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Old 17th October 2004, 03:29 AM   #1
amp_guy is offline amp_guy  United States
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Default pass 20 watt class a

has anyone built the 20 watt singe ended amp Nelson Pass designed abot 1977.
I wondering if c1 the input coupling cap is really supposed to be
1000uf. After a look thru my parts collection I find that I have
nearly every thing to build one
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Old 17th October 2004, 09:38 AM   #2
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Hi amp-guy.

I have, and yes, according to the original article it is 1000uF.
I did enjoyed my 20W ClassA right until my voltageregulator fried not only itself, but the ampboards as well

My ClassA amp

Good luck building it, but don't take lightly on the Pass' warning: "For this amplifier, there is no such thing as too much heat sinking for the output stage. Extravagance in this area is no vice, and good ventilation is similarly very important."
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Old 31st December 2014, 08:01 PM   #3
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Default 20W/8Ω - Class A Mosfet (Nelson Pass 1977 project)

Would be really interesting if someone else did it successfully...

https://www.passdiy.com/project/ampl...ss-a-amplifier

--

Last edited by Karl vd Berg; 31st December 2014 at 08:04 PM.
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Old 31st December 2014, 08:36 PM   #4
flg is offline flg  United States
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C1 should be 10uF or so, IMHO 10 years ago...
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Old 31st December 2014, 09:24 PM   #5
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Yes, I know. Was just curious.

OTOH it is still available at PassDiy.com page...
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Old 1st January 2015, 09:53 AM   #6
AndrewT is offline AndrewT  Scotland
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R3 is 10k.
if you use 1mF for the input filter you end up with an F-3dB of around 0.016Hz

The 10uF suggestion brings the F-3dB to around 1.6Hz. Much more sensible in limiting the passband and yet letting ALL the the lowest bass frequencies through.

1000uF would have to be an electrolytic and then you have to choose between Polar and non polar and polar+ralop and nonpolar+ralopnon.
Using 10uF allows you choose a plastic film. Then it's either FKP, or MKP or MKT
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Old 1st January 2015, 10:35 AM   #7
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If you refer back to Walter Jung's clasic article Picking Audio Capacitors he has
a little discussion about electrolitics used as coupling capacitors. It seems that
the closer you get to F-3 the more distortion you have in the signal. I read this
article more than 30 years ago I don't remember figures but i believe he showed
that even at 10x the F-3db point the distortion was still quite high and a lot of it
showing up in the higher frequencies hence the subjestion to use a cap much larger
than you need.
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Old 1st January 2015, 11:54 AM   #8
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by woody View Post
..........................
a little discussion about electrolitics used as coupling capacitors. It seems that
the closer you get to F-3 the more distortion you have in the signal............. he showed
that even at 10x the F-3db point the distortion was still quite high and a lot of it
showing up in the higher frequencies hence the subjestion to use a cap much larger
than you need.
yes.
1.6Hz is a factor of /12.5 less than 20Hz.
0.16Hz is /125 less
0.016 is /1250 less.

are we still talking about 10times?
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Old 3rd January 2015, 01:54 AM   #9
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I located that article and the effect I was talking about isn't as pronounced as I seem to remember. But heck I read that almost 40 years and many many beers ago. Even though its an old article it's still a worthwhile read. This is a link to the first part of the article but near the conclusion of the second part of the article a mention of runing the coupling cap into a lower resistance ofton makes the cap sound better. http://waltjung.org/PDFs/Picking_Capacitors_1.pdf
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Old 3rd January 2015, 02:37 AM   #10
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Quote:
Originally Posted by woody View Post
...

Even though its an old article it's still a worthwhile read. This is a link to the first part of the article but near the conclusion of the second part of the article a mention of runing the coupling cap into a lower resistance ofton makes the cap sound better.

http://waltjung.org/PDFs/Picking_Capacitors_1.pdf
That was the idea, woody. I like reading those old articles. Thank you for the link.
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