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Old 29th November 2020, 08:03 PM   #271
EUVL is offline EUVL  Europe
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I guess you meant R12a / R14a ?
https://www.diyaudio.com/forums/atta...s-img_2305-jpg

They are there to allow you to fine trim dynamic current ratio.
UDNeSS, or You don't need Semisouth's

The values provided in the schematics are based on the FQA FETs used by Morde.
You can change the current ratio by changing the ratio of R11//12 and R13//14.
Best to play with it in Spice first to get an idea.


Patrick
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Old 29th November 2020, 08:43 PM   #272
dBel84 is offline dBel84  United States
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Thanks Patrick.

I thought that was the case based on the table you added to the PDF. I was just surprised to see that R11a is listed in the BOM and based on the schematic it is in parallel to R11


Click the image to open in full size.

Click the image to open in full size.


Seems I cannot load images from my phone, will just leave it out and see if i need to trim. I used the same FETS as Morde so let's hope it all works without additional tweaking.

..dB

Last edited by dBel84; 29th November 2020 at 08:48 PM.
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Old 29th November 2020, 08:51 PM   #273
EUVL is offline EUVL  Europe
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R11a in the schematics = R12a on the PCB.

I did not have space to place it under R11.
And was too lazy to change document.


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Old 29th November 2020, 08:53 PM   #274
dBel84 is offline dBel84  United States
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I would never call you lazy, appreciate all the selfless work you do for our amusement
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Old 30th November 2020, 03:07 AM   #275
dBel84 is offline dBel84  United States
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boards populated and power supply assembled, now just to tap the heatsinks and we will have another UDNess in the wild

Click the image to open in full size.


I added a trimmer for setting the bias, hopefully I have given myself enough range.
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Old 30th November 2020, 07:49 AM   #276
EUVL is offline EUVL  Europe
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If that trimmer is parallel to R11,12,13, or 14 then it will not survive for long.
The easiest way to trim bias is to adjust R6.

R12a, 14a should be used to adjust current ratio.
And they want to be 0.4W minimum.
See Appendix 2.1 of post #89.


Patrick
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Old 30th November 2020, 01:23 PM   #277
dBel84 is offline dBel84  United States
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Hi. Yes, trimmer is parallel to R6. Allows adjustment from 150 to zero ohm.
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Old 1st December 2020, 11:25 PM   #278
EUVL is offline EUVL  Europe
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So you drilled through the PCB to install the trimmer for R6.

I strongly suggest you to replace it later with fixed resistors after trimming.
The power resistors next to it will get hot.
And trimmers will drift with temperature, even if they survive the heat.


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