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Old 14th November 2012, 08:08 PM   #311
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Yes, I change the Vr1-2 for trimpot 20 turn-off (10K), the offset value down much and is easier to control,... but was still very unstable depending of the temperature

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Old 14th November 2012, 09:17 PM   #312
oz7aff is offline oz7aff  Denmark
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In this Thread there have been talked about some errors on that PCB, have you corrected them ?
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Old 14th November 2012, 10:37 PM   #313
bkercso is offline bkercso  Hungary
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Quote:
Originally Posted by oz7aff View Post
Ok, I see that the schematic is the same as used in the blue PCB. There is an error on it C23 and C24 shall have + to GND.
C23/24 see AC voltage so it don't matter. You should use non polarized audio cap (or two series cap) here.
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Old 15th November 2012, 11:48 AM   #314
oz7aff is offline oz7aff  Denmark
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No, there is no AC voltage on C23/24. Standart elektrolytics are all right, but they shall be connected with + to GND.

By the way, at startup I measure about 100 mV Offset, and after 5 min. the Offset is down to 5 mV and stays in that area.
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Old 15th November 2012, 06:37 PM   #315
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I suppose that this pcb have some problem... I'm thinking buy the new version (blue PCB).

What do you think?

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Old 15th November 2012, 07:09 PM   #316
mkusan is offline mkusan  Croatia
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Originally Posted by blackdod View Post
I suppose that this pcb have some problem... I'm thinking buy the new version (blue PCB).

What do you think?

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I believe, apart from the color of course, they are the same, so there is no point doing that.

There is a build guide with pictures made by Miles Campbell in this thread, did you have a look at it and measured various voltages on the PCB as advised.

It would be also helpful if you post pictures of your build (both sides of the PCB).

Hope you will sort it
Marko
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Old 15th November 2012, 07:44 PM   #317
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Tomorrow I will take some pictures of this PCB

By the pictures, I suppose that the components are the "best" quality on the blue PCB ¿?

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Old 15th November 2012, 10:17 PM   #318
bkercso is offline bkercso  Hungary
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Quote:
Originally Posted by oz7aff View Post
No, there is no AC voltage on C23/24. Standart elektrolytics are all right, but they shall be connected with + to GND.
Hm, really, sorry! I misunderstood the schematic... But I used 2 series caps here and the sound is clear, detailed.
According to simulation there is really about -1V. I will not take apart it now for a measurement...

Quote:
Originally Posted by oz7aff View Post
By the way, at startup I measure about 100 mV Offset, and after 5 min. the Offset is down to 5 mV and stays in that area.
The offset is almost the same at my both JLH amps. I built these according to Miles Campbell's build guide, except I used multiturn cermets and trimmed the offset.

Last edited by bkercso; 15th November 2012 at 10:22 PM.
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Old 16th November 2012, 07:47 AM   #319
mkusan is offline mkusan  Croatia
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Originally Posted by blackdod View Post
Tomorrow I will take some pictures of this PCB

By the pictures, I suppose that the components are the "best" quality on the blue PCB ¿?

Regards
No, I don't believe that better quality parts come with newer blue PCB kit,
e-bay sellers use what is available to them and cheapest at a given time.

Replacing input caps with something better like WIMA as oz7aff suggested is worth doing, C23/24 could probably benefit from better parts as well.

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Marko
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Old 16th November 2012, 05:14 PM   #320
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Click the image to open in full size.

Rear:

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And I don´t like this (around C32 and C26)

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