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LJM 12-2 ver4 gnd short to signal gnd.
LJM 12-2 ver4 gnd short to signal gnd.
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Old 30th September 2020, 11:58 AM   #1
diegooo1972 is offline diegooo1972
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Join Date: Jul 2020
Default LJM 12-2 ver4 gnd short to signal gnd.

Hi all


My first post here so please be patient.


I have built my first diy ampli with LJM 12-2 ver4 amp. It's decent but doing FFT i see harmonics on 50hz, 150hz,250hz and so on.

I was trying to solve this but I found that speaker ground is shorted to signal ground. I wonder if this is normal. My main network work at 50hz but strangely to me I don't see 100hz harmonic. I think this is related to main 50hz but why I don't have 100hz harmonic ? 150hz and so on are even bigger then 50hz harmonic.
I did the same FFT on my NAD 902 and I don't have this problem.

Can someone help me clarifying this ?
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Old 30th September 2020, 04:27 PM   #2
duncan2 is online now duncan2  United Kingdom
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Join Date: Mar 2013
Location: Scotland.
Have a read of-


In most of the systems, we eliminate even order harmonics saying that their effect is negligible. How can this be physically interpreted?
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Old 30th September 2020, 05:30 PM   #3
diegooo1972 is offline diegooo1972
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Join Date: Jul 2020
I read it. You suggest I have cancellation of 100hz harmonic from another opposed harmonic present on my ampli ?
I'll read it again and think about it better.

On that case my problem is still 50hz.
The problem is that I can't get rid of it and I think the cause is that in this ljm 12-2 the ground is shorted to signal ground.

What do you think ? I don't understand why they did this in any case.
Is it a problem of this specific board ? I don't see that short between ground and signal ground in the riginal schematic.
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Old 1st October 2020, 06:17 AM   #4
Rallyfinnen is offline Rallyfinnen  Sweden
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Join Date: Jan 2017
They are all linked on LJM boards, but there are separate ground planes on the PCB. If I remember correctly, it's a via-hole linking them on the L12-2. You can drill it out and connect a ground break resistor between them if you have hum problems, but first thing to do is to look for ground loops in your setup, and minimize loop areas.
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Old 1st October 2020, 11:08 AM   #5
diegooo1972 is offline diegooo1972
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Join Date: Jul 2020
I'm going to dig in to that. Just found the hole you talk about but it's shorted to input signal. It's just up the connector and I see only that hole in the board. About ground loop i can exclude it atm. I don't have HUM I can hear but just pretty hign harmonic on 150hz and 250hz and so on going down. They are about the same db of the first 1khz harmonic of the signal I feed in the amplifier.

I don't see them on the NAD so I decided to investigate.



Really appreciatte.

I'll invesitgate that thanks.


I think I found the hole you talk about. It's up the signal connector but it's grounded to signal ground too and then to speaker ground.
I don't have HUM I can hear but the 50hz harmonic are quite big. Same db as the first harmonic of the 1khz signal to fft the amplifier.
I don't see that on my NAD so I decided to investigate if this is ljm 12-2 project related or to my specific amplifier project. Atm I can't find any ground loop related but can't exclude that at all.


Really appreciate.
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