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Old 13th March 2013, 04:44 PM   #1
shawnk is offline shawnk  United States
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Default MTX 81000D

Hey guys.

I've got this amp up and running again. The only isssue at this point is some audible noise (scratchy/screechy)

It's not terribly loud, but is audible.

Here is what the output looks like even with no signal on the inputs

Scope= 0.2v/div

Click the image to open in full size.

After a search, I don't assume anyone has a schematic?
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Old 14th March 2013, 11:12 PM   #2
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Location: Louisiana
What was the timebase setting?

Is the noise produced through the speaker?

Does it make the noise with no RCAs plugged in?
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Old 14th March 2013, 11:32 PM   #3
shawnk is offline shawnk  United States
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Hi Perry,

Time base was 20us

Yes, the noise is audible through speakers, and it is audible even without RCA cables plugged in

Thanks!
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Old 15th March 2013, 02:08 AM   #4
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Location: Louisiana
That appears to be well above what would be audible.

Set the timebase to 2ms and the vertical amp to 5v/div and post a photo.

What happens if you raise or lower the 12v power supply voltage?
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Old 15th March 2013, 04:12 AM   #5
shawnk is offline shawnk  United States
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The time base setting seemed odd to me as well, but I simply adjusted it until I could see the trace lock onto the signal. I'll try the 2ms in the morning again.

My power supply has a fixed voltage output.

Thanks
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Old 15th March 2013, 03:01 PM   #6
shawnk is offline shawnk  United States
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OK, here are the results of the new measurements:

2ms @ 5v/div
Click the image to open in full size.

2ms @ 0.5v/div
Click the image to open in full size.
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Old 16th March 2013, 12:07 AM   #7
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The noise that you see is mainly unfiltered carrier noise. It's common and inaudible.

Does the noise change in any way when you operate all of the various pots and switches on the amp?

Are you using a subwoofer or a full range speaker for testing?
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Old 16th March 2013, 12:13 AM   #8
shawnk is offline shawnk  United States
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Hi Perry,

I am using some bookshelf speaks.

The only pot that has any effect on the audible pitch is the gain pot. That being said, it's only barely a noticeable difference.

Thanks
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Old 16th March 2013, 12:18 AM   #9
shawnk is offline shawnk  United States
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I might be nitpicking here. It's certainly not anything one would here with subs mounted in the rear of a vehicle. This is the first time I've ever played with this specific amp so I'm not sure if its a common abnormality. It just didn't sound right and seemed like a further investigation was needed.
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Old 16th March 2013, 12:20 AM   #10
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Location: Louisiana
Class D amps (especially older class D amps or class D amps that don't have synchronized oscillators) are noisy. If you wouldn't hear the noise when it's finally installed, it's probably not anything that you need to be worried about.
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