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Old 31st March 2008, 07:52 PM   #1
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Default Strange sounds when power down gainclone

Hi all,

I am getting a very strange sound on one speaker channel of my stereo lm4780 when I power it down. It almost sounds like a ripping sound that is drawn out for about a second. I reconfigured my ground setup to a better star ground setup but am unsure if the strange sound was happening before or after i did that. Anyone know what is causing this and how I can fix it??

Thanks
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Old 1st April 2008, 01:30 AM   #2
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anybody??
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Old 1st April 2008, 03:15 AM   #3
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Hello synthius, if I understand your description correctly, it sounds kind of like capacitor discharge. I would think you would hear it in both channels. If the amp doesn't have any output muting, then when the amp is powered down, the caps discharge and cause that sort of sound in the speakers. My Adcom GFA-6000 does that when I shut it off. It's not something to worry about unless it gets really powerful. Otherwise it's just a nuisance. Remember, I said if I understand, I'm sure that others with more experience in amp design in general and chip amp in particular will offer other thoughts.

Peace,

Dave
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Old 1st April 2008, 06:44 AM   #4
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Thanks for the response dave. As long as its not something that will damage my speakers I guess i can forget about it. It would be nice to know if there was a resolution as this is my senior project and I don't want the professors to hear that sound when i turn off my amp at the end.
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Old 1st April 2008, 08:59 PM   #5
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Hello Synthius, Since this is a senior project, you should take some time to try and diagnose the source of the noise. That will look real good to the professor evaluating you for a grade. A start would be to get an o'scope and hook one channel to the output of your amp, in parallel with your speaker. Hook other channel(s) to the power supply cap(s) and watch both signals when you turn off the amp. This will give some idea if I am correct or talking gibberish. A 4 channel digital storage scope would really be good here. It is somewhat interesting that only one channel in your amp does this. Do you have separate filter caps for each channel?

Peace,

Dave

P.S. The reason I suggest doing actual measurements is that it shows you have good troubleshooting skills and can reason things through. A lot of teachers will give you a better grade for demonstrating that you know how to use what you learned, not just take tests.
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Old 1st April 2008, 10:06 PM   #6
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Do you have a schematic of your amp?
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Old 1st April 2008, 10:13 PM   #7
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yes

Stereo amp is IC 1 which is causing the strange sound

Click the image to open in full size.
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Old 1st April 2008, 10:47 PM   #8
BWRX is offline BWRX  United States
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Try putting some resistors from input to ground on each channel of IC1. Something on the order of 20k-50k should work fine.
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Old 1st April 2008, 10:53 PM   #9
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hey guys,

The strange sound is actually coming from both the left and right channel of the lm4780.

I will try putting the resistors from input to ground and get back to you on the results

Also, can someone explain the purpose of RB1 and RB2 on IC1? Are they used to set the input impedance to ~1kohm?


Thanks for the help
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Old 1st April 2008, 11:07 PM   #10
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the 'typical application' shows only a rudimentary circuit.
Better to use the 'auxiliary amplifier application circuit' for a standalone amplifier.
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