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Old 7th April 2009, 07:44 PM   #1
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Default Grounding chipamp 3875

I need help with connecting my chipamp-kit, LM3875. The problem i have is that my volume pot is not working. On no volume, i still hear normal level and if i raise volume it goes up very quickly.

I have a chipamp.com kit with a CT Transformer. I have enclosed a quick drawing of my setup. Since i have a wodden chassi, i have serial connected all input rca grounds and runs them pass the rotary switch directly to SG.

Any help for a newbie?

Please excuse my simple drawing...
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Old 8th April 2009, 10:47 AM   #2
AndrewT is offline AndrewT  Scotland
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I need more info.
can you colour or or identify the signal lines differently from the return lines?
Can you add on the other ground/return lines connected to the amp PCB?
Can you confirm how you have connected the pot/attenuator?

Is chipamp an inverting topology?
Can you link to a schematic?
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Old 9th April 2009, 08:42 PM   #3
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The problem is that my volume pot (50k log) is measuring 14k when fully turned to zero. So i still here music.

The ground from pot goes to SG on PCB, same as input gnd.

I dont have schematic for Audiosector kit.
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Old 9th April 2009, 10:34 PM   #4
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50k becomes 14k at zero position? Either the potentiometer is broken or you connected it in parellel to the 20k input resistor and the wrong way round.

Here is your schematic. How can you build an amplifier without a schematic?
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Old 9th April 2009, 11:31 PM   #5
AndrewT is offline AndrewT  Scotland
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Default NIGC = DC coupled

A newbie should not be building a DC coupled amplifier.

Be very careful you don't burn out your speakers.
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Old 9th April 2009, 11:45 PM   #6
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Your basic schematic seems to be fine. Why don't you look through this thread: http://www.diyaudio.com/forums/showt...97#post1516297 Everything has been explained there.
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Old 10th April 2009, 01:16 PM   #7
ratza is offline ratza  Romania
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Linear pot, maybe?
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Old 10th April 2009, 07:27 PM   #8
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The pot is fine, it measures fine when disconnected.

The main thing i supspect is the grounding of the amp boards.

Peter - when using a centertapped transformer, the centertap goes to PG+, Then i only have 2 PG- and one PG+ left. Should i only use one PG+ per board and thats it?
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Old 10th April 2009, 08:13 PM   #9
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With CT transformer you need to modify the bridges; it's been explained here: http://www.diyaudio.com/forums/showt...83#post1196283
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Old 10th April 2009, 10:10 PM   #10
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Quote:
Originally posted by marcus7601
The pot is fine, it measures fine when disconnected.
Quote:
Originally posted by pacificblue
you connected it in parallel to the 20k input resistor and the wrong way round.
50k || 20k = 14,3k

Desolder the 20k input resistor and check again. If it works, replace the potentiometer with a 20-25k type for better CMRR.
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