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-   -   pass 20 watt class a (http://www.diyaudio.com/forums/pass-labs/42714-pass-20-watt-class.html)

amp_guy 17th October 2004 03:29 AM

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

Hoffmeyer 17th October 2004 09:38 AM

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 :bawling:

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."

Karl vd Berg 31st December 2014 08:01 PM

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

--

flg 31st December 2014 08:36 PM

C1 should be 10uF or so, IMHO :D 10 years ago...:rolleyes:

Karl vd Berg 31st December 2014 09:24 PM

Yes, I know. Was just curious.

OTOH it is still available at PassDiy.com page...

AndrewT 1st January 2015 09:53 AM

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

woody 1st January 2015 10:35 AM

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.

AndrewT 1st January 2015 11:54 AM

Quote:

Originally Posted by woody (Post 4169619)
..........................
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?

woody 3rd January 2015 01:54 AM

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

Karl vd Berg 3rd January 2015 02:37 AM

Quote:

Originally Posted by woody (Post 4171603)
...

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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