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Aleph3 clone offset problem
Aleph3 clone offset problem
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Old 9th June 2018, 01:14 PM   #1
prakarn is offline prakarn  Thailand
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Default Aleph3 clone offset problem

Hello friends
I am a rookie, none electronic background. I am building Aleph3 clone based on an ebay PCB, All fet devices are matched. I changed R106 to 500R VR and R113 to 100K VR for adjusting DC offset and bias. A power supplies is about +/-23Vdc.
I face on Output offset problem. Right channel seems okay. It start at 70mv then I can easily adjust to 7 mV. But on left Channel it start at 3.7V. I try to adjust but not much effect. Moreover a LED will dim when turn too much.
I have checked many time on installed parts and quite sure they are correct as schematic. I measured voltage on both side to compare please see in picture. Please give an advice what I should do next. Thank you
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Last edited by prakarn; 9th June 2018 at 03:50 PM.
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Old 9th June 2018, 03:27 PM   #2
Zen Mod is offline Zen Mod  Serbia
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Aleph3 clone offset problem
you need more ohms at R108 position

both inputs shorted , no load , while setting
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Old 9th June 2018, 03:41 PM   #3
prakarn is offline prakarn  Thailand
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Thank you Zen MOD

No, I did not short input. I will do next time. Could I ask you why it happen on left channel only? and how much ohms should be added?
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Old 9th June 2018, 03:51 PM   #4
Zen Mod is offline Zen Mod  Serbia
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Aleph3 clone offset problem
leave R106 fixed - 220R

replace R108 with series string of 220R fixed value and 500R trimpot

it happened due to difference in mosfet Vgs

as long as values are in 10% bracket , everything is fine

offset of +3V7 means that you need to open lower half mosfets just tiny bit more
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Old 9th June 2018, 04:07 PM   #5
prakarn is offline prakarn  Thailand
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thank you. I will do and tell you then.
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Old 10th June 2018, 09:29 AM   #6
prakarn is offline prakarn  Thailand
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Hi Zen Mod,
I did an mistake, The offset is actually on minus side, -3.7V.

The solution is still same as above, isn't it?

Thank you
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Old 10th June 2018, 11:08 AM   #7
Zen Mod is offline Zen Mod  Serbia
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Aleph3 clone offset problem
hoping that you didn't make any mistake now , let's say that you need to decrease value of R108

put 500R trimpot there and go
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Old 10th June 2018, 02:39 PM   #8
prakarn is offline prakarn  Thailand
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I tried by parallel 390R with 5K POT, set at max first. At first offset be decrease when decreasing POT until about -2.6V it would be start to swing to 17V and LED dim down.

At that condition I tried to turn R106 (now 500R POT) to increase offset more minus like -3.0V. It would be back to stable again.

Then I back to decreasing 5K POT more but again when offset be around -2.6V it would start to swing again. Then I repeated step by tuning R106 many time but seem no improvement , I stopped.

It will good idea to ask you before burning it. Now R108 is about 77R and voltage at Q103 drain leg is about 7.4V,the best offset I can reached -2.3V.
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Old 10th June 2018, 03:08 PM   #9
Zen Mod is offline Zen Mod  Serbia
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Aleph3 clone offset problem
I told you - put proper value fixed resistor in R106 position

slapping your self with both hands simultaneously is simply looking nidicolous

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Old 10th June 2018, 03:13 PM   #10
mpmarino is offline mpmarino  United States
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What is the thing over the '610s? (see subtle arrow)?

Is it shorting the tabs together?

Otherwise, I'd go through the ebay board trace by trace to check for errors, and maybe make certain ALL the fets are matched. I've built countless Alephs, and never had an offset issue if ALL the fets were matched VGS. edit - never needed a pot to adjust
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