Super Simple Opto comp design - ELOP-inspired

If you look it "from the circuit input", the 100K pot "sees" the 50K load (from LM386). But, the 100K pot can be seen as two resistors, one in series with the load (386 input) and the "other" in parallel with the 386 input. IOW, if you want to squash, the actual load impendance will get heavier because you will turn the pot more "towards 10", so the "386input load" will be fully in parallel with 100K pot res. with no series resistance from the pot (cca. 33K-ish) - IOW you will load-down the 4K7 resistor from input wiht this 33K.. If you want only to compress gently, you won't turn the pot "to 10", but f.e. "to 2", meaning that some 70K series resistance from the pot will be combined with the parallel resistance of "386input" and the remaining 30K from the pot, loading-down the signal path less than the case just described above.... that's it, basically - fairly generalized but you get the picture..

Hi TV,


    Thank you very much for that explanation. NOW I understand! I hadn't considered each "load" other than in isolation. DOH!



        Many thanks again!



        ANdyP

Hi,


    Now I've got to thinking . . . Would a DOA(say 2520?) instead of LM386 deliver enough current to drive the Vactrols and the output? And could I put a meter in series(with a shunt) to derive some indication of GR? It could form part of the total of R4. I don't need much accuracy, just a needle waggling around . . . .


              Kindest regards,


              ANdyP

Not really. Because the "sidechain" is connected in such a way that some feed-forward action will take place ("1K" before the actual vactrol).

But then, with DOAs you will be (IMHO) better off playing with some other known topology.

For example, look here - you could adapt the "flatline" compresor circuit to a "shunt" variant like this "super-simple-opto" comp.
http://www.groupdiy.com/index.php?topic=38727.msg477540#msg477540

In the end, I think you will find yourself tweaking the "response" of your particular homebrew variant in any case anyway.

What I meant to say is that the original purpose of this particular circuit was (IIRC) to have a pure passive signal path, and so the low values of series-pass resistors were used - hence the need to have vactrols in parallel. But then again, I could easily have missed something important and obvious here... correct me if I'm wrong.

Hi tv,


  Many thanks for the reply. I get it being totally passive audio path. I was thinking if it might be possible to do it ALL in one go! I am fired up to have a good play about with this one. . . . . And I thought that having the two vactrols was to eliminate the need to rectify the signal as per normal, since the antiparallel LED's in the vactrol will stop each other from popping due to reverse voltage! I live and learn, all thanks to the generosity of people like you. You are most kind.

 
    Kindest regards,



      ANdyP

Actually,


  - Sod it! I've got an API 325 card already converted to standard EQ pinout, a bunch of Silonex NTL-32, and I am going to see just how well it works with 2520.



   Lash-up here we come! If it doesn't do anything usable, I'll just have to make it two stages, with an LM386n-4 to do the dirty work. I am actually relishing (eventually!) mucking about with this. I just might learn something! Something tells me that there is something in this . . . . Simplicity . . . .


   ANdyP

From the OP:

The inspiration for the design comes from discussion about the Manley ELOP. I have not seen the Manley schematic, however, and I imagine that the similarity ends with "driving a pair of vactrols with an LM386". Regardless, that basic idea stayed stuck in my head for months, and I finally had to give it a go.

Click to expand...

Well, what you describe would then be very vaguely similar to a solid-state a.p.i-m.a.n.l.e.y elop <cough>.

Hi,


  I love the idea of being "very vaguely"!!!!



    Kindest regards,


      ANdyP

….another lost in the "fogs" project ?  :'(