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-   -   LM3886 - Noisy signal unless inputs shorted (https://www.diyaudio.com/forums/chipamp/318235-lm3886-noisy-signal-unless-inputs-shorted.html)

Hogwild 31st January 2018 07:21 PM

LM3886 - Noisy signal unless inputs shorted
 
4 Attachment(s)
Okay, me again. Near-absolute beginner. Knowledge limited to mostly soldering and basic DMM / scope usage.

I now have two Chipamp.com LM3886 boards with stock resistors/gain and two I've modified with Linkwitz' values
after discussion here in another thread.

My question is: On both sets of boards, I get a lot of (noise?) on the scope unless the input leads are shorted. This seems to happen whether or not they are attached to an audio source. Is this normal?

Here is one board with Linkwitz values of resistors swapped in.
(I haven't yet swapped Rm, the Mute resistor with the Linkwitz value one yet). Input leads shorted.
Attachment 659856


Same board, (Linkwitz mods) with input leads not shorted or connected to anything.
Attachment 659857


Board with stock components. Input leads shorted.
Attachment 659858


Board with stock components. Input leads not shorted or connected to anything.
Attachment 659859

I forgot to put the scope into the measurement screen when taking the first
screenshots. The PtoP voltage on the flatter signal on that board is about 2.2 to 2.4 mV .

Nothing is connected to earth/ground. I wasn't sure how to do that for basic testing.
Without meaning to sound ungrateful, last time I asked about basic grounding connections, the discussion
devolved into a debate about the perfect method, and I never got an answer I could understand at
my level, so I just let it go.

In that earlier thread, coaxial cable was recommended for input leads, but using it just seemed to
make things worse.


Thanks.

adason 31st January 2018 10:59 PM

I see no images...

Hogwild 31st January 2018 11:02 PM

I'm seeing them. What am I missing? Are you logged on, Adason?

scottjoplin 1st February 2018 12:00 AM

Do you have a volume pot on the input? How much noise are you getting with it connected to a source?

Hogwild 1st February 2018 12:04 AM

No volume pot.

I can't remember the measurements. I tried with a Sony CD player directly connected. I could put it back on the scope if you can suggest what might be a good test. I have an audio test CD here, or I could download and burn something, if really necessary.


Quote:

Originally Posted by scottjoplin (https://www.diyaudio.com/forums/chipamp/318235-lm3886-noisy-signal-unless-inputs-shorted-post5326879.html#post5326879)
Do you have a volume pot on the input? How much noise are you getting with it connected to a source?


scottjoplin 1st February 2018 12:09 AM

Do you have it connected to a preamp? You need to attenuate the input, the noise on your scope is whatever is being picked up on the input amplified to the maximum.

Hogwild 1st February 2018 12:18 AM

Well, I was using a little test jig that a friend had created for me. RCA inputs on one end, and terminals on the other to connect to the amp inputs. This jig has a POT volume control, and it worked fine. Later, it seemed like there was a lot of noise on the scope, even with the jig (and its POT). Can you suggest some tests to run?

Would it be better to use a smartphone or something as a source? They have lower output voltage, don't they?


Quote:

Originally Posted by scottjoplin (https://www.diyaudio.com/forums/chipamp/318235-lm3886-noisy-signal-unless-inputs-shorted-post5326892.html#post5326892)
Do you have it connected to a preamp? You need to attenuate the input, the noise on your scope is whatever is being picked up on the input amplified to the maximum.


scottjoplin 1st February 2018 12:28 AM

You have proved that the amp is not making the noise by shorting the inputs. You really need to attach a volume control permanently on the input before you go any further and definitely before you connect a speaker. A 10K log pot would be ideal. Also before you connect a speaker check for any DC offset on the output

Hogwild 1st February 2018 12:54 AM

Which signal do I use to measure DC offset? I'll do that and post the pics back here. Oh and..um...does it matter for what I just posted above whether the scope is set to DC or AC coupling mode?


Quote:

Originally Posted by scottjoplin (https://www.diyaudio.com/forums/chipamp/318235-lm3886-noisy-signal-unless-inputs-shorted-post5326912.html#post5326912)
You have proved that the amp is not making the noise by shorting the inputs. You really need to attach a volume control permanently on the input before you go any further and definitely before you connect a speaker. A 10K log pot would be ideal. Also before you connect a speaker check for any DC offset on the output


scottjoplin 1st February 2018 01:12 AM

DC offset is the voltage measured across the speaker terminals with no input. Ideally it will be zero, but is more likely to be around +/- 10mV to +/- 50mV. Use a multimeter if you have one or your scope set to dc coupling, for your noise measurements ac coupling is fine


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