Amplifier with nonlinear gain curve?

[barefoot]

How would I go about designing an amplifier that has a rising gain curve like this?

I'd like to be able to program the "knee" and slopes, similar to a compressor.

Thanks!
Thomas

 

[bcarso]

Instantaneous or ~envelope, i.e., an expander?

 

[barefoot]

As instantaneous as possible.

 

[bcarso]

Sounds like, though, that you wouldn't want an outright memoryless nonlinear amp, which would cause mucho distortion of audio, but rather just a fast expander. So take a compressor and monkey with the feedback (or feedforward) so that above your threshold, your fast-as-possible detector is now used to increase the gain of your VCA or other controlled element rather than decrease it (like a compressor).

I would imagine there are some schematics floating around of d b x or even D*lby system blocks. In fact that meta thread with old manuals etc. I think includes some JBL/UREI relatively recent units that have expander modes alongside the compressor ones.

 

[PRR]

RDH4 Chap 16 pg 4

 

[clintrubber]

So take a compressor and monkey with the feedback (or feedforward) so that above your threshold, your fast-as-possible detector is now used to increase the gain of your VCA or other controlled element rather than decrease it (like a compressor).

Or if you don't want to mess inside your compressor, perhaps you can get away with this:
summing say 150% direct & 50% compressed signal (but of opposite polarity)
You'll need a non-digital compressor but that won't be a problem.