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Nelson Pass A40 Help
Nelson Pass A40 Help
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Old 8th February 2019, 04:43 PM   #1
Robbie010 is offline Robbie010
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Default Nelson Pass A40 Help

Hello all, new forum member here.

I am in the middle of a DIY project with a Nelson Pass A40 Power Amplifier. The amp has some age and was outputting a distorted sound so I decided to overhaul it and re-cap it etc. I have just gone through the list of resistors on the parts list and am struggling to match everything up. Please see the image attached comparing the published parts list and the parts I have noted in the amp.

I have a number of questions and any help would be appreciated:-

There is a variable resistor on my circuit board, has this been used in place of Resistors 11 & 12?

I am struggling to locate Resistor 5?

I cannot locate Resistor 21 on the circuit or output stage?

Resistors 16, 17, 18 & 19 seem to be out of spec, is this an issue?

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Thanks in advance.

Last edited by Robbie010; 8th February 2019 at 06:10 PM.
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Old 8th February 2019, 06:02 PM   #2
jan.didden is offline jan.didden  Europe
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Robbie your link doesn't open for me.

Do you have the original Audio Amateur article?

Jan
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Old 8th February 2019, 06:13 PM   #3
Robbie010 is offline Robbie010
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Quote:
Originally Posted by jan.didden View Post
Robbie your link doesn't open for me.

Do you have the original Audio Amateur article?

Jan
Hi Jan,

Thanks for the reply, I think ive fixed the link to the images.

I do have a copy of the original in pdf but the layout of the circuit does not match mine so Iím struggling a little.
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Old 8th February 2019, 06:57 PM   #4
jan.didden is offline jan.didden  Europe
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Do you have a schematic? I remember building it, should have the issue, which issue was it?

Jan
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Old 8th February 2019, 07:10 PM   #5
Nelson Pass is offline Nelson Pass  United States
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Nelson Pass A40 Help
It's on the FIRST WATT site as a pdf
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Old 8th February 2019, 07:16 PM   #6
drjbf1 is offline drjbf1  New Zealand
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http://www.firstwatt.com/pdf/art_a40.pdf

Schematic and original parts list does not show an R21, R5 should be 10k not 100R

R11 & R12 replaced by pot to adjust bias, I presume.
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Old 8th February 2019, 08:25 PM   #7
Robbie010 is offline Robbie010
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Quote:
Originally Posted by drjbf1 View Post
http://www.firstwatt.com/pdf/art_a40.pdf

Schematic and original parts list does not show an R21, R5 should be 10k not 100R

R11 & R12 replaced by pot to adjust bias, I presume.
Thanks. Thats what threw me, why have R19 & R22 but not R21!!?

The other potential issue is the values of R16, 17, 18 & 19, against the original spec. Could this be a potential issue?
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Old 8th February 2019, 08:36 PM   #8
jan.didden is offline jan.didden  Europe
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Quote:
Originally Posted by Robbie010 View Post
Thanks. Thats what threw me, why have R19 & R22 but not R21!!?
That's not unusual, sometimes a part gets deleted during the design process but normally one would not renumber everything else.

R16-19 are the same in the circuit and in the BOM. Same with R11, 12. R5 = 10k both in schematic and BOM.

I think you should follow that article, I build it at the time and don't remember any issues, worked at first switch on for me. Build 4 units.

Jan
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Last edited by jan.didden; 8th February 2019 at 08:40 PM.
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Old 8th February 2019, 08:53 PM   #9
Nelson Pass is offline Nelson Pass  United States
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Nelson Pass A40 Help
I don't recall complaints about the original....

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Old 8th February 2019, 08:58 PM   #10
Zen Mod is offline Zen Mod  Serbia
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Nelson Pass A40 Help
damn ...... how time flies

someone was already Designer of circuits in Yore 1978 , while someone had just 11 , not even thinking about girls, let alone all these manly things

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