JLM 60Watt Power AMP Kit
Moderator: Joe Malone
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Re: JLM 60Watt Power AMP Kit
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.
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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- Joe Malone
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Re: JLM 60Watt Power AMP Kit
The expected +/-voltages from 2 x 18vac are +/-25vdc (not 19vdc)? By Regulator do you mean diode bridge?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 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.My initial thoughts were that the chips were shorting against the heatsinks, but that does not appear to be the case.
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 lm3886I 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.
Joe
JLM Audio
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Re: JLM 60Watt Power AMP Kit
Hi Joe,Joe Malone wrote:The expected +/-voltages from 2 x 18vac are +/-25vdc (not 19vdc)? By Regulator do you mean diode bridge?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 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.My initial thoughts were that the chips were shorting against the heatsinks, but that does not appear to be the case.
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 lm3886I 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.
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
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.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.
Ok this is a fake reading from your meter because you have no capacitance to store the dc after the bridge.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.
The bridge would only be damaged after a high current surge like the short that seems to have happen to those ic legs.
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
What value mains fuse did you have in line with the power transformer and did it blow?
Joe
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Re: JLM 60Watt Power AMP Kit
Of course... It's obvious now that those DMM readings were invalid with out any capacitance.Joe Malone wrote: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.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.
Ok this is a fake reading from your meter because you have no capacitance to store the dc after the bridge.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.
The bridge would only be damaged after a high current surge like the short that seems to have happen to those ic legs.
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
What value mains fuse did you have in line with the power transformer and did it blow?
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
160VA should only need 1A slow blowI had a 2A slow blow fuse in there, it did not 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.
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.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.
Joe
JLM Audio
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Re: JLM 60Watt Power AMP Kit
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
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
Bottom view
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- Joe Malone
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Re: JLM 60Watt Power AMP Kit
Drop me a email so I can send you PCB layers and schematicfrazzman 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
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
JLM Audio
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Re: JLM 60Watt Power AMP Kit
Thanks Joe, I just sent you an email
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Re: JLM 60Watt Power AMP Kit
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
Hi the standard AMP kit is completely stable with or without input.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
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
Hi Joe,Joe Malone wrote:Hi the standard AMP kit is completely stable with or without input.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
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.
I'm running the unbalanced mod as per the instructions earlier in the build thread: 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
HI Richardrtchau wrote:Hi Joe,Joe Malone wrote:Hi the standard AMP kit is completely stable with or without input.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
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.
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
If the volume pot is turned fully down does the amp motorboat?
Joe
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Re: JLM 60Watt Power AMP Kit
Hi Joe,Joe Malone wrote:
HI Richard
If the volume pot is turned fully down does the amp motorboat?
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%.