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Old 31st January 2018, 06:21 PM   #1
Hogwild is offline Hogwild  Canada
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Default LM3886 - Noisy signal unless inputs shorted

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.
DS0000.BMP


Same board, (Linkwitz mods) with input leads not shorted or connected to anything.
DS0001.BMP


Board with stock components. Input leads shorted.
DS0002.BMP


Board with stock components. Input leads not shorted or connected to anything.
DS0003.BMP

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.

Last edited by Hogwild; 31st January 2018 at 10:42 PM.
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Old 31st January 2018, 09:59 PM   #2
adason is offline adason  United States
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Old 31st January 2018, 10:02 PM   #3
Hogwild is offline Hogwild  Canada
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I'm seeing them. What am I missing? Are you logged on, Adason?

Last edited by Hogwild; 31st January 2018 at 10:10 PM.
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Old 31st January 2018, 11:00 PM   #4
scottjoplin is offline scottjoplin  Wales
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Do you have a volume pot on the input? How much noise are you getting with it connected to a source?
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Old 31st January 2018, 11:04 PM   #5
Hogwild is offline Hogwild  Canada
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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.


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Do you have a volume pot on the input? How much noise are you getting with it connected to a source?
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Old 31st January 2018, 11:09 PM   #6
scottjoplin is offline scottjoplin  Wales
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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.
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Old 31st January 2018, 11:18 PM   #7
Hogwild is offline Hogwild  Canada
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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?


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Originally Posted by scottjoplin View Post
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.

Last edited by Hogwild; 31st January 2018 at 11:20 PM.
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Old 31st January 2018, 11:28 PM   #8
scottjoplin is offline scottjoplin  Wales
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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
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Old 31st January 2018, 11:54 PM   #9
Hogwild is offline Hogwild  Canada
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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?


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Originally Posted by scottjoplin View Post
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
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Old 1st February 2018, 12:12 AM   #10
scottjoplin is offline scottjoplin  Wales
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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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