nose pliers while heating the solder pad for that leg.
Joe :-)

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frazzman
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Re: JLM 60Watt Power AMP Kit

Postby frazzman » Thu Feb 23, 2017 7:43 am

Thanks Joe, I just sent you an email

rtchau
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Re: JLM 60Watt Power AMP Kit

Postby rtchau » Mon Apr 10, 2017 8:44 am

Is there any way to prevent oscillation in the LM3886 with no input? I made the mistake this morning of switching on the amp without checking that I had a source plugged in... BRRRRT

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Re: JLM 60Watt Power AMP Kit

Postby Joe Malone » Mon Apr 10, 2017 11:19 am

rtchau wrote:Is there any way to prevent oscillation in the LM3886 with no input? I made the mistake this morning of switching on the amp without checking that I had a source plugged in... BRRRRT


Hi the standard AMP kit is completely stable with or without input.

So your setup must be customized. So you will have to remind me exactly how you have it setup so I can see how to make this stable for you with no input.
Joe :-)

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Re: JLM 60Watt Power AMP Kit

Postby rtchau » Mon Apr 10, 2017 1:13 pm

Joe Malone wrote:
rtchau wrote:Is there any way to prevent oscillation in the LM3886 with no input? I made the mistake this morning of switching on the amp without checking that I had a source plugged in... BRRRRT


Hi the standard AMP kit is completely stable with or without input.

So your setup must be customized. So you will have to remind me exactly how you have it setup so I can see how to make this stable for you with no input.


Hi Joe,

I'm running the unbalanced mod as per the instructions earlier in the build thread:
60w amp unbalanced high gain.jpg


There are no modifications other than that - I do remember you mentioning that the amp will motorboat with an unbalanced input if the gain is under 10.

If it should still be perfectly stable even with the unbalanced mod, then I might need to go through my wiring and work out what I've missed! The only other suggestion you had at the time was to float the 0v with a 10R 1W resistor to eliminate hum when using a computer as a source.

Cheers,

Richard
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Re: JLM 60Watt Power AMP Kit

Postby Joe Malone » Mon Apr 10, 2017 2:28 pm

rtchau wrote:
Joe Malone wrote:
rtchau wrote:Is there any way to prevent oscillation in the LM3886 with no input? I made the mistake this morning of switching on the amp without checking that I had a source plugged in... BRRRRT


Hi the standard AMP kit is completely stable with or without input.

So your setup must be customized. So you will have to remind me exactly how you have it setup so I can see how to make this stable for you with no input.


Hi Joe,

I'm running the unbalanced mod as per the instructions earlier in the build thread: 60w amp unbalanced high gain.jpg

There are no modifications other than that - I do remember you mentioning that the amp will motorboat with an unbalanced input if the gain is under 10.

If it should still be perfectly stable even with the unbalanced mod, then I might need to go through my wiring and work out what I've missed! The only other suggestion you had at the time was to float the 0v with a 10R 1W resistor to eliminate hum when using a computer as a source.

Cheers,

Richard


HI Richard
If the volume pot is turned fully down does the amp motorboat?
Joe :-)

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rtchau
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Re: JLM 60Watt Power AMP Kit

Postby rtchau » Sat Apr 15, 2017 7:53 pm

Joe Malone wrote:
HI Richard
If the volume pot is turned fully down does the amp motorboat?


Hi Joe,

No - it only seems to go bananas after the volume pot is turned up past 50%, prior to that it starts to hum after 10-20%, and nothing at 0%.

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