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Old 2nd December 2006, 08:58 PM   #1
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Default high voltage on speaker terminal

Can someone help me with my first gainclone? I am using LM3875-kit from Audiosector. On right channel speaker terminal everything is fine. On left however i have -38V !! this is from my 25V transformer after rectification. How can this be?

I dont use a chassi so speaker ground goes to OG on pcb?
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Old 2nd December 2006, 09:51 PM   #2
Nuuk is offline Nuuk  United Kingdom
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Make sure that you have both power rails (positive and negative) connected properly.

If you can, measure the voltage on the pins of the chip but be very careful that you don't cause a short!

It is more than likely that one rail is not connected!
The truth need not be veiled, for it veils itself from the eyes of the ignorant.
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Old 3rd December 2006, 10:12 AM   #3
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ok, i will recheck my solderings.
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Old 5th December 2006, 06:33 PM   #4
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Ok, i have resoldered +V and -V on the rectifier and amp-pcb still same results. I need some help here...
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Old 5th December 2006, 06:38 PM   #5
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Originally posted by marcus7601
Ok, i have resoldered +V and -V on the rectifier and amp-pcb still same results. I need some help here...

Is the voltage on the chip output pin the same as on the speaker terminal? What is the voltage on each chip power supply pin? Measured?

Jan Didden
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Old 5th December 2006, 07:02 PM   #6
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No on output i have 0V

On -V powerpin on chip i have the same as on terminal -35V

I get -35V on output pin
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Old 6th December 2006, 06:54 PM   #7
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Hi, I had a quick look at the data sheet from National. On the faulty amplifier can you measure the voltage between ground and pin 7 as well as the voltage betwen ground and pin 8 of the LM3875.

We will establish if this is input related.
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