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FS: one pair of Nelson Pass A40 PCBs
FS: one pair of Nelson Pass A40 PCBs
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Old 30th October 2019, 04:10 AM   #31
gary s is offline gary s  Australia
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Thanks again Alex, shouldn't you be asleep by now?
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Old 9th November 2019, 02:30 PM   #32
alexcp is offline alexcp  United States
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FS: one pair of Nelson Pass A40 PCBs
I have been asleep for longer than expected But I did test a bunch of LM334, DN2450 and J113 as current sources in place of Q11.

LM334 produces 2mA with the R9 value of 33ohm, as per the datasheet. Reducing R9 to 22 ohm increases the current to 3mA or so.

DN2540 gives 2mA with R9 around 680ohm, with small variation among a bunch of these MOSFETs from the same bag. Note there can be larger variation between different batches, which may require adjusting R9 value, but then the current is not that critical in this particular application.

J113 gives 2mA with R9 around 800ohm, with the standard value of 750ohm making the current somewhat higher. Again, there is some small variation between JFETs from the same bag, with more expected for transistors from different batches. Note that the J113 is rates at 35V of drain-gate voltage, which is just a small margin below A40's -32V power supply.
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Old 17th November 2019, 02:12 PM   #33
alexcp is offline alexcp  United States
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FS: one pair of Nelson Pass A40 PCBs
Default Example build

This is an example build of the A40 front end with 2N5551/2N5401 bipolar transistors, a DN2540 as Q11 (R9=680ohm, I(D1,D2)=2mA), RN55 resistors, Nichicon FW capacitors. Tested so far without the output stage ("NFB" connected to "-DRV" to close the feedback loop).
Attached Images
File Type: png A40 1.png (244.9 KB, 228 views)
File Type: png A40 2.png (268.3 KB, 196 views)
File Type: png A40 3.png (242.9 KB, 231 views)
File Type: png A40 4.png (250.2 KB, 186 views)
File Type: png A40 5.png (245.4 KB, 220 views)
File Type: png A40 6.png (243.5 KB, 31 views)
File Type: png A40 7.png (239.9 KB, 29 views)
File Type: png A40 8.png (246.2 KB, 41 views)

Last edited by alexcp; 17th November 2019 at 02:17 PM.
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Old 8th March 2020, 02:51 PM   #34
alexcp is offline alexcp  United States
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FS: one pair of Nelson Pass A40 PCBs
Default LTSpice simulation of the A40

Attached is an LTSpice simulation of the A40 amplifier. The models for the output Darlingtons are from ONSemi, everything else comes with LTSpice. R11 had to be adjusted to keep the quiescent current of the output stage at approx. 1.5A as per the original article.

LTSpice can be downloaded from Analog Devices.
Attached Images
File Type: png A40 sch.png (77.3 KB, 79 views)
Attached Files
File Type: zip A40 LTSpice.zip (3.1 KB, 11 views)
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Old 16th March 2020, 11:36 PM   #35
Zentronics is offline Zentronics  Canada
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Quote:
Originally Posted by alexcp View Post
Attached is an LTSpice simulation of the A40 amplifier.
Awesome for sharing that, I really appreciate it. I love running a build through SPICE before assembly. Very good learning when these things combine...


It has been decades since I built my last set of PASS A40 mono block amplifiers. Covid-19 has slowed and cancelled some of my parts orders from abroad. I have everything on hand to build a dual mono block except the XFMR for each channel's pwr supply and Q11 - J111 in my build version. I think the J111's may show up tomorrow but my power supply transformer order looks like it was cancelled from Europe. I may need to shop local?


These Alexcp PCB's are looking great so far. Thank you Diy Audio!
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File Type: jpg IMG_20200316_191759.jpg (905.4 KB, 72 views)
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Old 16th March 2020, 11:46 PM   #36
alexcp is offline alexcp  United States
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FS: one pair of Nelson Pass A40 PCBs
As stated in the A40 part substitution article, you can use a 10kOhm or so resistor instead of Q11. The amp will have more noise but otherwise will work the same.
Quote:
You can substitute this part with a convention N channel JFET which has been tested to act as a current source in the circuit with R9. The value of R9 has been chosen so that the JFET passes about 3 mA or so (not critical). You can also substitute in a monolithic constant current source if you can find one, rated at about this current and 40 volts.
If you canít find any JFET for the current source, or are impatient while waiting for one to arrive, you can simply use a resistor as a poor manís current source by putting a 10 Kohm resistor where the JFET goes, connecting between R9 and ground. This will work, but is noisier that a constant current source. This can be improved with an electrolytic capacitor (say 100 uf @ 50V) between the negative rail and the junction between R9 and the 10Kohm resistor, with the positive pin of the capacitor between R9 and the 10K resistor.
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Old 20th March 2020, 08:04 PM   #37
Zentronics is offline Zentronics  Canada
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Here is one channel of Sanken 2SD2389 x 3pcs and 2SB1559 x 3pcs. Gonna run it on +/_25 VDC rails. I'm kind of stuck at home so I guess I will build this thing. I forgot to tap the holes for the thermal cut off when I was machining the threads for the darlingtons, I'll do that tomorrow when I go to work on the other heatsink.


I think the Sanken's will be a great match to the A40?
Attached Images
File Type: jpg Heatsink.jpg (962.4 KB, 56 views)
File Type: jpg 2SB1559.jpg (653.9 KB, 51 views)

Last edited by Zentronics; 20th March 2020 at 08:06 PM.
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Old 21st March 2020, 12:12 AM   #38
alexcp is offline alexcp  United States
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FS: one pair of Nelson Pass A40 PCBs
The output stage certainly looks promising
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Old 25th March 2020, 09:03 PM   #39
Zentronics is offline Zentronics  Canada
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I did end up using J111 for Q11 and adjusted the leads accordingly.
My heatsinks are not massive and considering supply line voltage variations, I settled on a bias of just over 1 amp per rail. Should provide near 30 watts-ish of class A from 27 volt DC rails.
R11 thus became two resistors in parallel to yield 1890 ohms-ish.
Each heatsink is idle at 42įC-ish

When it goes in the case with the actual power supplies, then I can take away the 'ish' factor... and hopefully soon.

When I get it all together, I may put a full description and complete build pictures in the Pass department of DIYAUDIO but I must give one final nod to alexcp. Cheers & thanks.
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File Type: jpg IMG_20200325_140127.jpg (813.5 KB, 32 views)

Last edited by Zentronics; 25th March 2020 at 09:09 PM.
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