LM3886 with full negative rail on the output? - diyAudio
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Old 31st May 2014, 05:20 AM   #1
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Default LM3886 with full negative rail on the output?

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?
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Old 31st May 2014, 06:32 AM   #2
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More probing has revealed 36volts on the audio input of the "bad" amplifier IC...

edit: this is only because there's a 20k resistor linking the output to the input

Last edited by ThyDntWntMusic; 31st May 2014 at 06:43 AM.
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Old 31st May 2014, 07:55 AM   #3
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This is in a Behringer B300 pa speaker, schematic can be found here:

BEHRINGER B300 Service Manual free download, schematics, eeprom, repair info for electronics

What I have now decided, is that the tweeter failed and took out the amplifier...
The tweeter in the other box is blown, but no dc offset from the amplifier

I'm going to connect some random, crappy tweeter to the second pa box, and see what happens before I source new tweeters.
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Old 31st May 2014, 09:33 AM   #4
AndrewT is offline AndrewT  Scotland
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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.
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Old 31st May 2014, 09:41 AM   #5
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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...
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Old 31st May 2014, 10:12 AM   #6
AndrewT is offline AndrewT  Scotland
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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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Last edited by AndrewT; 31st May 2014 at 10:19 AM.
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Old 31st May 2014, 10:13 AM   #7
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See, the first tweeter looked like this, and thats when I tested for dc offset on the output...

Click the image to open in full size.

Click the image to open in full size.

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

Click the image to open in full size.

However, the amplifier that was running it didn't have any noticeable dc offset... I did however only measure 6volts rms from it...
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Old 31st May 2014, 10:16 AM   #8
AndrewT is offline AndrewT  Scotland
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No pics attached. Should there be some?
Quote:
I did however only measure 6volts rms
do you have an rms reading voltmeter?
What frequncies are in that 6Vac signal? Is your voltmeter accurate at those frequencies?
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Old 31st May 2014, 10:20 AM   #9
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3-5khz, on a scope, and on a multimeter (that measured slightly lower, as I don't believe it's rated for those kind of frequencies)
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Old 31st May 2014, 10:21 PM   #10
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I think a lil bit more poking around is required.... there's eight different schematics listed for this amp?!?

http://dl.dropbox.com/u/35219914/-Au...GER%20B300.zip
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