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Old 10th January 2011, 03:34 AM   #1
Dr Zeus is offline Dr Zeus  United States
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Default Crossfire XP2400D

Hey. This amp powers and produces sound output (+-125 rail voltage); however theres a high frequency pitched wheezing sound coming off the speaker terminals. I'm really not sure where to look so I looked at a lot of things to start. This board is half double sided on some of the bigger components however the solder job wasnt the best. I went ahead a re-flowed al the large transistors, caps, and coils the best I could but it didnt help. Grounding the RCA shields changes the noise just slightly but does not make it go away. None of the crossover controls help or change the noise too much.

One thing I noticed is that the opAmps are only getting -7.5vDC and 10.5vDC. Is that enough voltage?

Last edited by Dr Zeus; 10th January 2011 at 03:37 AM.
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Old 10th January 2011, 03:41 AM   #2
Dr Zeus is offline Dr Zeus  United States
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Heres what the amp is:

Click the image to open in full size.

Click the image to open in full size.

Here is what comes out of the speaker terminals at all times: .1v/Div

Click the image to open in full size.

Next thing I noticed is that at higher volume the signal starts to distort badly like theres a bad driver.

Also, this unmarked 24 pin IC gets pretty hot after about 30 seconds. I measured 150F.
Click the image to open in full size.
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Old 10th January 2011, 07:51 AM   #3
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With no load, and your scope set to 10v/div, drive a sine wave into it that will make the trace deflect 3 major divisions. Set the timebase so that 2-4 full cycles are displayed.
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Old 10th January 2011, 12:50 PM   #4
Dr Zeus is offline Dr Zeus  United States
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Heres the output with no load, scope set to 10v/div with a sine wave driving up 3 divisions.
Click the image to open in full size.
Same:
Click the image to open in full size.

This probably tells you there is nothing wrong thus far. With a load the output sounds pretty terrible and its hard to see with a still image on the scope, so heres a quick 20 second video. This is with approximately 1-2v/Div, and I adjust things a little throughout. The frequency source is ~40-60hz, and throughout the whole time the test speaker crackles. Theres very little audible sound in this video.

jl audio :: VID_20110110_084612.mp4 video by unclemeat2010 - Photobucket
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Old 10th January 2011, 12:55 PM   #5
Dr Zeus is offline Dr Zeus  United States
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Also wanted to mention, this amp blew a couple of its owner's good speakers.

I'm really not too sure though. Its kind of like chasing a rabbit's tail

Last edited by Dr Zeus; 10th January 2011 at 01:01 PM.
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Old 10th January 2011, 01:59 PM   #6
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Are you sure that the speaker isn't damaged? I don't see anything there that would make a crackling sound.

I'm assuming the video was with a speaker load.
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Old 10th January 2011, 02:51 PM   #7
Dr Zeus is offline Dr Zeus  United States
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Like I said its like chasing a rabbit's tail. I changed out the test speaker and results are similar. The speaker is playing a high frequency hissing/whining and the scope shows what is in the second post. The hissing reminds me of an HVAC system inside of an airplane while sitting at the terminal.

I'm thinking one of the opamps might be dirty, then again they are only getting -7.5vDC and 10.5vDC
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Old 10th January 2011, 03:03 PM   #8
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You didn't state the timebase for the photo showing the noise but I'm assuming it's about 10uS. If so, that's normal for a lot of class D amps.

I think you first have to identify the static/noise. Then you can track it down.
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Old 10th January 2011, 03:12 PM   #9
Dr Zeus is offline Dr Zeus  United States
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Yes that was .1v/div at about 10uS on the scope.

This is something a little more promenent. Referencing amp ground, I get the following at both the positive and negative speaker output terminals with no source or load attached. .5v/div and 10uS

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Old 10th January 2011, 03:14 PM   #10
Dr Zeus is offline Dr Zeus  United States
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Also getting -1.5v dc on all speaker terminals referencing amp's ground.
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