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Old 14th March 2007, 06:11 PM   #1
Mayday is offline Mayday  Sweden
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Question chipamp PSU question

I'm building a dual-mono LM3886 kit from chipamp.com.
I've gotten to the point of testing the PSU-boards.
Both LED litght up just fine.
Both positive sides =31,6V (both mono boards the same)
Both negative sides = 37,4V (both boards the same)

Is this correct? Is negative supply supposed to be higher than positive? If not, what could be wrong?

Using a 500VA 2x24V~ Toroid trafo and a Y-splitter on each secondary.

Help is extremely appreciated!
/Mayday
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Old 14th March 2007, 06:26 PM   #2
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Hej, mayday!

It's more interesting to know the voltage when you play a little since the LM3886 consumes rather little at idle conditions. So a bit unequal voltage is nothing to worry about.
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Old 14th March 2007, 06:30 PM   #3
Mayday is offline Mayday  Sweden
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Tjena P-A!

I haven't connected the amplifierboards yet, I have to wait until tomorrow when the last components arrive.

So this is measured at the outs on the psu-boards.

/Mayday
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Old 14th March 2007, 06:37 PM   #4
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In that case it's probaly OK. It can be a bit tricky to measure with 10 MOhms voltmeter on only caps with no load. For example leakage could make the difference which is totally irrelevant.
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Old 14th March 2007, 06:40 PM   #5
Mayday is offline Mayday  Sweden
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Ok, thanks!
I will blame any smoke tomorrow on you then
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Old 15th March 2007, 04:07 AM   #6
Nordic is offline Nordic  South Africa
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Unless you put exactly equal resistors on the rails to actualy split them in half, it has no reason to stay that way, as you now learned... the capacitors vary greatly from each other, so they have no way of precisely splitting the rails either...

A power supply could still be built to achieve this, but as you will soon learn even the bare neccesseties... a rectifier and some caps, sound very good.

With nothing connected you would probably find the source of the diffirence between the rails at the transformer not haveing exact same windings...(exactly centre tapped)... vary rare to find one that is and very expensive.
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Old 15th March 2007, 05:31 PM   #7
Mayday is offline Mayday  Sweden
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unfortunately there was smoke, from both psu-boards.
It was a diode on each board that burnt.
Second diode to the right on the second row on left board, and the second diode to the left on the second row on the right board.
Click the image to open in full size.
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Old 15th March 2007, 05:34 PM   #8
Mayday is offline Mayday  Sweden
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Only thing changed since they were working yesterday is that I've connected the amp boards to the psu boards, the chassi gnd and the inputs on the amp boards.

EDIT: the picture is an early one before amp boards were fully populated, it's just to show the diodes.

EDIT2: Before the diodes went up in smoke, I measured 300mV on the outputs on the boards.
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Old 15th March 2007, 05:59 PM   #9
AndrewT is offline AndrewT  Scotland
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Hi,
5.8Vdc difference between +ve and -ve on a split dual polarity supply is enormous.

I see dual rectifiers (8 diodes on each PSU board). Have you got separate secondaries?

Have you connected the 0v commons?
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Old 15th March 2007, 06:02 PM   #10
Nordic is offline Nordic  South Africa
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explain what you mean by Y splitter.. where is this placed in the circuit...draw it if you can.

Don't sweat the smoke... it happens all the time... just get right back up on the horse...

I suspect you may hve connected the 2 "centre" taps...

Connect a bedlamp in series with the primary of the transformer and put a low wattage bulb in, if there is a significant fault the bulb will shine brightly and limit the power to the amp to some mA...
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