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Old 23rd February 2012, 02:03 AM   #1
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Default LM3886 Chipamp problems...

Ironic how I just made a thread saying how I was done with the amp. Obviously my amp finds it humorous to f**k me over at the most inopportune times.

I have a Yuanjing LM3886 x2 (+ Op amp preamp) board and a Jim's Audio LM4780 board (running in parallel for driving a subwoofer).

So I'll describe the sequence of events.

I finished my Sophomores speakers today (Wolf @ Parts Express). Hooked em up to the amp. and then hooked up my phone (for the music source). I've been using my phone for testing in the past. Both the phone and my amp were working fine. No issues.

I don't get any sound besides a really annoying buzz and then it shuts off. The only thing that would shut off the entire amp is a blown fuse. Checked the fuse. It's blown. Just to make sure, I throw another fuse in there. Blows it again.

I put a THIRD fuse in there and suddenly I have audio. (WTF?). I changed to my iPod for music source. So one would think- "It's your phone idiot", but I'm not so sure. I hooked up headphones to my phone and it drives them alright. However, it's kind of funny that my iPod suddenly makes the amp work...

Well I thought it was working...The left channel on the LM3886 board is very loud. Perfect by my ears. The subwoofer amp is working fine. The right channel does produce audio... however it is significantly quieter than the left channel and there is very little bass response. Mostly midrange and some treble.

Immediately I suspected faulty connections. I used the continuity function on my multimeter and I see no such bad connections. I also physically tightened down some of the connections.

The only things that I have discerned might be an issue are:

1.) Bad potentiometer (It seems that I get a "whirling/woosh" sound while rotating the knob sometimes).
2.) LM3886 dying
3.) Op-amp dying
4.) Bad solder joint (Though I was very meticulous with my soldering).


I know it's VERY difficult to troubleshoot something you aren't familiar with and have not actually seen, but perhaps someone has some pointers

Thank you.

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Last edited by FenderBender11; 23rd February 2012 at 02:08 AM.
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Old 23rd February 2012, 02:57 AM   #2
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Update- I decided to read the resistances on the potentiometers because that's the only thing I can really check right now. I have two pots for L and for R.

They are supposed to be 100k pots. The channel that works fine reads about 80k but the bad channel's potentiometer reads around 20k. Yes, these are the max resistances- probing pins 1 and 3. I'm not sure if these resistances are affected by things downstream, but 20k is definitely off.

It still does adjust resistance okay. I'm not completely sure. Perhaps I should try replacing that...Would a decrease in resistance value make it softer?

However, I'm still open to suggestions.
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Old 23rd February 2012, 03:08 AM   #3
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Quote:
Originally Posted by FenderBender11 View Post
Update- I decided to read the resistances on the potentiometers because that's the only thing I can really check right now. I have two pots for L and for R.

They are supposed to be 100k pots. The channel that works fine reads about 80k but the bad channel's potentiometer reads around 20k. Yes, these are the max resistances- probing pins 1 and 3. I'm not sure if these resistances are affected by things downstream, but 20k is definitely off.

It still does adjust resistance okay. I'm not completely sure. Perhaps I should try replacing that...Would a decrease in resistance value make it softer?

However, I'm still open to suggestions.
Nope, I do not think it is the potentiomers that are causing blowing the fuses.
When you read the potentiometers, Do you read them with source connected to them?
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Old 23rd February 2012, 03:11 AM   #4
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Can you jumper across both pot wipers and use the volume on your phone to test if the channels are then more of less equal?
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Old 23rd February 2012, 03:15 AM   #5
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Originally Posted by lanchile View Post
Nope, I do not think it is the potentiomers that are causing blowing the fuses.
When you read the potentiometers, Do you read them with source connected to them?
Yes. They were still wired.
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Old 23rd February 2012, 03:16 AM   #6
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Originally Posted by sofaspud View Post
Can you jumper across both pot wipers and use the volume on your phone to test if the channels are then more of less equal?
Sure, first thing tomorrow. However, I tried using my phone after the fact and I still didn't get sound...though the iPod worked..hm
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Old 23rd February 2012, 03:21 AM   #7
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Quote:
Originally Posted by FenderBender11 View Post
Yes. They were still wired.
Read them "without" inputs source connected.
The fuse that blows is it the main fuse or the outputs?
is your chassis made of wood, I asked this because I see the ground wire is connected to chassis and in this case it will be pointless in a wooden chassis
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Old 23rd February 2012, 03:27 AM   #8
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Originally Posted by lanchile View Post
Read them "without" inputs source connected.
The fuse that blows is it the main fuse or the outputs?
is your chassis made of wood, I asked this because I see the ground wire is connected to chassis and in this case it will be pointless in a wooden chassis
Oh yeah the ground thing. I was going to ground the heatsinks to that bolt but I never got to it...yes it is entirely wood.

The fuse is the main fuse. I think it was 4A slow blow.

I'm planning on desoldering the pots tomorrow when I get the chance. Try to get a reading.

I don't know why they would be so much different though. Perhaps that's the problem. Something is wrong on the board, not necessarily the pots...

Great.

Since I bought this board as a kit and I soldered it about...6 months ago, I don't know much about the LM3886 operation. Any test points that I could check?

Still unexplained fuse blowing...and then suddenly it works (partially).

Thanks.

Last edited by FenderBender11; 23rd February 2012 at 03:30 AM.
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Old 23rd February 2012, 03:29 AM   #9
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My bad... said phone when I should've said ipod. There's another recent thread concerning "works with this, not with that" inputs. I've no idea of an explanation.
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Old 23rd February 2012, 03:34 AM   #10
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Well, Maybe it can be inrush voltage. Did you check the blowing fuses were slow blow type too?
Disconnect the The right channel and try again! it can be problems with that channel shorting out fuse.
Check all connections all over again.

Last edited by lanchile; 23rd February 2012 at 03:39 AM.
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