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Old 29th December 2005, 08:00 AM   #1
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Default LM4835 Help Please!

Hi,
I am working on an upgrade for my portable PS2 (see original at www.freewebs.com/segasonicfan) however, I am having an audio problem I can't seem to fix and this is the only place I know of that can help...

The PSOne LCD Screen PCB has an integrated surface mount LM4835 chip amp. This chip produces 2W total, has headphone switch and output and it also has some other digital features.

The problem I'm having is whenever bass hits the speakers the display changes horziontal sync for a moment...kind of blinks along to the music. This doesn't go away unless I lower the volume to almost nothing (below 1W).

I found a partial fix when I took out the 7805 used to drive the amp and severed a trace from the regulator output to an LM1881 sync seperator. I then used a 5v AC adapter and I though my problems were solved.

However, I've now begun powering the system from a single source and so I used a high current (3A, LT1085) voltage reg to take the 8.5v input down to 5v. Then the problem showed up again >

I tried regulating the LT1085 every way I knew...3K resistors to ground, 10k resistors to ground for the audio lines, 1000uf caps, 22uf tantalum cap (required for stability it says on the datasheet). It seems to me it might be problem in the ground line.

Can anyone give me any advice on this problem? I would *really* appreciate it. This is one of the final obstacles I'm trying to overcome and I just don't think I can do it by myself. If anyone would like to see a video of the problem I would be happy to upload one if that would help. Thanks in advance!!

-Segasonicfan
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Old 29th December 2005, 01:48 PM   #2
lineup is offline lineup  Sweden
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Looks like you get some spikes in power supply rails.
Digital and analog ground rails/planes are best kept separate.
And only joined at one point in the supply.
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Old 30th December 2005, 01:19 AM   #3
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The LCD board combines them though...so you're saying I should seperate the amps ground from everything else and just join it to one point on the supply? That would take some time but I guess I can do that...any other suggestions? Thanks for your help,

-Segasonicfan
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Old 30th December 2005, 06:03 AM   #4
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maybe it would be better if you give the amp its own regulator with a fairly big capacitor as well.
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Old 30th December 2005, 11:16 AM   #5
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I DID give the amp its own regulator with a big capacitor....didn't help As long as the same supply is going to the system/screen and reg->amp the problem persists. I've noticed however the composite signal doesn't get as distorted from the bass.

Separating it from all other grounds is going to be *really* hard tho...it's surrounded by a bunch of fragile traces and that would take hours. I'll probably end up doing that anyway but I'm still hoping to find another solution..

-Segasonicfan
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Old 30th December 2005, 02:35 PM   #6
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Well...I spent the past 4 hours cutting and drilling the ground rails on the PCB and.............

IT WORKED!!! Thank you so much lineup! I really owe you one Members on the Benheck forums and I have been trying to solve this problem for a long time...it was both a ground and supply rail problem. I separated the 5v supply but the ground connection was still there. Oddly enough Sony tried to isolate the grounding around the chip a little bit, but only in one area.

Moderator, you can close this topic. Thanks again lineup

-Segasonicfan
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