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FS: one pair of Nelson Pass A40 PCBs

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FS: Nelson Pass A40 PCBs

UPDATE: a new batch of PCBs arrived! See post #17 below for photos of a bare PCB, post #28 for the schematic, post #33 for photos of a sample build.

The price is $10 per pair of boards plus shipping. Estimated USPS shipping prices are $5 for the continental U.S., $12 for Canada, $16 for Europe and Australia, and probably for most of the other countries. PM me if interested.

For details on the amplifier see the relevant article on FIRSTWATT web site and another one for substituting modern parts. The PCBs are double sided and follow the original 1978 design. Each 3"x3" PCB is for one channel front end only (power stage is off board), so you'd need a pair for stereo.
 

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What a great project! This was my very first DIY amp - I chose this one because Nelson's article about it was available on his webpage and this PCB was for sale on the Sound Colony web page.

My write up is a bit old and definitely crude by today's standards, but I have some notes about my a40 build for anyone wishing to build this amp here: my a40 build. Still use this amp fairly regularly and it sounds great.
 
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What a great project! This was my very first DIY amp - I chose this one because Nelson's article about it was available on his webpage and this PCB was for sale on the Sound Colony web page.

My write up is a bit old and definitely crude by today's standards, but I have some notes about my a40 build for anyone wishing to build this amp here: my a40 build. Still use this amp fairly regularly and it sounds great.

Eric, thank you for sharing your experience and for the link! I could not agree more - after 40 years, A40 is still a great project, remarkable in both its simplicity and the sound quality it achieves. I built an A40 myself some years ago but have had an extra pair of boards hanging around. Hopefully someone (with a pair of large heatsinks) will build another one :)
 
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Update: The boards have been SOLD. There was more than one person who wanted the boards, but I only had one pair.

If I get four or five people interested, I may be able to order another batch at about the same price. Until then, I am attaching the gerbers here. (I wanted to post them to my blog here on diyAudio.com, but could not do it for a reason unknown.)

I am also making the board available for order on OSHpark. Note that OSHpark is rather more expensive; they manufacture three boards at the price of US$5 per square inch of design, so with these gerbers, you'd get three boards for $45.

Acknowledgment: The A40 design, including the artwork of the PCB, was created by Nelson Pass. I only took his article (referenced in the first post) and produced an electronic version of Nelson's PCB artwork with the sole purpose of allowing PCB manufacture.
 

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I would be interested in a few boards. Probably 4 pieces.
I have an A40 that I built long time ago, got it out recently and found about 8 volts of d.c. on the output.
My boards are rather crude as they were made using the laserprinter film and iron on method.
The A40 can be made using lateral mosfets instead of the darlingtons, I have not tried it but others have. Would like to try it as I have a boxfull of the old Hitachi's (2sj49 and 2sk134)

I actually had started laying out a new board when I saw your post here. Didn't think anyone else had any interest in this old design.
Deceptively simple design and sounds great.
 
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Yes, you're right - thank you for spotting it.

There are two minor differences.

First, the original board uses the Source-Drain-Gate (SDG) pinout for the JFET, while my board uses the GSD pinout, which matches the LM334 current source that I used. As Nelson Pass wrote in "Part Substitution", his second article on A40: "note that the source pin of the Jfet is attached to R9, and there has been some confusion about that, with some different pinouts on the 2N5248".

Second, I added a Zobel network to the board, which has that extra GND terminal. Its other end is connected to the FB terminal (the right side of R5). The whole thing can be left out.

I attach the artwork copied from the original "The A40 Power Amplifier" article by Nelson Pass and the artwork of my board for comparison.
 

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Here is the document from Audio Amateur about the PCBs and the schematic.

Thank you Eric! Yours is a curious document.

It concerns the PCB that (I understand) was being sold in 1997. The amplifier input network includes the usual (although some say harmful to stereo imaging) low pass filter. The original article says:
... we start with C1 forming an input rolloff filter in conjunction with the typical 600 Ohm to 1k Ohm source impedance. Lower frequency rolloffs can be achieved using higher capacitance values and higher source resistance values.
However, the board mentioned in the document you provided was made inconsistent with the schematic and the above description: the rolloff filter was formed by C1, the source impedance, and R1. Not a big deal, but still an inconsistency.

My boards (which I still have a few left, BTW) are consistent with the schematic, including the input rolloff filter and the Zobel network, which is on the schematic but not on the original board.
 
Ah - I did more of a side-by-side comparison between the two circuits that you posted and spotted the differences. They appear to be fairly minimal. In the boards that I have, there was a trace error for which a correction was issued. At some point, Nelson indicated that the difference in the PCB from Audio Amateur was mostly irrelevant. Since I was unable to open the Gerber file you posted, I wasn't sure of the source for your board.

Either way, this is still a nice amp!
 
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New batch of A40 boards arrived

UPDATE: a new batch of A40 PCBs arrived. Unlike my first boards, these are double sided (eliminating the need for one wire jumper) with plated-through holes and solder mask and silkscreen on both sides.

These boards also follow the original article more closely; the only difference is the pinout of Q11, providing for the use of LM334 as the current source.

The price is the same: US$10 per pair of boards plus shipping.

PM me if interested.
 

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Attached is the schematic for the board only. The only difference from the original article's is that Q11 is replaced with an LM334 current source and R9 is adjusted accordingly.

Also, here is shared project at Mouser. It has all parts for one channel, including the output power Darlingtons, but not the power supply. All parts are currently available at Mouser. The project is just to give some guidance - if you wanted to build A40 for some time, you probably have some parts already.
 

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