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LM3886 - Noisy signal unless inputs shorted

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scottjoplin

scottjoplin

Banned
Joined 2015
You're welcome. Disconnect the green wire. The signal flows from the source to the pot as you have done, from the middle pot terminal to the IN connection on the board, then to the amplifier chip. It returns from the SG terminal back to the pot "earth" terminal (as you have marked it) and then back to the source. This completes the circuit from source to amp.
 
H

Hogwild

Member
Joined 2010
Still humming and no sound. However, a couple of things I just noticed:

1. My fluorescent magnifying lamp was adding a ton of noise.
(Like I said, I'm new to this). I will have to remember to always unplug it before testing.

2. This was not actually one of the boards I was intending to test right now. But since it was working (measurement and sound-wise) before I attached the POT, I still need to know why it's not working now.

I desoldered the green wire from the POT. I left it soldered at the Chassis Ground end on the amp board.

There is still hum and no music. It sounds like ground hum, but I'm too new to know. I assume the green wire connected only to CHGND is not the problem?

Here are a few scope pics to show what the waveform looks like now. IIRC, this was with the POT turned about half way.


View attachment DS0015.BMP



View attachment DS0016.BMP

If I touch the POT or the output wire from the RCA audio source, the buzzing gets louder.
 
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H

Hogwild

Member
Joined 2010
Sorry, my battery died. Then, by the time I could charge it, I hadn't noticed that the cables had shifted slightly and I was once again running out of light. If you zoom in on the full size photo, you can see everything quite well.

So everything is still the way it was, excepted I desoldered the green wire from the POT end. It's still attached to the CHGND on the PCB end.

DSCN2472.jpg
 
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H

Hogwild

Member
Joined 2010
Scott:

Which grounds? I need you to be more specific. Again, complete newbie. Please, spell it out for me.

Also, I started looking at breakout PCBs for POTs. The ones I'm seeing on ebay don't seem to have screwholes for mounting. Don't they usually have screwholes?
 
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H

Hogwild

Member
Joined 2010
Well, this is embarrassing. It turns out there was no sound cause I hadn't noticed there was a second set of RCA cables attached to the CD player, and the first set I was using were attached to nothing!

Now that the sound is back, I notice that the advice you all gave me has reduced hum to basically zero. I listened to some music, adjusted the POT and it's effect seems to be very smooth. I put a test CD in the player and played a 100Hz square wave test signal.

Here's what I got. Is something wrong? The waveforms look a little rounded off to me.

View attachment DS0017.BMP


View attachment DS0018.BMP
 
Bill_P

Bill_P

Member
Joined 2009
Nothing is wrong - reproducing a low frequency square wave with no discernible tilt requires exceptional low frequency bandwidth. The waveform might look more square at 1kHz. In any case there is nothing wrong with your waveform.

It seems you have solved all the outstanding problems, so put the amplifier in a case and enjoy the music.
 
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