compress/expand with one knob?

[Paul Perry (Frostwave)]

I'd like a single control that goes from heavy compression, through none, to expansion. I can't see how to marry the NE571 compress & expand circuits from the app notes together..
Now I'm thinking a ldr style voltage divider, using TWO ldrs, the bottom one for the compression part of the range & the top one for the expansion, with some logic to switch them over at the half-way point.
Must be an easier way, though!

[aaronkessman]

not exactly what you want, but maybe easier to do?

mark hammer has made john hollis's flatline compressor into a punchline expander with a flick of a switch. Not in one knob maybe, but its simple...

:|

[Johan]

how about if you place one LDR in the feedbackloop of an opamp and the other from the negative input to ground. then take an linear 1M pot with the pin 1 to ground pin 2 to the negaitve input and pin 3 to the output, so that each side of the pot from the wiper is parrallell with a LDR. now depending on where the pot is in its travel, different things will happen...this just comes to me as I write. never tested or even thought through yet, but something interesting should hapen..dont you think?..since the LDRs are allways parrallelled by part of the pot...oh I dont know..my head is hurting me...

Johan

[Mark Hammer]

Okay.  Let's combine the one knob goal, the Flatline/Punchline conversion and Johan's suggestion.

Essentially, the gist is this: if you can use a single control (single pot, or if need be dual ganged) to range between max influence of LDR/rectifier-output on gain-reduction in one direction and max influence of LDR/rectifier-output on gain-*increment* in the other direction, you'll have what Paul wants.

My guess is that it would take some trial and error and be cumbersome.  I suppose a better method might be to use a single control element like a 3080, and tinker with the direction of envelope signal (i.e., inverted/non-inverted).

[puretube]

output of env-foll., or LFO > one LED to plus, one to ground (with Rs)...

[lovekraft0]

...if you can use a single control (single pot, or if need be dual ganged) to range between max influence of LDR/rectifier-output on gain-reduction in one direction and max influence of LDR/rectifier-output on gain-*increment* in the other direction, you'll have what Paul wants...

How about using a guitar dual "blend" pot? It has the characteristics you need, since each half of the rotation affetcs only one pot (that is, CCW from ctr changes only pot A, while CW from ctr changes only pot B). Here's the info:
Blend Pot Wiring

The only possible downside is the audio taper - OK, I'll leave the implementation up to you guys with the talent!  :wink:

[stm]

Just start with the following circuit:



Which is contained and explained here, in the Application Note 174 by Phillips:

http://www.semiconductors.philips.com/acrobat_download/applicationnotes/AN174.pdf

And the datasheet (not so useful for design) is here:

http://www-us2.semiconductors.philips.com/acrobat/datasheets/SA571.pdf

Regrds,

STM

[Paul Perry (Frostwave)]

OK! Thanks everyone! now MY head hurts, too
And yes, i've got the NE571 data sheet, but it's the most opaque thing i ever saw :oops:  :x good one stm, how did i miss it...!

[stm]

Paul,

Sorry, I posted the link to the Datasheet instead of the Application Note 174, where the figure belongs.  Now I corrected the links above.

STM