LM3886 with full negative rail on the output?

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ThyDntWntMusic

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Joined 2008
#1
Am meant to be repairing a PA, active speaker.

It has three LM3886's in parallel driving a 3ohm 15inch midwoofer, and a single 3886 to run the 8ohm tweeter.

Now, the tweeter had failed.

I figured it was abuse, till I pulled the tweeter apart, and found it had literally POPPED!

Which is when I found -31volts of dc offset on the tweeter amplifier...

Thing is though, the preamp stage is on a separate board, and I was initially testing with this disconnected, and all four 3886s got HOT within seconds...

With the board reconnected, the three bass amps are cool, while the tweeter amp gets to over 70 degrees within a few seconds.

I also now have -38volts on the tweeter output 😱

But why? Sure, one could assume that the 3886 had simply failed... but I've been asked to fix TWO speakers, with non functional tweeters, and have been told of a third...

So, what's going on? How/why would the 3886 be doing this?
 
#3
#4
Maybe the reliability solution is to add a capacitor between the Treble amplifier and the Treble driver.
This blocks DC to prevent damage to the tweeter.
It also adds a single pole LF attenuator (high pass filter). This also protects the tweeter from excessive input LF voltage.

There are no input RF attenuators on any of the four 3886 amplifiers.
There are no input DC blocking capacitors on any of the four 3886 amplifiers.
Reliability is improved if these two filters are added to every power amplifier.
 
#5
The second speaker I've pulled apart doesn't have massive dc offset on the tweeter, however, I only measure about 6volts rms on the tweeter amplifier...

Now, 6volts doesn't seem much, but maybe that's all it ever had, to get about 111dB from the 105db 1w/1m tweeter...
 
#6
111dB @ 1m from a 105dB/W @ 1m requires 6dB more than 1W.
+3dB = 2W and +6dB = 4W

4W into 8r0 ~ 5.6Vac (= twice the voltage compared to 1W)

Have you worked out what frequencies are sent to the treble amplifier?
From there you can work out what DC blocking capacitor can be added to the amplifier input and to the amplifier output such that the caps do not attenuate any more than 0.1dB from the passed frequencies.
 
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#7
See, the first tweeter looked like this, and thats when I tested for dc offset on the output...

mxi7i.jpg


syr1.jpg


But then I got to the second tweeter, and found this?!

axuw7.jpg


However, the amplifier that was running it didn't have any noticeable dc offset... I did however only measure 6volts rms from it... 🙁
 
#11
8 schematics listed in the PDF.

D, E, F, G, H1, H2, I, and J...

But that means there's an A, B, and C that never got released...
And if that's not bad enough, one amp I have is a Rev K...

12 schematic revisions, and its still broken?!? for an amplifier IC that requires all of 5 external components to work?!

Most of the changes seem to be for the limiter circuit, possibly for using different tweeters... which may explain why the RevK only seems to do 6volts rms (possibly was meant to use a more sensitive tweeter) but had the treble boosted on the "eq" for some reason 😉
 
#12
I swapped the LM3886 in the bad board, its a rev G in the zip file on the previous page...

The amplifier, with a load, appears to limit itself to 4volts rms max....

One board is a rev G, the other is a J, the only difference appears to be one resistor on the voltage divider for the power limiter feedback circuit....
 
#13
In addition to the aforementioned deficiencies, the power supply decoupling caps are too small, generally, 470uF is the smallest that should be used there. There is no output Zobel. A 100nF film cap in series with a 2.7ohm 1W resistor should be connected as close as possible to the IC output pin, and the other end connected to a low impedance ground is what generally works well there.

Mike
 
#15
I replaced the shorted 3886, and now it seems to work.

The other amplifier, (the one that blew a hole through the tweeter) has a weird, random electrical crackle through both drivers, and may have something to do with the tweeter failure...

Both have 50Hz hum... mmm... sq... lmao.

And, I have concluded that the ~4volts rms is all the original 105+dB tweeter was getting....
 
#17
tomchr said:
Does the LM3886 provide a reasonable output if you provide it with a reasonable input? What happens if you ground the input to the LM3886? Do you get 0 V on the output?

~Tom
Click to expand...

not from the one with the negative rail shorted to the output 😛

I'm not 100% sure whats going on, but as the limiter circuit is monitoring both the woofer and tweeter amps, I think not having a woofer connected effected the rms voltage from the tweeter amp...

with a 3ohm dummy load, I ended up with 10volts rms! about 12watts into the 8ohm tweeter...

but this still raises the question... how did the tweeter end up with its guts inside out... (on the amplifier that didn't go shorted to the negative rail)
 
#18
One amp has constant hum...
I assumed the main filter caps were bad, but they measure ok (esr and capacitance)

The other amp, snaps, crackles and pops, while buzzing, and sounds like it would cook a tweeter pretty quickly...

And then it stops... no buzz, no crackles (while still outputting audio that is)
and then, it starts buzzing and crackling again at random...

hmm
 
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