LM3886 outputting few volts after shutdown? - diyAudio
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Old 22nd May 2013, 04:25 PM   #1
iggy111 is offline iggy111  Croatia
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Default LM3886 outputting few volts after shutdown?

After last tinkering with my chipamp.com lm3886 gc, something went wrong, don't know if I managed to short something, but this is what I get:

trying to measure DC offset (no source at input) I measured 0.0V on both channels, which was odd, but about 2sec after I powered down, the meter showed a burst at around 5V, and going slowly down to 0V after about 5sec, which means caps discharge.

Something is definitely wrong, and I didn't even want to connect my test speaker cause it would get fried.

Anyone have a clue what this behavior is showing?
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Old 22nd May 2013, 05:00 PM   #2
Mooly is offline Mooly  United Kingdom
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Try a reasonably low value load resistor and see what happens. Many amps can do strange things when rails collapse, particularly with nothing to draw any current. Try it with music and AC couple the speaker if you feel safer that way (use a bipolar cap).

Why is a DC offset of zero volts "odd" ?

There's really only the LM3886 itself that can easily fail unless the problem you had zapped a low value resistor or something.
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Old 22nd May 2013, 05:09 PM   #3
iggy111 is offline iggy111  Croatia
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DC offset of zero is odd because I never heard it being that low - at least a few mV is "normal", best I had with this amp is 30mV on this amp.

I will try a load resistor and check all the resistors on board.
I just hope the chips didn't die, cause it's worse to desolder them and put in new ones than buying a completely new kit...
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Old 22nd May 2013, 05:11 PM   #4
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Is your amp AC or DC coupled? What were you "tinkering" with when it went wrong? And I'm with Mooly on this, why is 0V offset odd? Did it have an offset voltage before that is now missing? If you could post a schematic of the circuit it might be helpful.

Mike
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Old 22nd May 2013, 05:47 PM   #5
iggy111 is offline iggy111  Croatia
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The amp is AC coupled, and even with a Ci (cap at the inverting input) had at least some mV of DC offset. Without the cap the offset was around 80mV.

I was kinda hoping someone would recognize this "symptom" and point me to the right direction, you know how it is when you spend a whole day soldering tiny bits and then you have to desolder everything again...

I'll do some measuring and post back with the schematic
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Old 22nd May 2013, 05:51 PM   #6
Mooly is offline Mooly  United Kingdom
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For a single sided PCB it should take under 30 seconds to cleanly desolder that chip. If its double sided with plated through vias then a couple of minutes if you sacrifice the IC and snip the leads
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Old 22nd May 2013, 06:08 PM   #7
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Another check it is to check the rails voltage when you powered down the amplifier. See if both discharge equally. My LM3886 with the input shorted the outputs read 1.9 mv.
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Old 22nd May 2013, 09:54 PM   #8
glenv6 is offline glenv6  United States
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Quote:
Originally Posted by iggy111 View Post
After last tinkering with my chipamp.com lm3886 gc, something went wrong, don't know if I managed to short something, but this is what I get:

trying to measure DC offset (no source at input) I measured 0.0V on both channels, which was odd, but about 2sec after I powered down, the meter showed a burst at around 5V, and going slowly down to 0V after about 5sec, which means caps discharge.

Something is definitely wrong, and I didn't even want to connect my test speaker cause it would get fried.

Anyone have a clue what this behavior is showing?
This post indicates that your 5V burst after shut down is normal...
Commercial Gainclone kit- building instructions
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Old 23rd May 2013, 11:08 AM   #9
iggy111 is offline iggy111  Croatia
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Ok, so I tested the amp with a source, and a resistor at the output - the 5V burst disappeared, so thanks Mooly for pointing that out.

The problem is my left channel, which is way quieter when playing music than the right, and I hear some very slight buzzing sound from the speaker.

I checked all the resistors and they are fine, but I checked the chip with my very crude method - I measure resistances between pins, and compare them to the healthy channel.
All things being equal the resistances should match, but between pin1 and pin3 I get a fixed 1040R on left channel, and changing resistance (pointing to a cap) on the healthy right channel.

I will check the caps later when I get a multimeter that can measure capacitance, but I'm pretty sure it's the chip.
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Old 23rd May 2013, 11:11 AM   #10
AndrewT is offline AndrewT  Scotland
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I find that in a two channel amplifier where the grounding is compromised by not being monoblock that one channel performs differently than the other.
Compare the wiring of the noisier channel to the quieter channel.
You might be able to spot a quite subtle difference in the wiring that could explain the noise difference.
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