Limit/ Compress switch?

I want ta add Limit/Compress switch to my vari gain (DIY) compressor, The side chain is similar to collins 26J. Post amp is similar to LA-1D, Any one can help me how to do it? Many-many thanks..

Any one? Please... :

that would be the bias voltage on the sidechain, which is already adjustable in the 26J.  Threshold is tied to ratio.  The 26J does not go above 6:1, I think, but you may see room in the bias voltage divider to go higher.

Thanks you very much Emrr, I'm really newbie in diy thing, So should I increase or decrease the two resistor value (R20,R21=510k) to get higher ratio? is that 100k pot(R25) is threshold control?

the pot is the voltage divider in question, along with any resistor on it's higher voltage side.    It's not something you'd find useful as a switch; expanding the pot range makes more sense. 

so  if i change to bigger pot value, can i get higher ratio? what should i call that pot? thershold control? or ratio control? or what? sorry for my stupidity...thank you

It is both threshold AND ratio....AND output level, if you have no adjustable output attenuator.  Bigger pot value; I don't have the circuit it front of me.  Don't know what room exists for that possibility there.  Post it if you can so everyone can see what we're talking about.  There's probably a fixed resistor as part of that overall voltage divider, and if so, that total value is the range you have to work with. 

Thanks Emrr, this is the schematic of Collins 26J, What i want is adding limit/compress mode switch like moonley varimu...many thanks for your help...

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There's 300K there to play with.  0-100K = compress, ?-300K = limit, assuming output has enough power to overcome the bias voltage.  It's not the actual 300K value, it's the specific DC voltage bias at various points across that 300K.  I don't know what Manley does, but if the output level remains about the same when switched, then they are doing something else. 

Thanks emrr, I will play with that. Just one more question if you don't mind, what the effect if i change the r20 and r21  to 270k like in LA-1D?

Someone step in; I don't recall ATM.  Lower Z increases potential current flow.  If ratio control all the way at the top of R24, then V4 could see full current drive of PSU, and also possibly have noise interaction with PSU/output stage.  r20-21 isolate and current limit.    I can't comment on the difference between the two.  You see many different values there; probably a matter of scaling.    Try all these things and see; faster than asking here. 

That's what i did, try and see, i'm very poor on theory so i really don't know what i'm doing by doing that. Thank you Emrr....