1977 20w class A little help please.

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Hello I need some help with the 1977 nelson pass 20w amplifier.
I am confused about the transformer.
In schematic it's 14v +- and in bom its writed signal 56-12
Signal 56-12 is 2x6.3v......
And someone can say me if I can use 2n5878 in place of 2n5877 because I don't find enough 2n5877 from the same brand....
The 2n5878 are PMC brand anyone have experience with this brand ?
Thanks
 
Official Court Jester
Joined 2003
Paid Member
you have picture/schematic of PSU , why on earth you need BOM specification of xformer?

xformer secondaries are 28Vac center tapped , per picture on top of page 5

so , two independent secondaries of 28Vac (CT) , where one goes per channel

that will give you something around +/-17Vdc for rails

and say that you need 250VA xformer

2N5878 is same part as 2N5877 , with higher voltage breakdown



PMC - dunno , doesn't ring a bell ...... but I'm probably younger than these parts

:rofl:

edit: you can probably use decent 2N3055 instead
 
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Thanks for the reply.
Sorry im not enough experienced with transistors for know what to choose as equivalent transistors if the reference from schematic is not the same as what is still available.
I follow your advice and order 16 rca 2n3055.
Let's try the two ! Rca 2n3055 and Pmc 2n5878.

About BOM and transformer I just follow the instructions from the pdf I find in pass diy site.....
In schematic it's yes -+14v but in bom you find signal 56-12 transformer...... Signal 56-12 is -+6.3v transformer....
Im not expert sorry for asking something that look stupid.
It's my first class A transistors amp I build it before trying alept F4 or F5 turbo.
:)
 

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About transformer yes I know they are two secondar or one center taped.
28v ac with middle point or two 14v ac.
Im sorry for my poor english it's not easy to explain well.
About transistors it's really not easy to find good brand from all this old transistors like 2n5877 from Motorola..... I need 16 pices...
Like the 6 RCA1A15.... Impossible to find them I have hardly find Motorola 2n5682....
I still need heatsink you have some advice for the size I need to use for one channel without using a cooling fan ?
Thanks
 
This was a good amp, but I think a smaller Aleph may be a simpler first build?

I find this one really simple....
The only problem is to find old stock of good brand vintage transistors....
I like old and experience myself what my ears find the best between old and new technology...
Until now the reference in beautiful amp for me is Quad 2....sounds beautiful but not strong enough....
 
The one and only
Joined 2001
Paid Member
Older designs with modern transistors work just fine, usually better.

Being a pretty simple circuit, not too fussy about exact parts.

Pick some nice bipolars from On-Semi or other from distribution, matching
or exceeding voltage, current, and dissipation limits.

:cool:
 
Older designs with modern transistors work just fine, usually better.

Being a pretty simple circuit, not too fussy about exact parts.

Pick some nice bipolars from On-Semi or other from distribution, matching
or exceeding voltage, current, and dissipation limits.

:cool:

Thank you for your advice Mr Pass.
I heard in France your mono blocks with preamp on Dynaudio confidence c2 it was really wonderful for me.
With the huge Proac and musicalfidelity huge amplifier it was the two best system I never heard until today.
Thank you
 
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