Nelson Pass A40 Help

[Robbie010]

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?

Dropbox - Amplifier - Simplify your life

Thanks in advance.

 

[jan.didden]

Robbie your link doesn't open for me.

Do you have the original Audio Amateur article?

Jan

 

[Robbie010]

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 I’m struggling a little.

 

[jan.didden]

Do you have a schematic? I remember building it, should have the issue, which issue was it?

Jan

 

[Nelson Pass]

It's on the FIRST WATT site as a pdf

 

[drjbf1]

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.

 

[Robbie010]

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?

 

[jan.didden]

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

 

[Nelson Pass]

I don't recall complaints about the original....

 

[Zen Mod]

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

 

[Robbie010]

I don't recall complaints about the original....

😃

Is this “THE” Nelson Pass!!??

No complaints here, I bought this amp as a used, faulty item from Germany.

It took me a while to work out it was an A40, I’m new to DIY electronics.

Really enjoying working on the amp and learning lots along the way but whoever made this amp didn’t follow the instructions word for word which makes it difficult for a newbie like me to properly compare it to the original schematics.

 

[mpmarino]

😃

Is this “THE” Nelson Pass!!??

Yup - pretty cool, huh?

 

[Robbie010]

Nelson Pass A40 anomolies

Hello all,

The project amplifier that I picked up turned out to be a Pass DIY A40, which is great as there is lots of info for me to use when trouble shooting. However, I have noticed a few anomolies as the amp is not 100% true to the published information. Please see the main schematic in the link below.

I have noticed that R4 seems to be in the wrong place as it is between R2/R3 and the positive side of C2, rather than between the negative side of C2 to C4. Please see the small schematic and image below.

Would anyone be willing to take a look at the schematic I have drawn-up via the link below and tell me whether this is a mistake by the person that built the amp or if it is an intentional change to the circuit design?

Dropbox - Schematic - Simplify your life

Thanks.

 

[Nelson Pass]

It's of no consequence, being that the cap and resistor are
just in series in both cases.

 

[Robbie010]

It's of no consequence, being that the cap and resistor are
just in series in both cases.

Thanks Nelson!

I must say, I think it is great that you take the time to read the forum and answer these questions. After 40 years I’m sure you’re fed up of some of these questions and queries.

 

[Nelson Pass]

No, not really. I like the easy ones...

 

[Robbie010]

No, not really. I like the easy ones...

Yes, after reading through the numerous threads I can see there have been many queries over the years..... most far beyond my comprehension

As mentioned, with my amplifier there a couple of deviations from the published design, if I message you with a copy of my schematic would you mind having a quick look over it and checking for any potential design flaws?

 

[Zen Mod]

why bothering him , post schm here , somebody will chime in

 

[Robbie010]

Pass A40 Schematic Review

I have managed to draw up a schematic of my project amplifier and while it is most definately a Pass A40, it is not 100 true to the published information.

The amplifier uses two centre tapped transformers, one for each channel, it has a triac transistor in the power stage and a thermostatic switch, so I was hoping someone with the skill and knowledge would be willing to review the schematic and provide some feedback, maybe point out any potential design issues?

Thanks.

 

[jazbo8]

Please don't start multiple threads for the same topic.