KrIVIUM2323
Well-known member
Hi,
I've been studying Rupert Neve design of output stage for some times now and i can't get my head over them.
I spent a bit of time looking at Focusrite Isa range and Amek 9098 but thanks to Whoops who recently posted 33114 schematic i would like to start from it before complexifying it further with Isa and Amek stage. It make a nice historical approach to R.Neve design evolution too imho.
https://groupdiy.com/threads/neve.44877/page-4#post-1165516
Here is how i understand things on 33114: it's a non inverting (discrete) 'opamp' ( ba440) with feedback taken from tertiary winding of output transformer.
Reference Level before output stage is -15db, post ba440 unbalanced output ( pre transformer) is -8db so there is 7db gain in the stage.
As it is non inverting stage, gain is equal to 20log( 1+(r64/r62))= 20log(1+(1600/1200))= 7,33db.
So far so good, from this we can deduce the tertiary feedback winding is 1:1 ratio. From brochure we know psu is +24v and max output level is +26db so here again we can assume the balanced output winding have a 1:2 ratio ( 24v rail should be able to provide +20dbu, 6db gain provided by transformer).
So out transfo is 1:2/1:1. Because i searched a bit i know it's the same ratio used in Focusrite Isa 110/red range and in the 9098 desk. And it makes sense R. Neve used the same transformer over the different successive designs ( why change something which work?).
What i don't get is the value of cap used in the feedback loop (c64, 470uf) and it's role. I suppose it's here to limit low end freq of feedback loop but how to calculate it/what parameters it define?
Could someone explain further and if i'm on right track?
Thanks in advance!
I've been studying Rupert Neve design of output stage for some times now and i can't get my head over them.
I spent a bit of time looking at Focusrite Isa range and Amek 9098 but thanks to Whoops who recently posted 33114 schematic i would like to start from it before complexifying it further with Isa and Amek stage. It make a nice historical approach to R.Neve design evolution too imho.
https://groupdiy.com/threads/neve.44877/page-4#post-1165516
Here is how i understand things on 33114: it's a non inverting (discrete) 'opamp' ( ba440) with feedback taken from tertiary winding of output transformer.
Reference Level before output stage is -15db, post ba440 unbalanced output ( pre transformer) is -8db so there is 7db gain in the stage.
As it is non inverting stage, gain is equal to 20log( 1+(r64/r62))= 20log(1+(1600/1200))= 7,33db.
So far so good, from this we can deduce the tertiary feedback winding is 1:1 ratio. From brochure we know psu is +24v and max output level is +26db so here again we can assume the balanced output winding have a 1:2 ratio ( 24v rail should be able to provide +20dbu, 6db gain provided by transformer).
So out transfo is 1:2/1:1. Because i searched a bit i know it's the same ratio used in Focusrite Isa 110/red range and in the 9098 desk. And it makes sense R. Neve used the same transformer over the different successive designs ( why change something which work?).
What i don't get is the value of cap used in the feedback loop (c64, 470uf) and it's role. I suppose it's here to limit low end freq of feedback loop but how to calculate it/what parameters it define?
Could someone explain further and if i'm on right track?
Thanks in advance!